DEVELOPMENT OF THE PIG DURING FIRST TEN DATS. 358 



At tlie same time his description can hardly be said to agree 

 with mine. He speaks of the cells being indistinct and 

 flattened, the cytoplasm "scarcely stainable " at all. In my 

 case they are mostly sharp and rounded, and they stain with 

 carmalum and haematoxylin as deeply as any others in the 

 embryo. A difference in methods of preservation and staining 

 may account for this. 



However this may be, there can be no question of the pre- 

 sence of a third complete layer outside the layer which he calls 

 extra-germinal ectoderm, and I call trophoblast; such a layer 

 never exists. 



Before the completion of the eleventh day all trace of the 

 loose fragments of the torn edge of the outer layer has dis- 

 appeared from the embryonal area and elsewhere. 



The formative epiblast is now continuous all round its 

 margin with the trophoblast, and forms a disc-like plate, 

 slightly thicker near its centre than at its edge. 



The hypoblast immediately beneath the epiblast is thicker, 

 and contains more nuclei than elsewhere. Its characters are 

 shown in figs. 36 — 42. 



There is one structure which is very puzzling. Weysse has 

 noticed this. Tliis is the membrane shown in figs. 30 — 33 and 

 36 — 39, M. H. L. I have not been able to determine its origin. 

 Weysse (25) gives a correct description of it on pages 297 and 

 298. It certainly has in my sections the appearance of being 

 part of the inner margin of the cells of the trophoblast and 

 the epiblast. It is structureless, but whether secreted by 

 the epiblast or trophoblast and the hypoblast I do not know. 

 Schafer (24) considers it of hypoblastic origin in the cat. 

 Its probable function would seem to be in some way con- 

 nected with the rupture of the outer layer, and the subsequent 

 fusion of its edges with those of the epiblastic disc. 



A time may very well occur when there would be weakness 

 in the general wall of the blastocyst during this process. The 

 raembrana hypoblastica limitans seems to serve to hold the 

 disc in its place while tliis fusion is effected. The membrane 

 is tightly fixed to the inner surface of the epiblastic disc, but 



