GAMMARUS. 863 



coming secondarily fused together in the interior of the 

 egg, so as to apparently fill the cleavage cavity. The 

 superficial cells finally contain only formative yolk, and 

 become separated from the nutritive yolk by a distinct 

 line. When the formation of the blastoderm is com- 

 pleted, a ventral thickening, the germ-disc, appears, the 

 ingrowing cells forming the mesoderm. According to 

 Eossiiskaya (18), the elements of the endoderm arise from 

 an immigration of separate blastomeres into the deeper 

 layers ; these, after dispersing in the food-yolk, soon 

 become arranged as two lateral endoderm bands, lying 

 superficially on the food-yolk. 



The germ-disc, at first round, spreads out into a 

 germ-band covering the whole ventral surface of the egg. 

 The anterior end becomes enlarged to form the cephalic 

 lobes from which the rudiments of the eyes and brain 

 arise, while the germ-band itself becomes divided up by 

 furrows into the separate body-segments. As the germ- 

 band lengthens, a ventral curvature of the abdomen is 

 brought about, and is seen throughout embryonic life. 

 During the differentiation of the germ-band, a dorsal 

 disc-shaped thickening of the blastoderm is formed. 

 When the larval integument develops, this dorsal organ 

 adheres closely to it, the central part becoming 

 invaginated to form a small cavity, which communicates 

 with the exterior by a small perforation, the micropyle. 

 It has been regarded by St. George (22) as a respiratory 

 apparatus. Later, when the heart develops, it degen- 

 erates. As the germ-band broadens, its lateral portions 

 grow up over the food yolk, forming the lateral parts of 

 the embryo, and ultimately coalesce in the mid-dorsal 

 line. 



The whole of the central nervous system arises as a 

 paired ectodermal thickening. Two longitudinal ecto- 



