SEGMENTATION OF THE OVUM OP THE SHEEF. 245 



converted into a diderniic vesicle, like those of other Carnivora, 

 we must conclude that it is brought about by the growth 

 round of the epiblastic disc cells. The process in the ferret 

 is, then, diametrically opposed to that by which an extremely 

 similar condition is brought about in the rabbit, Tupaia, 

 Talpa, &c. 



It is, of course, just possible that Robinson may have been 

 deceived by the appearances of the sections he examined. 

 The stage which he had is an extremely difficult one, and in 

 this case was quite isolated ; for the author had neither older 

 nor younger material. He had as many as seven specimens of 

 this one stage. 



If we are to suppose that Robinson was quite right, we must 

 conclude that in the ferret the course of development seems 

 to be very different from that of any other mammal. But a 

 more extended research in the ferret should be made before 

 this conclusion is adopted. 



The Two-layered Condition of the Hypoblast. 



With reference to the difficulty of the double-layered con- 

 dition of the hypoblast, I would point to my figs. 15 and 17 

 on PI. 16, and ask what else but a two-layered condition could 

 result upon the occurrence of the expansion of the blastocyst 

 without a corresponding extension of the epiblastic knob ? (v. 

 figs. 17 and 19). 



The simple expansion of the blastocyst would be sufficient 

 to set up the two-layered condition by drawing out the angles 

 as shown diagrammatically in the figure on p. 222. The two- 

 layered condition once set up, the more complete separation of 

 fig. 19 might arise owing to difference in tension and so forth, 

 under which the layers might reasonably be supposed to exist. 

 In some cases where the expansion of the blastocyst does 

 not occur in this way there is no double layer of hypoblast, 

 e. g. Cavia, Mus (Robinson). 



If, as suggested in the sequel, it is thought to be better to 

 compare the trophoblast more strictly with the yolk-bearing 



