REPRODUCTION 79 



estrous period. In mice the average interval between a sterile mating 

 and the next estrus is 11 days {t^^, 100); in rats the average interval is 14.5 

 days and the range 7 to 19 days (122). It has been shown that pseudo- 

 pregnancy can be induced in the rat by several forms of artificial stimu- 

 lation. These include the brief insertion into the uterine cervix of a fine 

 glass rod (83), electrical stimulation of the vagina (60, 119), and intense elec- 

 trical stimulation through the head (63). Rats stimulated by the probe 

 method while under ether anaesthesia show only ten per cent pseudo- 

 pregnancies as against sixty-nine per cent for the controls (94). Spinal 

 anesthesia completely prevents the induction of pseudopregnancy. In the 

 rat, copulation without plug formation is a much less effective stimulus than 

 copulation with plug formation, and the chance that pseudopregnancy will 

 be induced seems to be still further increased if several completed matings 

 each with plug formation are permitted (9). 



Pseudopregnancy is accompanied by important changes in the uterus 

 paralleling those that occur during the corresponding stages of pregnancy 

 and serving to prepare the uterus for the implantation of embryos. His- 

 tologically, the changes in the rat and mouse uterus are not as striking as 

 those occurring in the rabbit, but definite progressive changes in the 

 epithelium and stroma have been noted (7). More significant than the 

 histological changes is the capacity of the uterus during the early part of 

 pseudopregnancy to respond to appropriate stimuli by local growth of the 

 decidua, giving rise to swellings called deciduomata. Any slight local 

 injury to the uterus will incite their formation; a common practice is to use 

 a silk thread inserted through the uterine wall (83). In the pseudopregnant 

 mouse, the maximum capacity for deciduomata formation following local 

 injury of the uterus occurs about three days post coitum; by five days post 

 coitum the sensitivity is almost lost (103). The sensitive period thus corre- 

 sponds to the period of normal implantation. 



The mammary gland undergoes development during pseudopregnancy. 

 The changes parallel those of pregnancy for the first nine days following 

 copulation. At the end of this period the pseudopregnant development of 

 the mammary gland reaches its peak, and regression sets in (30). 



The available evidence, though not conclusive, seems to indicate that 

 the remarkable causal chain by which a stimulus applied to the uterine 

 cervix prepares the uterus to receive the young embryo involves a nervous 

 impulse from the cervix to the pituitary, an endocrine effect of the pituitary 

 on the corpora lutea, and a second endocrine effect of the corpora on the 

 uterus and mammary glands. 



