THE RELATION BETWEEN CHROMOSOME-NUMBER 

 AND SPECIES IN NOTONECTA. 1 



ETHEL NICHOLSON BROWNE. 



After working for several years on the spermatogenesis of 

 Notonecta, some very interesting facts have recently come to 

 light which I wish to present in a preliminary paper. The ma- 

 terial, collected at Woods Hole, consists of three species, Notonecta 

 undulata Say, N. insulata Kirby and N. irrorata Uhler. The 

 character of the chromosome groups is quite distinctive for each 

 species, TV. insulata representing a transition stage between TV. 

 undulata with a larger number of chromosomes and N. irrorata 

 with a smaller number. 



In all three species there is present a pair of idiochromosomes, 

 which divide separately in the first spermatocyte-division, as 

 described by Wilson 2 for Euchistus, Ccenus, etc. Accordingly, 

 there is one more than the reduced or haploid number in the first 

 division. Owing to the fact that the conjugation of the idio- 

 chromosome-pair is often still further delayed in Notonecta, the 

 two unequal mates frequently lie side by side on different spindle 

 fibers in the second division (Figs. 7, 9, 16, 18, 33, 35). They 

 subsequently pass to their respective poles without ever having 

 conjugated. As a result of the second division, two classes of 

 cells are produced, one containing the small idiochromosome and 

 the other the large one (Figs. 10, 19, 36). Accordingly, the 

 spermatozoa are of two kinds which, from analogy with other 

 insects, are the male-producing and the female-producing sperm- 

 atozoa respectively. 



In N. undulata there are 14 chromosomes in the first spermato- 

 cyte-division, consisting of a ring of 12 chromosomes with two 

 small ones in the center (Figs. 1-3). These small ones frequently 



J I wish to express my thanks to Professor E. B. Wilson for his helpful sugges- 

 tions and criticism during the course of the work, to the director of the Marine 

 Biological Laboratory and the Wistar Institute of Anatomy for the facilities 

 offered me at Woods Hole, and to Mr. E. P. Van Duzee for identifying the species. 



2 Wilson, E. B., 1905, "Studies on Chromosomes," I., Journ. Exp. ZooL, II., 3. 



'9 



