1902.] NICHOLS — SPERMATOGENESIS ONISCUS ASELLUS LINN. 79 



membrane for the secretory cells. It is continuous with the layer 

 of cells which line the narrow portion of the vas deferens and is 

 similar to it in structure (Fig. 3). The narrow portion of the vas, 

 as a rule, is covered externally by dark pigment, thus forming a 

 marked contrast in the fresh state to the milk-white walls of the 

 anterior portion (Fig. i). Between the pigment layer and the 

 lining cells, delicate muscle-fibres are occasionally discernible. 



The three lobes of the testis are seen in section to be three folli- 

 cles (Fig. 2). Each follicle is covered by a thin membrane which 

 is provided with delicate muscle-fibres (Fig. 5W, /). The margins 

 of the follicle are occupied by large nuclei of unsymmetrical outline, 

 containing irregular blocks of chromatin interspersed with finer 

 granules. Cell boundaries between these nuclei are not visible. 

 They can sometimes be seen to be undergoing amitotic division, of 

 a character similar to that described by vom Rath (1891) for 

 Astacus (Fig. 5/, c). In ibllicles of a certain stage of development 

 these nuclei, as will be explained later, are subject to degeneration. 



The strands of tissue, by means of which the follicles are 

 suspended from the body wall, are made up of cells which also 

 divide amitotically and which are similiar in appearance to the 

 follicle nuclei, inasmuch as their outlines are irregular, but the 

 blocks of chromatin are of larger size and the nuclei are separated 

 from each other by distinct cell walls (Fig. 6). 



The interior of the follicle, except during the migration of the 

 follicle cells, is occupied entirely by the germ cells, which are in 

 differing stages of development in the three follicles of one side. 

 Corresponding follicles of opposite sides of the body contain, how- 

 ever, germ cells which have developed to very nearly the same 

 degree. 



Fig. 2 shows, in a typical case, the comparative degrees of devel- 

 opment to which the cells of the three follicles have attained. Each 

 follicle may be divided into two principal regions of growth, com- 

 posed of cells of different generations and of different degrees of 

 development. Thus, in the most posterior of the follicles (a), the 

 apical third is occupied exclusively by spermatogonia, some of 

 which can be seen in mitosis ; the basal region, on the other hand, 

 by spermatids in a not very advanced stage. Follicle cells occur 

 on the outside of the follicle, being especially abundant in the basal 

 region. In the adjacent follicle (^), the apical two-thirds is occu- 

 pied by cells in the synapsis stage, the remaining portion by sper- 



