98 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



of the pituitary gland is an established fact. Consequently, the words estrus, 

 rut, sex excitement, and heat basically designate the same thing throughout 

 the vertebrate series — namely, a period during which the physiology and 

 metabolism of the parental body is prepared to undertake the reproductive 

 functions. In this sense, the words estrus, anestrus, heat, etc. also may be 

 applied to the male as well as to the female when the male experiences peri- 

 odic expressions of the sexual state. 



Although the reproductive cycle in all vertebrates represents basically a 

 periodic development of the reproductive functions, there is a marked dif- 

 ference between the estrous cycle in the female mammal and the reproductive 

 cycle in most of the other female vertebrates with the exception of viviparous 

 forms among the snakes, lizards, and certain fishes. This difference is due 

 to the absence of a true luteal phase in the cycle. The follicular phase and 

 elaboration of estrogen appears to be much the same in birds, amphibia, and 

 fishes as in the mammals, but the phase of the cycle governed by progesterone 

 secretion, associated with a gestational condition in the accessory reproductive 

 organs, is found only among those vertebrates which give birth to their 

 young alive. 



The reproductive cycles in certain vertebrates may be changed by selective 

 breeding and domestication. For example, the domestic hen is derived from 

 the wild jungle fowl. The jungle fowl conform to the general stimuli of nature 

 as do most wild birds, and the reproductive cycle is associated with a par- 

 ticular season of the year. However, domestication and selection by man of 

 certain laying strains have altered the original hereditary pattern of seasonal 

 laying. Consequently, good layers will lay eggs over an extended period of 

 the year, although there is a strong tendency to follow the ancestral plan by 

 laying most of the eggs during the spring and summer months; during the 

 fall and winter months, a smaller number of eggs are laid. Some of the vari- 

 eties of the domestic hen conform more closely to the ancestral condition 

 than do other strains. Similar changes may be produced in the buffalo, which 

 in nature breeds in middle to late summer but in captivity has estrous periods 

 three weeks apart throughout the year (Asdell, '46). 



G. Role of the Ovary in Gestation (Pregnancy) 



1. Control of Implantation and the Maintenance of 

 Pregnancy in Mammals 



The ruling power of the ovary over the processes involved in pregnancy 

 is absolute, particularly during its earlier phases. In the first place, the corpus- 

 luteum hormone, progesterone, is necessary to change the uterus already con- 

 ditioned by the estrogenic hormone into a functionally active state. The latter 

 condition is necessary for the nutrition and care of the embryo. A second 

 change which the gestational hormone imposes upon the genital tract of the 



