APPEARANCE OF THE SOMITES IN THE CHICK. 



131 



the upper or under surface of the mesoderm (Fig. 11,/). As 

 seen in section, the forming somite is at first elongated, with its 

 long axis coinciding with that of the embryo (Fig. 11, p). How- 

 ever, it soon rounds up (Fig. 10, p), and in so doing becomes 

 separated from the posterior mesoderm (Fig. 12, g). The cleft 

 thus made is never vertical, but is so formed that the posterior 

 edge of the last formed somite is in the shape of a wedge, which 

 fits into a corresponding concavity on the anterior edge of the 

 unsegmented mesoderm (Figs. 10-12). That the above process 



/ / 



r a b 



Figs, ii and 12. Sections of six and seven somites respectively, r, rudimentary 

 somite. X x 57- 



of cutting off somites is a rapid one, is shown in Fig. 10, in which 

 it will be noted that the somite is completely formed before there 

 is any indication of one succeeding it. 



E. Discussion and Conclusion. 

 Since Miss Piatt's account of the formation of the incomplete 

 somite is not essentially different from that of mine, it follows 

 that we differ only as regards one cleft. According to Miss 

 Piatt's view one cleft slowly forms in front of the first one, and 

 hence one complete somite arises anterior to the first formed 

 somite. 



