Chap. TIL] 



THE GASTRULA. 



33 



the segmentation cavity ; the cells bounding 

 this cavity then undergo a further change, by means 

 of which the single becomes replaced by a double 

 layer, one of which is interior to the other. 



This two-layered condition is brought about in 

 one of two ways ; either the cells of one half of the 

 sphere are pushed into the contained space, and, by 

 approaching the other half, more or less completely 

 obliterate the 

 segmentation 

 cavity, or the 

 cells undergo a 

 transverse and 

 concentric clea- 

 vage, by means 

 of which each 

 cell becomes 

 two, and the 

 single is con- 

 verted into a 

 double layer. 

 Whether the 

 former process 

 (that of in- 

 vag ination) 

 or the latter (delamiiiation) takes place, the cell- 

 layers are regarded as comparable, and receive the same 

 names ; the outer is known as the epiblast (Fig. 6, ep\ 

 the inner as the hypoblast (hyp). Similarly the con- 

 tained cavity, which is clearly the segmentation 

 cavity in the latter mode, and an altogether new 

 formation in the former, is spoken of as the 

 arcliciiterou, while the narrow opening to the ex- 

 terior is the blastopore (o). The whole organism is 

 now said to be in the Gastmla stage (Fig. 6). 



No known animal remains at quite the low and 

 undifferentiated condition of a Gastrula; and, indeed, 

 D 16 



Fig. 6. Diagram of a Gastrula. 

 o, Blastopore; ep, epiblast ; hyp, bypoblast. 



