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The early development of the thyroid gland in the dog with especial reference to the origin and position of accessory thyroid tissue within the thoracic cavity.

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Department of Histology and Embryology, Cornell University, Ithaca
The early development of the thyroid gland in the dog is
of especial interest because of the precocious appearance of
branching cell cords from the thyroid plate. This peculiar
mode of development leads naturally into two main lines of
investigation which have been made the subject of this paper.
The first of these was to determine what light this peculiar
manner of development might throw upon the interpretation
of the thyroid as an endocrine gland. The second was to
determine the manner of origin of accessory thyroid tissue
within the thorax especially that upon the aortic arch.
The thyroid gland of Petromyzon has been shown by
Schneider (1879) to arise from the endostyle organ during the
metamorphosis of the larval Ammocoetes. Marine ('13 a )
has shown that one or possibly two of the five types of
epithelial cells of the endostyle organ take part in the formation of thyroid follicle: The other cell types degenerate.
Dohrn (1886) found pharyngeal grooves in early development
of rays and sharks which he believed to be remnants of the
endostyle organ. Studies of fishes, amphibians, reptiles, and
birds reveal no endostyle organ but show that the thyroid
develops as a median downgrowth of the pharyngeal floor.
Studies of various mammalian forms show a similar origin
with the possibility in some animals of addition to the median
thyroid material by the inclusion of the ultimobranchial body.
The direct origin of the gland from the endostyle organ of
Ammocoetes, the lowest form having a thyroid, as well as its
mode of origin from the mid-ventral pharyngeal epithelium of
higher forms indicates that the thyroid was never an exocrine
gland with an excretory duct.
The secretory polarity of the thyroid has been and is still
a problem. Bensley ('16) stated his belief ". . . .that the
thyroid cell exhibits a true reversal of polarity. " Norris
( '18) found intraglandular spaces in the human thyroid which
he suggested might ". . .appear in response to a tendency
to reproduce the ancient lumen or duct of the ancestral
gland. " These intraglandular spaces were invaded by mesenchyme and a reversal of the epithelial mesenchymal relation
was thus effected. Such a mechanism would make Bensley's
hypothesis unnecessary. Cowdry ( '21) believed the primary
secretory polarity was toward the follicular cavity and not
toward the mesoderm as Norris suggested. Cowdry ('22) in
a study of the position of the Golgi apparatus finds that
temporary physiological reversal of secretory polarity occurred in the thyroid. Recent work by Gillman ('34) who
also studied the position of the Golgi apparatus finds that
even temporary physiological reversal as determined by this
method does not occur. The mode of development of the
thyroid in the dog strongly indicates that the primary secretory polarity of the cell is toward the center of the follicle.
In embryological literature there are many references to
the presence of accessory thyroid material located above the
level of the thyroid gland. No developmental study dealing
with thyroid tissue found caudal to the thyroid seems to have
been made in any form. The occurrence of accessory thyroid
nodules within the thorax of dogs is well known. They are
commonly found in the anterior mediastinum from the thyroid
to well caudad upon the aortic arch. Muller (1896) and Kohn
(1895, 1896) give a careful description of thoracic thyroid
tissue. Wolfler and Wagner (1879) found accessory thyroids
about the size of a pinhead quite constantly upon the aortic
arch of dogs. Halstead (1896) and Marine ('32) find accessory thyroids within the thorax almost constantly in the
dog. The latter author states that thyroid tissue may be
found as far caudally as the esophageal opening in the
diaphragm. The present study describes the manner in which
thoracic thyroid material reaches its definitive position.
The prenatal material used in this study consists of fortyone series of dog embryos of various breeds ranging from
4 to 28 mm. crown rump length and cut in the customary three
planes. The study of the embryos was supplemented by the
gross examination of a number of puppies2 and adult dogs.
Many pieces of accessory thyroid tissue were cut and mounted
serially. Serial sections of a number of puppy and adult
thyroid glands were prepared and stained with haematoxylin
and eosin.
For convenience of description the early development of
the thyroid gland in the dog will be considered in four stages.
I t is obvious that the division is purely arbitrary and that
a certain amount of variation and overlapping must occur.
