82 NICHOLS— SPERMATOGENESIS ONISCUS ASELLUS LINN. [April 4, 



beginning the task of producing a new generation, individual cells 

 often equal the spermatocytes in size (Figs. lo and 13). In the 

 cytoplasm are occasionally seen irregular masses of a dull brown 

 tint (yolk ?), but neither sphere substance nor centrosomes are 

 apparent in the resting cell. 



In nuclei preparing to divide, the chromatin is seen to be 

 arranged in the form of slender, elongated threads, which, so far as 

 I have been able to discover, in no case form a continuous spireme 

 (Fig. 7). In the cytoplasm surrounding a nucleus of this character 

 are visible two minute black specks joined by a delicate thready 

 presumably the centrosome undergoing division. The nuclear 

 membrane at this stage begins to fade. Figs. 8, 9 and 10 show 

 stages immediately succeeding the stage shown in Fig. 7. The 

 threads have become shorter and thicker, the nuclear membrane has 

 entirely disappeared, and the centrosomes have become more widely 

 separated. The amount of segmentation of the thread varies in 

 different cells. In Fig. iib is shown a nucleus in which very little 

 segmentation has taken place, although the thread is considerably 

 thicker than that shown in Fig. 7. The linin threads joining the 

 chromosomes are of extreme delicacy and difficult to discover. 

 Occasionally, however, (Fig. 9) fine fibres may be seen stretching 

 from one chromatin thread to the next, The shortened and thick- 

 ened chromosomes then arrange themselves into an equatorial plate 

 (Fig. 12). The appearance of the plate, both in side and in pole 

 view, is irregular. The division of the chromosome into chromo- 

 meres and their longitudinal division is visible only in very thin 

 sections, which have been stained with iron haematoxylin and 

 rather strongly decolorized (Fig. 14). The centrosomes and 

 spindle-fibres of the spermatogonic, mitotic figure are not quite so 

 prominent as those of the spermatocytic divisions. The same is true 

 of the polar radiations. Central spindle-fibres are apparently entirely 

 lacking. After splitting of the chromosomes the halves diverge, in 

 the manner of the two legs of a pair of compasses, the divergence 

 commencing at one end, while at the other end the two halves 

 remain in contact (Fig. 15). 



A still later anaphase is shown in Fig. 17. The chromosomes 

 have become massed together, the spindle-fibres are beginning to 

 disappear and the centrosomes are almost lost to sight. The con- 

 striction of the cell body, observable to a slight degree at this 

 stage, becomes more marked and a membrane comes to separate 

 the two daughter cells (Fig. 19). 



