140 RICHARD ASSHETON. 



The circumference of this plane and of all planes taken 

 parallel to it are, I think, as yet circles. There is nothing so far 

 to indicate which will be anterior or posterior end of the embryo. 



The blastodermic vesicle of about the 140th hour is in shape 

 of the same character, but more markedly so. . 



In the blastodermic vesicle of the 175th liour sections taken 

 parallel to the equator are now no longer circles, but are 

 ovoidal. The fact that there are now a long and a short axis 

 may be entirely due to the pressure upon the vesicle exerted 

 by the walls of the uterus; but I want to point out more 

 especially that the horizontal sections are not true ellipses, 

 but have one end larger than the other. The large end corre- 

 sponds approximately, but by no means always exactly, with 

 the future posterior end of the embryo. 



A median longitudinal vertical section of this stage is very 

 instructive, for the asymmetry is very marked. One end, the 

 future posterior end, is much more bulky than the anterior 

 end. The embryonic area is always placed nearer to the 

 anterior than to the posterior end. 



Fig. 42 shows four stages, namely, at the 100th hour, the 

 125th, 140th, and 175th. A. is the more anterior, P. the more 

 posterior end. The thick black line in each case is the outer 

 layer of cells or epiblast ; the thin black line represents those 

 cells of the inner mass which become separated to form the 

 hypoblast. It is quite clear that a very great change in shape 

 as well as size has occurred between the 96th and 168th hours. 

 How has this been produced ? 



I have argued that there is a hydrostatic pressure within the 

 blastodermic vesicle causing it to expand. The pressure is 

 sufficient to cause the stretching and flattening out of the cells 

 of the wall of the vesicle, and also to cause the stretching and 

 expansion of the very tough albumen layer. On the other 

 hand, we know that the pressure is not so great as to rupture 

 either the vesicle wall or the albumen layer ; in other words, 

 the vesicle wall and albumen layer are sufficiently strong to 

 resist the hydrostatic pressure for, at any rate, some con- 

 siderable time. The albumen layer becomes continually more 



