CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 249 



RAY LANKESTER assumes, with considerable inherent probability, that the 

 vitelline membrane owes its origin to cells (or nuclei) freely distributed in the 

 yolk, these being at once comparable to the yolk-cells and merocytes in the 

 eggs of the Arthropods and Vertebrata which are also very rich in yolk. 

 These nuclei are said to shift to the surface, to become surrounded with 

 protoplasm and to unite to form the vitelline membrane. 



Other investigators (e.g., Ussow) have been inclined to derive the cells of 

 the vitelline membrane rather from the deeper layers of the germ-disc, and 

 thus from the "mesoderm." In any case, this enveloping membrane of the 

 yolk has the same significance as the vitellophags in the Arthropoda and the 

 Vertebrata. As the terms vitelline membrane and yolk-integument are not 

 specially happy, being commonly used in another sense, we shall give this 

 cellular integument another name (also applied to it by RAY LANKESTER), 

 calling it the yolk-epithelium. 



During the formation of the yolk-epithelium, the superficial cell-layer, the 

 ectoderm, has spread over the whole of the food-yolk, which thus, beyond the 

 germ-disc, is covered by two cell-layers. Besides this, there is, as far as the 

 germ-disc extends, the "mesodermal" cell-mass lying between the ectoderm 

 and the yolk-epithelium. The three germ-layers are thus apparently repre- 

 sented, if we may for the time assume the yolk-epithelium to be the entoderm. 

 The difficulty arises, however, that the }-olk-epithelium is not found to be 

 connected with the formation of the enteron, which owes its origin rather to 

 the breaking up of the middle layer. 



Having now arrived at some comprehension of the germ-layers 

 from which the Cephalopodan body is built up, we must trace the 

 way in which the latter originates, as described in the most recent 

 accounts. 



We traced the origin of the germ up to the point at which the 

 animal pole of the egg is covered by a unilaminar plate of polygonal 

 cells, and at which the irregular peripheral cells of this plate begin 

 to detach themselves from it (Fig 112). Almost simultaneously with 

 this process, the thickening of the edge of the plate already mentioned 

 occurs, i.e., the plate here becomes multilaminar through more active 

 increase in number of its cells (Figs. 112, vd, and 113). This is the 

 process which was described by earlier authors as the formation 

 (dela mi nation) of the mesoderm. 



Before the layer which has formed in this way loses its close connection 

 with the superficial layer of the germ-disc, the cells previously detached from 

 that disc undergo, according to VIALLETON, an essential alteration. Their 

 cellular character disappears, they are no longer distinctly although irregularly 

 bounded, but now appear as a syncytium, i.e., as nuclei lying in the thin 

 layer of protoplasm that surrounds the food-yolk. There can be no doubt 

 that we have before us, in them, the same nuclei which, according to 

 LANKESTER, give rise to the yolk-epithelium. This last significance actually 

 belongs to these cellular structures which, according to VIALLETON, arise 



