154 EICHAKD ASSHETON. 



by differeut authors ; indeed, very opposite views have beeu 

 held during many years. It is an interesting question from 

 the extreme difficulty of the investigation and from the morpho- 

 logical problems connected with its solution. As regards the 

 actual facts, there have been three quite distinct accounts. 

 Most observers have noticed three layers at this stage : (1) an 

 outer thin layer ; (2) a middle thick layer ; (3) an inner thin 

 layer. These accounts are very briefly as follows : 



(1) Van Beneden maintained that the inner layer is not 

 epiblast at all, but is mesoblast, and that the outer layer 

 becomes thickened over the embryonic disc area, and gives rise 

 by itself to the epiblast of the embryo. 



(2) Rauber, Lieberkuhn, and later Kolliker and others, 

 hold that the outer layer is quite transitory, and tha,t during 

 the seventh day it splits up, degenerates, and disappears 

 entirely, taking no part in the epiblast formation of the embryo. 

 The epiblast, they hold, is wholly derived from the inner layer 

 of ovoid cells, which van Beneden took to be mesoblast. 



(3) Balfour and Heape contend that both layers persist as 

 the epiblast of the embryo, but the two layers fuse together 

 by the growth downwards of the outer layer cells in amongst 

 the inner layer cells. 



There can be no doubt now that van Beneden was wrong. 

 The middle layer certainly forms part if not all of the perma- 

 nent epiblast, and this is acknowledged by van Beneden himself 

 {' Arch. Biologic,' vol. v). 



It requires some careful examination to determine whether 

 E,auber, Lieberkuhn, and Kolliker, on the one hand, or Bal- 

 four and Heape, on the other, were right, or whether all were 

 wrong. 



The surface views given by van Beneden (pis. v and vi) and 

 Kolliker (pi. ii, figs. 13, 15, 16, and 19) are, I think, quite 

 correct. Tlie interpretation van Beneden put upon his figure 

 he no doubt now admits to be wrong. The interpretation put 

 upon them by Kolliker I believe to be correct, namely, that the 

 large areas are cells belonging to the outer epiblast layer, and 

 the smaller ones are cells belonging to the inner epiblast layer. I 



