SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 63 



As has been pointed out, the acinus or spermatocyst is the end 

 product of the tubule formation and the beginning of spermato- 

 genesis. The definitive germ cells in the acini approach the size 

 and appearance of the primordial germ cells before spermatogenesis 

 begins. At this period the nuclei of these cells measure from 

 7.2 to 9ju, in diameter. This is the largest size they ever reach. 

 The cell dimensions are hard to measure because the entire acinus 

 presents the aspect of a syncytium (Fig. 36). On account of the 

 size and what is to follow, it would seem proper to term these cells 

 primary spermatocytes. The next thing that happens to the acini 

 is the doubling of their volume and the number of their cells. 

 Evidently cell division has taken place. The picture has changed 

 radically (Fig. 37, ssc). The nuclei no longer show a clear aspect 

 with more or less thread-like chromatin ; the stain is darker, the 

 chrotnatin is broken up into a great many rod-like structures ; the 

 nuclear membrane is absent and the cytoplasm is greatly dimin- 

 ished. The nuclei now measure on the average 5.96^ in diam- 

 eter. This might be considered a secondary spermatocyte stage. 



Apparently another division takes jplace as the number of nuclei 

 doubles. This time there is no appreciable increase in size of the 

 acini. The size of the nucleus, however, has diminished to 2. 5 p., 

 which is practically one half of the previous stage. The color has 

 not changed, but the cytoplasm is reduced to a minimum ; in fact, 

 it can hardly be demonstrated with general methods of staining 

 (Fig. 37, sds}. It is probable that we are dealing with the 

 spermatid stage. 



In the next stage the metamorphosis of the spermatids into 

 spermatozoa is quite evident. For the first time the acinus be- 

 comes luminated as the developing spermatozoa move toward the 

 very thin, homogeneous wall of the acinus, where they form a com- 

 plete layer of one layer of cells thick. The tail differentiates and 

 the spermatozoa are all oriented so that the tail is free in the lumen. 

 The head, or rather the nucleus, gradually decreases in size as it 

 elongates. The final dimensions are 1.35 x 2.55. There is also a 

 decrease of size in the entire acinus or spermatophore, which meas- 

 ures about 50 ft in diameter, or half that of the secondary spermato- 

 cyte stage. The heads of the spermatozoa are so closely pressed 

 together that they appear like a single layer of epithelial cells (Fig. 



