96 SPECIFIC EXAMPLES OF SLIDE-MAKING 



physiological saline and laid out one by one about an inch apart on the paper. 

 This must be done as rapidly as possible to prevent fixation firom taking place 

 before an additional layer of paper is placed on top, saturated with fixative, 

 and the second sheet of glass placed on top of this. Assuming that the sheets 

 of glass employed are of the size of a sheet of typewriting paper, it is sug- 

 gested that about a two-pound weight be placed on the upper sheet of glass. 

 The whole should now be left for at least 12 hours before removing the glass 

 and upper paper, picking the worms up one by one on a glass section lifter 

 (metal cannot be used because of the presence of mercuric chloride), and trans- 

 ferring them to a large jar of fixative where they may remain for anothei: day 

 to another week at the discretion of the technician. 



At the conclusion of fixation the worms should be washed in running 

 water for at least 24 hours. It has not been the author's experience that this 

 fixative, followed by such washing, requires the use of iodine for the final 

 removal of the mercury. At this stage, moreover, iodine tends to render the 

 worms brittle, and the author would strongly recommend its omission. After 

 thoroughly washing in water, the worms may be stained; the formula selected 

 for the purpose of this example is the well-known carmalum of Mayer. 

 Objects of this type are better stained by the additive process than by a 

 process of differentiation; that is, they are better placed in an exceedingly 

 weak solution and allowed to absorb the stain slowly than placed in a strong 

 solution which requires subsequent differentiation. The best diluent for the 

 stain is a 5 per cent solution of potassium alum. The extent of the dilution is 

 dependent upon the choice of the operator and the size of the object which is 

 to be stained. In the present instance a dilution of about 1 part of the stain 

 to 100 parts of 5 per cent potassium alum would be correct. It is far more 

 dangerous to have the solution too strong than it is to have it too weak, and, 

 since it is an excellent preservative, the worms can remain in it for an indefi- 

 nite period. The worms are merely placed in this diluted stain and left there 

 until their internal structures have become clearly visible. It is suggested that 

 they be examined at the end of a week, and subsequently every three days, 

 until such time as examination with a low-power binocular microscope, using 

 a bright light from beneath, shows the testes to be darkly stained. At this 

 point the worms are removed to a fresh clean solution of 5 per cent potassium 

 alum and rinsed for a short time to remove all the adherent color. However, 

 they will still be pink on the outside. Since the purpose of the stain is to 

 demonstrate the internal organs, it is obviously desirable to bleach this outer 

 layer in order to produce bright scarlet internal organs against a white back- 

 ground. In the experience of the author this may be done most readily with 



