278 CEPHALOPODA. 



by three cell-integuments. Yolk-cells are not found in the Cephalo- 

 poda, and the yolk thus attains, in them, a greater independence 

 than in the eggs of most Arthropoda and Vertebrata, which are also 

 very rich in yolk. . 



Very different conclusions have been arrived at as to the signifi- 

 cance of these cell-layers. We shall consider first the inner layer, 

 the so-called yolk-epithelium. 



The yolk-epithelium is formed of large flattened cells which are 

 swollen in the region of their nuclei. The only function of this layer 

 is to yield an envelope for the yolk, and to bring about its utilisation 

 by the embryo. In later stages it is found surrounding the yolk still 

 in the same state as before. The yolk, especially in the typical forms 

 of Cephalopodan development in which it is plentiful, is accumulated 

 principally in the external yolk-sac. This latter is directly continuous 

 with the mass which lies now within the developing embryo (Fig. 

 132); at a later stage, in consequence of the processes of growth 

 that take place in the embryo, a constriction forms in the region of 

 the arms (Fig. 120, p. 262), and a rather narrow duct here arises 

 connecting the external with the internal yolk-sac (Fig. 133, a, d#, 

 and i, ds). In this latter, again, various parts may be distinguished 

 as lying respectively in the cephalic, the pallial, or the nuchal region. 

 The part lying in the head gives off' two outgrowths into the optic 

 stalks, the nuchal portion narrows later and leads to the voluminous 

 pallial portion. The embryo no doubt absorbs the yolk in the follow- 

 ing way : the external yolk-sac passes on its contents to the internal 

 sac, partly in consequence of the rhythmic contractions of its wall and 

 partly in consequence of the processes of growth in the embryo itself, 

 then the nutritive masses are conducted to the embryo from the inner 

 yolk-sac through the intervention of the yolk-epithelium. Since, so 

 far as we know, there are no vessels in the external sac, its dis- 

 appearance cannot be accounted for in any other way. 



It has already been pointed out that the yolk-epithelium is a 

 closed unilaminar layer of cells extending round the whole yolk. In 

 consequence of its close connection with the yolk, the yolk-epithelium 

 must certainly be regarded as the inner layer of the Cephalopod 

 germ. The question now arises, What is its relation to the fttfun: 

 entodenn ? This latter first becomes perceptible in the following 

 way. 



About the time when the first rudiments of organs appear ex- 

 ternally on the embryo, there is seen, on the ventral side, contiguous 

 to the yolk, an epithelial plate which at first consists of only a few 



