350 RICHARD ASSHETON. 



Some time ago (1) I supported Balfour's (3) and Heape's 

 (14) views upon the fate of Rauber's cells in the rabbit. 



It must be remembered that the conditions in the pig are 

 very different from those of the rabbit. In the pig the 

 epiblastic disc is a compact mass of small area^ and has no 

 investing membrane. In tiie rabbit the epiblastic disc is a 

 loose layer of cells, stretching over a large area, and outside 

 the trophoblast layer is a firm investing membrane, the 

 albumen layer, against which all the cellular layers are pressed 

 by the force of internal fluid. So whereas in the pig there 



is nothing to prevent the broken fragments from being brushed 

 away or otherwise lost, except such original attachment as 

 they may have to the underlying epiblast, in the rabbit these 

 fragments are kept tightly pressed down to the surface of the 

 epiblast, and may even be forced into the interstices between 

 the loosely arranged cells as shown in my figures of the rabbit 

 (1) (PL 16, figs. 30, 32, 33). 



Although there is very good evidence (Balfour [3], Heape 

 [14], Lieberkiihn [21], &c.) that in certain mammals a fusion 

 occurs, it is equally clear that in other cases (Verspertilio 

 murinus, Duval [13], Tupaia, Hubrecht [17], and in the 

 pig) most if not all of these cells are lost. It is only in those 

 cases in which there is, at the time in question, a well- developed 

 albuminous or other investment that we find evidence of a 

 fusion between these layers. 



So perhaps we may say that there is no inherent tendency 

 for the "Bauber" cells to fuse with the formative epiblastic 

 disc (except, of course, round its margin), but that in certain 

 cases (e. g. the rabbit, mole, and perhaps Sorex [Hubrecht, 16, 

 17]) in which extraneous conditions in the form of zona 

 radiata and albumen layer occur — and in the case of the 

 rabbit, the existence of interstices between the epiblast cells — 

 many of the cells are included with and may form part of the 

 permanent epiblast. 



It may be asked. Is the rejection of the cells in the pig, 

 Tupaia, and bat, due to the loss of continuity caused by the 

 physical and mechanical conditions of the earlier development. 



