250 



CEPHALOPODA. 



from the germ-disc ; they increase greatly in number and at first unite to 

 form a cell-layer which not only penetrates beneath the disc (Fig. 114 A-C r 

 de), but also extends beneath the ectoderm over the whole of the yolk. The 

 formation of this yolk-epithelium and the thickening of the edge of the 

 disc which affects the whole of its periphery are clearly illustrated in Pig. 

 114 A-C. These figures show, at the same time, how the yolk-epithelium 

 presses in towards the middle of the disc, as assumed by both Vialleton and 

 Watase. 



31^v®\5? 



*~ a. ^ 



V 



1 mt\®T:~-Jis. 

 ! — d. 



; : "'.- — d. 



■ 





Fig. 114. — Sections through a marginal portion of the germ-disc (ks) of Sepia officinalis 

 (after Vialleton). d, yolk ; de, yolk-epithelium ; /•, the thickened edge of the 

 germ-disc. 



The separation from the superficial or ectodermal layer of the cell- 

 mass, formed by the thickening of the margin of the disc, gives rise 

 to the layer claimed by earlier authors as the mesoderm. So as to 

 understand the real significance of this layer, we must follow its 

 further fate. Since, to do so, we should have to pass on to some- 

 what later stages, it will be advisable first to turn our attention to 

 the external changes taking place in the germ-disc or in the whole 

 blastoderm. 



3. The Development of the External Form of the Embryo. 



While the formation of the yolk-epithelium and the simultaneous 

 thickening of the edge of the germ-disc are taking place, another rapid 

 increase in number of cells occurs in the superficial layer of the disc, 

 and results in the gradual extension of this upper cell-layer over the 

 whole egg. This layer is known as the blastoderm, although this 

 term is not cpiite correct for, apart from the fact that a differentia- 

 tion of this layer has already taken place round the animal pole, it 