The first stage will include all embryos in which the
thyroid is in the form of a thickened plate of pharyngeal
epithelium. The second stage will include embryos which
show the development from the plate of branched cell cords
and the formation of a narrow stalk by which the thyroid still
remains attached to the pharyngeal floor. The third period
will include those stages which show the lateral expansion,
plate formation, and partial descent of the unattached thyroid
'This investigation was carried on under the direction of Dr. B. F. Kingsbury
to whom I wish to make grateful acknowledgment.
'1 wish to thank Prof. J. A. Dye of the Department of Physiology for placing
five thyroidectomiced puppies at my disposal.
material. The fourth period includes the stages during which
the ‘caudal descent ’ to approximately the definitive position
of both the thyroid and the heart is completed as well as the
great expansion of the lateral thyroid lobes resulting in their
fusion with the ultimobranchial bodies. During this period
the aortic sac and associated vessels undergo a relative shift
caudally into the thorax thus loosing their intimate relation
to the thyroid.
First period. The thyroid anlage at the earliest stage
considered is a thickened area of the epithelium in the midpharyngeal floor extending from slightly caudad to the furrow
formed at the mid-ventral junction of the mandibular arches
to the caudal limit of the aortic sac (fig. 3). The thyroid
plate is in intimate relation to the aortic sac. The plate
is thickest near the cephalic edge and projects ventrally into
the angle formed by the union of the first pair of aortic
arches with the aortic sac. A similar thickened portion of
the plate projects into the angle formed by the union of the
first and second arches with the aortic sac. The thyroid plate
is slightly depressed in the median portion (fig. 5); this
slight concavity gradually fades toward the cephalic and
caudal edges of the plate. The cells on the surface are
columnar and seem to be laterally compressed. The surface
of the anlage is ragged and irregular; this appears to be due
to the squeezing out and loss of some of the cells (fig. 4).
I n slightly older stages the plate thickens and buds of
cells more or less separate from one another may be distinguished. The buds which project lateral to the aortic sac
between the first and second aortic arches are particularly
marked (fig. 6). These are formed immediately under the
depression in the pharyngeal floor caused by the medial ends
of the furrows of the first branchial pouches. The rapid
growth of the anlage sometimes causes its surface to become
convex and project into the pharyngeal cavity.
Secorcd period. The small buds which earlier began to
grow down from the thyroid plate elongate rapidly during the
second period and form branched cell cords which retain
tlicir attt~chmentto the plate. A s the dowinvard cxtciixion of
cell corcls proceeds the pharyiigcal surface of’ the thyroid plate
decreases rapidly. Tlie morl)Eiology of the thyroid illaterial
~v-hilcstill rather broadly at tacliecl (fig. 7 ) to the pharyngeal
floor is shown in a photograph of a niodel of a 6-inm. eiribryo
(fig. 1). The first pair of aoi*ticarclies have ‘movctl out’ u p i i
I’ig. 1 I’liotograph of ti niodel of :I 6-nini. embryo showing thc mueh br:mclied
thyroid attached to the phnryngcal floor at thc levcl of the medial rnds of thc first
poue11 furrows. A peculiar bud of tissue is present at the lercl of the second
pouch furrows. The second arch arises froin the anterior surfacr of thc aortic sac
~ r n dp:isses hetwccn thj roid cords. Tlie first aortic. arch arisc.8 a short distance
distally along the second. The third, fourth, and sixth aortic arches a r e shown
arising from tho aortic sar.
the secoiid and distally have become broken up into small
irregular vessels. The second arches arc rather small vessels
which leave the aortic sac and pass forward and laterally
through the branchiiig cell cords of the thyroid. Iii slightly
older stages the tlttaclimciit of the thyroid to the pharynx has
become narrowed to a slender stalk which is situated at the
point of jiiiiction of llic caudo-medial lirrilns of tlie first pouch
flll.i*owFS. 1;s11illly th (1 th yroi tl stttlk ( S O - C ~ I ~ ~t ChC~~ ~ * ~ > ~ l ~ ) ~ s ; t l
tlud ) is solid with ;i siiiglc layer of cells rutlially :ii*i*tiiigvcl
ahout ti pol eiiti;il lunieii. However, iii certain inststnccs iiii
acatual lumcii is prcsent 21s in figure 8. Also the stalk niay he
solid at the attachment poilit but liarc? n sinall cavity i i i the
thyroid material a short tlistaiicc. ctiuclad a s iii figuw 9.
