CHROMOSOMES OF MOSQUITO 551 



certain changes are going on in each that make them darkly- 

 staining at one time, not at another. That the latter theory is 

 preferable would seem to be the case in the chromosome-nucleoli. 

 In many forms the unpaired or accessory chromosome appears 

 condensed in the resting nucleus, but becomes diffuse in meta- 

 phase or anaphase. Other odd chromosomes or parts of chromo- 

 somes condense when the others are diffuse. 



b. Heterochromosomes 



Stevens ('11) found in Anopheles an unequal pair of chromo- 

 somes attached to the smaller pair of euchromosomes in sper- 

 matogonial metaphases and in first spermatocytes. In ovogonia 

 the corresponding elements are each equal in size to the larger 

 member of the unequal pair. In resting spermatogonia the 

 condensed pair is associated with a plasmosome and in growth 

 stages the two members can often be distinguished. They are 

 condensed and attached to the still diffuse smaller pair of euchro- 

 mosomes. The first maturation division separates the members. 

 The interkinesis condition was not made out, but the dimorphism 

 of the spermatids was declared evident, each having a single 

 nucleolus, which might be either large or small. 



Unequal xy pairs were found by Stevens ('08) in nine Myo- 

 daria, unassociated with the euchromosomes. The larger mem- 

 ber of each pair was shown to be the sex chromosome by com- 

 parison with the female in Musca, Calliphora, Sarcophaga, 

 Scatophaga, Tetanocera, and Eristalis, but not in Lucilia and 

 Phorbia. It is probable that the same condition obtains in the 

 latter two genera as well; but in Drosophila the reverse seems 

 to be the case according to Bridges ('16). Stevens supposed 

 that in Drosophila the larger bent member was the rr-element. 



Stevens ('09) has made considerable search for heterochromo- 

 somes in Culex. Especial attention was given to the smaller 

 pair, as it seemed to condense at some times earlier than at 

 others. I have found no evidence for this in my material. No 

 inequality could be found by Stevens. 



My own evidence in regard to heterochromosomes in Culex is 

 not at all conclusive and applies to only a few of the spermato- 



