CRUSTACEA. 431 



ventral plate does not extend nearly so far round to the dorsal side 

 as in Asellus, in consequence of which the ventral surface is not nearly 

 so convex as in other Isopoda. At the same time the telson is early 

 formed, and is bent forwards so as to lie on the under side of the part 

 of the blastoderm in front. In having this ventral curvature of the 

 telson Cymothoa forms an exception amongst Isopods ; and in this 

 respect is intermediate between the embryos of Asellus and those of 

 the Amphipoda. 



Amphipoda. Amongst the Amphipoda the segmentation is 

 usually centrolecithal. In the case of Gammarus locusta (Ed. van 

 Beneden and Bessels, No. 503) it commences with an unequal 

 but total segmentation like that of the Frog (vide p. 80), and the 

 separation of a central yolk mass is a late occurrence ; and it is 

 noticeable that the part of the egg with the small segments event- 

 ually becomes the ventral surface. In the fresh-water species of 

 Gammarus (G. pulex and fluviatilis) the segmentation is more like 

 that of Insects ; the blastoderm cells being formed nearly simultane- 

 ously over a large part of the surface of the egg. 



Both forms of segmentation give rise to a blastoderm covering 

 the whole eofff, which soon becomes thickened on the ventral surface. 



OO " 



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 con- 

 trast 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 em- 

 bryo 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 



