134 KIOHARD ASSHETON. 



Van Beneden (p. 29) describes the process thus : — " La 

 masse cellulaire endodermique s'aplatit a la face profonde de 

 I'ectoderme et la surface de contact entre les deux feuillets 

 s'etend peu a peu." 



This is a quite correct account of the appearances but in that 

 the description tends to suggest an inherent inclination on 

 the part of the inner mass cells to flatten themselves ; it 

 is, I think, right to look for some other explanation. 



The inner mass is attached in some way to the outer layer, 

 and accordingly it seems only probable that this mass should 

 be drawn out with the stretching of that part of the outer 

 layer to which they are attached, during the expansion of the 

 whole blastodermic vesicle by the increasing hydrostatic pres- 

 sure within. 



Figs. 22 — 27 are all sections of preserved specimens, and so 

 mostly crumpled to a certain extent. 



If, however, the angle subtended by the arc of the wall of 

 the blastodermic vesicle to which the inner mass is attached 

 is measured in optical sections of fresh specimens of ages 

 corresponding to my figs. 22 to 27, it will be found that the 

 angle is nearly constant. This angle is about 80° (see my 

 diagram, fig. 41). Up to this time there is no indication of a 

 separation into what one may call epiblast and hypoblast. 

 There is a separation into outer layer and inner mass, but this 

 I believe to be of no palingenetic importance whatever. 



The separation I have tried to show is due to the individual 

 circumstances of ontogeny entirely. So also, and as I shall 

 endeavour to show in the next chapter, I believe the ultimate 

 separation of epiblast and hypoblast is also devoid of all palin- 

 genetic influences such as any form of invagination, or epibole, 

 which is usually spoken of as " gastrulation." 



The several segments of the inner mass change their shape 

 in response to the change of their individual environment. 

 In figs. 20, 21 they are compressed and hence polygonal. As 

 the pressure is removed during the growth of the cavity C BL., 

 those towards the cavity become rounded (figs. 22 — 27), which 

 character tends to become universal in figs. 28, 29, with excep- 



