EAELY STAGES OF DEVELOPMENT OP THE EABBIT. 141 



and more stretched, and quite late it does, as a matter of fact, 

 rupture, but at a time (the ninth day) that does not con- 

 cern the present question. No addition is ever made to the 

 thickness of the albumen layer after the embryo leaves the 

 Fallopian tube ; it is an inanimate structure. The cellular 

 wall of the blastodermic vesicle is, on the contrary, living, and 

 capable of adding to itself at any part of its area. Like the 

 albumen layer, it becomes greatly stretched and becomes very 

 thin (v. figs. 31 — 29), and unless it received additional matter 

 (i.e. multiplication of the cellular units) it would rapidly thin 

 out altogether. After about the 100th hour the cellular wall 

 ceases to get any thinner. Up to this moment we must suppose 

 that the rate of increase of hydrostatic pressure has been in 

 excess of the rate of addition of material to the cellular wall of 

 the vesicle and so has stretched it, but from now there is no 

 appreciable thinning out of this cellular wall (v. figs. 27, 28, 

 29, and 34). 



This means, I believe, that the increase of cellular tissue 

 just balances the increase of hydrostatic pressure, and so the 

 vesicle grows in size, the thickness of the cellular wall remaining 

 unaltered. Now there is no reason, as far as I can see, to 

 doubt the albumen layer being equally tough on all sides of 

 the embryo. The albumen layer is not secreted by the embryo, 

 but is applied by the Fallopian tube. 



Although preserved specimens and sections are not well 

 adapted for this purpose, still my figures show that there is no 

 regularity at all in these preserved specimens, such as a thicker 

 part of the albumen layer being present over any one part of 

 the embryo in stages up to the 96th hour (v. figs. 16, 20, 21, 

 22, 23, and 24), and in the fresh specimens the outer and 

 inner limits of the albumen layer present in optical section 

 true concentric circles. Therefore I do not think we can 

 attribute any subsequent difference in thickness of the albumen 

 layer to a difference in texture acquired by that albumen layer 

 during its deposition. At a later stage we do find a difference 

 in thickness occurring as a constant character. 



The difference I find is as follows : 



