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Tables for the normal development of Rana sylvatica.

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TABLES FOR THE NORMAL DEVELOPMENT O F
RANA SYLVATICA
ARTHUR W. POLLISTER AND JOHN A. MOORE
Department of Zolology, Columbia University
INTRODUCTION
To any student of anuran embryology it is probably uiinecessary to point out the need f o r a description of a series
of normal stages in the development. The usefulness of such
data is shown by the frequency of references to the widely
known, though unpublished, series of Amblystoma punctatum
by Harrison. The need for a similar ‘common language’ for
anuran development has been keenly felt in this laboratory
and in others f o r some time, and the present study is an attempt to meet this demand.
It has been found most useful to define all stages in the
series by at least two characteristics : the external morphology
and the age a t a constant temperature (hours after fertilixation at 18°C.). The changes in external form are relatively
simple up to the beginning of the growth of the tail. Accordingly no additional data, beyond age and form, are needed on
stages 1 to 17, which comprise the successive phenomena of
cleavage, gastrulation and neurula formation. I n the later
period of embryology, however, there a r e many simultaneous
processes that are readily identifiable externally. The most
obvious are the following: growth of tail; elongation of the
body ; growth of external gills, development of operculum ;
development of transparency of epidermis ; coiling of gut.
Furthermore, there are physiological features of development
that appear as the embryo approaches larval condition, eg.,
beginning of muscular movement and of swimming ability,
onset of heart beat, of gill circulation, and of tail fin circula489
T H E ANATOMICAL RECORD, VOL. 68, N O . 4
490
A. W. POLLISTEE AND J. A. MOORE
tion, and spontaneous hatching of the embryo. There are
interspecific differences in the time of occurrence of these
various features of later development. F o r example, the
operculum is nearly completed in a Rana pipiens embryo
which, as classified by other features, is at a stage when it
is just beginning to grow backward over the gills i n Rana
sylvatica. Because of such differences one cannot describe
a series of stages for even these two species of R a m , which
will be strictly alike in every one of the features mentioned
above. Hence it has been found more widely useful t o define
each of stages 18 to 23 by a group of characteristics.
The total length of' the embryo is widely used as an exclusive
basis for definition of stages in later development of Anura.
I n wide practice this is not very reliable because of variability of egg size, both iiitraspecifically and interspecifically.
A much more nseful criterion, likewise based on measurcmciits, is the ratio of tail length (measured from the anus)
t o the length of the remainder of the embryo, the body. Data
on total length are, nevertheless, included in the tables. They
are based on measurements of eggs of approximately average
size.
The study has been made partly from eggs collected in
nature and partly from those artificially fertilized after orulation had been induced by pituitary administration. There
are no differences in the development of eggs obtained by the
two methods. I n the r i c h i t y of New York, R a m splvatica
tisnally lays its eggs about the middle of March in water a t
a temperature of about 11". I n the laboratory eggs will
develop normally a t from 2 to 22". At the lower limit development is extremely slow, yet when eggs are removed to
higher temperatures they develop normally in every respect.
This unusual tolerance of such a low temperature is of practical advantage in that one can place eggs collected in the field
in heat-insulated containers with ice cubes and thus bring
the embryos to the laboratory in very nearly the stage in
which they were collected. Data on the time f o r development
at different temperatures between stages 3 and 20 are given
N O R M A L DEVELOPMENT O F R A N A SYLVATICA
491
in table 4. Such data are very accurately reproducible for
the early stages, but from stage 18 onward differences between embryos become increasingly significant. The total
elapsed time for development to stage 23 may vary as much
as 10% among embryos from a single batch of eggs.
DESCRIPTION OF STAGES *
Stages 1 t o 17 (table I)
1. E g g at fertilization.
2. Establishment of gray crescent area a s first external
evidence of development, sharply defined at 1 hour.
3 t o 6. Age given is time of appearance of cleavage furrow
that establishes the number of cells drawn for the stage.
7 to 9. Later stages in cell multiplication, best determined
by comparison of size of cells a t vegetal pole.
