476 M. Louise Nichols. 



not difficult to obtain. The follicles of Astacus and Homanis, on 

 the other hand, present comparatively few stages at any one time. 



Summary and Conclusions. 



The foregoing review and comparison shows that within the limits 

 of the Class Crustacea differentiation of the chromatin occurs to a 

 degree appreciable to our vision and in a way that is to a certain 

 extent characteristic of the orders included in the class. The dif- 

 ferentiation takes place as a divergence of form and relative size. 



Thus in the Amphipod little difference is noticeable in the size or 

 shape of the chromosones, but in the Isopoda two types are discov- 

 erable in the spermatocytes, those in which the components are joined 

 end to end forming rods or dumb-bells and those in which they are 

 joined side by side forming rings, these differences being accom- 

 panied by a somewhat more marked difference in size. The chromo- 

 somes of the Decapoda, though differing from each other but little 

 in shape, show some difference in size. On the whole, however, the 

 Crustacea as compared with that other large class of the Arthropoda, 

 the TIexapoda, exhibit less differentiation in the form and size of 

 the chromosomes, nothing comparable to the accessory chromosomes 

 having been found and none exceptionally large such as McClung 

 has described for the Orthoptera. 



As regards the number of chromosomes but little relation is evident 

 to the degree of development. In the lower forms they may be very 

 numerous, eighty-four (reduced number) in Artemia as described 

 by Brauer (1893) or comparatively few, eleven or twelve in Cyclops 

 (Ruckert, 1894). In the Decapoda they are likely to be numerous, 

 sixty in Hippa, fifty-eight in Astacus (Prowazek, 1902), while in 

 the Isopoda and Amphipoda there are not so many, Oniscus sixteen. 

 Idotea twenty-eight, Talorchestia eighteen. Braun, however (1907), 

 writing about the specific chromosome number of the genus Cyclops, 

 draws a parallel between the reduction in number of chromosomes 

 (from twenty-two in C. strenuus to six C. gracilis) and a gradual 

 reduction in the development of the rudimentary swimmeret. Accord- 

 ing to his account also, in C. viridis two of the chromosomes are much 

 smaller than the others and in C. prasenus one is smaller than the 

 others. 



