26 BIOLOGY 



age, pure and simple, or depends on the bearing of previous 

 litters has not been determined. 



In extracted Albinos and in Norways the litter size is close 

 to that for the stock Albinos, while in extracted Norways and 

 in Piebalds it is somewhat higher: 6.7 and 6.8 respectively 

 (King, '24). 



Greenman and Duhring ('23) find however in their exercised 

 and particularly well fed series, breeding at four months, the 

 first litter to be the largest. This is possibly another instance 

 of precocity which may be thus induced, or it may be the result 

 of the later age at which the female bred. 



Mortality. From a study covering five years and based on 

 the litters from 415 rats, King ('21) determined that 1.3 per 

 cent of the young were still born. The still born males were 

 markedly in excess of the females (129 males to 100 females') 

 due apparently to a greater intrinsic weakness of the male fetus. 



Young are also lost during the early days of lactation, through 

 injury or neglect. Only a small number are born dead in the 

 case of normal mothers. Where abnormal conditions exist, the 

 number of dead born may be high. 



Birth weight. The birth weights as determined by King ('15) 

 are shown in table 7 according to the increasing age of the 

 mother and in table 8 according to the increasing body weight 

 of the mother. 



On the average the birth weight of the males exceeds by 0.3 

 grams that of the females. 



King ('15) reaches the following conclusions: Increasing age 

 or increasing weight of the mother (the two being correlated) 

 give a heavier birth weight, while the increase in the number 

 in a litter tends to diminish the individual birth weight. There 

 is to be observed also a diminution in birth weight in those lit- 

 ters born of mothers below the standard size, or suffering from 

 infectious disease. 



Precise data for the birth weight require great care to secure 

 the selection of young which have not suckled. This is most 

 readily made by the use of the so-called obstetrical cage, de- 

 scribed by Long and Evans ('22), by which the young are automat- 



