232 RIOHAUD ASSHETON. 



and origin of the epiblast is exactly comparable to that of van 

 Beneden for the rabbit, which, however, has been abandoned 

 by van Beneden himself. Duval believes that it is neverthe- 

 less true for the rabbit. It follows in that case that the per- 

 manent epiblast in that animal has a different origin from that 

 which at present it is held to have. 



Duval suggests that its true origin has been missed by all 

 the workers on the rabbit hitherto, and that the whole of the 

 inner mass becomes flattened out as hypoblast (v. Duval [26], 

 fig. 28, p. 163). 



The rabbit has formed an object of research for so many 

 persons that such an important stage is very unlikely to have 

 been missed. Duval suggests that it is a stage which occupies 

 so short a time that no one has chanced to catch it. But 

 think what the process must involve ! Cells which are ex- 

 tremely attenuated and under considerable tension must sud- 

 denly grow so rapidly as to bud off large cells in a direction 

 contrary to that in which they have hitherto given rise to cells, 

 and then again resume their attenuated form ; and while this 

 is going on, the large rounded cells previously existing between 

 the outer layer and the hypoblast have to pass into the hypo- 

 blast and become suddenly stretched and flattened, like those 

 already there. These would surely be changes more profound 

 than anything that occurs during the first seven days of deve- 

 lopment ! It is difficult to believe that they could have 

 escaped notice if they had existed. If there is a true " meta- 

 gastrula^^ in the rabbit, and if the outer layer of this meta- 

 gastrula gives rise to the epiblast in a way similar to DuvaFs 

 description of the process in bats, the thickening of the outer 

 layer to form the permanent epiblast must occur at a much 

 earlier stage, and possibly before the formation of the cavity 

 of the blastodermic vesicle, — as, for instance, the thickening 

 of the outer layer does in the similar stage in the sheep 

 (fig. 9). 



Here is, at first sight, another possible interpretation of ray 

 sheep specimens, if the differences in colour indicated in my 

 drawings of the later specimens (figs. 12 — 15) are held to 



