338 G. F. DOWDESWIOLL. 



described cannot be supposed to exist without some purpose 

 recent researches at once suggest that this is to attach the 

 spermatozoon to, and enable it to penetrate into the ovum in 

 the early stages of fertilisation, as has been shown to occur by 

 Fol and others ; and we should expect to find a similar forma- 

 tion in other spermatozoa. In those, however, which I have 

 hitherto examined I have not detected it, and the structure of 

 many, as that of the toad, and of most mammalia, does not 

 appear to admit of its existence. 



To prepare the spermatozoa for the examination of this 

 object the first essential is to get them as nearly as possible in 

 contact with the cover-glass and flat upon it ; this requires 

 some care, to avoid their drying, by which they are materially 

 altered. They may be preserved by several methods, either 

 by treating for twelve to twenty-four hours with a concen- 

 trated solution of picric acid, a dilute solution of chromic 

 acid, by Dr. Klein's method with a 5 per-cent. solution of 

 ammonium chromate, by iodine, by silver nitrate, or by osmic 

 acid or gold chloride ; the latter are convenient as being 

 quicker. I have myself most usually employed picric acid. 

 For staining I have found glycerine magenta^ the best method, 

 as it stains all parts as strongly as desired. To show the 

 sreneral structure alcoholic carminate of ammonia is the most 

 satisfactory, but it does not stain the barb deeply. Other 

 aniline dyes I have not found answer so well. If it be in- 

 tended to examine the preparation with a homogeneous, or 

 " oil "-immersion objective, it should be mounted in Canada 

 balsam, the objective having, as is generally known, no advan- 

 tage, and, indeed, being inferior to dry glasses for objects be- 

 tween which and the cover-glass there is either a film of air, 

 or of any fluid the refractive index of which is much different 

 from that of glass. 



The use of glycerine as a mounting fluid for preparations 

 stained with any of the aniline dyes is at best trouble- 



' 1} Magenta crjsi., 1 part; glycerine, 200 parts; alcohol, 150 parts; aq., 

 150 parts; immerse the preparation in the solution for from two to four 

 minutes according to the dopth of colouring required, and then wash. 