Small groups of tliyroid cells frecyueiitlp becoinc loosenet1
f‘iwni tlic ti1)s of thc tliyi.oic1 cords wliich arc in iiitiniittc rclatioii to tlic doi-sal, aiitcrior, and yeiit rul 1)ortioii of tlic aortic
sac (fig.10). Such picccs arc dcstiiiccl to descciitl with tlie snc*
aiid forni accessory thyroid tissue upon the ascc~i~liiig
p ~ r 1of the tr;iiisvers;c portion of the arch of the adult (lox.
Ocwisioiidlp siicli pieces a r c fouiicl upon the pulnioiiary a i ~ l i .
While the thyroid is still attaclietl to tlic pliaryiigcwl
cl)itlielium tlic c*auchl tips of some of tliv bruiiclictl wI1 cwds
bcc*onirltrtiiisformccl into cellular plates two cells in Ihickiiess
(fig. 11). This change iii thc structure of tlie thyi*oicl c*oivls
tho cxteiisive plate formutioil w7hicli occiirs during tlio nest developmental pcriod.
711 two ~ i n h r p o sa small bud o f cclls projects fi*oiii the
i r i c d i m fioor of thr l)liaryis tit the level of the mccIii-ll ciitls of
tht. sc,contl poucl1 flli*l*ows(figs. 1 , 7 ) .
7‘71 i d period. During Ihis clevcloymicntal epoch llic thyroid
mutwial cspaiicls latcrdly ; its latcral portions then sharc in
tho cephalic and Ititel-a1 cxpausioii of this port ioii of tlic iicck.
The most cautltll ciitls of the thyroid coids a1.c thus rliroctc>tl
hit crully t h c h i i u p - a r d mid f t m l to form tlic i q ) p ( portioiis
tho 1:itcr;il thyroid I o h of tliv ;idinlt (fig. 2 ) . ‘I’lie i i i o i ~ l
nictliaii niatcrial i-cii~ains<*losclyadjacelit to thc aortic szic
a i i d sliarcs in its ctiuclal descent. ‘l‘hc nicdial i ~ y i o nis tlius
dcstiiicd to form a ti.ansitory isthmus. Not infrcclueiit1;v thc
he obscrrcd iwojecting cephdad from
small thyroid stalk 1~1;ty
tlic isthmic rexion. 111 soiiie cases it is a small cdlulai~cord
tliscoiincc~tctlfroni thyi*oiti matt.i*itil.
Biiriultaneouslp with tho Ijrocess of lateral i i i i t l wj)lialic
ex1)atision, the thyroid coi*cIsai-c traiisforrned into 1)latcs two
Fig. 2 3’1gu1es1 to 5 :lie :I tli:igiiiiiiiii:itic interpretatioil of the growth p ~ o c c s s
iii\olvcd in the formation of two crll thick platcs in tJw th~roicl of the dog.
1) T h ) roid plate slightly clcprcssc~d. 2 ) Buds grow cuudad from the thyroid pl:?tc>.
3 ) Buds rapid13 elongate whilr, tlic surface area of the plate decreases. Arrows
iirdicnte the direction of growth sliiftings. 4 ) Thyroid is attac1rcb.d to the
l)lraryngeal floor ~ J Ja i i a i i o w stalk. The line in tlic ccJnter of the stalk and c o ~ d h
rrpresents tlie original surface of tlie thyroid plate. B ) Thr roid corcls have
Iwcomr tranxforiiicd into two ccll thick plotcs. The line rrprcrsents the original
sui f:ic*e of the thyroid platr mtl tlic interiial sur
of thc future th? roid follicles.
Figure 6 is a tlingram of t h r aortic and pulnronary arches and associ:ited wsscls
i n :I 28 iiirn. clog cbrrrbryo. The 1inc.s :i(*iossthe :rrc*hrs lirnlt the rrgion withut which
:iccvmorj thyroid tissue mag Ire piesent. The stippled rcgion indiwtes t h e area
witlnn which nccrssory tliyro~tl tissucl is most com1non. A., aortic ;jrvil. K.,
I~rachiocephalic artery. T,.C.C., left ~ w ~ i i ~ n carotld.
R.(’.C”., 1 ight conininn
raiotid. K.S., right sul)cl:iriwn. L.R., left subc*la\inn. P., pulrnorrar~awh. P..4.,
pu1nion:irj artrrieq.
