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On teaching the germ layers.

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Zoological Laboratory o j W a s h i n g t o n U n i v e r s i t y
The course in Comparative Embryology for premedical students as usually organized around the chick and the pig seems
to lack on the laboratory side material for certain fundamental
conceptions of the earliest development, without which the
student finds himself at sea when attempting to study later
stages, such as the 33-, 48-,and 72-hour chick and the 10-mm.
pig. That this lack is felt is evidenced by the introduction at
the beginning of many courses of the frog's egg or the egg of
some teleostean form for the study of the cleavage stages and the
rise of the germ layers. That these are sufficientfor the cleavage
stages is undoubted, but when the beginning student attempts
to unravel the germ layers of the amphibian embryo and keep
track of the history of the various cavities therein (segmentation,
archenteron, and coelome), his conclusion is apt to be that the
germ-layer theory is more theory than fact.
The writer wishes to call attention to Balfour's stages A to H
of the shark embryo as probably the best material available for
teaching the germ layers. In our Comparative Embryology
class at Washington University the cleavage stages are taught
from Amphioxus slides, also the blastula and gastrula. Then
the shark, Balfour's stages, are taken up in toto mounts and
transverse sections. The transverse sections are made up into
sets, each set containing a slide from each stage through the same
region of the body. For instance, one set (figs. 1 t o 5 ) is through
the middle of the body; another set gives the stages of the brain;
a third near the anterior end of the foregut, and the last through
the tail. Four sets are used at these four levels, others could be
added or substituted.
In the set herein illustrated there is never any confusion in
the student’s mind as to the identity of the germ layers, for
there is no confusion of the germ layers themselves. They are
perfectly separate and distinct, lie a t considerable distance
from each other, and the first three cavities of the embryo are
always definitely demarkated and followed from stage to stage
without difficulty.
The origin of the mesoderm from the entoderm of the gut is
conclusively shown, for this form at least. The schizocoele is
formed before the student’s eyes, and a few words of explanation
contrasts this with an enterocoele. The development of the
spinal cord in its earliest stages, the infolding of the neural plate,
its origin from the ectoderm, are all told simply, quickly, and
clearly in the set of five slides indicated.
The notochord is seen t o arise out of the dorsal midline of the
entoderm of the digestive tract, and its subsequent history is
all contained within this same set of slides.
The shark has been used here for several years and with uniformly good results. A diagrammatic series of figures could
hardly be devised that would more clearly show the steps of
these early and important stages, while these have the advantage
of being the sections of an actual animal. From this an easy
transition is made t o the chick and later the pig.
It has been the writer’s experience that, judging from the
final examination papers, this part of the work relating to the
rise of the germ layers based upon shark material has left one of
the most vivid impressions of the entire course. It has been
likewise his experience that when only amphibian material was
used, t’his remained the muddiest part of the course. By the
use of specific colors for each germ layer, the differentiation and
contrast is heightened, and the student soon comes to associate
automatically each germ layer and its respective color.
Balfour’s stages of the Elasmobranch embryo may be secured
from several dealers in such supplies, and are not expensive if
one cuts his own sections. To install sets of the stages outlined
above costs approximately one dollar per student. Since the
same slides are used year after year, in subsequent years there is
no expense, the student paying for all breakage.
Figures 1 t o 5 are camera-lucida drawings of the set of slides
through the midsomites. It has been found advantageous to
have the student study successively sections from all the stages
at the same level of the body, rather than study the different
levels in each stage before passing to the next.
Resumido por el autor, Frank Blair Hanson.
El coracoides de Sus scrofa.
El proceso coracoideo falta en el cerdo. Existe una porci6n
subcoracoidea que participa en la constituci6n de la fosa glenoidea
y es hom6loga del subcoracoides del hombre. El subcoracoides
se h a considerado como una simple epifisis, pero puede muy bien
corresponder a la epifisis del metacoracoideo, como ha indicado
Gregory. E n la escipula del cerdo hay solamente un centro de
osificaci6n en la parte coracoidea.
Translation by Jose F. Nonidez
Columbia University
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germ, teaching, layer
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