I. LIFE PROCESSES 15 



uray and the albino race of Norways, however, the average 

 birth weight of the males, as a rule, exceeds that of the 

 females. 



Birth weight in the rat is influenced by a number of differ- 

 ent factors, such as the age and physical condition of the 

 mother, the size of the litter and its position in the litter 

 series, and the length of the gestation period (King, '15). 

 Environmental and physiological factors, therefore, may be 

 largely responsible for the differences in the birth weights of 

 UTJIV and of albino young. To decide this point definitely will 

 necessitate comparison between the birth weights of offspring 

 from a large number of gray and albino females of various 

 ages, and from litters of the same size. The collection of 

 data for this purpose has been in progress for some years. 



GROWTH IN BODY WEIGHT 



There are two distinct phases of body growth in mamma I >. 

 each of which consists of several growth cycles which merge 

 into each other at transitional periods. The first of these 

 phases comprises the period between conception and puberty, 

 and in it the time rate of growth increases with the increase 

 in body size. The second phase covers body growth during 

 adult life, and here the time rate of growth decreases while 

 the size of the organism increases (Brody, '26). 



Our knowledge of the initial stage of the first growth phase 

 in the rat, that occurring during fetal life, is based on the 

 observations of Stotsenburg ('15), who studied the weight 

 increase in albino fetuses from the thirteenth to the twenty- 

 first day of gestation. During this period the percentage rate 

 of growth per day was found to be approximately 53 per cent. 



X<> study has been made, as yet, of embryonic growth in the 

 gray rat. Data for the postnatal growth in individuals of 

 the fir>t ueiieratiou of these captive Grays were published 

 some years ago (King, '23), and they are here reproduced in 

 tables 4 and 5. Points of interest in connection with the>e 

 data \vill be discussed when comparison is made between them 

 and the findings in later generations. 



