DEVELOPMENT OF THE ALBINO RAT 249 
ment of the rat has received especial consideration by Fraser, 
Christiani, Selenka, Duval, Robinson, Widakowich, and as 
concerns maturation and ovulation, by Sobotta and Burckhard, 
Kirkham and Burr. The pertinent literature will be considered 
in connection with the presentation of my own results. 
MATERIAL AND METHODS 
The material on which this investigation is based was obtained 
from albino rats (Mus norvegicus albinus, Donaldson)! taken 
from the extensive rat colony of The Wistar Institute of Anatomy 
and Biology. The experience gained in the breeding, feeding, 
and growth experiments, extending over many years, conducted 
by Donaldson and his associates and resulting in numerous ex- 
cellent publications, was at my disposal while collecting this 
material. The material used was all carefully timed, so that 
sequence of stages was obtained with some degree of certainty. 
With care and experience, it is possible to regulate and observe 
insemination, so that stages may be approximated quite accu- 
rately. Kirkham and Burr state that “on several different 
ocezsions we have observed actual pairing” of the albino rat. 
Widakowich states that he was unable to obtain accurate data 
as to the age of the embryos except by observing coitus. Ac- 
cording to this observer, a female rat permits many males to 
copulate in the course of several hours, receiving males 30 times 
or oftener, when suddenly she drives them away. Sobotta and 
Burekhard, on the other hand, admitted males a few hours 
after parturition, depending on the fact that many mammals 
ovulate soon after parturition. Though attempts were made, 
they were unable to observe pairing, and they state that the 
‘Dieners’ charged with the care and feeding of the rat colony 
were only seldom able to observe attempts at pairing. At 
The Wistar Institute no difficulty is experienced in pairing albino 
!Melissinos and Widakowich state having used as material the albino rat, 
variety Mus rattus albinus. Donaldson has conclusively shown, that by reason 
of physical characters—blood crystals, shape of the skull, ete.—the albino rat 
kept as pet or laboratory animal cannot be Mus rattus albinus, but must be Mus 
norvegicus albinus. 
