142 ETCHAED ASSHETON. 



The most marked difiPerence concerns the part of the albu- 

 men layer adjoining the embryonic disc. This is in the 

 later stages very much thicker than elsewhere. Figs. 30 and 

 31 are portions of the upper and lower poles of an embryo 

 taken from a rabbit of the one hundred and forty-fourth 

 hour. 



The albumen layer is at the upper pole about twice as thick 

 as it is at the lower pole. This is a perfectly constant feature, 

 and becomes more and more marked the later the stage. 



Hydrostatic pressure exercises its influence equally in all 

 directions. If it encounters less resistance in one direction 

 than in another it will cause greater effects in that one direc- 

 tion. I have argued above that at any one moment (between 

 one hundredth and one hundred and sixty-eighth hours) the 

 hydrostatic pressure within the vesicle on the one hand, and 

 the living cellular wall of the vesicle together with the non- 

 living albumen layer on the other hand, are in a state of 

 equilibrium. The blastodermic vesicle is always taut, but does 

 not rupture. But the next moment the hydrostatic pressure 

 has increased on the one hand, and the albumen layer has 

 become thinner and the cellular wall has increased its material 

 — and still the equilibrium is maintained. It seems to me to 

 be clear that the degree of rapidity of increase of the cellular 

 wall must be a factor in the resistance afforded to the hydro- 

 static pressure by the two walls of the vesicle. 



If this is so, then it follows that at any area where the 

 increase of cellular tissue is greatest, there the hydrostatic 

 pressure will exert its greatest influence, and there the albumen 

 layer will, as a consequence, be thinnest. This is, of course, 

 dependent on the close attachment of the cellular layer to the 

 albumen layer. I am bound to confess I do not find it easy 

 to prove that there is no sliding of the albumen layer over the 

 cellular layer; but, on the other hand, I see no evidence to 

 suggest that there is such a sliding. 



Accordingly, I take it that the great thickness of the albu- 

 men layer adjoining the embryonic disc over that adjoining 

 the lower pole shows that there has been less stretching and 



