MALE GERM CELLS IN NOTONECTA 69 



They are more nearly equal in size than in N. undulata, in this 

 respect resembling those of N. irrorata. The invariable number 

 12 is accounted for on the assumption that the second small 

 chromosome which in the first division is sometimes separate and 

 sometimes associated with the large one, has fused with it in all 

 cases before the second division. Additional evidence is given 

 by the fact that in most cases two of the chromosomes are con- 

 siderably smaller than the others, one of these corresponding to 

 the small (ientral one, and the other to the small peripheral one 

 of the first division (figs. 35-39). The large chromosome, how- 

 ever, presents an unexpected appearance. It gives no evidence 

 whatever of its real composition of two very unequal parts, but 

 it appears in the metaphase as a large quadripartite chromosome, 

 as though each part into which it divides were composed of 

 two equal parts (p. 89). The longitudinal split which was very 

 noticeable in the first division in polar view marks the division 

 plane of the second division. In figure 4:0 A, B are shown tw^o 

 sister plates of an anaphase; the groups are identical except for' 

 the middle chromosome, and it is evident that on this account 

 two kinds of cells are produced which give two kinds of sper- 

 matozoa. 



Unfortunately no spermatogonial groups have been found of 

 which a satisfactory count could be made. The expectation 

 would be either 26 single chromosomes, or 24 including two 

 compound ones. 



4. Notonecta glauca (Pantcl and Sinety). According to the 

 account of Pantel and Sinety ('06), there are in N, glauca some- 

 times 12, sometimes 13 chromosomes in the first division. They 

 state that they are unable to account for this difference, but they 

 also say, ''elle (la couronne equatoriale) comprend un anneau 

 peripherique, plus une ou deux unites situees au centre" (op. cit., 

 p. 139). No figures of polar views are given but it seems probable 

 from the statement that the discrepancy is here due to the pres- 

 ence or absence of a second small chromosome in the center, as 

 in the case of N. insulata. They do not state in the text the 

 number present in the second division, but figure 12 chromosomes. 

 The writers mention no unequal A"F-pair in the second division. 



