250 EICHARD ASSHETON. 



cells themselves. It is an actual sliding of the free eclg;e of 

 the epiblast over the yolk, and is therefore quite different from 

 the extension of the epiblast over the amphibian egg, where it 

 is not a sliding of one layer over the yolk, but a conversion in 

 situ of nuclei from the yolk into epiblast nuclei. 



Let us, on the supposition that the mammalian ancestor had 

 a large yolked egg like that of a reptile or bird with vitelline 

 membrane, consider the probable steps which converted such a 

 condition to the one suggested in my paper. 



Firstly, it must be remembered that the epiblast is cut off 

 from the rest of the ovum at a very early period — in the bird 

 about the time of laying. From this moment it never receives 

 any nuclei or cells from the yolk, but spreads by its own 

 interstitial growth. It does not of itself form a closed vesicle 

 until quite late in embryonic life. 



On the other hand, the lower layer cells, after their separa- 

 tion from the epiblast, become continuous with yolk along the 

 germinal wall (bourrelet entodermo- vitellin — Duval), 

 and form at once a closed vesicle, which now rapidly fills with 

 fluid and expands. 



This closed vesicle, which is formed by true hypoblast and 

 yolk-containing hypoblast, has two functions in addition to 

 that of supplying actual cell material, — namely, of providing 

 nourishment, and of supplying a roomy cavity for the reception 

 of the embryo while forming. 



If the same condition appertained in the large-yolked egg of 

 the mammalian ancestor, is it not more likely that the same 

 group of cells should have continued during the substitution 

 of maternal fluids for yolk the offices of supplying nutriment 

 as before, and of the maintenance of a large cavity for the 

 reception of the embryo, than that the other group of cells 

 should acquire them ? 



Duval (24) has described a thickening, and a series of villous 

 processes on the edges of the extending epiblast of the bird's 

 egg, which according to him form with the allantois a ''pla- 

 centa/' by means of which nourishment is obtained from the 

 albumen towards the end of incubation. Nevertheless by far 



