GESTATION OF THE ALBINO RAT. 383 



time, i. e., when 15 days old. The prolonged period of gestation 

 did not make these young rats more precocious in any way as 

 far as could be determined. 



In the rat, as in the mouse according to the observations of 

 Sobotta ('95) and of Long and Mark ('n), ovulation followed 

 by copulation must normally take place within a few hours after 

 parturition, since in all of the cases recorded in the above tables 

 the male was removed shortly after the birth of the litter and 

 the female with her young occupied a cage inaccessible to the 

 entrance of other rats. In none of the cases cited in Table I. 

 could lactation have delayed ovulation, since in each instance 

 the period of gestation was of normal length. The manner in 

 which the experiments were conducted seems to preclude the 

 possibility that the prolonged periods of gestation found in many 

 cases were due to a delay in ovulation caused by lactation, since 

 presumably mammalian spermatozoa are not functionally active 

 for more than two or three days after insemination, although, 

 according to Schultze ('66), they can live in the uterus for as much 

 as six days. It seems probable, therefore, that lactation pro- 

 longs the period of gestation by influencing the nutritive condi- 

 tions of the developing embryos. In the lactating albino rat 

 the physiological conditions affecting nutrition seem to be so 

 adjusted that the suckling of a small number of young does not 

 interfere at all with the development of a second litter which also 

 contains a small number of young. When the number of rats 

 suckled equals or exceeds the number that represents the average 

 size of the litter in the species, then lactation appears to lessen 

 the amount of nourishment that the developing embryos receive. 

 It seems to be more essential for the welfare of the species that 

 the suckling young should be well nourished than that the fcetal 

 young should develop at the normal rate. When a pregnant 

 female is suckling a large number of young, the embryos receive 

 less nourishment than is usually given by a non-lactating mother 

 and consequently they develop more slowly. In cases of this 

 kind the period of gestation is prolonged until the embryos have 

 reached the proper stage of maturity for birth. When a female 

 is suckling a small number of young and carrying a large litter, 

 there is apparently no readjustment of the nutritive conditions 



