BREEDING HABITS OF ALBINO RAT 295 



before, does the mother assemble the young, cleaning them up 

 with her tongue, after which they lie close together under her 

 to keep warm. From this time on until the young are able to 

 crawl around by themselves the mother never leaves the nest 

 until she has carefully covered her litter. On returning she 

 always looks around for any that may have rolled or crawled 

 out in her absence, and such offenders are quickly seized in her 

 jaws and hauled back into the nest. 



The albino rat becomes sexually mature, at least in some cases, 

 as early as fifty-five days after birth, since in one instance a 

 litter was born to rats that were only seventy-seven days old. 



MATURATION AND OVULATION 



The paper by Sobotta and Burckhard ('10) on the matura- 

 tion and fertilization of the albino rat is by far the most com- 

 plete account of the subject that has so far been published. 

 However this report left a number of things to be cleared up. 



Working with material from 81 rats we have attempted to 

 investigate and make clear the following points: (1) the early 

 development of the egg previous to the formation of the first 

 polar spindle, (2) the formation of the first polar body, (3) the 

 condition of the egg at o\nilation, (4) the process of fertilization 

 and second polar body formation. 



At first the rats were watched and killed at short intervals 

 up to forty-eight hours after pairing. This gave no data that 

 could be depended upon for determining the stage which either 

 the ovarian or the tube eggs had reached. However, by relat- 

 ing the time of killing the female to the time of parturition it 

 was found that tiie approximate development of the egg could 

 be predicted without much difficulty. We say approximate be- 

 cause even though the exact hour of parturition be known it is 

 impossible to say that at a given interval of time the eggs are 

 in a given stage of development. 



The parturition of a female caged with a male having been 

 observed, she was killed twenty-four hours later. This female 

 yielded unfertilized tube eggs, indicating that ovulation had re- 



