20 



BIOLOGY 



4. Period of gestation. The gestation period of the non-lac- 

 tating albino rat is usually from 21-22 days. 



TABLE 2 

 Table of observed instances of oestrous cycles of various lengths 



TABLE 2A 



On the proportion of normal oestrous cycles occurring in two colony groups of Albinos 

 and in the gray-white strain of Long and Evans (Evans, '24 MS.) 



. Careful observations made by Long and Evans ('22) using 

 so-called "obstetrical cages" which make possible the deter- 

 mination of the exact time of birth show that 90 per cent of 

 the'gestation periods fall between 21f and 22 days. 



