THE SEPARATION OF THE GERM-LAYERS, ETC. 281 



At a later stage, as already described, the lowest cell-layer, that 

 which is in contact with the yolk, becomes differentiated and yields 

 the provisional and the final entoderm (Fig. 171 G and I), de, iml). 

 The cell -material which remains between these two and the ectoderm 

 corresponds to the mesoderm, which, in its development is, like the 

 entoderm, greatly influenced by the excessive abundance of the yolk. 

 During these processes, the germ-disc has extended far over the egg 

 (Fig. 131 (J and D). 



We need hardly point out that the formation of the germ-layers 

 in the Cephalopoda, as compared with that in other Molluscs is much 

 modified ; transitions between the two types of formation may, how- 

 ever, be found in the eggs of many Gastropods that are rich in yolk 

 (e.g., Nassa, Fig. 95, p. 207) although, in these latter cases, the 

 modifications undergone by the process are not nearly so great. 

 The entoderm, indeed, was there seen forming in a disconnected 

 manner and the rudiment of the enteron was open towards the yolk 

 (Figs. 123-135), as in the Cephalopoda, except that, in the 

 latter, the yolk is still covered by the thin epithelial layer. 



The alimentary canal now develops, the enteron (Fig. 132 A, nul) 

 increasing in size and soon dividing into two parts, as is seen in Fig. 

 132 C which represents a later stage. The lower part, which appears 

 sac-like (tb), represents the rudiment of the ink-sac, and the upper- 

 part, which is open towards the yolk-epithelium, the actual stomach 

 and intestine. Where this comes into contact with the ectoderm (a), 

 fusion subsequently takes place and results in the anus. There is 

 at this point only an inconsiderable depression of the ectoderm, and 

 there is therefore no proctodaeum of any size, as is evident from the 

 fact that the ink-sac, which originates from the entoderm, opens into 

 the intestine in the adult quite near the anus. 



The enteron grows out towards the apex of the yolk-sac, this 

 being still more marked in later stages. It is growing toward the 

 stomodaeum which is approaching it from the other side of the 

 yolk-sac. This latter rudiment arose as an ectodermal depression 

 on the opposite side of the blastoderm and farther down, i.e., more 

 anteriorly. 



The positions of the mouth and the anus have already been men- 

 tioned and are made clearer by the sections now before us (Figs. 1 ■">'_' 

 and 133, also Figs. 116-120, pp. ii55-i ) 62). The oral invagination arises 

 below the ectodermal thickening depicted on the left in Fig. 131 A, 

 ''</. Another depression (sp), at first lying outside of the stomodaeum, 

 represents the rudiment of the posterior or large salivary glands. At 



