Fig. 7. 
Fig. 8 
Fig. 9. 
Fig. 10 
Fig. 12. 
Fig. 13. 
Fig. 14. 
Fig. 15. 
Fig. 16. 
Fig. 17. 
NEAL. — Nervous Syst. Squalus, 
PLATE 3. 
All figures drawn from cleared specimens and magnified 43 diameters. The 
neural tube seen in optical sagittal section. Neural-crest cells (ganglionic Anla- 
gen) colored in blue. In all cases the embryo is viewed from the right side. 
An embryo with 14 to 16 somites. Six vesicles only appear, and these are 
included within the limits of the cephalic plate. Neural crest (trigemi- 
nus Anlage) differentiated in the region of encephalomeres IJ and III. 
The mesodermic constrictions dividing somites 1, 2, 3, and 4 have ap- 
peared. Two visceral pouches (1 and 2) are in the process of breaking 
through the lateral plates (splanchnic mesoderm). 
Embryo with 18 or 19 somites. A thickening of the lateral zones in the 
posterior part of encephalomere III (not shown in figure) appears in 
sections of this stage. The acustico-facialis Anlage has become dif- 
ferentiated in the region of encephalomere V. The “anterior cavity ” 
(Platt's) begins to be cut off from the mesoderm of the Ist somite (van 
Wijhe’s). 
Embryo with 19 or 20 somites. A dorsal expansion now appears behind 
VI, as the first indication of encephalomere VII. Posteriorly it is 
bounded by somite 6. The constriction between van Wijhe’s 3d and 
4th somites has become obscured by the migration of cells from both 
sides of the constriction to meet the advancing Anlage of the acustico- 
facialis. 
Embryo with 21 or 22 somites. The conditions remain practically un- 
changed. 
Embryo with 24 or 25 somites. A ventral migration of neural-crest cells in 
the region of encephalomere VI has now begun, and the crest is now 
differentiated in the region of encephalomere VII and posteriorly. 
Embryo with 26 or 27 somites. A continuous neural crest extends from 
encephalomere V into the trunk region. Thalamic portion of the tri- 
geminus Anlage clearly differentiated. 
Embryo with 28 to 80 somites. At this stage all of van Wijhe’s somites 
appear clearly differentiated. The Anlagen of the acustico-facialis 
and the glossopharyngeus, differentiated from encephalomeres V and 
VI, appear topographically related to the constrictions between van 
Wijhe’s somites, 3-4 and 4-5. 
Embryo with 83 or 34 somites. Trochlear portion of trigeminus Anlage 
(compare Fig. 21, trch.) clearly differentiated. The commissure con- 
necting acustico-facialis and glossopharyngeus appears dorsal to the 
auditory invagination. 
Embryo with 88 or 89 somites. 
Embryo with 42 somites. Platt's “anterior somite” (cap. a.) clearly 
differentiated. The anterior cells of the vagus Anlage, proliferated 
from encephalomere VII, have become clearly differentiated in the 
3d visceral arch as the Urvagus Anlage. Two visceral clefts have 
appeared. 
Embryo with 48 somites (7.5 mm.). The fifth and seventh nerves have 
assumed fibrillar relation with the neural tube. The main branches 
of the trigeminus begin to appear. 
