MOLLUSCA. 



243 



mences by the segregation, at the narrow pole of the ovum oppo- 

 site the egg-stalk, of the greater part of the protoplasmic forma- 

 tive 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 symme- 

 trical 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 segments continue 

 to be smaller than the remainder: so that throughout the seg- 

 mentation 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 

 covering one pole of the egg, but, unlike the vertebrate blasto- 

 derm, 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. 1 10 ms). The origin of the meso- 

 blast 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 seg- 

 mentation 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 ring-like series in advance of the 

 margin of the blastoderm, but subsequently appearing indiscrim- 

 inately over all parts of the egg. They take no share in form- 

 ing the epiblast, but would seem, according to Lankester, to 

 assist in giving rise to the lower layer cells, and also to a 

 layer of flattened cells which eventually completely encloses the 

 yolk, and may be called the yolk membrane. The cells of the 

 yolk membrane first of all appear at the thickened edge of the 



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. 



2 I do not know the relation of this axis of symmetry to the future embryo. 



1 6 2 



