ANIMAL BREEDING 203 



the discouraging proportion of 50%. During the 

 period post mortems, bacterial cultures, and similar 

 data had yielded no information concerning the high 

 death rate, and the conclusion was drawn that death 

 was due to "natural causes" incident to breeding under 

 domesticated conditions. 



As ordinarily used, the term "death from natural 

 causes" signifies little short of a confession of igno- 

 rance. As it seemed reasonable to suppose that the 

 high death rate was due to cytost intoxication, experi- 

 ments were begun with this in view. 



For this purpose mouse cytost was prepared from 

 autolyzed mouse muscle in a manner identical with 

 that used for the preparation of guinea pig cytost, as 

 described in the preceding chapter. Initially the ex- 

 tract was prepared by the extraction of the autolyzed 

 muscle with an equal weight of water. Such prepara- 

 tions proved too toxic for the experiments, for upon 

 the injection of 0.5 cc. of such an extract subcutane- 

 ously or intraperitoneally, the mice always died 

 within 12 to 48 hours. When the dosage was halved 

 (0.25 cc), the mice were found to withstand a series 

 of five or six such injections spaced a week or ten days 

 apart. In this instance, the usual birth rate prevailed, 

 but 95% of the young died within two months after 

 birth. 



After considerable experimentation of this sort it 

 was finally discovered that it was necessary to employ 

 a considerably more dilute tissue extract in order to 

 achieve any distinctly beneficial effects upon the 

 breeding of such animals. In our final series of ex- 

 periments, which we wish to discuss here, it was found 

 that a cytost preparation obtained by extracting the 



