240 CEPHALOPODA. 



spreads to right and left for an equal distance (Fig. 106 A, r and /). 

 A comparison of Fig. 106 B with the median sagittal sections of 

 later embryonic stages (Figs. 132, 133, p. 282) shows that the 

 animal pole of the egg (d) corresponds to the area of the dorsal 

 surface, the vegetative pole (v), on the conti'ary, to the ventral 

 surface. 



In the germ-disc we find the egg-nucleus and later the first cleav- 

 age-nucleus. Neither of these, however, quite coincides with the 

 animal pole of the germ-disc (Fig. 105 A) but occupies a somewhat 

 excentric position, i.e., it is slightly nearer the posterior edge of the 

 disc. The nucleus appears to be surrounded by a more hyaline zone 

 of protoplasm which passes externally into a more granular proto- 

 plasm. The protoplasm of the germ-disc is in any case distinguished 

 by its granular character from the thin protoplasmic layer that 

 surrounds the whole egg. The thickness of the disc appears to vary 

 in different cases, so far as can be gathered from the statements of 

 authors. This is perhaps to be accounted for by the various stages 

 of development at which the disc is represented. 



A vitelline membrane is apparently never developed in the eggs of' 

 the Cephalopoda. The membrane, which is perforated by the micropyle 

 and is often very tough, is secreted by the follicular epithelium (Ussow, 

 Vialleton) and may therefore be termed the chorion. Between this 

 egg-shell and the surface of the egg there is a somewhat wide space 

 filled with clear albuminous fluid. This enables the embryo to extend 

 considerably within the chorion, which is itself said to be extensible, 

 and thus to admit of further growth on the part of the embryo. 



At the animal pole, in the mature egg, there are two polar bodies, 

 one of which has been observed to divide (Loligo and Argonemtd) and 

 give rise to a third body (Fig. 105, rk). Such division is said by 

 Vialleton not to occur in Sepia, but the fact that one of the two 

 polar bodies possesses two nuclei shows that these bodies, even in 

 Sepia, are quite normal. Vialleton denies that there is any con- 

 stant relation between the polar bodies and the first cleavage-plane, 

 but the deviations from such a relation are very rare, and from his 

 own figures and those of other authors this line of cleavage is seen to 

 occur in the closest proximity to the polar bodies. 



2. Cleavage and Formation of the Germ-layers. 



The cleavage of the egg is incomplete, a fact connected with the 

 great abundance of the food-yolk in it and the distribution of this 



