DeveIopmeiit<al history of priiriary segments of the vertebrate head. 443 



Eig. 13. Right half of the anterior portion of a divided embryo 

 with 39 somites , 26 days old. Internal encephalon surface exposed. 

 Compare this figure with Fig. 8, which shows the external surface of an 

 encephalon of the same age. 



Fig. 14. Left half of the anterior portion of a divided embryo 

 with 44 somites, 28 days old. Internal view of encephalon. 



Fig. 15. Left half of the anterior portion of a divided embryo 

 with 54 somites, 36 days old. Internal encephalic surface exposed to 

 view. Segments of the medulla (7 — 11) have passed ventrad. The 

 posterior commissure has appeared at c. The transverse groove that 

 formed the dividing line between segments 3 and 4 has disappeared. 

 Secondary folds have formed in the lateral walls of the infundibulum. 



Plate 29. 



All figures are of the chick embryo. Figures 16 — 19 are X 27 

 diameters; the remaining figures (20 — 33) are X 50 diameters. 



Fig. 16. Dorsal view of an embryo with 1 somite, 21 hours old. 

 The neural segments can be traced not only across the wide neural 

 groove, but some distance laterally into the adjacent tissue. 



Fig. 17. Dorsal view of an embryo with 2 somites, 22 hours hold. 

 The neural groove is better defined than in the preceding figure. 



Fig. 18. Dorsal view of an embryo with 3 somites, 23 hours old. 

 The segments are very pronounced along the neural crests, but can be 

 traced across the base of the neural groove. 



Fig. 19. Dorsal view of an embryo with 4 somites, 23 ^/.^ hours 

 old. The neural groove is about to close. Along the margins of this 

 groove the segments appear more distinctly than they do in younger 

 forms. 



Fig. 20. Anterior portion of the neural groove of the same em- 

 bryo as represented by Fig. 19. Right profile view. The optic eva- 

 gination covers the first 3 segments. Transverse external and in- 

 ternal grooves form the dividing lines between adjacent segments. 



Fig. 21. Left half of the anterior portion of the divided neural 

 groove of an embryo with 3 somites, 23 hours old. Internal surface 

 exposed to view. This surface is distinctly divided into joints by 

 transverse depressions or constrictions that pass across the divided 

 neural axis. 



Fig. 22. Anterior portion of the neural tube of an embryo with 

 5^/2 somites, 24 hours old. Dorsal view, a, h, C etc. mark the seg- 

 mental grooves, and 7, 5, 3 etc., the neural segments. The optic eva- 

 ginations distinctly cover the three anterior segments. 



Fig. 23. Right surface view of the same encephalon as represent- 

 ed by Fig. 22. The segments are found to encircle the neural axis. 

 The third segmental groove divides the optic evagination. 



Fig. 24. Left half of the same neural tube represented by Figs. 22 

 and 23. Internal surface exposed to view. The segmentation is es- 

 pecially distinct along the thick dorsal margin. Two transverse ridges 



