542 Edmund B. Wilson. 



or may not pair at the time of general synapsis to form a bivalent; 

 in the former case they appear in the growth-period as a single 

 bivalent chromosome-nucleolus, in the latter case as two separate 

 univalent chromosome-nucleoli. In either case they undergo 

 equal division as separate univalents in the first maturation- 

 mitosis, their products conjugating at the close of this division to 

 form an asymmetrical dyad the two constituents of which are, 

 without fusion, immediately separated in the second division. 



4. The heterotropic chromosome is without a mate in the 

 spermatogonia (which accordingly show an odd number of chro- 

 mosomes) and hence fails to undergo synapsis. Its behavior is 

 throughout that of a univalent body. It divides only once in the 

 course of the two maturation mitoses, this division taking place 

 usually in the first, but in some species in the second, mitosis. 

 It has probably arisen by the reduction and final disappearance 

 of one member of a symmetrical chromosome-pair, this process 

 having taken place in the male only. 



5. The w-chromosomes are always associated with a hetero- 

 tropic chromosome, while the idiochromosomes and heterotropic 

 chromosomes are known to coexist in only a single case (Banasa). 

 This case indicates that the formation of heterotropic chromo- 

 somes may have taken place more than once in the history of the 

 species and possibly represents one mode of change from a higher 

 to a lower number of chromosomes. 



6. In forms possessing the idiochromosomes two classes of 

 spermatozoa exist in equal numbers, which receive the same 

 number of chromosomes but differ in respect to the idiochro- 

 mosome. In forms possessing a heterotropic chromosome two 

 classes of spermatozoa likewise exist, one of which possesses one 

 more chromosome than the other. When both idiochromosomes 

 and heterotropic chromosomes are present (Banasa) four classes 

 of spermatozoa are formed, two having one more chromosome than 

 the other two, each of these groups again differing in respect to 

 the idiochromosome. 



7. The facts support the general theory of the individuality 

 of chromosomes, the theory of Montgomery in regard to synapsis, 

 and that of Sutton and Boveri regarding its application to Men- 

 delian inheritance; and they point toward a definite connection 

 between the chromosome-group and the determination of sex. 



Zoological Laboratory, Columbia University, 

 July 29th, 1905. 



