516 FORMATION OF THE LAYERS. 



In Asellus aquaticus there is a centrolecithal segmentation, 

 ending in the formation of a blastoderm, which appears first 

 on the ventral surface and subsequently extends to the dorsal. 



In Oniscus murarius, and Cymothoa the segmentation is 

 partial [for its peculiarities and relationship vide p. 120] and a 

 disc, formed of a single layer of cells, appears at a pole of the 

 egg which corresponds to the future ventral surface (Bobretzky). 

 This layer gradually grows round the yolk partly by division of 

 its cells, though a formation of fresh cells from the yolk may 

 also take place. Before it has extended far round the yolk, the 

 central part of it becomes two or more layers deep, and the cells 

 of the deeper layers rapidly increase in number, and are destined 

 to give rise to the mesoblast and probably also to part or the 

 whole of the hypoblast. In Cymothoa this layer does not at 

 first undergo any important change, but in Oniscus it becomes 

 very thick, and its innermost cells (Bobretzky) become imbedded 

 in the yolk, which they rapidly absorb ; and increasing in 

 number first of all form a layer in the periphery of the yolk, and 

 finally fill up the whole of the interior of the yolk (fig. 241 A), 

 absorbing it in the process. 



It appears possible that these cells do not, as Bobretzky believes, origin- 

 ate from the blastoderm, but from nuclei in the yolk which have escaped 

 his observation. This mode of origin would be similar to that by which yolk 

 cells originate in the eggs of the Insecta, etc. If Bobretzky's account is 

 correct we must look to Palsemon, as he himself suggests, to find an explana- 

 tion of the passage of the hypoblast cells into the yolk. The thickening of 

 the primitive germinal disc would, according to this view, be equivalent to 

 the'invagination of the archenteron in Astacus, PalEemon, etc. 



Whatever may be the origin of the cells in the yolk they no 

 doubt correspond to the hypoblast of other types. In Cymothoa 

 nothing similar to them has been met with, but the hypoblast 

 has a somewhat different origin ; being apparently formed from 

 some of the indifferent cells below the epiblast, which collect as 

 a solid mass- on the ventral surface, and then divide into two 

 masses which become hollow and give rise to the liver caeca. 

 Their fate, as well as that of the hypoblast in Oniscus, is dealt 

 with in connection with the alimentary tract. The completion 

 of the enclosure of the yolk by the blastoderm takes place on 

 the dorsal surface. In all the Isopods which have been carefully 



