MOLLUSC A. 201 



special capsules formed in the oviduct, which differ considerably in 

 ttie different members of the group. 



In the case of Argonauta each egg is enveloped in an elongated cap- 

 sule provided with a stalk. By means of the stalk the eggs are attached 

 together in hunches, and these again are connected together and form 

 transparent masses, which are placed in the back of the shell. In Octopus 

 the eggs are small and transparent : each of them is enclosed in a stalked 

 capsule. In Loligo the eggs are enveloped in elongated sack-like gelatinous 

 cords, each containing about thirty or forty eggs. The cords are attached 

 in bunches to submarine objects. In Sepia each egg is independently 

 enveloped in a spindle-shaped black capsule, which is attached to a stone 

 or other object. 



In a decapod form with pelagic larva?, described by Grenadier (No. 280), 

 the eggs were enclosed in a somewhat cylindrical gelatinous mass. In each 

 mass there were an immense number of eggs arranged in spirals. Each 

 ovum was enclosed in a structureless membrane, within which it floated in a 

 colourless albumen. 



The ovum itself within the capsule is a nearly homogeneous 

 granular mass, without a distinct envelope. Development com- 

 mences by the segregation, a f the narrow pole of the ovum opposite 

 the egg-stalk, of the greater part of the protoplasmic formative 

 material 1 . This material forms a disc equivalent to the germinal 

 disc of meroblastic vertebrate ova. The germinal disc in Sepia and 

 Loligo does not, however, undergo a quite symmetrical segmentation 

 (Bobretzky, No. 279). When eight segments are present, two of them 

 close together are much smaller and narrower than the remainder; and 

 when, in the succeeding stages, small segments are formed from the 

 inner ends of the large ones, those derived from the two smaller seg- 

 ments continue to be smaller than the remainder : so that throughout 

 the segmentation one pole of the blastoderm is formed of smaller 

 segments, and the blastoderm exhibits a bilateral symmetry 2 . The 

 partial segmentation results in the formation of a blastoderm cover- 

 ing one pole of the egg, but, unlike the vertebrate blastoderm, formed 

 of a single row of cells. This blastoderm very soon becomes two or 

 three cells deep at its edge, and the cells below the surface constitute 

 the layer from which the mesoblast and hypoblast originate (fig. 

 110 ms}. The origin of the mesoblast at the edge of the blastoderm is 

 a phenomenon equivalent to its origin at the lips of the blastopore 

 in so many other types. The external layer forms the epiblast. 



The whole blastoderm does not take its origin from the segmen- 

 tation spheres, but, as was discovered by Lankester (282), a number 

 of nuclei arise spontaneously in the yolk outside the blastoderm, 

 around which cell-bodies become subsequently formed. They make 

 their appearance near to, but not at the surface, extending first in a 



1 In Octopus and Argonauta (Lankester) as soon as the blastoderm is completed the 

 egg reverses its position in the egg-shell ; the cleavage pole taking up a position nearest 

 the stalk. 



a I do riot know the relation of this axis of symmetry to the future embryo. 



