SPERMATOGENESIS OF CANCER MAGISTER. 285 



distinctly opened up at one end and temporarily fused at the 

 opposite end. The two pairs of these threads continue to diverge 

 in opposite directions until they ultimately form X's (Fig. 23). 



The pairs of threads on opposite sides of the central fusion 

 point of each X now migrate closer to each other until they come 

 to lie almost parallel. The central fusion point next disappears, 

 and four thin threads arranged in two parallel rows are produced. 

 A hasty glance at this arrangement makes it appear as if the 

 geminus from which the four threads were derived, was traversed 

 by a longitudnal and a transverse cleavage plane (see Fig. 23). 

 However, prolonged and careful study reveals the true nature of 

 the case, that the threads under discussion are the results of two 

 longitudnal splits of each geminus. 



The tetrads are soon formed. Each thin thread shortens and 

 thickens into a spherical chromatin mass (Figs. 23 and 24), so 

 that very soon every geminus is changed into four spherical 

 chromosomes, representing a tetrad (Figs. 23-25). Each tetrad 

 thus contains four univalent chromosomes. In the next step 

 pairs of these univalent chromosomes fuse, resulting in two large 

 bivalents attached to each other by linin strands. This con- 

 dition is particularly well shown in Figs. 24 and 25. The con- 

 densation of the bivalents continues until they are changed into 

 dumb-bell shaped structures (Fig. 25). The growth process is 

 now completed and the cell is ready to undergo reduction. 



It is quite evident from the above discussion that in Cancer 

 magister we apparently have to deal with a case of parasynapsis, 

 or side by side conjugation of the chromosomes. This con- 

 clusion was reached after prolonged observations upon numerous 

 excellent smear preparations, which contained an abundance of 

 synapsis material. The essential steps closely resemble those of 

 Cambarus virilis (Fasten, '14). 



(6) Reduction Division.- In the last stages of the growth 

 period, the nuclear wall begins to break down (Fig. 24). The 

 two centrosomes, which have been formed by a division of the 

 original centrosome, migrate to opposite poles, and thin spindle 

 fibers make their appearance between them and the chromosomes. 

 The chromosomes are soon pulled to the equator of the cell to 

 undergo reduction. In Fig. 25, the dumb-bell shaped bivalents 



