DEVELOPMENT OF THE PIG DUlflNG FIRST TEN DAYS. 351 



or is there really a more profound difference in the nature of 

 the cells which prevents trophoblast and epiblast cells from 

 mingling ? If (as I have argued in a discussion on the 

 development of the sheep) the trophoblast is really to be 

 regarded as hypoblast, and if there is in the pig so real a 

 difference between the two layers, and dating from so early a 

 period as appears to be the case in the sheep, one may well 

 consider that the trophoblast cells overlying the epiblast are 

 incapable of permanent incorporation as epiblast cells ; and 

 that fusions which are brought about by the presence of 

 investing layers as described in the cases of the rab\)it, mole, 

 and Sorex are secondary phenomena, or may even be only 

 apparent, and are not permanent fusions. 



Weysse took a quite different view of the homology of the 

 layer he termed extra-germinal outer layer. The innermost 

 layer of cells [H., fig. 36, &c.) he calls entoderm. The outer 

 layer (7^. R.) he did not regard as the original outer wall of the 

 blastodermic vesicle, but called it the extra-germinal ectoderm, 

 and considered that it was continuous at all times with the 

 ectoderm of the germinal area (£.). 



Outside this he believed that a third layer, the true Rauber 

 layer, had existed during an earlier stage, and he describes the 

 remains of it as appearing at intervals over the surface of the 

 " extra-germinal ectoderm." 



It is clear that this was a rather rash supposition, as 

 the youngest embryo which he possessed was ten days old, 

 — that is to say, the same age as those of which my figs. 10, 

 36 — 39, are drawings. That certain cells existed outside 

 the " extra-germinal ectoderm " he was quite convinced, and 

 describes and gives drawings of them. Hubrecht (17) con- 

 sidered that these cells must be cells derived from the uterine 

 walls. 



Weysse is perfectly right in saying that at the time of the 

 process of " bridge" formation on the embryonic area, there are 

 at certain places cells outside the outer wall of the vesicle. 

 For instance, in the specimen (fig. 10) there are no less than 

 forty-three perfectly distinct cells with nuclei^ scattered at 