(&ellsin thickness (fig. 13). The plates rapidly become cu1q)etl
t i h u t the common carotid arteries (as they may now be
callcti) aiid tlie ccplialic eiid of tlie aortic sac (fig.12). During this l)ei.iod small clusters of thyroid cells 11i~l;v be fonntl
r.aiIic3r. cwiistaiitlp low clown upoii tho i*ai)idlp~loiigtitiiigaoi*tic
r i
1 h e w accessory ccll gronl’s may be fouiicl ul)oii tlit.
wiitral? lateral, or tIoi*sal (fig. 13) asl)ect of the sac; rarely N
ma)- he fouiitl upoii tlie portion of the pulmonary arc11
wliich forms from the aortic sac.
Period four. I h r i i i g thc fourth 1)ei’iotl tlic thyroid loscs
its close relation to the coinmoii cai-oticl arteries latcrally 1))i*e~soiiof Clieir sliiftiiig dorsally. The separatioii fl*om tlic
carotids is also aitlcd by the ra1)id espaiisioii of thc tracheii
aiitl l a r p s .
Tlie lateral lobes uiidergo a pci*iod of rapid
esl)aiisioii duriiig which they ftisc witlt the l i k c w i s ~rapidly
o q ) ~ i d i i i , qiiltiiiiobriliicliial hotlics. Tlic silt a i i t l assoc4titetl
veswls tlcscciicl from their iutimatc relatioii t o tliv
istliiiiiis i*e:ioii of the t Iiyroid. A s thch caudal shifting occurs
the (~oiiiiiioiicarotid arteries untlcrgo a rapid loiigituclind
growtli but mmaiii close togcltlicr at their caudal ends wlicre
they j o i i t a short coiiimoii triirik-tlic~ future bmchiocephulic
ai*tery--.r\-hicli in tiii.11 joins the aortic arch. This tlcsceiit is
1)ut a I’ortioit of tho growth shifting of the wliole region wliicli
has I ~ c i isumiiicd up uiiclw tlic tcrtii ‘(leswiit of tlic licari ’
1)no to tlic cnlqjiiig of tlic tliyi*oitl almit the aortic S H C ‘ tiiitl
iihso(+itcd 1)ortioris of tho commoii wmtids their clcscciit n i a ~
rcsult i i i pic~ccsof thyroid matcrial beiiig draggctl caudall?(fig. 14). Such picccls may become fiscd tit varying levels in
t I i c iiictliastii~um-riiore common1;v about the caudal ends of
coniiiioii cwrotitl art wits aiitl in the angle forriictl hy the
uiiion of t h e arteries with tlie 1,racliiocel~lialic artcyv
Acccsso~;\-thq’iwid m a t c ~ i a lw n s found upoii the aortic arc11
iii dl tlic tbmbiys of this 1)wiod. ‘I’liey are niost coiiimoii1~fouiitl lwtn-iwi the lmiiit of origiii of the I~rticliiocc~,lialic
:ii*tcii*yf i w n tlic :ioi*tic ;iixali a i d tlic lmirit of junc.tioii of the
descending limb of the arch with the pulmonary arch (fig. 16).
Infrequently thyroid tissue was observed upon the pulmonary
arch. There is wide variation in both size and number of
pieces present-as many as ten having been found in a single
specimen. They may be embedded in the muscular coat of the
vessel or in the loose connective tissue between the vessel
wall proper and the epithelium of the pericardial cavity as
in figure 17. The blood supply of the pieces is quite abundant.
The cell groups are usually rounded masses with the nuclei
peripherally placed. A potential and at times actual cavity
is present in the center. No signs of degeneration were ever
observed in any of the accessory pieces.
The early thyroid plate in the dog since it is apparently
quite similar to that of other mammalian forms will require
no extensive comment. The relation of the thyroid plate to
the aortic sac in the dog as indeed throughout the vertebrate
series leads one to suspect that the sac may be the inducer
of the thyroid plate. I n addition to the slightly concave form
it assumes-possibly a reflection of the hypobranchial groove
of Ammocoetes-the loss of cells from the anlage is of
interest. Certain cell types present in the hypobranchial
groove of Ammocoetes are known to degenerate (Marine,
'13 a, '13 b) at the time of formation of the thyroid gland from
the groove. Whether or not the loss of cells from the thyroid
plate of the dog bears any relation to the cell degeneration
of Ammocoetes cannot be stated. It might be more reasonable
to assume that the cells are squeezed out and lost as a result
of pressure due to growth of the mid-ventral portions of the
mesodermal arches and also rapid growth of the anlage itself.
