.")S0 ROBERT T. HANCE 



but unfortunately material favorable for the study of develop- 

 ment is usually hopeless for cytological work, and the reverse 

 is true. Some success which I have had with mosquito eggs 

 leads me to believe that improved technique would make this 

 insect a favorable form for such a study, but until such a tech- 

 nique has been developed, the study of the division figures 

 found in the larvae of this form will prove, I think, both inter- 

 esting and valuable. Because of an interest in somatic chromo- 

 somes and because of the contradictory reports that have been 

 published on Culex this animal was selected for study. The 

 results are complementary' to Mr. Whiting's thesis on the 

 spermatogensis. 



In this paper only what may be termed the normal somatic 

 divisions will be considered, as Miss Holt has made a study of 

 the multiple complexes. 



It is a pleasure to take this opportunity of thanking Dr. 

 McClung for the many facilities he has placed at my disposal 

 and for his constant interest and advice. Discussions with my 

 fellow workers Dr. Caroline Holt and Dr. P. W. Whiting have 

 been of great assistance to me. 



TECHNIQUE 



An abundance of material was found in tubs of water stand- 

 ing in the greenhouses on the Campus. Larvae and pupae have 

 been fixed, but only the larvae show any number of somatic divi- 

 sions. The question of the method of fixation is an exceedingly 

 important one, as the reasons for the various conclusions reached 

 by other investigators of Culex chromosomes are quite evident 

 from the results obtained with various fixatives. Eight different 

 killing fluids were tried, each at three temperatures, 5°C, room 

 temperature and 38°C. Of these the best has proved to be 

 Flemming's solution to which had been added a little urea and 

 used at room temperature. The urea was added to increase the 

 rapidity of penetration. Another fluid which gave good results 

 for some stages was what is designated in this Laboratory as 

 P. F. A. No. 3 which was used at 38°C. The formula for this 

 fluid is as follows. 



