MUSEUM OF COMPARATIVE ZOOLOGY. 189 



EXPLANATION OF THE PLATES. 



LETTERING. 



d, first cleft in the mesoderm. 



o, i,j> ^, h indicate respectively successive clefts in the mesoderm posterior to the 



first cleft, d. 

 f, g, position of first and second clefts anterior to d. 

 1-6, successive protovertebrte posterior to the first mesodermic cleft, d. 

 a, b, first and second protovertebrae anterior to the first cleft, d. 

 III., v., VII., VIII., IX., X., third, fifth, seventh, &c. cranial nerves. 



A. Prosencephalon. 



B. Thalamencephalon. 



C. Mesencephalon. 



D. Metencephalon. 



E. Myelencephalon. 

 and. auditory capsule. 

 ect. ectoderm. 



h. heart. 



inf. infundibulum. 



m. p. muscle plate. 



n. c. neural crest. 



n. ch. notochord. 



n. sp. spinal " neuromeres." 



p. V. protovertebrae. 



sp. g. spinal ganglion. 



V. fourth ventricle. 



X. section broken. 



1', 2', 3', 4', 5', neuromeres of the medulla. 



PLATE L 



Sagittal sections, showing the development of the protovertebraB. 



Fig. 1. Thefirst mesodermic cleft is seen anterior to the first protovertebrse. X60 



Fig. 1 a. Same, enlarged. X 180. 



Fig. 2. Section from an embryo with two protovertebrae, showing the peculiar 

 shape of protovertebra o, which is due to the less rapid proliferation of cells in its 

 anterior portion. X 330. 



