REPRODUCTIVE CYCLE IN FEMALE VERTEBRATES 97 



in many birds. Here the incubation period for the young is short and a 

 plentiful supply of food awaits the parents and young when it is needed. In 

 other words, the factors which induce the onset of the reproductive state 

 are correlated with the conditions which enhance the end to be achieved, 

 namely, the production of a new individual. 



Let us consider next the internal factors which induce the breeding state 

 in the female mammal. The commonly held theory regarding the pituitary- 

 ovarian relationship governing the control of the reproductive periods in the 

 mammal which ovulates spontaneously is as follows (figs. 53 and 59): 



( 1 ) FSH of the pituitary gland stimulates later follicular growth. This factor 

 probably is aided by small, amounts of the luteinizing factor, LH, to 

 effect an increased production by the ovarian tissues of the estrogenic 

 hormone. Early follicle growth probably occurs without FSH. 



(2) Estrogen output by the ovary rises steadily during the period previous 

 to ovulation. 



(3) Old corpora lutea or other ovarian tissue possibly secrete minimal 

 amounts of progesterone under the influence of luteotrophin, LTH. 



(4) As the quantity of estrogen rises in the blood stream, it inhibits the 

 production of FSH and together with small quantities of progesterone, 

 increases the output of LH from the pituitary gland. This combination 

 also may cause an increased outflow of the luteotrophic factor. 



(5) An increased amount of LH aids in eff'ecting ovulation and the sub- 

 sequent luteinization of the follicle. As the follicle becomes converted 

 into the corpus luteum, the presence of the luteotrophic factor brings 

 about the formation of increased quantities of progesterone and main- 

 tains for a time the corpus luteum and the functional luteal phase of 

 the cycle. 



(6) In those mammals possessing a series of repeating sexual cycles, it 

 is assumed that the fall of estrogen in the blood stream after ovulation 

 suppresses the LH outflow and permits a fresh liberation of FSH 

 from the anterior lobe of the pituitary gland, thus starting a new cycle. 

 The lowering of the estrogen level may be particularly and immedi- 

 ately effective in forms such as the rat and mouse, which have a 

 short metestrus or luteal phase in the estrous cycle. 



e. Reproductive Cycle in Lower Vertebrate Females 



While the words estrus, heat, or rut are generally applied to the mammalian 

 groups, the recurrent periods of sexual excitement in lower vertebrates are 

 fundamentally the same sort of reaction, although the changes in the repro- 

 ductive tract associated with ovarian events are not always the same as in 

 mammals. However, similar cyclic changes in the ovary and reproductive tract 

 are present in the lower vertebrates, and their correlation with the activities 



