Keal. — 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. 



Fig. 7. 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 II 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 (splanciinic mesoderm). 



Fig. 8. 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" 

 (Piatt's) begins to be cut off from the mesoderm of the 1st somite (van 

 Wijhe's). 



Fig. 9. 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 

 4tli somites has become obscured by the migration of cells from both 

 sides of the constriction to meet the advancing Anlage of the acustico- 

 facialis. 



Fig. 10. Embryo with 21 or 22 somites. The conditions remain practically un- 

 changed. 



Fig. 11. Embryo witli 24 or 25 somites. A ventral migration of neural-crest cells in 

 the region of encephalomere VI lias now begun, and the crest is now 

 differentiated in the region of enceplialomere VII and posteriorly. 



Fig. 12. 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. 



Fig. 13. Embryo with 28 to 30 somites. At this stage all of van Wijhe's somites 

 appear clearly differentiated. The Anlagen of the acustico-facialis 

 and the glossopharj-ngeus, differentiated from encephalomeres V and 

 VI, appear topographically related to the constrictions between van 

 Wijhe's somites, 3-4 and 4-5. 



Fig. 14. Embryo with 33 or 34 somites. Trochlear portion of trigeminus Anlage 

 (compare Fig. 2], trch.) clearly differentiated. The commissure con- 

 necting acustico-facialis and glossopharyngeus appears dorsal to the 

 auditory invagination. 



Fig. 15. Embryo with 38 or 39 somites. 



Fig. 16. Embryo with 42 somites. Piatt's "anterior somite" (cav. a.) clearly 

 differentiated. The anterior cells of the vagus Anlage, proliferated 

 from encephalnmere VII, have become clearly differentiated in the 

 3d visceral arch as the TJrvagus Anlage. Two visceral clefts have 

 appeared. 



Fig. 17. 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. 