If this were true however loss of cells from the epithelium
near the thyroid plate would be expected but this is not the
As development proceeds cords of cells project from the
ventral surface of the thyroid plate into the mesenchyme
cephalad to the aortic sac. During this process the surface
area of the plate is rapidly reduced until the much branched
thyroid remains attached to the pharyngeal floor by a slender
stalk (thyreoglossal duct). The shifting morphology of the
gland during this period occurs in such a manner that the
surface of the plate becomes the central part of the cell
cords while the mesenchymal surface of the plate retains this
relation (fig. 2).
In two embryos a small bud of cells projects from the floor
of the pharynx at the level of the second pouch furrows. The
possibility of accessory thyroid tissue arising at this point
must not be overlooked but it seems more probable that the
true explanation is a mechanical one. The expanding
pharyngeal epithelium probably causes a downward buckling
at the second pouch furrows resulting in a small bud of cells
being pinched off. No clue to the fate of such pieces has been
The thyroid stalk frequently contains a small cavity; the
surface of this cavity represents the peripheral surface of
the thyroid plate. The part of the stalk attached to the
pharyngeal floor would represent the boundary of the thyroid
anlage in the plate stage. Study of postnatal material indicates that epithelium at the edge of the thyroid anlage which
is not determined to form thyroid may be present in the
thyroid stalk. Certain spaces-& times somewhat cystic-are
occasionally found in the caudal portion of the thyroid lobes.
Such spaces may be lined by stratified squamous, columnar, or
pseudostratified columnar epithelium and bear an intimate
relation to the thyroid parenchyma. I n two puppies, whose
thyroids had been removed some weeks previously, a small
thyroid mass containing a persistent .thyroid stalk was found
upon the trachea at the level of the isthmus. One was lined
by stratified squamous epithelium; the others by pseudostratified columnar. Thyroid material had a lobular arrangement about the duct (fig. 18) ; many culs-de-sac opening into
the duct were lined by typical thyroid epithelium. The
presence of a patent thyreoglossal ‘duct’ does not mean that
the thyroid was once an exocrine gland. Similar ‘ducts’ may
be found associated with the thymus and parathyroids.
The thyroid stalk is best regarded as the mechanical result
of the mode of origin of the gland.
Intraglandular spaces in the thyroid gland of the human
were extensively studied by Norris (’18) who also reports
their presence in a number of widely different animals. He
states that a casual examination has shown them to be present
in the dog. The author has been unable to confirm this
observation on the dog. I n the central part of the thicker
masses of thyroid material the cells often are rather loosely
joined and potential or even small actual spaces may occur.
These spaces do not open and become related to mesenchyme
as Norris describes in the human and which by implication he
considers to be true for the dog. He speculates that the
purpose of the intraglandular cavities is to effect a reversal
of the relation of the secretory pole of the thyroid cell to the
mesoderm. Instead of facing toward the lumen of the
follicles the secretory pole would by this mechanism be
brought to face the mesoderm. Norris says that this would
make unnecessary the hypothesis of Bensley (’16) ‘ b . . . .that
the thyroid cell represents a true reversal of polarity.’’ Since
the author did not observe this process in the dog it cannot be
accepted for this form. A second suggestion made by Norris
for the human thyroid that b b . . .the immediate purpose
and significance of these cavities is to be found in the formation of the two-celled plates from which the follicles are later
to be derived,” would seem to be a more satisfactory interpretation. The precocious branching from the thyroid plate in
the dog apparently obviates the necssity for such a mechanism
in this form.
Studies upon the secretory polarity of the thyroid by
determining the position of the Golgi apparatus indicate that
the secretory pole is toward the central part of the follicle
(Cowdry, ’22). Recent work by Gillman (’34) indicates that
even temporary reversal of the secretory pole-as determined
by the Golgi apparatus-does not occur as previously believed.
I n the dog cell plates form simultaneously with lateral growth
along the third aortic arch. The writer believes that the
fundamental relation of the cells to the lumen-now represented potentially within the two-cell thick plates-is not
disturbed by the change in morphology (fig. 2). Actual
follicle formation, which will not be considered here in detail,
would therefore occur by a breaking up of the plates into
follicles whose cavity represents the original surface of the
thyroid plate.
