HARRIS : DIMORPHISM OF CAMBARUS. 59 



the sexual elements takes place, but that the limits of this period are 

 not definitely determined by the condition of development of the 

 germ -cells. In other words, the sexual condition of the animal is not 

 the only determining factor to be taken into consideration. This is 

 well shown by the variation observed in the testes of individuals in 

 the same form as well as by the similarity of those from individuals 

 of different form, and also by the occurrence of individuals in which 

 the alternation of forms does not regularly occur. 



SUMMARY. 



The principal points of this paper may be briefly summarized as 

 follows : 



1. The alternation of forms, which Faxon observed for (J. rusticiis 

 Girard and C. pt'opinquus Girard has been shown for C. immutiis 

 Hageri, and without doubt occurs in C. virilis Hagen. 



2. Exceptions to this alternation of form are to be found in adult 

 indviduals. 



3. No difference can be detected between the testes of first- and 

 second-form males taken at the same time of year, either as regards 

 gross anatomy or microscopic structure. So far as the presence of 

 sexual elements is concerned, therefore, the second-form male is as 

 capable of copulation as is the first-form. 



4. The second-form condition probably represents, in the adult in- 

 dividual, the period in which the greater part of the regeneration of 

 the sexual elements takes place. 



5. The parallel between the first- and second-form males in Cani- 

 harus and the spring and fall exuviation of Astacus is probably not 

 without significance. 



Laboeatoey of Zoology and Histology, 



University of Kansas, May 20, 1901. 



