1905.] NATURAL SCIENCES OF PHILADELPHIA. 175 



whole abdominal visceral mass, then dissecting out the testes in distilled 

 water. 



The spermatogonia form the inner epithelium of the organ, and by 

 their last divisions the spermatocytes formed lose their connection with 

 this ceU layer and come to lie free within the lumen of the testis ; in this 

 cavity are found all stages of the growth period, the maturation 

 mitoses, the spermatids and spermatozoa. In any transverse plane of a 

 testis one finds the same series of stages. Not only in the arrangement 

 of the cells within the testis, but also in the process of spermatogenesis 

 this spider shows close resemblance to Peripatus; in both, e.g., the 

 longitudinal split of the bivalent chromosomes is very clear, and during 

 the synapsis stage the chromatin loops are not so densely massed but 

 that each may be distinguished. 



Only two clear cases of pole views of the equatorial plate of spermato- 

 gonia were found. On one of these (Plate X, fig. 41) exactly twenty- 

 eight chromatin elements could be distinctly counted. In the other 

 case the chromosomes were more densely grouped, and I could not be 

 certain whether there were twenty-eight or thirty of them ; it w^as possi- 

 ble that tw^o of them were already dividing in metakinesis. Tw^o of these 

 spermatogonic chromosomes are very small (S.) ; the subsequent history 

 of these could not be ascertained with any degree of certainty. There 

 are accordingly twenty-six larger chromosomes, all of which can be 

 recognized in the thirteen bivalent chromosomes of the first spermato- 

 cyte. All of these appear to be longitudinally halved during the ana- 

 phase, so that each first spermatocyte receives twenty-six daughter 

 chromosomes. 



There is no rest stage at any period of the spermatocytic history. 

 Shortly after the last spermatogonic mitosis commences the synapsis 

 stage (figs. 42-44). At its beginning (fig. 42) the daughter chromo- 

 somes are elongated threads, already commencing to join into pairs (at 

 the points lettered x). But two of them (N. 2) differ in maintaining 

 the dense contour and smooth outline characteristic of mitosis; these 

 are the heterochromosomes, and there is clearly one pair of them. 

 Accordingly, of the twenty-six large chromosomes of the spermatogonia 

 two are heterochromosomes, though they cannot be recognized in the 

 spermatogonic monaster stage nor yet in the preceding rest stage. In 

 following synaptic stages (figs. 43, 44) the twenty-six chromosomes 

 unite to form thirteen bivalent pairs. This takes place, as in Peripatus, 

 by an approximation or even close fusion of every two chromosomes of 

 similar length at their ends directed toward the central pole of the 

 nucleus (that one farthest removed from the greatest mass of cyto- 



