MALE GERM CELLS IN NOTONECTA 75 



suggests that some sudden mutation has taken place, involving a 

 new segregation of the nuclear material, and causing a change 

 in number and size relations of the chromosomes, but not in their 

 essential quality. 



A fourth method by which either a slight or a radical change in 

 the chromosome number might take place is by an abnormality 

 occurring in mitosis, as has been suggested by several authors, 

 either by an unequal distribution of the chromosomes to the 

 daughter cells, or by an arrest of cell division after a division of 

 the chromosomes. The former abnormality has actually been 

 observed in the case of Metapodius (Wilson '09 a), and the Oeno- 

 theras (Gates '08, et al.). The possibilit}- of the occurrence of 

 the second abnormality is shown by the experiments of Gerassi- 

 mow ('01) on Spirogyra, of Nemec ('04) on Pisum and of Boveri 

 ('05) on sea-urchin eggs, in which a monaster was produced 

 instead. of an amphiaster, the chromosomes dividing but not the 

 nucleus, and the double number of chromosomes remaining in 

 subsequent divisions. To this cause has been attributed the 

 occurrence of triploid and tetraploid mutants in Oenothera (Gates 

 '09, Lutz '12), and it seems probable that many of the cases of a 

 double number of chromosomes occurring in closely related forms 

 of some animals (e.g., xlscaris megalocephala), and many plants 

 have been brought about in this way. 



2. Temijorary association and separation of chromosomes. The 

 condition of temporary association and separation of particular 

 chromosomes which occurs in N, insulata is of especial interest 

 in comparison with other forms. In the first place, there are 

 cases where the union and separation concerns the sex-chromo- 

 somes only. In these cases the X-element may consist of two or 

 more components — in Acholla (Payne '09) and Ascaris lumbri- 

 coides (Edwards '10) as many as five — that appear as separate 

 chromosomes in the diploid nuclei but become associated in the 

 spermatocyte divisions and behave as a single accessory. 



In a second category may be placed those forms where there is 

 a temporary or permanent association of the sex chromosomes 

 with other chromosomes. Sinety ('01) was the first to describe 

 a case of this sort in the phasmids Menexenus and Lept3'nia. 



