DEVELOPMENT OF THE ALBINO RAT 5 



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) 1 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 

 occasions 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 

 Burckhard, 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 



1 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, etc. the albino rat 

 kept as pet or laboratory animal cannot be Mus rattus albinus, but must be Mus 

 norvegicus albinus. 



