HELEN DEAN KING. 



so that each foetus will get the nourishment that will enable it 

 to develop in the usual time. The development of ten or of a 

 dozen embryos on nourishment normally used by six means, 

 necessarily, that the embryos will develop more slowly and 

 therefore that the period of gestation will be prolonged. 



The causes that determine whether the period of gestation in 

 non-lactating albino rats shall be 21 days or 23 days are as yet 

 unknown. Presumably the size of the litter is a determining 

 factor in these cases as well as in those in which a female is 

 already suckling young. The age and physical condition of the 

 female are also factors that are probably of importance in this 

 regard. A very young female or one in poor condition would not 

 be able to nourish her foetal young as well as would a more mature 

 female in good condition, consequently the period of gestation 

 .would tend to be longer in the former case than in the latter. 



The extension of the period of gestation in the albino rat from 

 21 days to 34 days is not a remarkable phenomenon, since the 

 early observations of Tessier, quoted in detail by Collins ('86), 

 show that the duration of the gestation period in various mam- 

 mals is subject to considerable variation. 



The data furnished by Collins and Daniels give the extremes 

 of gestation periods shown in Table IV. 



TABLE IV. 



In none of these cases is the duration of the gestation period 

 relatively as long as in some of the cas^s recorded in Table III., 

 where the gestation period for the albino rat is shown to have 

 been extended more than one half of the minimum normal period. 

 The relatively greater length of the gestation period in the rat 

 may not, however, be of any significance, since it is very probable 

 that the periods of gestation given in Table IV. are not the longest 



