214 N- M- Stevens 



of Figs. 8 and 9. In all of these cases I should suppose that the 

 pairs were equal if the question had not been raised by the seven 

 spermatogonia! plates represented by Fig. Ii, and also by the 

 occurrence of an unequal pair of heterochromosomes in each of 

 the nine species of Muscidae previously described ('08). In Fig. 

 52, a to /, the same pair of chromosomes is shown in metaphase 

 and metakinesis. It is possible in each case that one chromosome 

 is slightly larger than its mate, but the difference is certainly not 

 conspicuous, and, if present, is obscured by the fact that the outer, 

 free ends of the elements usually turn in different directions. 



DISCUSSION 



Heterochromosomes 



If we define a heterochromosome as one that remains condensed 

 through the growth stages of oocytes or spermatocytes, then we 

 must say that such are not present in Culex. We have seen, 

 however, that one pair of chromosomes may be condensed in 

 advance of the other two pairs, in an early prophase of the first 

 maturation mitosis, and in the spermatogonia of two individuals 

 we have found evidence that an unequal pair of small chromosomes 

 is combined with a larger equal pair, which, we would suggest, 

 may control the behavior of the smaller unequal heterochromo- 

 some pair, preventing it from remaining condensed during the 

 growth stage. On the other hand the tendency of the hetero- 

 chromosomes to remain condensed may account for this pair of 

 chromosomes sometimes appearing in condensed form earlier 

 than the other two pairs in the prophase of the first maturation 

 division. It is certain that in most cases the heterochromosomes, 

 if present, are so intimately fused with another pair of chromo- 

 somes that it is rarely possible to detect their presence, the slight 

 difference in length of a pair of long, twisted chromosomes being 

 difficult to determine. 



The case of Culex is an interesting one in connection with that 

 of several species of Lepidoptera (Stevens '06, Dederer '07), 

 Nezara (Wilson '05) and Forficula'^ (Zweiger '06) and Anisolabis 



' In the case of Forficula auricularia, the author finds an unequal pair of heterochromosomes entirely 

 distinct from the lagging pair described by Zweiger as present in some individuals and absent in others. 



