420 WALTER HEAPE. 



continuous layer exhibits these phenomena (fig. 21), and then 

 become separated from the remainder of the mass (fig. 22). 



The remaining cells of the inner mass increase in number, 

 and assume a columnar form, at the same time becoming sepa- 

 rated by a narrow cavity in the centre from the outer layer. 

 The cells of the latter layer in the region of the cavity also 

 increase in number and become thicker than their fellows (fig. 

 23). 



The further development of the hypoblast may be stated in 

 a few words ; it extends laterally by virtue of the multiplica- 

 tion of its cells, which at the same time become considerably 

 flattened. Later on, as may be seen in fig. 28, the cells are 

 again more rounded, and, indeed, at different stages during 

 the formation of the layers, they assume various proportions. 

 It is to be noted that this layer, after being once completely 

 separated off from the inner mass (vide fig. 23), remains sepa- 

 rate until the mesoblast is formed, and increases, therefore, 

 wholly by the division of its own cells. 



- The hypoblast eventually completely surrounds the whole of 

 the blastodermic vesicle. 



The changes which take place in the remaining portion of 

 the inner mass and in the outer layer adjoining it are some- 

 what more complicated. 



1. The inner mass increases in size and its columnar cells, 

 arranged in a double row, form an hemispherical plate, the edge 

 of which rests upon and is continuous with the cells of the 

 outer layer. In consequence of this the narrow cavity men- 

 tioned above assumes considerably greater proportions ; it is 

 bounded below and at the sides by the plate, and above by the 

 outer layer ; at the same time it becomes partially filled up by 

 branched stellate cells, which are derived from the cells of the 

 outer layer. 



This cavity may be termed the secondary cavity of the blas- 

 todermic vesicle in contradistinction to the cavity which is 

 formed at the close of stage a, and ■svhich also arises, altliough 

 at the opposite side, between the outer layer and inner mass. 



Fig. 5 is a drawing of an ovum at this stage of growth. 



