6 CALIFORNIA ACADEMY OF SCIENCES. [3D Ser., 



and the head is clipped off just above the heart. Blood is 

 then caught by the curved points of the forceps, which 

 must be closed. The prongs are then quickly passed over 

 the cover glass, always in the same direction and never 

 twice over the same place, as the blood cells would then be 

 disturbed. A zigzag movement over the glass is best when 

 it is desirable to cover the whole surface. The forceps must 

 not be lifted at the margin, but simply be pushed back; 

 the quicker this is done the better the blood will be spread. 

 The blood coagulates with great rapidity, and even if the 

 blood supply would hold out it is hardly possible to procure 

 more than two or three good cover glass preparations from 

 the same animal. With some practice it is not difficult to so 

 spread the blood that the film is only one corpuscle thick, 

 and so that the individual corpuscles are not distorted. 

 Great haste is necessary as a second's delay may result in 

 failure. Furthermore, the corpuscles should be so far apart 

 that the small plasmocytes are entirely free, as, if massed 

 together, they cannot be properly studied. 



The cover glasses are then at once placed with the film 

 downwards on clean, dusted, blotting paper, and covered 

 with a bell glass. This is absolutely necessary, as even 

 under well closed bells some dust will penetrate and settle 

 on the upper side of the glass. Afterwards these foreign 

 substances may be mistaken for centrosomes, experience 

 having shown me that through some cause or other these 

 specks of dust frequently settle in just those places where 

 a centrosome is to be expected. 



After twelve hours or more of air drying, the cover glass 

 is dropped into a shallow dish containing absolute alcohol, 

 and allow r ed to remain two hours or longer. It may then 

 be taken out and dried between blotting papers, after which 

 it is ready for staining. 



Two points are important to observe : the blotting 

 paper must be smooth and not corrugated; and after the 

 glass has finally become dry it must be brushed off with a 

 fine, clean, soft brush, in order to remove all the dust, which 

 settles with astonishing rapidity, even in a few seconds. 



