GESTATION OF THE ALBIXO RAT. 387 



An instance recently came under my observation in which a male 

 rat in a litter about three months old weighed only 60 gm., while 

 his brothers and sisters all weighed from 130 gm. to 190 gm. 

 each. This small individual was unquestionably a runt, and he 

 was far less vigorous and active than the other rats in the litter. 

 Runts that are found in litters about two weeks old appear 

 fully as mature as do the other members of the litter, but they are 

 much less active. In litters like those described above the 

 smaller individuals are always less mature than the other rats, 

 although equally as vigorous and well nourished. 



In cases of this kind all members of the litter must have de- 

 veloped from ova belonging to the same period of ovulation, since 

 it is very improbable that a second period of ovulation ever 

 follows immediately after the first. There are two possible 

 explanations for these cases. If at some period of ovulation one 

 or more of the ova were unusually slow in maturing they might 

 not be liberated until two or three days after the rupture of the 

 more mature follicles, and so would not be fertilized at the first 

 mating. Such ova might, however, be fertilized at a subsequent 

 mating; for the period of heat in the rat extends over several 

 days and copulations take place frequently during this time, as 

 the investigations of Miller have shown. As the minimum 

 gestation period in the albino rat is 21 days, ova that were fer- 

 tilized late could not reach the proper stage of maturity to be 

 born with the embryos that developed from the first ova fertilized, 

 and consequently there would be a considerable interval between 

 the births of different members of the litter. According to this 

 explanation such litters are good examples of superfecundation. 



There is another possible explanation for these cases, namely, 

 that all of the individuals in the litter were developed from ova 

 that were fertilized at the same mating, but that for some reason, 

 possibly on account of faulty implantation, some of the embryos 

 received less nourishment than the rest and so reached maturity 

 later. Such conditions, it seems to me, would tend to produce 

 runts, and not merely to retard normal development. 



On dissecting pregnant females one frequently finds one or more 

 embryos that are much smaller than the rest. While in some 

 instances such small embryos appear normal and are presumably 



