538 Scientific Proceedings," Royal Dublin Society. 



ploblastic sixth stage. Althougli van Beneden makes this assertion, 

 he does not prove it, and he gives no sections in support of his 

 statement ; so I take it we are at liberty to doubt this identifica- 

 tion, as it lands us in so much difficulty. The upper layer is formed 

 of the flattened outer cells of the blastodermic vesicle, and the lower 

 layer, which is the true hypoblast, also consists of flattened cells, 

 which are continuous outside the embryonic area with the zone of 

 amoeboid cells which is creeping round the blastodermic vesicle. 

 Between these two layers is a third layer of rounded cells (Meso- 

 blast of van Beneden), which is now recognized to be the embry- 

 onic epiblast. 



Kolliker (12) regards the outer-layer as protective and belong- 

 ing to the blastodermic vesicle, and the middle layer to be the true 

 epiblast, and the deepest layer the hypoblast. He states that the 

 outer layer disappears, the epiblast becomes columnar, and that the 

 mesoblast develops later in connexion with the primitive streak. 



Rauber (16) also believes that the outer layer of the blastoderm 

 of the rabbit (" Deckschicht " or covering layer) early disappears, 

 and that the epiblast is derived from the inner mass. 



Balfour and Heape (2, ii., p. 181), found on the sixth day that 

 the embryonic area is throughout composed of — 



1. *' A layer of flattened hypoblast cells. 



2. " A somewhat irregular layer of more columnar elements, 

 in some places only a single row deep, and in other places two or 

 more rows deep. 



3. '.' Flat elements on the surface, which do not, however, form 

 a continuous layer, and are intimately attached to the columnar 

 cells below," 



They believe they have in many instances been able to trace 

 the flattened elements in the act of becoming columnar. Later, 

 Heape (10) has afforded additional evidence for the rabbit, and 

 has also shown it for the mole. 



Lieberkiihn (14), from his researches on the dog and mole, also 

 believes that the epiblast is formed from the superficial portion of 

 the inner mass, together with the overlying outer layer, after the 

 differentiation of the hypoblast from the former. 



Kupffer (13) finds that in the field-mouse (Arvicola) the cover- 

 ing layer (Deckschicht), which is by-the-by attached to the uterine 



