Spermatogenesis in spiders. 563 



have made it clear, that the first maturation division is the reduction 

 division, as far as the sex-chromosome is concerned, and were a 

 Y-element present in the germ cells of spiders, it would separate 

 from the X-element during- this division. That is, the Y-element 

 would pass undivided to one pole and in the second maturation 

 division it would divide, presumably. The planosomes may divide 

 in the first maturation division and in the second division they may 

 appear bipartite although they do not usually divide. The fact 

 that the planosomes may be bipartite in the second division, has 

 been interpreted as indicating that they probably did divide at one 

 time, in the history of the spider in question. Now were we to 

 derive the planosomes from a Y-element, we should expect that they 

 would pass undivided to one pole during the first division and that 

 during the second division, they would divide. As a matter of fact, 

 their behavior is just the reverse of this and in order to explain 

 their origin from the Y-element, we should have to assume that 

 the Y-element secondarily acquired the division mechanism for the 

 first maturation mitosis and lost this mechanism for the second 

 division. At the present time, such an assumption would not seem 

 justifiable. 



In the case of the ctetosomes, Wilson's suggestion would not 

 have the objections offered for the planosomes. For during the 

 first division the ctetosomes do not divide. In the second division 

 they seem to divide. This behavior is much like that of the 

 Y-element, such as is found among many of the Hemiptera. The 

 fact that the ctetosomes show a close association for the accessory 

 chromosome during the growth period might also be interpreted 

 as indicating the close relation of the two. Were this explanation 

 correct, we should expect to find the Y-element persisting in some 

 primitive spider. Aside from the fact that no Y-element has been 

 tound as yet, there is no reason for not accepting it as the origin 

 of the ctetosomes, at least for the present. 



2. We turn to the second possibility that the planosomes 

 are derived from the microchromosomes. The behavior of these two 

 types of bodies is so closely similar that when Amaurobius sylvestris 

 was first examined, it was believed that a large number of these 

 m-chroraosomes was present. Further study made it clear that 

 the planosomes were too irregularly distributed to be regarded in 

 this way. 



The behavior of the m-chromosomes has been described by many 



