CASTIM LATION 



43 



between the two cavities so that they form a continuous space the 

 broader front part of which is derived from the segmentation cavity, 

 the narrower posterior part from the true archenteron (Fig. 27, (- 



FIG. 27. Sagittal sections illustrating the process of gastrulatiou in Hypogeophit. 

 (After Brauer, 1897.) 



eft, ectoderm ; i-nt, au-lienteiic cavity ; ;/./, gastrular lip; s.c, seKmentation cavity. 



and D). The two sections of the cavity remain for a time clearly 

 distinguishable by the character of the cells which form the roof 

 those of the archenteric portion being composed of fine-grained 

 protoplasm like that of the ectoderm while those of the portion 

 derived from the segmentation cavity are typical yolk-cells. 



