FUBTHEK DIPPEBENTIATION OK THE GERM-LAYERS, ETC. 277 



junction of the two posterior funnel-folds has taken place but these 

 folds have not yet fused in the median line. On the mantle, the 

 tins are visible, these apparently forming late in Sepia. The mouth 

 lies at the opposite side; its position can be ascertained from 

 Fig. 130 of a later stage. The yolk-sac is still very large, and is 

 directly connected with the internal mass of yolk. As the yolk-sac 

 is gradually absorbed, the large cephalic section decreases in size, 

 approaching the final shape of the adult Sepia. By the time that 

 the embryo is mature and ready to hatch, the greater part of the 

 external yolk-sac has been absorbed. Only after its complete reduc- 

 tion can the arms occupy their final position round the month. At 

 hatching, they are still rather short, and only attain their full length 

 during free life. The prehensile arms which, as rudiments in Sepia 

 also, are not specially distinguished from the others (Figs. 128 and 

 129), become characterised at later stages not only by their length 

 (Fig. 1:50, a) but also by their bases sinking into the body. The 

 depression thus caused is, in older embryos, visible even externally 

 in the form of a horse-shoe-shaped fold lying above the first pair 

 of arms. 



4. The further differentiation of the Germ-layers and the 

 formation of the Organs. 



A. The separation of the Germ-layers and the formation of the 

 Yolk-epithelium and the Alimentary Canal. 



Only after the external form of the body is known is it possible to 

 enter in detail into the further development of the germ-layers. 

 These, in the Cephalopods, develop very late, or rather, they do not 

 appear so distinctly here as in the other divisions of the Mollusca. 

 As we have already remarked, the great accumulation of yolk in the 

 embryo has, in this direction also, essentially modified the ontogenetic 

 processes. 



Our description of the body-layers was interrupted at the stage at 

 which there had formed, at the margin of the germ-disc which covered 

 only a small part of the egg, a peripheral thickened ring (Fig 112, 

 p. 247 and 131 J) and cells became detached from the edge which, 

 according to the view adopted by Vialleton, wandered beneath the 

 superficial cell-layer (Fig. 114, p. 250) there to give rise to a con- 

 nected layer. This lower cell-layer then extends over the whole yolk, 

 and is followed by a middle layer * so that the yolk is now surrounded 



* See also p. 126, etc. 



