Studies on Chromosomes. 59 



"idiochromosomes" (which occur in such forms as Lygseus, 

 Euschistus, Coenus, Brochymena, etc.) are typically unequal in 

 size, and differ from all other known forms of chromosomes in 

 the fact that their union in synapsis gives rise to an unequal or 

 asymmetrical bivalent. The spermatogonial groups correspond- 

 ingly show but one small chromosome, since the larger idio- 

 chromosome is not noticeably smaller than the ordinary chromo- 

 somes. The second type includes the equal paired "chromatin 

 nucleoli" of such forms as Anasa, Alydus, Syromastes or 

 Archimerus. Since the latter are almost always markedly 

 smaller than the others they may conveniently be called the 

 paired microchromosomes, or better, in order to avoid all 

 ambiguity, simply the m-chromosomes; and these are distin- 

 guishable in the spermatogonial groups as an equal pair of 

 especially small chromosomes. The most obvious difference of 

 behavior between these two types, so far as is now known, is that 

 the idiochromosomes divide as separate univalents in the first 

 maturation-mitosis, which accordingly always shows one more 

 than half the spermatogonial number of separate chromatin 

 elements, while the m-chromosomes, like the other chromosomes, 

 always unite to form a bivalent before the first mitosis which 

 therefore shows the same number as in the second division. 

 Other no less characteristic differences are described beyond. 

 These two forms are not yet known to coexist in the same species; 

 and, as a rule, forms that possess the idiochromosomes do not 

 have an "accessory" or heterotropic chromosome, while as far 

 as now known such a chromosome is always associated with the 

 m-chromosomes. 



The confusion that has grown out of the failure to observe these 

 differences arose in the first instance from two conclusions both of 

 which I shall show to be untenable reached by Paulmier in his 

 valuable, and, as far as the general history of the maturation- 

 process is concerned, very accurate, study of the spermatogenesis 

 of Anasa tristis ('99), and was increased by the subsequent efforts 

 of Montgomery ('01, '04, '05) to reduce the behavior of the 

 "chromatin nucleoli" to a uniform scheme. Paulmier, who was 

 the first to reexamine the history of the "accessory" chromosome 

 since its discovery by Henking, was also the first to describe the 

 m-chromosomes (in Anasa) as two very small chromosomes of 

 equal size in the spermatogonial metaphase-groups. These two 



