34 AKT. H. — P. HATTA. 



in the neighbourhood of the cavity lose their mutual union, 

 thus disturbing the sharp contour of the cavity ; and furthermore, 

 some scattered cells come to fill up the last remnant of 

 the cavity, which is thus entirely obliterated {Figs. 21 and 

 30). 



The loosening of the cellular union of the macromeres, 

 which is doubtless a very remarkable occurrence, is not confined 

 to the neighbourhood of the segmentation cavity where it first 

 takes place, but eventually extends over a wider extent and 

 especially to the ventral part. Even the whole mass of the cells 

 may be more or less influenced by the process {Fig. 21 and 

 22). The main mass of the hemisphere formed of macromeric 

 elements has hitherto been in firm union, so as to give them a 

 polyhedral shape by mutual pressure and to cause the sharp 

 contour against the segmentation cavity. This close union has 

 been, I believe, kept uj) owing to the pressure exerted by the 

 liquid contents of the segmentation cavity upon the cell-mass. 

 The pressure is probably intensified by stages with growing 

 reduction of the cavity. Upon this j^i'^ssure, as before stated, 

 depends also the external form of the boundary groove during 

 invagination, it having the effect of distending the animal 

 layer. The liquid is at length forced to escape in some way as 

 the cavity is by stages reduced ; it is probably pressed out through 

 the outer micromeric layer when the pressure attains the maxi- 

 mum. As soon as this happens, the pressure is suddenly much 

 weakened, and the loosening of the cellular union should set in. 

 The segmentation cavity is thus obliterated. 



In the gastrula thus brought forth, the invaginated parts of 

 the ovum, including the micromeric and macromeric parts, will 

 be called the entoderm, while the micromeric epithelium which 



