THE SEPAKATION OF THE GEKM-LAYERS, ETC. 285 



The yolk-material thus does not pass directly into the intestinal 

 cavity, but has first to pass through the yolk-epithelium. The latter 

 consequently promotes the absorption of the yolk by the embryo, this 

 being its principal function. A specially active inception of nutritive 

 substance probably takes place at the open part of the enteron by 

 means of the yolk-epithelium. This view is confirmed by the rise of 

 the liver in this region, this organ showing in other Molluscs also 

 a close relation to the nutritive substance of the egg, and also by 

 the appearance of the large rhizopod-like cells in the yolk-epithelium 

 which separate the cavity of the enteron and the yolk-sac. 



When the gap in the intestinal epithelium closes as development 

 advances, the internal yolk-sac appears to lie in the body-cavity 

 quite unconnected with the enteron. The external yolk-sac gradually 

 decreases in size as its contents are transported to the internal sac 

 whence they are absorbed and, when completely taken up, the internal 

 sac itself shares the same fate, the yolk-epithelium being the last to 

 disappear, its function being now fulfilled. 



The function of the yolk-epithelium resembles that of the yolk-cells of the 

 Artln*opoda and the Vertebrates, described above. This comparison has 

 already been made by RAY LANKESTER, VIALLETON, WATASE and others, and 

 the resemblance is heightened by the fact that the yolk-cells may also appear 

 as a peripheral layer, the outer merocyte-layer of the Selachians or the so- 

 called periblasts of the Teleosteans. Some difference between these and the 

 yolk-epithelium of the Cephalopoda still, however, remains, since, in the 

 Vertebrates, these cells are at first distributed throughout the yolk and then 

 collect into a continuous layer, whereas, in the Cephalopoda, the cells are 

 never found in the yolk, the yolk-epithelium being produced direct from the 

 cells of the germ-disc. The type of the meroblastic egg is thus specially 

 marked in the Cephalopoda. 



Iii order to complete our description of the development of the 

 alimentary canal, some reference must be made to the ink-sac. We 

 have already seen that it arises from the rudiment of the enteron as 

 a vesicular structure (Fig. 132 (7, md\ It soon deepens, and grows 

 out as a tube which is surrounded by mesoderm-cells. In this tube, 

 two sections can soon be distinguished, the inner blind end, the walls 

 of which are much folded (GmoD), and the superficial part which 

 opens externally and becomes greatly dilated, but remains lined with 

 a simple epithelial layer. The inner part of the ink-bag represents 

 the glandular secreting part, while the dilated portion which finally, 

 through a long efferent duct, opens into the intestine near the anus, 

 forms the reservoir for the secretion. As has already been pointed 

 out (p. 281), the fact that this entodermal structure opens close to 



