CRUSTACEA. 519 



surface. There is formed, as in the Isopoda, a larval membrane 

 at about the time when the blastoderm is completed. Very 

 soon after this the egg loses its spherical shape, and becomes 

 produced into a pointed extremity the future abdomen which 

 is immediately bent over the ventral surface of the part in front. 

 The ventral curvature of the hinder part of the embryo at so 

 early an age stands in marked contrast to the usual condition of 

 Isopod embryos, and is only approached in this group, so far as 

 is known, in the case of Cymothoa. 



At the formation of the first larval membrane the blastoderm 

 cells separate themselves from it, except at one part on the 

 dorsal surface. The patch of cells adherent at this part gives rise 

 to a dorsal organ, comparable with that in Oniscus, connecting 

 the embryo and its first larval skin. A perforation appears in it 

 at a later period. 



The segments and limbs of the Amphipoda are all formed 

 before the larva leaves the egg. 



Cladocera. The segmentation (Grobben, No. 455) takes place on the 

 normal centrolecithal type, but is somewhat unequal. Before the close of 

 the segmentation there may be seen at the apex of the vegetative pole one 

 cell marked off from the remainder by its granular aspect. It gives rise 

 to the generative organs. One of the cells adjoining it gives rise to the 

 hypoblast, and the other cells which surround it form the commencement 

 of the mesoblast. The remaining cells of the ovum form the epiblast. By 

 a later stage the hypoblast cell is divided into thirty-two cells and the genital 

 cell into four, while the mesoblast forms a circle of twelve cells round the 

 genital mass. 



The hypoblast soon becomes involuted ; the blastopore probably closes, 

 and the hypoblast forms a solid cord of cells which eventually becomes the 

 mesenteron. The stomodaeum is said to be formed at the point of closure 

 of the blastopore. The mesoblast passes inwards and forms a mass ad- 

 joining the hypoblast, and somewhat later the genital mass also becomes 

 covered by the epiblast. The proctodreum appears to be formed later than 

 the stomodaeum. 



The embryo as first shewn by Dohrn passes through a Nauplius stage 

 in the brood-pouch, but is hatched, except in the case of the winter eggs of 

 Leptodora, in a form closely resembling the adult. 



Copepoda. Amongst the free Copepoda the segmentation and 

 formation of the layers have recently been investigated by Hoek (No. 512). 

 He finds that there is, in both the fresh-water and marine forms studied 

 by him, a centrolecithal segmentation similar to that of Palasmon and 

 Pagurus (vide p. 112), which might from the surface be supposed to be 



