REPRODUCTIVE CYCLE IN FEMALE VERTEBRATES 93 



on the other hand is controlled by the activities of the corpus luteum, which 

 has replaced the Graafian follicle under the influence of the luteinizing hor- 

 mone. The production of progesterone by the corpus luteum is eff'ected as 

 stated previously by the pituitary hormone, luteotrophin (LTH). OVulation 

 is the pivotal point interposed between these two phases. The follicular phase 

 may occur without ovulation, but the true luteal phase of a normal or fertile 

 reproductive cycle is dependent upon the ovulatory phenomena Certain luteal 

 conditions may be elaborated in an anovulatory cycle, but we are here con- 

 cerned with the normal events of the fertile reproductive cycle. 



The follicular phase includes that portion of the reproductive cycle known 

 as proestrus and a considerable part of estrus. Proestrus is the period of 

 rapid follicular growth and elaboration of the estrogenic substance which 

 precedes the period of estrus. Estrogen stimulates developmental changes in 

 the cellular structure of the accessory reproductive organs, particularly the 

 vagina and the uterus (figs. 52, 53). Estrus represents the climax of the fol- 

 licular phase. As such, it is a period of sexual receptivity of the male, and, 

 in spontaneously ovulating forms, of ovulation. During other periods of the 

 cycle the female is indiff'erent or even antagonistic to the male. The period 

 of estrus is often called period of heat, or period of rut. Estrus is followed 

 by pregnancy if mating is allowed and is successful, or, in many species, by 

 a period of pseudopregnancy if mating is not permitted or if the mating is 

 sterile (figs. 53-57). In some animals, such as the dog, pseudopregnancy is 

 a prolonged normal event even if mating does not occur, continuing over a 

 period almost as long as that of normal pregnancy (fig. 54). In other animals, 

 such as the opossum, pseudopregnancy forms but a brief episode. 



Pseudopregnancy is, generally speaking, intermediate in duration between 

 that of a normal luteal phase of the cycle and that of gestation. In those female 

 mammals where it does not occur normally, it is aroused by such procedures 

 as sucking of the nipples, stimulation of the vagina and cervix by the natural 

 mating process, or by artificially stimulating these structures. In some forms, 

 such as the rabbit, pseudopregnancy is aroused by mere handling or even by 

 sight of a male. (For discussion, see Selye, '48, p. 813.) 



The general changes of growth and development of the accessory organs 

 which occur during pregnancy and pseudopregnancy are controlled largely 

 by the secretions of the corpus luteum. The conditions thus imposed by the 

 corpus luteum comprise the luteal or progestational phase of the cycle (fig. 57). 



In most mammals, if pregnancy does not occur, the ovary and acces- 

 sory organs again gradually return to the sexually-resting condition known 

 as diestrus (fig. 53). In man and other primates the changes within the uterus 

 are not gradual but are precipitous, and most of the endometrial lining, to- 

 gether with considerable amounts of blood, is discharged to the outside (figs. 

 53, 59). This phenomenon is called menstruation. The causes of menstruation 

 are largely problematical; it is related to the fall of the level of either or both 



