The Reproductive System 



263 



Pregnancy. — Fertilization of the human ovum usually occurs about 

 the fourteenth day of the menstrual cycle. It takes the fertilized ovum 

 approximately ten days to reach the uterus and to begin implantation. 

 In the meantime, the endometrium has proliferated in preparation for 

 the reception of the ovum. If both fertilization and implantation are 

 successful, the corpus luteum continues to grow and secrete the hormone 

 progesterone. The secretion of this hormone seems to be necessary for 

 the maintenance of pregnancy for at least the first three months in 

 human beings. Normally, in human beings, parturition or birth of 

 the young occurs 280 days after the beginning of the last menstrual 

 period. Menstruation, of course, does not occur during pregnancy. 



OVARIAN CYCLE 



f-.' ;v,P^ CORPUS LUTEUM 



m . , 



CORPUS LUTEUM 

 o| PREGNANCY 



Fig. 79. — Diagram of the changes that occur in the endometrium of the uterus 

 during an ordinary menstrual cycle and in one terminating in pregnancy. These 

 changes are correlated with the ovarian cycle. (Modified from Patten: Human 

 Embryology, The Blakiston Company.) 



Estrus.— While man and a few other primates have a mating sea- 

 son which lasts throughout the year, other mammals are receptive to 

 copulation only at certain periods. These periods of sexual drive known 

 as heat or estrus occur with varying frequency in different mammals. 

 The recurring cycles may vary from five days in the rat to about sixteen 

 days for the hog and guinea pig. Many wild animals have but one 

 cycle a year; others, such as the dog, have two. Very often estrus 

 is accompanied by a vaginal discharge, and usually is associated with 

 the release of eggs from the ovary. Some animals such as the cat 



