342 RICHARD ASSHETON. 



point on the inner surface a small group of cells is loosely 

 attached to it, which is the epiblast, and lying along the inner 

 surface of the epiblast is a loose network of irregular-shaped 

 cells, the hypoblast. 



In figs. 21 and 24 the hypoblast (H.) forms a continuous 

 layer beneath the epiblast {K). I do not find that the embry- 

 onic hypoblast in the pig has a later origin from the inner 

 mass than that which the extra-embryonic hypoblast has in 

 the way described by Hubrecht (17) for Tupaia. 



The smallest specimen which I have, from which the zona 

 radiata has gone, is one (figs. 22, 23) in size intermediate 

 between fig. 19 and fig. 21. Its anatomy is similar to the 

 latter specimen. 



The increase in size between the stages of fig. 19 and fig. 

 22 or 23 is by no means great, which leads me to suppose 

 that the latter specimens are only slightly advanced on the 

 former. 



The epiblast cells in the specimen represented by figs. 22, 

 23, number twelve or fourteen, and the hypoblast cells seven. 



The actual " ages '' of the specimens, figs. 18 — 24, are as 

 follows : 



We may say that, as a rule, the zona radiata is lost during 

 the seventh day, and on the same day the separation of the 

 inner mass into epiblast and hypoblast occurs. Even if this 

 separation is not so sudden as I have supposed, there can be 

 no doubt that the two inner cell layers are derived from the 

 inner mass of figs. 19 and 20. 



From this time — that is to say, during the eighth and until 

 the closing hours of the ninth day — the shape of the blasto- 

 dermic vesicle becomes extraordinarily irregular. 



