DEVELOPMENT OF THE EXTERNAL FORM OF THE EMBRYO. 251 



is soon followed in its circumcrescence of the egg by a second layer, the 

 yolk-epithelium. Two areas can now be distinguished in the egg. 

 The germ-disc, which tonus the embryonic rudiment and which now 

 increases by the thickening of its margins (Figs. 112 and 113) only 

 later extends over the yolk, and a second, the yolk-sac, which at 

 first is bounded by the two cell-layers, the ectoderm and the yolk- 

 epithelium. A middle layer is added to the two cellular integuments 

 which at first cover the yolk-sac, this third layer appearing either 

 simultaneously with them or very soon after them (p. 277). 



The extension of the part known as the germ-disc over the yolk 

 varies greatly in different Cephalopods. In the egg of Sepia, which 

 is unusually rich in yolk, the embryonic rudiment is represented only 

 by a small cup-shaped region of the spherical egg. Here, therefore, 

 we can best speak of a germ-disc, and the yolk-sac is from the first 

 very large. In other Cephalopods, indeed, so far as is yet known, in 

 the majority, the embryonic rudiment and thus also the so-called 

 germ-disc spread over a much larger portion of the egg (Fig. 115), 

 but as, at a later stage, the embryonic rudiment again draws back 

 more towardsHhe animal pole, a yolk-sac is formed in these cases also 

 (Figs. 116- US). In the Cephalopod investigated by Grenacher, 

 there is hardly any development of an external yolk-sac (Fig. 126, 

 ]). 270) and the yolk-mass which is comparatively small is here found 

 enclosed by the embryonic rudiment at a very early stage. This 

 form would thus have to stand at the end of a series, the starting 

 point of which would be formed by Sepia with its unusually large 

 development of yolk. Transitionary forms between Sepia and the 

 Cephalopod described but not identified by Grenacher would be 

 found in such forms as Loligo, Octopus, Argonaida, in which the yolk- 

 sac is more and more reduced and the embryonic rudiment at the 

 first contains the larger part of the yolk. 



If we may judge from the large amount of yolk in the egg (p. 237) and the 

 large size of the yolk-sac in older embryos (No. 26) the condition of Eledone 

 in this respect ma} - resemble that of Sepia. 



It should here be mentioned that the cleavage and the formation of the 

 germ-layers in forms which differ somewhat in their later development is, so 

 far as is known, very much alike, and takes place in the way described above. 



In forms in which the rudiment of the embryo can early be dis- 

 tinguished from the large yolk-sac, the ectoderm, in the region of 

 the germ-disc consists of cubical cells while in the yolk-region it is 

 formed of flat cells. During its gradual extension over the yolk, 

 the blastoderm becomes ciliated. The filiation may extend over the 



