248 CEPHALOPODA. 



these cells, according to YIALLETON, wander beneath the cell-material 

 of the germ-disc, there becoming arranged into a connected cell-layer 

 which is said gradually to spread over the whole of the food-yolk. 

 But this brings us to the formation of the germ-layers, and in order 

 to comprehend this, we must first refer to another process and also 

 mention the views which have hitherto prevailed on the subject. 



The problem of the formation of the germ-layers in the Cephalopoda 

 must be regarded as exceedingly complicated. The fact that the 

 significance of the different cell-layers forming in the germ has not 

 been recognised and that it has not yet been possible satisfactorily to 

 trace back the method of their formation to corresponding processes 

 in the other Molluscs or in the members of other animal groups, is 

 evidently due to the highly modified conditions under which the 

 Cephalopodan egg develops on account of the large amount of food- 

 yolk deposited in it and the marked distinction between the food- 

 yolk and the formative yolk. 



ms 



FIG. 113. Longitudinal section through an egg of L<>li(/o at the stage in which the 

 edge of the germ-disc becomes thickened (alter BOBUETZKY from BALFOUR'S Text- 

 book), c, peripheral cells ; d and ms, the thickenings of the edge. 



The following is a brief statement of the view hitherto held as to the formation 

 of the germ-layers. The germ-disc consisting of a single layer of cells which 

 at first covered only a small part of the animal pole of the egg, at a certain 

 time, undergoes at the periphery a thickening of the cell-layer (Fig. 113). 

 The layer which thus arises and which soon gains considerably in size by the 

 active increase in number of its cells, and spreads out beneath the whole of 

 the upper layer of the disc, has been derived either through delamination 

 from the cell-layer already present (METSCHNIKOFF, Ussow) or else through 

 the bending in of this layer (BOBBETZKY). 



In contrast to the upper (ectodermal) cell-layer, the massive cell-accumula- 

 tion has been claimed as chiefly mesodermal although the mid-gut epithelium, 

 which elsewhere is always entodermal in origin, is said to be derived from this 

 accumulation. Beneath the latter which, for the present, we must regard as 

 mesoderm, a new layer is now found which, in its origin and relation to the 

 other layers is of special interest. This is the " vitelline membrane " of authors 

 which, starting from the periphery of the germ-disc, no doubt spreads below 

 the disc (and the " mesoderm ") as well as over the whole food-yolk, surround- 

 ing it as a unilaminar cell-integument. 



