218 RICHARD ASSHETON. 



continuous sheet extending nearly halfway round the inner 

 surface of the vesicle (v. fig. 22). 



This is the oldest stage which I shall describe. My next 

 specimens are twelve and thirteen days old^ and already much 

 elongated. 



The specimen, fig. 20, is younger than the youngest de- 

 scribed by Bonnet. I regret very much that I have nothing 

 between this and Bonnet's, for in Bonnet's specimen the 

 Rauber layer had gone. 



That the layer H is the same as Bonnet's layer Ei (fig. 3) 

 there can be no reasonable doubt. Nor can there be much 

 doubt that the mass E in my fig. 16 gives rise to most if not 

 all of the " ectoblast " E in Bonnet's fig. 3. But of the fate 

 of the Rauber cells overlying this in my fig. 16 I have no 

 record (vide note on p. 216). 



In the complete absence of zona radiata I cannot think that 

 their fate can be very different from what it is in the pig or 

 tupaia, with whose development the sheep has many points in 

 common. The separation between the trophoblast cells and 

 epiblast is quite complete in my specimens (figs. 19, 21). 



Observations on the Mammalian Blastocyst. 



The interpretation which I believe may be placed upon 

 the specimens described above and in my paper on the pig 

 may be briefly stated. At an early stage, in the sheep per- 

 haps as early as the eight-segment stage, the future epiblast 

 and hypoblast are differentiated. 



The hypoblast surrounds the epiblast, so that at the morula 

 stage the embryo consists of a few epiblast cells surrounded 

 by hypoblast cells, which at one pole form a thicker investment 

 than elsewhere. In the middle of this thickened mass of 

 hypoblast the cavity of the blastodermic vesicle arises. 



Subsequently, by the rupture of the hypoblast over the 

 epiblast, the latter comes to the surface. 



Therefore the greater part of the wall of the blastodermic 

 vesicle is hypoblast. The hypoblast is, owing to mechanical 



