242 EIOHAED ASSHETON. 



mass of densely granular segments^' (29^ p. 166) had, accord- 

 ing to his description^ a strictly physiological significance only, 

 and was due, he suggested, to the transmission of yolk con- 

 tained in the outer segments to the inner segments, this 

 transmission being performed in order that the changes about 

 to '' take place in the constitution of the ovum may more 

 readily be performed'^ (p. 167). And the inner mass as it 

 appears in his figure of this stage (fig. 1) gives rise to definite 

 epiblast, definite hypoblast, and a part of the walls of the 

 blastodermic vesicle (vide 30, p. 418). 



It is, however, possible that a stage similar to my figs. 8, 9, 

 11, of the sheep may have been missed, and that the metagas- 

 trula stage of the mole may be open to the same interpretation 

 as that which I offer for the sheep. 



In Sorex the earliest specimen described by Hubrecht (34) 

 is one in which the cavity of the blastocyst is well established. 

 The segmentation stages are unfortunately not known. 



It is, I think, by no means difficult to interpret Hubrecht's 

 youngest stage (PI. 36, fig. 5) in the way that is indicated in 

 diagram Z (PI. 18). There is, moreover, a distinct difference 

 in size between the cells comprising the embryonic knob. 

 Two are very much larger, and two more are larger still than 

 the others. Are these the epiblast, and all the rest hypoblast ? 

 (vide Hubrecht's remarks upon it, pp. 506, 507). 



The Rat, the Mouse. 



Robinson's hypothesis, to which I have already alluded, was 

 based chiefly upon his interpretation of rat and mouse em- 

 bryos. I have suggested only a slight modification of his views 

 in connection with these animals. His idea with regard to 

 other animals was that the epiblast subsequently grew round 

 a pre-existing hypoblastic vesicle. It was in connection with 

 this inference that he met with so little sympathy. 



In a former paper on the rabbit (1) I wrote, " There is no 

 evidence in support of Robinson's speculations concerning the 

 existence of a hypoblastic wall to the blastocyst surrounded 

 subsequently by the epiblast." 



