Studies on Chromosomes 9 



somes in these processes. Since the oogonia contain fourteen 

 equally paired chromosomes, synapsis in the oocyte must result 

 in the formation of seven symmetrical bivalents /. e., seven 

 couples of equal chromosomes and each egg after maturation 

 contains seven univalent chromosomes, one of which is the 

 maternal representative or mate of the heterotropic chromosome 

 of the male. This group contains one chromosome of each of the 

 original pairs, and is precisely similar to the group present in 

 those spermatozoa that contain the heterotropic chromosome 

 (Fig. I, <:). Fertilization by such a spermatozoa doubles this 

 group, giving the condition observed in the female /. ^., fourteen 

 chromosomes equally paired, the largest pair consisting of the 

 heterotropic chromosome and its maternal mate (/, J, Fig. I, g, h~). 

 Fertilization by a spermatozoon that lacks the heterotropic chro- 

 mosome will give the condition observed in the male, namely, 

 thirteen chromosomes, of which twelve are equally paired, while 

 the thirteenth is the large unpaired one which is obviously derived 

 from the egg. There is therefore no escape from the conclusion 

 that both forms of spermatozoa are functional, that females are 

 produced upon fertilization by spermatozoa that contain, and 

 males upon fertilization by spermatozoa that lack, the hetero- 

 tropic chromosome. Since the two classes of spermatozoa are 

 equal in number, fertilization will in the long run produce males 

 and females in approximately equal numbers. 



b. Anasa tristis 



A comparison of the nuclei of the two sexes in this species gives 

 a precisely concordant result, though the size-differences do not 

 allow of so exact an identification of the differential chromo- 

 somes. In the preceding study I showed that the number of 

 chromosomes in the male (spermatogonia) is twenty-one, not 

 twenty-two as stated by previous observers. Study of the sper- 

 matogonial metaphase groups shows that twenty of the chro- 

 mosomes may be equally paired, two by two, while the remaining 

 one is, of course, without a mate (Fig. 2, <?, /). The unpaired 

 heterotropic chromosome is one of three largest chromosomes, 



