CRUSTACEA. 429 



area the blastoderm is formed of at least two layers of cells an 

 external columnar epiblast, and an internal layer of scattered cells 

 which form the mesoblast and probably in part also the hypoblast 

 (Oniscus, Bobretzky; Cymothoa, Buttar). 



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. 99] 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 num- 

 ber, 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 

 verv 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, 

 originate 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 explanation 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 invaginatioii of the archenteron in Astacus, 

 Palsemon, 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 cseca. Their fate, as well as that of the hypo- 

 blast 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 studied, there appears before any other organ a provisional 

 structure formed from the epiblast and known as the dorsal organ. 

 An account of it is given in connection with the development of the 

 organs. The general external changes undergone by the larva in its 

 development are as follows. The ventral thickened area of the 

 blastoderm (ventral plate) shapes itself and girths nearly the whole 



