44 INTERNAL SECRETIONS OF THE OVARY 



of cycle of 5-4 days for 1,999 cycles. Of these 82 per cent, had 

 an average of 4-6 days. In the mouse the length of the cycle is 

 given by Allen (6) as between four and five days, while the 

 present author (498) found a mean length of 6-2 days for 1,000 

 cycles. The first oestrus occurs in the rat when about ten to eleven 

 weeks old, while in the mouse the cycle begins at the age of 

 about seven weeks. The earlier cycles appear to be slightly 

 longer than those occurring in the fully mature animal. In the 

 mated mouse, in the absence of pregnancy, the interval between 

 sterile copulations is about twelve days, and this increase is due 

 to the occurrence of a pseudo-pregnant period. Long and Evans 

 found the interval in the rat to be about thirteen days and also 

 showed that the mechanical stimulation of the cervix by the 

 vaginal plug results in the ovarian changes associated with the 

 pseudo-pregnant period (see also p. 171). Since copulation was 

 originally used as the criterion of oestrus, this lengthening of the 

 cycle after sterile copulation explains the error in the original 

 estimates. 



(Estrus occurs within twenty-four hours after parturi- 

 tion and is then in abeyance during the whole of lactation, 

 provided that a normal-sized litter is being suckled. In the 

 mouse this dioestrous period during lactation lasts for about 

 three weeks (491). In the rat, it lasts rather longer, twenty-five 

 to forty days according to Long and Evans, varying with the 

 number of young suckled. This disappearance of oestrus during 

 lactation is correlated with ovarian changes which result from 

 lactation. The abnormal prolongation of lactation does not 

 result in further prolongation of the dioestrous period. By 

 foster-mothering young litters, it is possible to prolong lactation 

 in the mouse for forty to fifty days without materially extending 

 the length of the lactation dioestrus (495). If one or two young 

 only are being suckled, inhibition is not set up by lactation and 

 the cycle after parturition is normal. If pregnancy supervenes 

 from the immediate post-partum oestrus the female mouse and 

 rat may be both pregnant and lactating at the same time. 

 Under these conditions the length of pregnancy in the mouse 

 is increased from the normal nineteen or twenty days up to as 

 much as twenty-eight days. According to Kirkham (323) this 

 is due to delay in implantation of the embryos. 



