GESTATION OF THE ALBINO RAT. 389 



short time before as they were bright red in color and weighed 

 less than 5 gm. each. Between the birth of the first four members 

 of the litter and of the last seven there was an interval of about 

 fourteen days. 



Female 6A 6 2 3 gave birth to her first litter of three young on 

 February 26, 1912. Thirteen days later the nest was examined 

 and found to contain seven newborn rats that weighed from 4 

 gm. to 4.5 gm. each. The older members of the litter weighed 

 at this time 10.1 gm., 10.2 gm. and 10.5 gm. respectively. The 

 interval between the birth of the first and of the last members of 

 the litter was in this instance about twelve days. In each of these 

 cases the female with her young occupied an entire cage, so there 

 was no possibility that the newborn rats could have belonged to 

 another female. None of the small rats in these litters were runts, 

 and, with the exception of one that was killed for a museum 

 specimen, all of them were raised to maturity and some were 

 used for breeding purposes. 



Female 4^29 was not examined after her death for any possible 

 malformation of the genital organs. A careful autopsy was made 

 of female 6 A 6 2 3 , however, and there was no evidence whatever of 

 any abnormality either in the ovaries or in the uterus. This 

 rat had three other litters besides the one described above, and 

 in each of these litters all of the individuals were born during 

 the one period of parturition. 



The cases of superfecundation described in the second section 

 of this paper indicate that a single period of ovulation may ex- 

 tend over three or four days, but it is difficult to see how embryos, 

 born at intervals of two weeks, could possibly have developed 

 from ova belonging to the same period of ovulation. To assume 

 that some of the ova liberated at a given period of ovulation 

 were fertilized at once while others remained in the tubes or in 

 the uterus for nearly two weeks before they were fertilized seems 

 unwarranted; for while it is not known how long mature ova 

 can live without fertilization, it is improbable that they can live 

 for more than a few days at most. That part of the ova belonging 

 to a certain period of ovulation should produce embryos that 

 would be born after a normal gestation period of about 21 days 

 while the rest of the ova had their development so delayed that 



