8 HELEN DEAN KING AND HENRY H. DONALDSON 



In this paper the observations on the life processes are 

 discussed in part I by King and those on the changes in the 

 size and weight of organs and of the bones in part II by 

 Donaldson. 



Preceding these descriptions a statement is made concern- 

 ing the conditions in the colony during the period under con- 

 sideration. These conditions have an important bearing on 

 the changes that have been observed. 



The general care of the rats was not equally good during 

 the period of seven years (1919-1926) which elapsed before 

 the tenth generation was examined. In order to get at any 

 effects of colony conditions on the captive Grays, the time 

 relations of important events to the time relations of the 

 several generations have been plotted in chart 1. In this 

 chart each generation is represented by a vertical line run- 

 ning between the average date of birth, above, and the aver- 

 age date of killing, below. In individual cases, of course, 

 birth preceded and killing followed the average date, but it 

 was thought that this method of presentation would answer 

 the present purpose of showing to what generations any 

 particularly unfavorable conditions applied. 



In the middle of 1922, the rats became infected with lice, 

 and in September, 1922, were twice treated with larkspur 

 .solution for disinfection. This is a severe ordeal and, as the 

 chart shows, affected younger animals in G 5 and G 6 and pro- 

 spective mothers in G 7 .* In the summer of 1923, great heat 

 prevailed for a time in the colony and could have affected 

 most readily G 6 , G 7 , G 8 , as these individuals were at this time 

 only half grown or younger. 



From the middle of 1923 to the latter part of 1925, the care 

 of the animals was not satisfactory, the caretaker being 

 inefficient. 



The observations to be reported show retardations in 

 growth which apparently coincide with the unfavorable con- 

 ditions represented by the louse infection, the larkspur treat- 



1 The successive generations are designated by G with a subscript giving the 

 number of the generation in the series. 



