10 



THE MATURATION OF THE EGG OF THE MOUSE. 



pointers will inscribe lines in the position of the lines between a and b, 

 fig. E. A pregnant female placed on the floor (MF) causes its pointer 

 (FP) to go up, as at b. As long as she remains on the floor the record 

 is like that between b and c. When she enters the nest, the nest pointer 

 (NP) goes up and the floor pointer (FP) down, the record being that 

 between c and d. When she leaves the nest and goes directly to the 

 food, the record becomes that between d and e, as at first. The record 

 at e shows that she again enters the nest, but on her way jumps to the 

 main floor (vertical mark on line FP). If, before again making an exit 

 (as she must for water and food), she gives birth to a litter of little ones, 

 on the one hand her weight will still be sufficient to depress the floor 

 (MF) , as at /, and on the other, the young will be heavy enough to keep 

 the nest down, so, that no matter how often she goes in and out, the nest 

 pointer (NP) will make an unbroken line, the floor pointer alone making 

 vertical marks. 



Knowing the time when the record began or ended, it is an easy 

 matter to ascertain the limits of a period of time, of day or night, within 

 which parturition occurred. The length of the period depending on 

 the frequency of the excursions which the mouse makes may vary 

 from about 15 minutes to 6 hours, but is usually from J hour to 2 hours. 

 Table i, based on the observation of 147 mice, is interesting as showing 

 the degree of precision of these observations. 



TABLE i. Observations. 



From this it is seen that in nearly one-third of the cases the period 

 is not over i hour; in nearly two-thirds (about 60 per cent) it is not 

 over 2 hours, and in nearly 85 per cent it does not exceed 3 hours. In 

 all subsequent calculations the middle point of the period is adopted 

 in each case, so that the greatest inaccuracy as to time can not exceed 

 half the length of the period, and assumably will be on the average 

 much less. 



