THE SEPARATION OF THE GERM-LAYERS, ETC. 279 



cells. This is the first indication of the enteron, which soon increases 

 somewhat in size (Fig. 131 D, md) and finally separates from the 

 yolk and appears sac-like (Fig. 132 A, md). Beneath it, the yolk- 

 epithelium is now seen, which, in earlier stages, was wanting at this 

 spot (KORSCHELT, No. 25). 



The sac-like rudiment of the enteron was known long ago and it 

 was assumed, with great probability, that it might be connected 

 with the yolk-epithelium, and thus to a certain extent might be 

 regarded as an outgrowth of the latter (RAY LANKESTER, VIALLETON, 

 BRUCE). The two together would represent the entoderm, the 

 vesicle being regarded as the permanent and the yolk-epithelium as 

 the provisional part of the entoderm. BOBRETZKY, on the other 

 hand, considered that the vesicle arose as a mere differentiation of 

 the lowest cell-layer of the middle layer, i.e., of the so-called 

 " mesoderm." There, indeed, appear to be no yolk-epithelium at the 

 stage when the epithelial plate above described as the first indica- 

 tion of the enteron forms, either beneath that plate or in its near 

 neighbourhood, and this has led to the assumption that the cells of 

 the yolk-epithelium as well as the enteric plate arose as differentia- 

 tions of a cell-layer which grew from the edge of the germ-disc 

 towards its centre. 



This last view seems not without justification because it is very 

 difficult to decide whether the yolk-epithelium presses beneath the 

 germ-disc (Fig. 114, p. 250) or whether these cells are differentiated 

 from the whole cell-mass. The cells which are directly in contact 

 with the yolk actually bear a very close resemblance to those of the 

 middle layer. 



These questions are of importance as determining the manner in 

 which the germ-layers form. We have to imagine that the cell-mass 

 pressing from the edge towards the centre of the germ-disc (Fig. 131 

 A and B) represents the meso-entoderm. The whole process is then 

 to be regarded as a much modified invagination. The edge of the 

 -erm-disc corresponds to the blastopore which is filled by the large 

 yolk-plug. The yolk-mass also fills the whole of the archenteric 

 cavity (Fig. 131 Ji). 



In the Molluscs considered earlier in this work, especially in the Gastro- 

 poda, we have already found the mouth related to the blastopore. In GREN- 

 ACHER'S Cephalopodan embryo, we saw that the oral aperture arises in the 

 neighbourhood of the aperture of the blastoderm which closes only at a very 

 late stage (p. 270). Since we regard the latter as the blastopore, relations 

 1 etween it and the mouth may exist here also. 