10. Appearance of dorsal lip.
11. Blastopore approximately a semicircle.
12. Complete blastopore (yolk-plug) stage.
13. Slit blastopore or neural plate stage.
14. Neural fold stage.
15. Beginning of closure of neural folds, beginning of
elongation. Cilia begin to rotate the embryo a t about this
stage.
16. Closurc! of neural folds comnleted.
17. Beginning of development o'f tail bud, marlicd off from
body by ventral notch when embryo is viewed laterally.
Stages 18 t o 23 (tables 2 and 3 )
The figures are ventral and lateral views of all but stage 22,
which shows dorsal instead o€ ventral aspect.
18. Stage begins with development of capacity for mnscular movement, i.e., simple unilateral flexure in response to
mechanical stimulation. This is very suddenly acquired and is
closely correlated with attainment of the external form
figured.
The stages illustrated and defined by age are of course esseiltially a n arbitrary
series of readily identifiable points in the coiitinuous process of development.
Defined in terms of these points, the total developmelit is comprised i n a series
of periods, each extellding from oiic stage until the nest. In practice the points
and the periods are iiot sharply distinguished and it is convenient t o describe
each period in terms of the stage t h a t initiates it. Thus the development from
onset of the heart beat to the beginning of gill circulation would be the period
of stage 19.
492
A. W. POLLISTER A N D
J. A. N O O R E
19. Time given indicates onset of heart beat which appears
very suddenly and is accordingly a most useful marker for
this stage. (Use of strong reflected light is necessary f o r
identification of this early pulse.) Tail equals one-third the
length of the body.
20. Beginning of circulation of blood corpuscles through a
capillary loop of anterior gill is closely correlated with gill
morphology, and is the best indication of the beginning of
this stage, Shaking will hatch embryos early in this stage;
they hatch spontaneously late in 20. Swimming ability is
acquired in the latter part of this stage. Tail equals one-half
the body length.
TABLE 4
H o u r s f r o m first cleavage required l o reach variozts s l a p s at different teniperatzlrrs
STAGE
3
4
5
30.4"C.
0
2+
5
ti
7
8
9
10
11
12
13
14
15
16
17
18
19
20
15.4"C.
0
1.3
2.2
3.0+
11
24
36
45
60
72
96
112
124
141
168
180
216
27.5
4.7
14.0
19.5
24.0
32.0
37.0
52.0
.XO
63.0
72.0
83.0
90.0
108.0
130.0
18.5'C.
0
1.0
2.0
3.0
3.5
9.5
13.5
16.5
21.0
25.0
33.0
3 7.0
42.0
47.0
55.0
62.0
72.0
87.0
21. Cornea becoming transparent so lens is visible as light
spot. Body and tail nearly equal in length.
22. Developmeiit of posterior beiid in gut niakes trunk appear asymmetrical from dorsal aspect. A few capillary loops
are fiinctioiial in the tail fin. Epidermis rapidly becoming
trans parent.
23. Triiiik and head liave rounded out and embryo assumes
trne lnrval or 'tadpole ' shape. Horny larval teeth developed.
Posterior limb bud identifiable. Opercixlar fold beginning to
develop. Active spoiitaiieous swimming begins.
X O R N A L DEVELOPMENT O F R A N A SYLVATICA
TABLE 1
493
494
A. W. POLLISTElZ A N D J. A. MOORE
TABLE 2
NORMAL DEVELOPMENT O F R A N A SYLVATICA
TABLE 3
495
3-96
A. W. POIJ,ISTEit
A N D J. A. MOORE
In t e r nal a n at o ~1 y
It has been found useful to have some meails of readily
identifying sectioned material in terms of the series of stages
described above. There arc, of c’ourse, no difficulties in doing
this with cnibryos up to the time of closure of thc n e u i d folds
TABLE 5
EYE
EAR
EYE
EAR
6 6
__
7 12
--
8 18
(stage 15). F o r recognition of stages 16 t o 21 table 5 was
constructed. The number of somites was counted from frontal
or sagittal sections. Tlie development of the eye and ear, as
seen in (TOSS sections, are shown by the series of drawings
made at a commoii magiiification by a projection method.
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