1902.] NICHOLS— SPERMATOGENESIS ONISCUS ASELLUS LINN. 81 



the vas deferens, the spermatogonia again fill the apical region, and 

 the cycle is repeated. 



The invasion of the follicle cells begins, as a rule, when the germ 

 cells are in an advanced prophase and may continue later. Many 

 of the germ cells likewise degenerate, and they, together with the 

 follicle cells, form a disintegrated mass in which the spermatids lie. 

 In young follicles, which have not as yet matured sperm, the basal 

 region is filled with follicle cells, the apical region with spermato- 

 gonia. This is sometimes true also of older follicles which have 

 recently discharged the sperm. 



It will thus be seen that a series of stages, illustrating the com- 

 plete history of the changes through which the germ cell pass, can 

 be obtained only by an examination of numerous testes. Dupli- 

 cates are often obtained and some of the stages occur very infre- 

 quently, probably owing to a greater rapidity of development at 

 certain periods. 



This study was begun in the latter part of February. In March 

 or April, according to the rigor of the weather, the land Isopods 

 in the vicinity of Philadelphia commence to breed. The breeding 

 season continues during the summer months. There are, in a single 

 year, several cycles of development of the reproductive elements ; 

 the exact number I have not determined. It is therefore pos- 

 sible, at almost any time of year, by examination of a sufficient 

 number of individuals, to procure a complete series of develop- 

 mental stages. 



II. Spermatogenesis. 

 I. Spermatogonia. 



The resting spermatogonia are distinguished from the follicle 

 cells by their smaller size, the distinctness of the cell walls, and by 

 the fact that in their nuclei the chromatin masses are of smaller 

 size and show indications of an arrangement into a network (Fig. 

 \\d). They possess a prominent true nucleolus of more or less 

 rounded form. Some cells contain one or more smaller nucleoli. 



It is impossible to determine the exact number of spermatogonic 

 divisions. They are probably numerous, since it must require a 

 considerable number of divisions of the spermatogonia remaining 

 in the follicles to fill the space left vacant by the discharged sperm. 

 The cells vary somewhat in size. When the apical region of the 

 follicle is filled with spermatogonia the individual cells are small, 

 but when the follicle is not well filled and the spermatogonia are 



PROC. AMER. PHILOS. SOC. XLI. 168. F. PRINTED MAY 7, 1902. 



