CHROMOSOME-NUMBER IN NOTONECTA. 21 



the largest chromosome has been lost. It has a decided quad- 

 ripartite appearance, as though each element into which it divides 

 were composed of two equal parts. Its appearance suggests that 

 of the large chromosome described by Wilson 1 in the second divi- 

 sion of Nezara, except that in Nezara the two parts are somewhat 

 unequal. Unfortunately, no satisfactory spermatogonial groups 

 have been found, but the expectation would be either 26 or 24, 

 in the latter case, two of them being compound in character. 



DISCUSSION. 



The chief interest in Notonecta lies in the fact that there is 

 a definite relation between the chromosome-number and the 

 species, and that the change in number can be attributed to the 

 behavior of a particular chromosome. In N. undulata this chro- 

 mosome is present as a separate element, together with another 

 small one, in the center of the spindle in the first division, and in 

 the peripheral ring in the second division. In TV. irrorata, this 

 chromosome is lacking throughout both divisions, there being 

 only one small one in the center of the spindle in the first division. 

 TV. insulata gives a transition stage between the two; the small 

 chromosome is present in the first division, either free in the 

 center as in TV. undulata, or fused with the largest chromosome; 

 and it is apparently absent in the second division as in TV. irrorata. 

 Every chromosome cannot be homologized individually in the 

 three species, for the size relations are different, especially in the 

 case of the idiochromosomes. The largest chromosome and one 

 small one can be followed throughout in the three species. Like- 

 wise, we may safely compare, I think, the other small chromo- 

 some in TV. undulata with the one of like size and position that 

 sometimes occurs in TV. insulata in the first division; and since 

 we can follow the steps of its fusion with the large chromosome 

 in TV. insulata, we may, I think, reasonably attribute its absence 

 in TV. irrorata to its permanent fusion with the large chromosome. 

 A schematic representation follows, only the chromosomes that 

 are comparable being separated from the ordinary chromosomes 

 or autosomes, which are designated A. The large and small 



'Wilson, E. B., 1910, "Note on the Chromosomes of Nezara," Science, May 20, 

 1910. 



