(ESTRUS 227 



In other animals, as the frog, fertihzation is also external, hnt is 

 aided by a closer proximity of the male and female in an embrace 

 called am])lexns, a psendocopulation. When the females become 

 turgid with mature eggs, the male clasps a female tightly and as 

 the eggs are discharged sperm are poured over them. Among the 

 reptiles, birds, and mammals, sperm are introduced into the female 

 cloaca or vagina, and fertilization later occurs in the oviducts. 

 Most vertebrates, including the lower animals, show mating interest 

 at the time of ovulation, and their outward behavior is generally 

 influenced by this internal condition. There are some accomjjany- 

 ing changes in the oviducts and uterine regions. In the lower 

 forms these changes involve an activity of the glands adding 

 allnnnen and shells to the eggs, but in those forms where de\-elop- 

 ment is internal there are additional preparatory changes in the 

 uterus. 



In most of the lower mammals, ovulation occurs during the 

 period known as heat, rut, or oestrus. In the rat, cycles of heat, or 

 oestrus, occur e^•ery four days, in the guinea-pig every sixteen days, 

 in the pig every twenty-one days unless fertilization occurs. In 

 each of these periods there is some uterine preparation for ovulation 

 and also changes in the vaginal condition. The oestrus cycles in 

 these forms can be followed by changes in the vaginal cells to the 

 time at which the follicle is mature, at which time the animal is 

 said to be in heat. A desire for mating is manifested at this time, 

 and at the time of or following copulation, ovulation occurs. Should 

 copulation not be effected, ovulation usually occurs,, the egg degen- 

 erates, and the cycle is begun anew with the maturing of other 

 follicles. In man and the primates the main evidence of a regular 

 oestrus cycle is the phenomenon known as menstruation, which 

 occurs with the failure of fertilization. In these forms the uterine 

 preparation for fertilization is much more extensive and involves a 

 remarkable thickening and softening of the mucosa of the body of 

 the uterus for reception of the fertilized egg. In case the egg is 

 not fertilized l^efore it reaches the uterus such preparation is not 

 utilized, and there follows a loss of the major portion of the uterine 

 mucosa together with considerable blood. Menstruation represents 

 a failure of the re])roductive process and is followed by regeneration 

 and a return to the resting stage before there is a renewal of prepa- 

 ration for the next o\'uIation. Maturation of follicles usually ceases 

 with fertilization and is renewed after birth of the young. 



