132 THE AMPH1OXUS. 



gastrula-mouth, with the two large round pole cells 

 of the mesoblast. The lumen of the mesoblast folds 

 still remains in open communication with the lumen 

 of the archenteron (Fig. 143) ; and it is only at the 

 conclusion of this period of development that we 

 have a complete separation of the undifferentiated 

 mesoblast folds from the archenteron, and it is then 

 that they present themselves as formations fully 

 separated from the hypoblast. 



The alterations which the mesoblastic somites 

 experience in this period concern (1) alterations of 

 form, and (2) histological differentiations. 



So far as concerns the alterations of form, we 

 must mention in the first place that the mesoblastic 

 somites gradually grow forward until they reach 

 the ventral middle line. At the same time not only 

 do the outer surface and archenteron grow, but 

 also the dissepiments, so that the mesoblastic somite 

 cavities in their ventral part also become separated 

 by the dissepiments into segmental divisions (Fig. 57). 

 At the end of this period of development, but not 

 till then, the dissepiments are curved backwards in 

 the ventral portion, and remain confined to the dorsal 

 part of the body. 



