Studies on Chromosomes 191 



23 chromosomes (egg n + spermatozoon 12) including two 

 large idiochromosomes hence presumably a female and one 

 small. The eggs produced by such a female should after matura- 

 tion be of two classes, having respectively n and 12 chromosomes. 

 The 12-chromosome class would contain both a large and a small 

 idiochromosome, and if fertilized by ordinary n-chromosome 

 spermatozoa would produce individuals with 23 chromosomes, 

 male or female according to the class of spermatozoon concerned. 

 Such females would, as before, contain two large idiochromosomes 

 and one small. The males would contain one large and two small, 

 and would accordingly produce spermatozoa having either II or 

 12 chromosomes. 



Now, such an additional small idiochromosome in the male 

 would be indistinguishable from a single "supernumerary chro- 

 mosome" as it appears in the 23-chromosome individuals in my 

 material. The resemblance is shown not only in size but also in 

 behavior; for, as I have shown, the supernumerary, like the 

 idiochromosome, forms a chromosome-nucleolus during the growth 

 period, it divides as a univalentin the first division, and in the sec- 

 ond is usually associated with the idiochromosome bivalent. A 

 single such supernumerary chromosome, once introduced into the 

 race would lead to the presence of additional ones in succeeding 

 generations. Thus, 12-chromosome eggs fertilized by 12-chromo- 

 some spermatozoa would give individuals (male or female) with 



24 chromosomes, including two supernumeraries; and from these 

 might arise, through irregularities of distribution such as I have 

 described, gametes with n, 12, or 13 chromosomes, giving in the 

 next generation 22, 23, 24, 25 or 26 chromosomes according to 

 the particular combination established in fertilization." If this 



11 Since the presence of an unpaired idiochromosome in some individuals and of supernumeraries 

 in others is assumed to be traceable to the same initial cause, we should naturally expect to find the two 

 conditions coexisting side by side, and in approximately equal numbers; but in point of fact the former 

 is very rare and was only found in one locality, while the latter is very common. This may constitute 

 a valid objection to my interpretation. It should be borne in mind, however, that abnormal divi- 

 sions of the kind assumed to form the starting point are very rare, and that an extremely minute propor- 

 tion of the total number of spermatozoa produced ever actually enter the eggs. The chances against 

 fertilization by either class of the original modified spermatozoa are therefore very great. Since only 

 sixty individuals have been examined it need not surprise us that one of the two conditions in question 



