422 



yet completely risen to the surface (Samassa says that so far as obser- 

 vations went at the time he wrote (1893) in all Cladoceran eggs the 

 blastomeres rise to the surface in the 8 celled stage). 



The figure also shows the fat globule, the edge only of which is cut. 

 In sections which have been treated with xylol it, appears of course as 

 a space. 



The presence of a thick cuticle at this very early stage is a note- 

 worthy point. 



In the next stage obtained a complete blastoderm has been formed 

 all over the surface of the egg (Fig. 2). The yolk instead of being in the 

 form of small spheres embedded in protoplasm, is in larger blocks, 

 spherical, or polygonal from mutual pressure. No protoplasm can be 





£'■■ 8 9 ^-- ANTENNA 2 





Fig. 3. Section transverse to the antero-posterior axis of the embryo. 



seen between the yolk blocks, but it appears to be all aggregated in the 

 blastoderm. The protoplasm of the blastoderm however (in which cell 

 outlines cannot be made out) contains a certain number of small yolk 

 granules. None of these occur in the section figured. The section shows 

 that there is only one row of nuclei on the future dorsal surface of the 

 blastoderm, but on the ventral side the nuclei are in many layers. The 

 fat globule and cuticle are as in the previous stage. 



In the next stage figured (Fig. 3) the mesendoderm is beginning to 

 develop. As before, the dorsal and lateral walls of the embryo are thin, 

 with the nuclei in one layer, while ventrally the wall is very much thicker. 

 On the antero-dorsal surface of the embryo we find external to the thin 

 dorsal wall a layer of tall columnar cells appearing. This is the very 

 early appearing rudiment of the carapace. 



