18 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
a“ spiel ee zz zz1!dEttterttrt 
No. | Spinal a ke ern Nerven, Eye Nose Ear Integument, Lateral Line Mouth 
I a ‚et . = 2 
2 a EN N ei u. 
3 — — — — Be — 
4 au m a == 3 er 
5 RE —— ee I == — 
6 = — _ — Transition from skin-epithelium to neur-epithelium = 
very gradual. Ectoderm at base of medullary 
plate 40 u thick, showing two layers of nuclei in 
cross section. Ectoderm over blastodisc thickly 
squamous. 
7 | Chordal portion of medullary plate — — — Ectoderm of blastodisc a little thicker than in = 
(myelon plate), .75 mm. long, .25 preceding stage (No. 6). 
mm. wide. Indented by a well 
defined median groove. 
8 "Medullary canal deeply V- or U-|Optic vesicles a pair of very shal- _ — Sharp transition between neur- and skin-ectoderm == 
shaped in cross section. Edges low depressions in anterior half of in the tail and trunk region. In the pharyngeal 
of medullary folds turned a little | cephalic plate, separated at median and cephalic regions distinction less marked. 
ventrally just posterior to cephalic |line by a low ridge. Ectoderm of tail and trunk region thickly squa- 
plate. mous, with large closely packed nuclei. In cephalic 
region epithelium 35 u to 55 u thick, with two 
rows of nuclei in cross section: ventrally, where 
it is somewhat thinner, one row of nuclei present. 
On lateral surface of pharyngeal region epithelium 
35 u thick, slightly thinner above. 
9. |Medullary canal broadly V-shaped | Optic vesicles form a single deep — — Much the same as in preceding stage (No. 8). = 
in cross section anteriorly. semicircular depression in anterior 
part of cephalic plate. A little 
evaginated laterally, ending ab- 
ruptly posteriorly. 
10 ‚Like preceding stage (No. 9). Optic vesicles expanded laterally, — — Much as preceding stage (No. 9). Cells of cephalic _ 
Medullary folds in contact an- 
teriorly for 52 sections of 6 p. 
and extending backward from pos- 
terior wall of fore-brain for three 
sections of 6 u. 
Ventral part of optic vesicles ex- 
panded laterally. 
epithelium have a clear inner zone. Epithelium 
ventral to pharynx low cuboidal. 
As preceding stage (No. 10). 
