28O F. B. SUMNER. 



tion ranges from 14! to 28| hours. Their observations have 

 thrown little light, however, upon the periodicity of ovulation 

 at other times, though they do not seem to believe that this is 

 governed by any great regularity. Sobotta (1895) states that 

 ovulation recurs 21 days after parturition, basing his conclusions 

 upon the examination of several females. He regards as im- 

 probable the occurrence of intermediate periods, at intervals 

 smaller than this. 



On the other hand, it is stated by Heape (1900) that "the 

 usual length of the dicestrous cycle for rodents is ten to twenty 

 days," that of the rat and mouse (in England) being given as 

 approximately ten days (p. 26). Since it is also stated (Marshall, 

 p. 135) that "in the mouse, the rat, and the guinea-pig, ovulation 

 occurs spontaneously during 'heat,' and generally, if not in- 

 variably, during oestrus," it would follow that in these animals, 

 ovulation likewise occurs at intervals of about ten days. This 

 figure approximates much more nearly than Sobotta's to the 

 thirteen-day periods indicated by my observations. Presumably, 

 pregnancy would ordinarily interrupt these periods of ovulation, 

 and initiate a new cycle at its close, though the phenomena of 

 superfetation and deferred fertilization are pretty good evidence 

 that this is not necessarily the case. 



If, as seems probable from my data, the second brood of off- 

 spring resulted from a later period of ovulation, during or after 

 the first pregnancy, we must suppose that the spermatozoa were 

 retained in an active condition for many days and in some cases 

 even for weeks. Fertilization was deferred until the arrival of 

 viable eggs in the fallopian tubes or the uterus. If we consider 

 the intervals (Table I.) between the last possible opportunity 

 for copulation and the birth of the second litter, we have: 1 no. i : 

 13 days ; no. 2: 26 days; no. 3: 27 days; no. 4: 27 days; no. 5: 

 47 days (); no. 6: 28 days; no. 7: 37 days; no. 8: 36 days. 



Thus in only a single case (the first) could copulation have 

 occurred as late as 22 days (i. e., the normal period of gestation) 

 before the birth of the second brood. In case no. 5, if we suppose 



1 In all these cases except no. 5 and perhaps no. i, the first brood was born 

 unexpectedly, and it is likely that the male was still in the cage at the time of 

 delivery. 



