42 CAUDAL GILLS UP' ZYGOPTERID LAkW?:, 



cuts across the larva with a pair of fine scissors; the first across 

 the three gills at a level about one-third of their total length 

 from the base, and the second across the seventh abdominal 

 segment of the lar\a. For this method, I am indebted to Dr. 

 Calvert, of Philadelphia. Its great advantage is that in one 

 series one can study the general structure of the gills, while in 

 the other series one combines the structure of the rectum, which 

 is of great importance in the general question of the respiration 

 of these larvte, with the structure of the basal portions of the 

 gills themselves. For longitudinal sections, I find it best to 

 sever the gills at the breaking-joint. If the gills are too long, 

 they may be cut across the middle. The last three segments of 

 the larval abdomen may also be cut off separately, if it is desired 

 to stud}' the rectum and basal pieces in longitudinal sections. 



If it is desired to employ the fixative hot, it should be heated 

 in a test-tube to just on boiling-point. 8ome of the hot fixative 

 is then poured into a large watch-glass, over which the larva is 

 held, while its gills are cut ofi' with the scissors. The cut off 

 portions usually sink immediately; if any of them fioat, they 

 should be at once submerged by means of a camel's-hair brush. 

 Leave the gills in the fixative until it has cooled to about the 

 temperature of the room. They should be transferred to 70 per 

 cent, alcohol (either directly, from C-arls" Fixative, or indirectly, 

 from Gilson's Fixative, through grades of 17, 35, and 50 })er 

 cent). In addition, if Gilson's Fixative be employed, the mercuric 

 chloride must be thoroughly removed by means of iodised alcohol, 

 in the usual manner. In both cases, wash well in several changes 

 of 70 per cent, alcohol over a period of 24 hours. 



If used C(>ld, the fixative should be poured into a crystal dish, 

 and the gills cut ofi' and allowed to fall into it as described aboAc. 

 The dish must then be co\ ered over, and the gills left in the 

 fixative for from 12 to 24 hours (not longer) according to the 

 state of the cuticle. Transfer to 70 per cent, alcohol as describes 

 above, and wash well. 



Sojteniny : — Before dehydrating, the gills should always be 

 subjected to a certain amount of softening, by immersion in a 