While the branched thyroid is narrowly attached to the
pharynx and soon after this connection has been lost small
clusters of cells in close relation to the dorsal, anterior, and
ventral aspects of the aortic sac become free from the thyroid
cords. As development proceeds the position of these accessory pieces is a very material aid in following the growth
shiftings of the aortic sac. I n embryos in which the adult
condition of the aortic arch and associated vessels has for
our purpose been reached the greater number of pieces is
found upon about the middle third of the portion of the aortic
arch which extends from the origin of the brachiocephalic
artery to the aortic valves (fig. 2). The movement and
position of the accessories indicates that the aortic sac
rapidly elongates during the descent just as Congdon ('22)
found to be the case in the human. The brachiocephalic
artery, from which first the left and then the right common
carotid arteries arise (Ellenberger and Baum, '26 ; Sisson,
'11) is formed from the more anterior portion of the sac.
Rarely pieces of accessory thyroid material were observed
upon the pulmonary arteries and near the point of fusion of
the proximal portions of the aortic and pulmonary arches.
This finding supports the statement of Congdon for the human
. . little of the sac is removed when they (pulmonary
arches) separate off."
The cupping of the thyroid plates about the aortic sac and
adjacent vessels may result in caudal displacement of thyroid
tissue. Such pieces may be located a t any level of the anterior
mediasthum, more commonly in the region of the angle
formed by the union of the common carotid arteries with the
. .
brachiocephalic artery and about the caudal portions of the
common carotids. Thyroid tissue in the mediastinum could
very easily become closely associated with the rapidly expanding thymus I11 as stated to be the case in post-natal dogs
by Marine ( '32).
As previously stated a number of investigators have reported accessory thyroid tissue in the dog in approximately
the same positions as those outlined above for the embryo up
to 28 mm. Wolfler and Wagner (1879) comment upon accessory thyroid tissue being suspended grapelike from the
aortic arch by a stalk consisting of little more than an artery
and vein as in figure 19. Growth shiftings and displacements
combined with the variations which may occur, adequately
account for any discrepancies in position which might be
found between the embryo and adult.
1. The precocious formation of branching cell. cords from
the thyroid plate provides evidence in support of the hypothesis that the fundamental polarity of thyroid cells is
not changed during development. The central end of the
cells of a thyroid follicle is the primary secretory pole and
represents the pharyngeal surface of the thyroid plate.
2. Intraglandular spaces which open and are invaded by
mesenchyme have not been observed in the dog.
3. The thyroid stalk (thyreoglossal duct) may persist and
form certain cysts within or without the thyroid lobes. Such
cysts may bear an intimate relation to thyroid parenchyma;
they may be partially lined by typical thyroid cells.
4. Accessory thyroid tissue is almost constantly found upon
the aortic arch of the embryo as a result of the close association of thyroid cells to the aortic sac while the branching
thyroid is still attached to the pharynx.
5. Accessory thyroid tissue is found at variable levels in the
anterior mediastinum as a result of caudal displacement of
thyroid tissue by the descent of the aortic sac and adjoining
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~ ,8. 1896 An experimental study of the thyroid gland of dogs
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1913 b The evolution of the thyroid gland.
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A. 1879 Thyroidea von Ammoeoetes. Beitrilge z. vergleich Anat.
u. Entwieklungsgesch der Wirbelthiere, Berlin, p. 85.
Srsso~,S. 1911 A textbook of Veterinary Anatomy, p. 633. Saundere co.,
~ 1879
Die Aortendrnse und der Aortenhropf.
Medizinische Wochensehrift., Nr. 8, S. 198-199.
3 Mid-sagittal section through the thyroid anlage of a 5-mm. embryo. The
section is through the furrow a t the ventral ends of the mandibular arches.
The aortic sao is beneath the thyroid plate and co-extensive with it. x 84.
4 High power view of the thyroid plate of figure 3. Partially free cella are
present upon the surface of the thyroid plate. X 154.
5 Transverse section through the slightly grooved thyroid anlage of a 5-mm.
embryo. The surface cells are columnar and appear to be laterally compressed.
Cell fragments appear at the surface of the thyroid plate. X 154.
6 Parasagittal section of a 4-mm. embryo showing a large bud of thyroid
cells projecting caudad between the first and second aortic arches. X 84.
