NON-DISJUNCTION OF SEX CHROMOSOMES 605 



is directly comparable to the production of two-X and no-X 

 eggs. 



Wilson ('10) found an individual of Metapodius in which there 

 were three homologous m-chromosomes. He explained this indi- 

 vidual in the same manner as the result of the union of an ex- 

 ceptional two-m gamete and the regular one-m gamete. 



In* the case of Diabrotica, Miss Stevens ('08) found a varia- 

 tion in the number of supernumeraries from zero to four, with 

 inequalities of the distribution in gametogenesis such as Wilson 

 found in Metapodius. 



The closest parallel, is perhaps in the case of Ascaris mega- 

 cephala as reported by Frolowa ('12). Here the X is attached 

 to the end of an autosome, as it perhaps is in Drosophila. How- 

 ever, Frolowa found in exceptional eggs that both sex chromo- 

 somes became attached to one member of the pair of autosomes 

 instead of one X to each member. During maturation, there 

 will arise two-X" and no-X eggs determined by which autosome 

 of the two, passes out into the polar body. Her diagram for 

 Ascaris fulfils all the requirements for the case of non-disjunction 

 in Drosophila, and, from cytological grounds, she reaches nearly 

 the same conclusion with respect to the fate of the two-X and 

 no-X eggs, as I do from the side of genetics. 



The phenomenon of non-disjunction is most readily discovered 

 in any form when the non-disjunction occurs in the sex chromo- 

 some, but it might occur with respect to any autosome, where it 

 would only be detected in certain cases. I have a case which 

 may be non-disjunction of the third chromosome of Drosophila, 

 and probably cases of so called '^monolepsis" or ''faux hybrides" 

 will be found to depend upon this phenomenon. Some forms 

 of parthenogenesis probably differ from non-disjunction only in 

 that the whole number of chromosomes is concerned and not 

 one, or a limited number. 



Columbia University 

 July 7, 1913 



