spermatogenesis ^'Jl 



that year we find the first statement that one chromosome behaves 

 differently from the others (Henking '90). Unfortunately there is 

 the greatest confusion in the results for the next decade; but since 

 Montgomery's suggestion ('01 a) that synapsis means the conjuga- 

 tion of homologous maternal and paternal chromosomes, and its 

 confirmation by Sutton's work on Brachystola ('00, 02, '03), there 

 has been greater accord. As a consequence of this, certain funda- 

 mental theories are coming to rest on a firm foundation. The chro- 

 mosomes are shown to keep their individuality from one cell gener- 

 ation to another. The real reduction in number is proved to be 

 brought about by the joining of each paternal to a corresponding 

 maternal chromosome in synapsis. It is found to make no differ- 

 ence whether the reducing or equational division comes first, but 

 the distinction between these two divisions is constant, the one 

 being the separating of the individual spermatogonial chromo- 

 somes, the other a simple splitting of these univalent chromosomes. 

 In addition to this, recent work indicates that there is usually 

 present throughout the Tracheata an odd chromosome in the 

 spermatogonia, which behaves differently from the other chromo- 

 somes throughout its history. Still later work seems to establish 

 the fact that this chromosome has no paternal mate, does not join 

 any other chromosome in synapsis, divides in only one sperma- 

 tocyte division, and enters only half of the spermatozoa. In some 

 forms, a small chromosome is present as the paternal mate of this 

 odd chromosome, but dimorphism of the spermatozoa results in 

 either case. 



The following review takes up the different observations on the 

 Tracheata since 1890, and attempts to show how each helps to 

 establish, or differs from, the above mentioned theories. 



Arachnida 



Wallace ('05) finds an even number of spermatogonial chromo- 

 somes, 40, two of these being larger than the others and different 

 in behavior. They are condensed in the spermatogonial rest 

 stage, and take an eccentric position in the equatorial plate. 

 They remain separate from each other in the spermatocyte growth 



