92 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



accessory reproductive structures added normally to the estrogenic effects 

 during the reproductive cycle constitutes the luteal phase of the cycle. In this 

 phase of the cycle the uterine glands elongate and begin secretion, and the 

 uterus as a whole is prepared for gestation as a result of the action of the 

 progestational hormone, progesterone, associated with estrogen. (See figs. 

 53, 59.) 



F. Reproductive State and Its Relation to the Reproductive Cycle in 



Female Vertebrates 



The changes in the female reproductive organs resulting in structural growth 

 and development referred to above (70-74, 85-88) are consummated in the 

 ability of the female to fulfill the reproductive functions. The phase of the repro- 

 ductive events characterized by the ability to reproduce is known as the repro- 

 ductive climax. This period of culmination remains for a brief period, to be 

 followed by recession and involution once again to a resting condition. This 

 developmental progression to a state of reproductive climax followed by re- 

 gression to a resting condition constitutes a cycle of changing events. When 

 conditions again are right, the cycle is repeated. Each of these cyclic periods 

 is known as a reproductive or sexual cycle (figs. 53-59). The reproductive 

 life of all female vertebrates is characterized by this series of cyclic changes. 



In most vertebrate species, the female experiences one sexual cycle per 

 year, which corresponds to the seasonal cycle in the male. However, in various 

 mammals and in certain birds, such as the domestic hen, several or many 

 reproductive cycles may occur during the year. The male, under these con- 

 ditions, is a continuous breeder; that is, he produces sperm continuously 

 throughout the year. 



1. Sexual Cycle in the Female Mammal 



a. Characteristics and Phases of the Reproductive Cycle 



The estrous cycle in mammals is a complex affair composed of a number 

 of integrated subcycles. The changes occurring in the ovary are called the 

 ovarian cycle; the cellular changes in the uterine (Fallopian tube) form a 

 cycle; the responses in the mammary glands constitute the mammary cycle; 

 the cyclic events in the uterus make up the uterine cycle, while those in the 

 vagina form the vaginal cycle (figs. 53, 54, 57). 



The entire estrous cycle may be divided by ovarian changes into two main 

 phases: the follicular phase and the luteal phase (fig. 53). The former is under 

 the immediate influence of the enlarging Graafian follicle, which in turn is 

 stimulated by the follicle-stimulating and luteinizing hormones of the pituitary 

 gland, with the subsequent production of estrogen. It is probable that the 

 luteinizing hormone, LH, is mainly responsible for estrogen secretion. (See 

 Evans and Simpson in Pincus and Thimann, '50, p. 355.) The luteal phase 



