MALE GERM CELLS IN NOTONECTA 65 



second, thus making the total number of separate chromatin 

 elements one greater in the first than in the second division. In 

 N. undulata, there are 14 chromosomes in the first division, 13 in 

 the second, including two small chromosomes. In N. irrorata, 

 there are 13 in the first and 12 in the second, including onh^ one 

 small one. In N. insulata there are either 14 or 13 in the first, and 

 12 in the second; when there are 14 in the first, there are two small 

 ones, when 13 there is only one free small one, but the other small 

 one can often be detected attached to the largest chromosome. 

 This species thus appears to be intermediate in respect to the 

 chromosomes between N. undulata with a larger number, and N. 

 irrorata with a smaller number. 



1. Notonecta undulata. In N. undulata, the typical, and I 

 am inclined to believe, the invariable, arrangement in the first 

 spermatocyte division is a ring of 12 surrounding two very small 

 chromosomes. This is shown in polar view in figures 1 and 2, 

 and in side view of a spindle from two adjoining sections in figure 

 3 A, B. Very freciuently side views present the appearance 

 shown in figure 4 B, the two pairs of small chromosomes lying in 

 a straight line, as though on the same spindle fiber (A, B, C are 

 serial sections of the same spindle). This is probably due to 

 the fact that they lie very close together and the smaller of the 

 two pairs usually precedes the other in division. In the periph- 

 eral ring can be distinguished one chromosome larger than the 

 rest, one very small one slightly larger than the central ones, 

 and ten of intermediate and intergrading sizes. 



In the second spermatocyte division, side views clearly show 

 the presence of an unequal A'F-pair (fig. 5 B). Since these chro- 

 mosomes have divided separately in the first division, as is the 

 case in many other Heteroptera, there should be one chromosome 

 less in the equatorial plate of the second division. That there 

 are 13 chromosomes in this division is shown in side view in 

 figure 5 A, B, C (from the same spindle), and in polar view in 

 figures 6 and 7. ^In the latter figure the X-chromosome is seen 

 at a lower focus). In this division, X and Y always take up their 

 position in the center of the spindle, as they do in other Hemip- 

 tera. A rather interesting phenomenon occurs in Notonecta 



THE JOURNAL OF EXPERIMENTAI^ ZOOLOGY, VOL. 14, NO. 1 



