282 F. B. SUMNER. 



been more than doubled. Now the first brood consisted of only 

 two individuals, so that any retarding effect due to lactation 

 would have been almost at a minimum. Daniel found that this 

 amounted to only about a day for each young mouse suckled. 1 



If it be contended that the retardation probably resulted from 

 the gestation of the first litter rather than their subsequent 

 nursing, it is relevant to point out that this species of Peromyscus 

 may give birth to 5, 6, or even 7 young, with little, if any, 

 prolongation of the period of gestation beyond the normal. 2 

 Again, in cases 7 and 8, the birth of the second brood occurred 

 37 and 39 days respectively after the first. In both of these 

 cases it is possible that copulation occurred during or shortly 

 after the first pregnancy, since the male was not removed until 

 after the delivery of the first brood. Thus, at the least, we must 

 assume that the second gestation was prolonged more than two 

 weeks, owing to the suckling by the mother of her three earlier 

 young. 



To me it seems much more credible that the cases here dis- 

 cussed resulted from the fertilization of recently liberated eggs 

 by spermatozoa which had been received many days previously. 

 I do not think it helps us at all to assume that the term of 

 development, after fertilization, was prolonged to any such 

 extent as we should here have to suppose. Nor do I believe 

 that it is necessary to assume a second copulation to account for 

 the later brood, though this possibility is not excluded in my 

 experiments. To refer again to case 5, of my first table, the last 

 opportunity for copulation was 19 days before the birth of the 

 first brood, i. e., only 3 days after the conception which initiated 

 the development of this brood. Is it not as easy to admit the 

 retention of living spermatozoa from this first effective copulation 

 as it is to admit their retention from another copulation occurring 

 only three days later? 



At first glance, it is not clear why these cases of deferred 

 fertilization should, in every instance, have followed a previous 

 pregnancy. In other words, why should not this phenomenon 



1 For rats, indeed, Miss King found that retardation was not certain to follow 

 unless more than five young were suckled. 



2 One brood of 6 was born 23 days after the earliest possible fertilization, while 

 one brood of 6 and one of 7 were born 24 days after the earliest possible fertilization. 



