Fig. 240. Two stages in the development of Palæmon.
A. Nauplius stage.
B. Stage with eight pairs of appendages. op. eyes; at1. and at2. first and second antennæ; md. mandibles; mx1, mx2., first and second maxillæ; mxp3. third maxillipeds; lb. upper lip.
In all the forms the segmentation is followed by the formation of a blastoderm, completely enclosing the yolk, and thickened along an area which will become the ventral surface of the embryo. In this 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, Bullar).
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, 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 Palæmon, 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 invagination of the archenteron in Astacus, Palæmon, etc.
