190 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 



Fig. 3. Embryo with three protovertebrae. The shape of protovertebra a much 

 the same as in the embryo of two protovertebrae. X 330. 

 Fig. 4. Section from a chick with four protovertebra?. X 180. 

 Fig. 5. " " " five " X 160. 



Fig. 5 a. " " " five " X 160. 



Fig. 6. " " " six " X 160. 



Fig. 7. " " " seven " X 165. 



Between /and g is seen the beginning of the protovertebra anterior to o. 

 Fig. 8. Sections from a chick with eight protovertebrae. 



PLATE II. 



Fig. 10. Horizontal section showing the origin of the V., VII., and VIII. cranial 

 nerves in a chick with fifteen protovertebrae. 



Figs. 11, 11a. Two sections from the same series. Fig. 11 shows the origin of 

 the IX. nerve from the neural crest back of the auditory capsule (aud.). Fig. 

 11 a is a more ventral section, showing the relation of the ear capsule to the 

 neural concavity dorsal to which the IX. nerve arises. Horizontal sections. 



Fig. 12. Shows the neuromeres between which the nerves arise, as indicated. 

 Sagittal section. 



Fig. 12 a. Sagittal section, showing the two roots of the VII. and VIII. cranial 

 nerves. 



Fig. 12 b represents the right medullary wall from a horizontal section, show- 

 ing the cell arrangement of the neural ridges connected with the VII. and VIII. 

 cranial nerves. 



Figs. 13, 13 a. Sections from two 4^-day chicks. They show the relation of 

 the spinal ganglia (sp. g.) to the muscle plates (m. p.), and to the neuromeres of 

 the spinal cord (n. sp.). 13 a is sagittal; 13, horizontal. 



Fig. 14. Median sagittal section, showing the relation of the five main divisions 

 (A, B, C, &c.) of the brain, after the cranial flexure. 



Fig. 15. Median sagittal section, showing their relations at the beginning of the 

 cranial flexure. 



Fig. 16 shows the neural ridges in the medulla of a five-day chick. The roof 

 of the fourth ventricle and mid-brain have been removed. 



Fig. 17. Diagrammatic representation of the relation of the successive cranial 

 vesicles at the time of their first appearance. 



Fig. 18. Sagittal section through the lateral wall of the medulla in a five-day 

 chick. In hardening, the mesoderm has broken away from the neural wall. 



Fig. 19. Sagittal section near the middle line of the brain of a four-day chick. 



