I 28 JAMES O. FOLKV. 



During the transformation of such spermatogonia into spermato- 

 cytes they undergo an increase of 15 to 20 per cent, in size by 

 the time they have attained the late pachytene stage. 



2. Leptonema, Pachynema and Zygonema. At the beginning of 

 maturation, the chromatin nucleoli seen in the spermatogonia 

 disappear. The leptotene threads are formed by a condensation 

 of the chromatin granules on the linin network of the nucleus. 

 The nucleus becomes filled with a mass of fine intertwining 

 threads (Figs. 18 and 19) of which it is impossible to determine 

 the exact number. From this step the succeeding stages of pre- 

 synizesis follow each other in rapid succession. Fig. 20, an early 

 zygonema, shows some irregular pairing of the threads. Figs. 

 21 and 22, judging from the increasing proportion of thick 

 strands, show further advancement in the pairing of these 

 threads. Fig. 23 represents a late zygonema or early pachynema 

 in which the fusion has been completed in the mid regions of 

 the threads, but in which the ends still remain single. Fig. 24 

 shows a stage, the pachynema, in which the fusion has been 

 completed and a definite spireme is formed containing approxi- 

 mately one half the number of threads of chromatin found in 

 preceeding stages. Although a definite count of these threads 

 has not been attempted, by comparing this stage with the 

 previous leptotene stage one can readily see it displays fewer 

 threads. 



Synizesis and Diakenesis. Soon after the formation of the 

 spireme of pachytene threads, the latter begin to concentrate 

 toward the periphery of the nucleus (Fig. 25), leaving the center 

 clear, with only a few chromatin strands connecting the peripheral 

 masses. This massing of the pachytene loops around the nuclear 

 membrane marks the beginning of synizesis (Fig. 25). There 

 seems to be a definite tendency for the chromatin material to 

 concentrate (Fig. 26) into one or the other of two areas at opposite 

 sides of the nucleus, thus indicating a marked polarity. Fig. 2~ 

 shows a still more advanced stage of synizesis in which the 

 chromatin is collected in the form of loops, principally toward 

 one pole of the nucleus. The chromatin strands at the opposite 

 pole are concentrated and do not show the looped condition. 

 Two loops or rings (Fig. 27) can commonly be identified in this 



