514 



PHYSIOLOGY OF GONADS 



such as basal body temperature fluctuations 

 which may bear some intrinsically closer 

 relationship to the event in question. (See 

 Hartman, 1936, and Buxton and Engle, 

 1950, for discussion of this very practical 

 ]iroblera. ) 



Among mammals generally, si)ontaneous 

 ovulation takes place sometime during es- 

 trus (Asdell, 1946) . It is found during early 

 estrus in the opossum, red fox, dog, mouse 

 and hamster. In the rat some authors have 

 placed it early (Young, Boling and Blandau, 

 1941) and others late (Long and Evans, 

 19221 with respect to vaginal estrus. In the 

 writer's colony both relations hold, in 4-day 

 and 5-day cycles, respectively. Ovulation in 

 late estrus is reported for the cotton rat, 

 bank vole, guinea pig, pig, horse, and ass. 

 Sheep usually ovulate shortly before the 

 end of heat, sometimes a few hours after- 

 ward. As stated earlier, ovulation may even 

 occur in guinea pigs, rats, sheep, and cattle 

 without overt estrus. The cow usually ovu- 

 lates several hours after the end of heat. The 

 marsupial cat is said to ovulate 5 days 

 afterward (Hill and O'Donoghue, 1913). 

 The extreme is represented by certain bats 

 (Asdell, 1946) which copulate in autumn 

 and ovulate in the spring after a prolonged 

 state of subestrus. These variations prob- 

 ably express several factors. 



Among reflex ovulators there is consider- 

 able interspecies variation in the interval 

 between the stimulus that invokes release 

 of gonadotroj^hin from the hypophysis and 

 the eventual rupture of the Graafian follicles 

 (rabbit, ca. 10 hours; ferret, ca. 30 hours; 

 cat, 24 to 54 hours; 13-lined ground sciuirrcl, 

 8 to 12 hours; mink, 36 to 50 hours) . Among 

 spontaneous ovulators the comparable in- 

 terval is clearly defined for only the rat, 10 

 to 12 hours (Everett, Sawyer and Markce, 

 1949) . In the cow the data obtained by Han- 

 sel and Trimberger (1951) and Hough, 

 Bearden and Hansel (1955) i)lace the out- 

 side limit at about 30 hours. Here again, 

 threshold differentials among tlic vari- 

 ous tissues of the individual are pioba- 

 bly of gicat importance. 'I'hus in one 

 species the threshold foi' gonadoti'ophin 

 release may be lower than that foi' es- 

 trous behavior with the result tliat by 

 the time the latter makes its ai)pearance 

 the former has already transpired and 



ovulation will shortly take place. The rat, 

 for example, releases LH during the after- 

 noon, begins to show estrous behavior 

 around 8 p.m., and ovulates around 2 a.m. 

 (Everett, 1948, 1956b). In other species 

 these time relationships may be reversed. 

 In the cow, activation of the hypophysis ap- 

 parently occurs several hours after the onset 

 of estrus (Hansel and Trimberger, 1951). 

 The cow remains in heat 10 to 18 hours and 

 ovulates 13!/2 to 151/2 hours after going out 

 of heat (Asdell, 1946). The early termina- 

 tion of estrus apparently reflects a refrac- 

 tory state which sets in after estrogen 

 activity has continued for a time, for cas- 

 trates receiving continued estrogen therapy 

 remain in estrus for similarly brief periods. 

 In the mare, ovulation is delayed until a few 

 hours before the end of estrous periods that 

 may extend for 5 to 10 days or longer. This 

 suggests a relative refractoriness of the LH- 

 release mechanism in this animal. Such a 

 state of affairs approaches that in persistent 

 estrus or in the anovulatory cycle. 



B. OVARIAN STEROmS AND OVULATION 



1. Estrogens 



Chronic administration of estrogen to the 

 intact animal eventually produces ovarian 

 atrophy by suppression of gonadotrophin 

 secretion. However, some moderate basic 

 level of continuous estrogen secretion must 

 be compatible with normal function of the 

 hypophyseal-ovarian system; witness the 

 fact that blood estrogen assays in normal 

 women (]Markee and Berg, 1944) indicate 

 only a 2-fold increase at midinterval above 

 a base value of considerable magnitude. 



Induction of corpus luteum formation by 

 injected estrogen was first demonstrated by 

 Hohlweg (1934) in prepubertal rats^ and 

 the phenomenon has been repeatedly ob- 

 served by other woi'kers (Desclin, 1935; 

 Mazer, Israel and Aljjcrs, 1936; Westman 

 and Jacobsolm, 1938b; Herold and Effke- 

 mann, 1938; Price and Ortiz, 1944; Cole, 

 1946). The fact that the effect was not ob- 

 tained in rats younger than 30 to 36 days 

 l>y Piice and Ortiz, whereas Cole observed 

 it in the age-range of 21 to 28 days, demon- 



' Tlic eH'cct was later ol)1aiii(^(l witli androgens 

 (Holilwpg, 1937; Salmon, 1938; Xathanson, Fian- 

 spen and Sweenev, 1938). 



