FOURTH PERIOD OF DEVELOPMENT. 131 



information with regard to the differentiations of the 

 rnesoblastic somites. He has only told us that they 

 serve for the formation of the muscular system. 

 His conjecture, moreover, is to be mentioned that the 

 cavity of the rnesoblastic somites becomes the body 

 cavity. I will proceed to give my own experi- 

 ences. 



The multiplication of the mesoblastic somites 

 proceeds more slowly in this period of development. 

 The elongation of the body is also to be referred 

 not to a growing out of the posterior end, but 

 especially to extension of the body segments which 

 we already have, and of them, chiefly those which 

 are anterior. The number of the mesoblastic somites 

 up to the perforation of the mouth opening, which 

 for us marks the end of this period, only increases to 

 fourteen (Figs. 59, 61). 



The formation of these mesoblastic somites follows 

 in the same way as before. The mesoblastic somite 

 cavities remain for some time in open communica- 

 tion with that of the archenteron (Figs. 128, 141). 



Posteriorwards the fourteenth mesoblastic somite 

 is followed by a quite short and undivided mesoblast 

 fold, which ends off at the posterior end of the 



