164 PREPARATION OF THE SUBSTRATES 



made use of abnormal ones. The question 

 whether the organs of animals may be used will be 

 discussed later. 14 



The organ must be absolutely .freed from blood, 

 a condition that can be attained in the case of 

 different organs with varying facility. Placenta and 

 lungs, for instance, can be easily washed so as to free 

 them from blood, or the blood may be rinsed out 

 through the large blood-vessels ; whilst the liver, 

 kidneys, and particularly the uvea, are freed from 

 blood with great difficulty. With the latter there is 

 scarcely any other means of proving its suitability, 

 than by experimenting comparatively with serum 

 from individuals with healthy and diseased uvea 

 respectively. The pigment prevents us from dis- 

 covering the last traces of blood. 



The fresh and still warm placenta is first freed 

 from blood clots by mechanical means, the mem- 

 branes and the umbilical cord being removed at the 

 same time. Then the placenta is cut into small pieces, 

 about one inch square or less, and these are crushed 

 in a current of water, for which purpose they are 

 best placed on a sieve. Water is allowed to run 

 continuously upon the pieces of placenta, each piece 

 being pressed between the fingers. From time to 

 time the pieces are placed in a cloth and squeezed 

 in it. The washing of the placenta must never be 



14 See also p. 27. 



