CHROMOSOMES OF MOSQUITO 543 



B. TECHNIQUE 



q. Experiments and observations 



Among all of the causes of difference of observation and inter- 

 pretation in cytology, the matter of technique is of prime im- 

 portance. 



The class in cytology under Dr. Ale Clung undertook during 

 the winter of 1914-15 to investigate Culex pipiens. The abdo- 

 mens of larvae and pupae were cut off near the thorax and the 

 tips of the abdomens also removed for better penetration. In 

 some cases parts of the thoraces of larvae were also preserved. 

 Division figures were found, not only in the gonads, but scat- 

 tered through various parts of the somatic tissue as well. They 

 were especially abundant in the developing leg and wing buds 

 of the thorax, and were also found in the nervous system. In 

 certain of the pupae the degenerating larval intestines also 

 showed division figures, mostly multiple complexes. 



The best fixation seemed to be given by a formula of Dr. 

 Ezra Allen, consisting of saturated aqueous picric acid, 75 per 

 cent; formol, 15 per cent; glacial acetic acid, 10 per cent. The 

 results, however, were not very satisfactory, which seemed to 

 be due in part to slow penetration, in part to the distorting 

 action of the fixative. Flemming's fluid has been employed by 

 Metz in his investigations on the Diptera. It does not give 

 good results, however, unless the tissue to be fixed is dissected 

 out in minute pieces and immediately placed in the fixative. 

 An attempt to fix in Flemming without dissecting out the tissue 

 resulted in shrunken and blackened sections, clearly the result 

 of poor penetration and delayed fixation. 



At the suggestion of Dr. Metz and Dr. Wenrich, therefore, 

 in the fall of 1915, I dissected out the gonads of larvae and pupae 

 in tap water and transferred immediately to strong Flemming. 

 The gonads were drawn up in a capillary pipette and transferred 

 to the fixative. After some experimenting it was found that 

 the dissection may be done with greatest facility by cutting off 

 the tip of the abdomen just posterior to the gonads and then 

 pressing out the latter. 



