498 



PHYSIOLOGY OF GONADS 



dividual and infertile ("menstmal") cycles 

 are the rule. Inasmuch as corresponding 

 factors operate among domesticated ani- 

 mals, the expression "female reproductive 

 cycle" commonly refers to those truly 

 abortive cycles that succeed one another in 

 the absence of insemination. The term is 

 used in that restricted sense in this chapter. 



With even that restriction, the female cy- 

 cle is actually a multiplicity of interlocking 

 cycles, in which the rhythmic interplay be- 

 tween hypophysis and ovary is fundamen- 

 tal. Attention must therefore be focused 

 on the physiology of the ovary and on the 

 hormonal and neural mechanisms that in- 

 tegrate hypophysis and ovary as a func- 

 tional system. Cyclic alterations in sex ac- 

 cessories and other nongonadal tissues are 

 considered mainly as indicators. The 

 "menstrual cycle," being strictly a uterine 

 cycle, comes in this category, together with 

 changes in behavior. 



No attempt is made to present an ex- 

 haustive description of the varied adaptive 

 modifications of the ovarian cycle among 

 the several mammalian orders. The reader 

 may consult works of the late F. H. A. 

 Marshall whose full bibliography is given 

 by Parkes (1949 1. Asdell's Patterns of 

 Mammalian Reproduction (1946» is an- 

 otlu'i' A-alual)lo source. 



II. Cycles Spontaneously Interrupted 



Cycles in the natural state are only im- 

 perfectly known, from random and often er- 

 ratic sampling. One may safely assume that, 

 as a rule, under optimal conditions they 

 are complete, fertile cycles. There are, then, 

 relatively few subhuman species in wdiich 

 the characteristics of incomplete cycles have 

 been studied. These species are necessarily 

 the very ones that have been amenable to 

 some form of human restraint. 



Segregation of the sexes or any other 

 interference with insemination should be re- 

 garded as a first experimental approach 

 to understanding the complete cycle. Such 

 factors unriuestionably operate in nature on 

 occasion. Controlled changes of environ- 

 mental conditions afford another approach 

 in which natural factors are simulated. 



The statement was made earlier that the 

 complete cycle may conceivably be inter- 

 rupted at almost any point. It has been 



learned that in different species segregated 

 females interrupt their cycles at different 

 stages and that usually the point of inter- 

 ruption is species-characteristic. These facts 

 have been of great service to the study of 

 reproduction, first, by arousing the curi- 

 osity of the investigator and, second, by 

 supplying a variety of ready made condi- 

 tions individually appropriate for particular 

 experimental studies. 



Examples of mammalian cycles are 

 schematically diagrammed in Figure 8.1. 

 It is customary to state that the usual, or 

 standard, infertile cycle is like that in pri- 

 mates or the guinea pig. The follicular 

 phase culminates in spontaneous ovulation, 

 after which corpora lutea are organized and 

 become spontaneously functional for a 

 period of time that is usually considerably 

 shorter than in pregnancy. 



In a few animals (rat, mouse, hamster) 

 the cycle terminates shortly after ovulation 

 before the corpora lutea become fully func- 

 tional. Such corpora lutea are said to be 

 inactive, in the sense that they cannot pro- 

 duce a decidual response to uterine trauma 

 (Long and Evans, 1922). Sterile mating or 

 analogous stimulation induces a luteal phase 

 which corresponds to that of the "standard" 

 mammal. This phenomenon is not entirely 

 limited to the small rodents, having been 

 described in the European hedgehog 

 (Deanesly, 1934). 



To this writer's knowledge there have not 

 been described any mammalian species in 

 which it is the rule that in isolated females 

 the process of ovulation begins (follicle ma- 

 turation, prelutein changes in granulosa, 

 secretion of secondary liquor folliculi, and 

 so on) without proceeding to eventual rup- 

 ture of the follicles. Many cases could l3e 

 cited, however, in which this has occurred 

 "abnormally." Characteristically, some de- 

 gree of luteinization occurs in the wall of 

 such a follicle and a lutein cyst is formed. 



On the other hand, there are numerous 

 species (reflex ovulators) in which the pre- 

 ovulatory maturation of follicles and ovu- 

 lation nearly always fail in the absence of 

 tlie male. The known species in which this 

 is true are widely distributed among the 

 mammalian orders and are often closely 

 iclated to other species in which spontane- 

 ous ovulation is usual. The domestic rabbit 



