SY R. .1. TILLYARD 43 



solution of soft soap in 70 per cent, alcohol. A good rule to 

 follow is to make the time of softening equal to the time of 

 fixation, e.g., in the majority of cases, 24 hours for both. The 

 soap will remove the fat from the cells of the fat-bod v^ in the 

 larval abdomen, but will not affect the gill-tissues in the slightest? 

 except perhaps by a removal of fat from a wandering fat-cell or 

 two in the blood-canals. In the case of very tough gills, taken 

 from larvee just before ecdysis, a little warming (by standing the 

 solution on the bath) will expedite the softening. . 



After softening, the gills must be thoioughly washed in several 

 changes of 70 per cent, alcohol over a period of at least two days. 

 The last change, when poured off into a small tube and shaken 

 up, should not show the slightest sign of forming a lather. 



Dehydration: — Pass up into 90 per cent, alcohol for 6 hours, 

 then into absolute alcohol for 6 hours at the most. 



Embedding : — In studying the rectal gills of Anisoptera, ex- 

 cellent sections were obtained by single-embedding in paraffin. 

 This method is not reconnnended for the caudal gills of Zygoptera. 

 Owing to the thickness of the cuticle, the method of douhle- 

 evibeddlng should always be employed. 8ome of the lai-ger and 

 tougher gills resist considerably the penetration of celloidin. 

 Gills of ordinary thickness should be left '1\ hours in each suc- 

 cessive stage, beginning with a 1 : 1 mixture of alcohol and ether, 

 and passing up through i per cent., '11 i^qy cent., and 5 per cent, 

 celloidin. Very tough gills may be left 18 hours in each suc- 

 cessive stage of celloidin solution. The hardening of the celloidin 

 block in chloroform vapour must be very carefully carried out, 

 the block being cut out and shaped when, if anything, it is 

 slightly harder than it would be allowed to l)ecome if it contained 

 softer tissues, but still not hard enough to cause any contraction 

 in the gill. Hardening is then completed by immersion in liquid 

 chloroform, and the hardened block is infiltrated with, and em- 

 bedded in, paraffin in the usual manner. The cutting and 

 mounting of the sections present no features of special difficulty, 

 other than those usually attendant on dealing with somewhat 

 tough material. 



