Spermatogenesis in spiders. 565 



different origin for the ctetosomes and planosomes, the former 

 being- derived from a Y-element and the latter from degenerating 

 m-chromosomes. 



Function of ctetosomes and planosomes. 



Whether the supernumeraries have any function in the germ 

 cells of spiders cannot be determined by a pure cytological study. 



As will be pointed out, however, it seems probable that the 

 ctetosomes may play a part in heredity. 



With regard to the planosomes there is no evidence that they 

 are concerned with inheritance. No variation has been noted in 

 the spiders which carry a different number of these bodies but, of 

 course, it is impossible to say whether or not they may have a 

 function. 



5. Autosomes. 



Wilson, in his recent paper (Study VIII), has given such a 

 comprehensive discussion of the general problems relating to the 

 growth period, to the question of synapsis and the time of the 

 reduction division, that it will be unnecessary to discuss these subjects 

 in a detailed comparative way. From the telophase of the last 

 spermatogonial division up to the formation of the spermatids, the 

 changes going on in the autosomes may be conveniently divided 

 into a number of periods for spiders, as was done by Wilson 

 for Oncopeltus and Lygaeus. A full description has been given of 

 these stages in the foregoing pages, so we shall only give an out- 

 line here. 



a) Immediately following the prophase of the last spermatogonial 

 division, there is a rest period for all spiders, when the autosomes 

 become diffuse and lightly staining, while the hétérochromosome 

 elements retain their compact form. This stage is followed by the 

 gradual collection of the autosomes into chromatin knots. 



b) Formation of the chromatin knots from the reticulum of the 

 resting stage. These knots are massive bodies, more or less irre- 

 gular in outline, and are connected to each other by very fine linin 

 threads. The accessory chromosome remains as a double nucleolus 

 and can be distinguished by its large size and smooth outline. 



c) No distinct "unravelling stage" has been found for any of 

 the spiders studied. The leptotene-threads seem to arise from the 

 chromatin knots, by a process of elongation. 



