510 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tlie appendages, with special reference to those of NympJion stromii and 

 PJioxichilidiiim femoratum. In a second chapter the author discusses the 

 larval stages of the last-named species. 



€. Crustacea. 



Development of Amphipoda.*— Madlle. M. Eossiiskaya has studied 

 the development of Orchestia littorea. The egg, which is deep-violet in 

 colour, and oval in form, is covered by a single membrane, the chorion. 

 The tirst two blastomeres differ slightly in size, and the later segments 

 are still more unequal. Segmentation stops when thirty-two blastomeres 

 have been formed ; protoplasm is then detached from the yolk in the 

 form of amoeboid cells, which become scattered over the whole surface 

 of the egg. The blastoderm is formed by the approximation of from 

 four to ten cells, which contract their pseudopodia, become polyhedral, 

 and form a small, irregular, white spot. Around this, cells elongate aijd 

 become divided in the direction of the radii of a circle, whose centre is 

 the blastodermic spot. Although the cells on the dorsal surface multiply, 

 their number does not increase ; this shows that they migrate to the 

 ventral surface, where they aid in enlarging the blastodermic spot. 



After the blastoderm has completely covered the ventral surface it 

 elongates at one pole much more rapidly than at the other ; the former 

 of these poles is the oral, and the other the aboral. At last the whole 

 surface of the egg is covered by the blastoderm. 



During the formation of the endoderm, very interesting sections 

 were obtained ; these showed that each blastodermic cell of the ventral 

 surface consists of two parts ; the external portion has a condensed pro- 

 toplasm which stains well, while the internal part stains feebly, and 

 seems to contain yolk ; in several of these cells there are two nuclei. 

 These sections show exactly the mode in which nutrient matter is taken 

 in. "When the blastoderm covers about two-thirds of the surface of the 

 egg, a dorsal organ is formed on one of its sides ; this has the appearance 

 of a funnel, and is made up of large pyriform cells with large nuclei. 

 When the blastoderm completely envelopes the nutrient yolk, it secretes 

 the larval tissue, which is very delicate, transparent, and structureless. 



As in Oniscus murarius, the endodermic cells arise from a small part 

 of the blastoderm. When the dorsal organ has taken up its definite 

 position on the median line of the dorsal surface, the endodermic cells, 

 which, till now, have been multiplying in the interior of the yolk, 

 migrate towards its surface, and form two lateral bands, which are 

 applied against the abdomen ; these bands are the walls of the mid- 

 intestine. Shortly after the intestinal tube is completely closed, the 

 cells which form it change in appearance ; instead of being flattened and 

 solid they become large, prismatic, and so charged with vacuoles that 

 the protoplasm only forms a delicate layer on their walls. There then 

 appear three grooves, two of which are dorsal and one ventral ; the 

 former cut off, so to say, the true intestine from the intestinal sac, the 

 latter divides the rest of the intestinal tube into two hepatic sacs. 



The gonads are formed thus ; in the dorsal wall of the intestine, at 

 the two lateral points at which they touch the hepatic sacs, the epithelial 

 cells beconie cylindrical in form, and multiply rapidly ; the cells of the 

 hepatic sacs, where they touch the intestine, simultaneously undergo the 

 same changes. Thus there are formed two solid masses of cells, placed 



* Bull. Soc. Imp. Nat., 1888 (1889) pp. 561-81 (2 pis.). 



