452 Messrs. W. Cramer, A. H. Drew, and J. C. Mottram. 



and of Bordet (2) had demonstrated, led us to examine the blood for 

 platelets. 



Method of Counting Platelets in Rats. — The animal is deeply etherised, and 

 the tail placed in a watch-glass filled with a solution of 2 per cent, sodium 

 citrate in - 6 per cent. NaCl solution. After cutting the tail and allowing 

 the blood to flow until free bleeding is established, the tail is transferred 

 quickly to another watch-glass containing about 1 c.c. of the citrate solution 

 or of Toisson's fluid. Blood is allowed to flow so that a mixture of the 

 solution and blood convenient for counting the red blood corpuscles and the 

 platelets is obtained. After the first few counts it is easy to recognise 

 when the mixture is of convenient concentration. The tail is then wiped 

 dry and the pipette of the hamiocytometer is then filled with blood in the 

 usual way, so as to obtain a count of the absolute number of red corpuscles. 

 In order to count the number of platelets, the citrate-blood mixture is 

 thoroughly stirred, a standard drop placed on a large slide and covered with 

 a cover-glass, which is then cemented with melted paraffin. After allowing 

 the cells to settle, the proportion of red cells to platelets in each field is 

 counted, until about thirty platelets have been counted. From this the 

 absolute number of platelets can be calculated. A helpful device for 

 counting the platelets, which has proved very useful, is as follows: A coarse 

 grating of about 1 mm. squares is ruled on a piece of ground glass with a 

 lead pencil. This is mounted in balsam under a cover-glass, and placed 

 close up to the source of light at right angles to the beam. The image of 

 the grating is then focussed by means of the substage condenser, after the 

 preparation has been focussed with the objective, and conveniently divides 

 the field for counting. 



Bed Cells and Platelets of normal Bats. — The following Table gives these 

 data for nine normal rats. The counts for the first five rats were made in 

 one laboratory, those of the last four in the other. The figures obtained show 



Table I. — Normal Rats. 



"Weight in 

 grm. 



Red cells. 



Platelets. 



150 

 150 

 150 

 100 

 120 

 50 

 50 

 60 

 60 



10,400,000 

 11,840,000 

 10,660,000 

 10,480,000 

 9,960,000 

 8,194,000 

 10,928,000 

 10,128,(100 

 9,888,000 



960,000 

 730,000 

 845,000 

 720,000 

 660,000 

 786,000 



1,050,000 

 912,000 



1,000,000 