7 Mid-sagittal section of the broadly attached, much branched thyroid of
an embryo of 6 mm. A narrowly attached bud of cells is present in the mid-line
a t the level of the second pouch furrows. X 96.
8 Frontal section through the thyroid stalk of an embryo of 7 mm. The
first pouch furrow lightly touches the stalk on the left. The second and third
branchial membranes are shown on both sides. X 44.
9 Transverse section of an Sf-mm. embryo showing the hollow caudal end of
a disconnected thyroid stalk. The second pair of aortic arches are arising from
the anterior end of the aortic sac. X 154.
10 Transverse section of a 63-mm. embryo showing two small isolated pieces of
thyroid material upon the anterio-ventral surface of the aortic sac. The third
aortic arch is arising on the right and has just been passed upon the left. X 44.
M l I r Y l N CIlA\V+ORI)
Tr;insversr seution through the c:iud:tl tips of the a t t a r h r d thyroid of a
eiiibryo. Two of thc. tips h:lw l)cco111(1tr;insformtd into plates two cells
i n lhickiiws. The pwxinial ends of thc third aortic arelies are cut nil both sides.
x 5y.
12 S:igitt:il srction of a 10-mni. embryo showing thyroid platrs two cells in
thickness cnpped about the antrrior end of the aortic s:ic. Three small isolated
pieces of thyroid material arc present upon the dorsal side of the aortiv sac
just cc~pha1:iil of the prricardial cavity. The pulriionary artrry i s prrsent lateral
to tlir scerssory thyroid piecrs. x 541.
13 Froiit:lI sec*tion of n 124-mni. cwibryo showing extonsivc. thjroicl p h t w two
c ~ l l siu thickness along tlic aortic siie and coilinion earotict artcrics. The thymus
1 1 I :inlagc. shows kitrrally on the right and somewhat caudally on the left. X 40.
11 Frontal sertion of a 13-nini. rmbryo showing two pieces of thyroid matrrial
Iwiiig clraggetl c~iudallyby t h r drscent of the aortic m c and assnc*iated vessrls.
‘Fhymus TTI is nearly attaehed t o parathyroid TIT on the right; on the left
thyinus ITT is ninrc e:iud:rI. x 48.
1.5 ‘l‘r:tnsr(Jrs(t scction of ;I li-inin. rinbryo Rhowing threc pieces of thyroid
matcrial situated in the m g l c foriwd 1)y the union of the common carotid arteries
with the ~,raiirhioeepIialic artery. Thr right subclavian artery is cut longitudinally. Thymus I I I is present on both sides just crplialad of tlw perirmdial
cavity i i i i d is in rlosc association with thc accessory thyroid tissue upon t h r left.
x 54:.
16 Sagittal sec*tion of a 15-mni. cmhryo showing three pieces of accessory
thyroid tissue within thv perirardisl w v i t y qriitc low upon the descrnding pnrtion
of the aortic. :irvh, ‘1‘h)iiluR IT[, coiiirnoii rarotid artery, and the \:igus n e r w
show a t the uppcr lrft. x ,544.
1i Frontal swtion of a 13-mm. tmhryo showing B 1:rrge p i c w of iiccrssory
thyroid tissue low down upon the descrntliiig pnrtion of the aortic arch. The
aortic iiiid pulmoi~aryarches a r r fused at this level. T h r aecrssory thyroid is
partially surroundrd by :I sinusoidal blood channel. X 154.
18 Swtion through a pi(.(*<.
of thyroid tissue renioved from the anterior surface
of the tlachtls a t the lex el of tlir i ~ t h n i ~
ofi ~thv thyroid gland of ii puppy from
uhich tht3 thyroid 1ol)es had bcen reniovrd 6 weeks e:irlier. Thv thyroid stalk i H
liiird hy typiwl pscwlostmtificd columnar epithelium. Thyroid tissur i s zirranged i i i Iohules about a part of t h r duct. X 12.
19 Section of a piece of :ieerssory thyroid tissue found upon the clescending
portion of the aortic arch of :I puppy from which t h e thyroid lobm had hrrn
reriitnrd 6 wrrks previously. The thyroid ‘grape’ was suspended from thc
iioitic areh by a stalk cwnsisting of littlc morr than a sniall artrr:- and rein. x 12.
M l I r Y l N CIlA\V+ORI)
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development, cavity, references, thoracica, origin, tissue, early, especial, accessory, gland, within, thyroid, dog, positional
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