THE RAT- FLEA, CERATOPHYLLUS FASCIATUS BOSC. 445 



cover-glass and letting the salt-solution run off, or, if the nervous 

 system shows a tendency to run off with the fluid, by holding the 

 cover-glass flat and carefully mopping up all superfluity of fluid 

 with a small piece of filter-paper. The object to be attained is to 

 leave the specimen stranded on the cover-glass and to drain off as 

 much of the salt-solution as possible, in order that by capillary 

 attraction the object may be pressed against the cover-slip ; but 

 on no account must the fluid be allowed to dry completely. When 

 this has been done the cover-slip is inverted, so that the object is 

 on its lower side ; and then it is dropped face downwards quite 

 flat on to the surface of some fixative fluid. 



Various fixatives can be used, but I have nearly always made 

 use of 5 per cent, sublimate-acetic that is to say, saturated 

 solution of corrosive sublimate in distilled water, 95 volumes, 

 mixed with glacial acetic acid, 5 volumes. When fixing the 

 preparation some of the fixative is put into a large watch-glass 

 or clock-glass and the cover-glass with the adherent object is 

 dropped on to it and remains floating on the surface of the 

 solution. In nine cases out of ten the object remains firmly 

 adherent to the under side of the cover-glass, if one has hit the 

 happy medium in draining off the fluid in which it was dissected 

 out. If superfluity of the salt-solution remains, the object will 

 come off; if it has been allowed to dry up altogether, the 

 preparation is ruined. 



The cover-glass with the adherent organs can now be mani- 

 pulated just as if it was a smear, lifting the cover-slip with an 

 ordinary forceps and transferring it from one liquid to another. 

 After the preparation has been fixed in the sublimate-acetic for 

 some time, say from 10 minutes to an hour, it can be brought 

 up through successive strengths of alcohol in watch-glasses 

 (10 per cent., 30 per cent., 50 per cent, and 70 per cent.) to 

 90 per cent, alcohol, in which it should be left for a longer time 

 (preferably over night, or as long as is convenient) in order that 

 the preparation may be well hardened and the corrosive sublimate 

 thoroughly dissolved out. In the stronger alcohols the cover-slip 

 will sink, but it rests on its corners on the rounded bottom of 

 the watch-glass and there is no contact or pressure on the object, 

 which of course is on the under side of the cover-slip. It is now 

 apparent why square cover-slips must be used, since they rest on 

 their corners and can be easily picked up with the forceps : 



