Studies in Crustacean Spermatogenesis. 471 



As to the manner in which the chromosomes arise, the Decapoda 

 and Talorchestia follow one method, the Isopoda another. In the 

 former no spireme is fonnd, nor even separate elongated threads; 

 instead proehromosomal areas appear in the nucleus and the network 

 becomes less clear and regular. The prochromosomes gradually in- 

 crease in size until they are fully developed chromosomes. (PI. 1. 

 T, 2-3 ; Hi, 1-^ ; A, 1-2 ; Ho, 1-2.) The chromosomes of the Isopoda, 

 on the other hand, arise as more or lestS elongated threads, a coarsen- 

 ing and breaking up the network as it were, which gradually shorten 

 and condense to form mature chromosomes. Those of Idotea are 

 generally crescent-shaped and are from the first somewhat shorter and 

 more compact than those of Oniscus. (Figs. I, 2-3; O, 1-2.) The 

 latter difference persists even in mitosis. (I, 4; O, 2.) The centro- 

 some of Idotea appears to arise within the nucleus and that of Onis- 

 cus outside it, and correlated with this is a greater tendency towards 

 polarity in the spermatogonia of Idotea, the bends of the horseshoe- 

 shaped chromosomes pointing towards the center of the cell (Fig. 

 1,2.) ■ _ 



Synapsis. — Some difference is noticeable in the mode of transi- 

 tion from the last spermatogonia to the synapsis. In Idotea, Hippa 

 and Homarus there is evidence of an approach of chromosomes in 

 parallel pairs. (Figs. I, 5-6; Hi, 6-7; Ho, 4.) In Hippa and 

 Homarus this is not at first accompanied by a recession of cytoplasm 

 from the nuclear wall as is the case with Idotea. Gradually, how- 

 ever, the cytoplasmic connections become fewer and the chromosomes 

 more closely associated. Those of Homarus and Astacus crowd to- 

 gether at one side of the nucleus, but I have not observed this condi- 

 tion in Hippa. (PL 1. Ho, 5; A, 3.) There the chromosomes 

 mass together near the center of the cell for a very short period and 

 then scatter more or less. During this time the members of each 

 pair come into intimate union. Fig. Hi, S represents this union, 

 Figs. 9 and 10 the immediately succeeding stages in which the 

 ruembers of each pair commence and continue to diverge. It is prob- 

 able that a longitudinal splitting of the threads occui's at this time, 

 although it is not always evident. The chromosomes of Idotea first 

 appear as parallel and elongated thickenings of the nuclear network. 



