Perry has suggested an ingenious method of getting rid of the surface 

 tension. It is due to the presence, even on the most carefully cleaned 

 slide, of a fine film of grease which is probably in- 

 corporated in the substance of the glass, since the Use of Blle '" 



.. r . dissections 



ordinary solvents for fat will not remove it. Now bile 



is miscible with both water and fat, and if the slides are smeared with 

 a trace of it before the dissection is commenced, the organs will stay in 

 any position in which they are placed, and do not follow the movements 

 of the needles at all. A few drams of bile from the bladder of a sheep 

 are all that is required for a large number of dissections, as the merest 

 trace suffices for a slide, the surplus being wiped off with a handker- 

 chief. 



For the dissection of large flies, and especially for ticks, the paraffin 

 trough suggested by Christophers is invaluable. It is made as follows. 

 A glass dish of considerable depth and of a convenient size, say two 

 inches by three, is taken, and filled to a depth of half to one inch with a 

 mixture of paraffin wax and lamp-black, poured in while hot and allowed 

 to set hard. The specimen is pinned down in this by means of entomo- 

 logical pins, and the whole flooded with saline. As the dissection 

 proceeds the various parts can be drawn out of the way and secured 

 with pins ; tracheae, Malphigian tubes, etc., can be fixed by merely 

 pressing them into the wax. The dissection is readily cleaned by 

 agitating the saline with a pipette. 



The method of dissection by pulling the organs out of the exo-skeleton 

 is perfectly satisfactory in the majority of cases when looking for para- 

 sites, or merely examining a particular piece of tissue, but it is not to be 

 relied on when studying the minute anatomy of the parts, on account of 

 the stretching of the organs which must necessarily take place. In the 

 case of the mosquito the shape of the gut is entirely altered when it is 

 pulled out in the ordinary way. To get a true picture of the organs in 

 their natural relations it is necessary to free them by dissecting off the 

 external wall piecemeal, without pulling on the body contents at all, and 

 then to cut, not pull, the organs which conceal the part to be studied. 

 This is a very laborious and difficult process, and need only be adopted 

 when some special end is in view. 



Permanent stained preparations of the internal organs mounted whole 

 are very useful both for demonstration and for the study of the minute 



structure of the parts, and are in many ways superior 



,.-.*_.''. L j -i n^u -11 Permanent prepara 



t o the fresh unstained material. They are especially tions of di88ection8 



useful for making drawings of the parts, as one can 



