80 LEONARD W. WILLIAMS 



It is not my purpose to follow in detail the development of each 

 segment, but merely to mention the most conspicuous points of 

 difference, most of which are already known, between the somites 

 of the head, neck, trunk, and tail, and to determine as far as pos- 

 sible the relation of the somite to the sclerotome and of the scle- 

 rotome to the vertebra. 



THE TENTH SOMITE 



The tenth somite will serve as a type of the somites of the neck, 

 just as the second has served as an example of an occipital seg- 

 ment. The structure of this somite at the time of its origin (fig. 

 10) needs no description, except to note that the intersegmental 

 fissures which bound it are inclined slightly forward below\ 



While the three following segments are forming, the tenth so- 

 mite of each side grows considerably and, owing apparently to the 

 pressure of the surrounding structures, becomes somewhat pen- 

 tagonal, with nearly equal dorsal, medial, and ventro-lateral sur- 

 faces and smaller lateral and ventro-medial surfaces. The single 

 important structural change is the thinning out of the medial 

 and anterior walls and the formation at the top of the former of 

 the upper myotomic groove. 



An ejctensive fusion of the core with the antero-ventral angle of 

 the floor of the tenth somite is present in embryos of fifteen seg- 

 ments and in embryos of seventeen and eighteen segments the 

 lower myotomic groove appears on both the medial and the an- 

 terior walls of the somite. The newly formed notochordal proc- 

 ess now replaces the ventro-medial surface of the somite and the 

 ridge between the dorsal and lateral surfaces has disappeared. 

 Consequently the somite is now triangular. A mesenchymal 

 mass, the aortic process, projects downward between each aorta 

 and the corresponding posterior cardinal vein, but it apparently 

 arises entirely from the nephrotome, not from the sclerotome. 

 The anterior end of the notochordal process projects some dis- 

 tance forward and consequently the medial portion of the inter- 

 segmental fissure is inclined forward ventrally so as to enclose 



