REPRODUCTION 



155 



and in the previous tables 2, 2a, 3, and 4 these have been given, 

 together with a description of the vaginal smear which char- 

 acterizes each stage. 



The sexual behavior of the young male has been studied in 

 great detail by Stone ('22). 



Long and Evans ('22, pp. 70-74) also describe the behavior 

 during mating. There is left in the vagina, after copulation, 

 a so-called vaginal plug, formed by the male. This may remain 

 in place from 12-24 hours and is a trustworthy sign of in- 

 semination. 



At about the 14th day of pregnancy a deposit of blood may be 

 seen in the upper part of the floor of the vagina. This occurs 

 only at this phase and persists some three days. It is known as 



TABLE 113 



Age in days at opening of vagina and at first ovulation, and length in days of first 

 four oestrous cycles in a group of 200 rats. 



the "placental sign" and is the earliest infallible sign of preg- 

 nancy which may be detected in the living animal. 



In the series studied by Long and Evans ('22) there were 

 found for one ovulation 5.2 corpora lutea in one ovary or 10.4 

 in both. The average number of eggs was 9.6 in both oviducts 

 and the average number of young born 7. Thus there has been 

 a loss at each step between ovulation and parturition. 



Weight of ovaries during gestation and lactation. The changes 

 in the weight of the ovaries during gestation and lactation have 

 been followed by Stotsenburg ('23). The change in weight 

 during gestation for rats weighing 200 grams is illustrated by 

 chart 17. 



