82 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



deer, moose, fur seal, etc., ovulate one egg per year over a life span of a 

 few years. As stated previously, the human female might ovulate as many 

 as 400 eggs in a lifetime. In some species the reproductive life is brief. For 

 example, in the Pacific salmon (Oncorhynchus) females and males die after 

 their single spawning season, and a similar demise occurs in the eel (Anguilla). 



9. Spontaneous and Dependent Ovulation in the Mammals 

 AND IN Other Vertebrates 



Spontaneous ovulation without apparent stimulation from external sources 

 occurs commonly throughout the vertebrate series. However, dependent ovu- 

 lation conditioned by psychic or other nervous stimuli also is found exten- 

 sively. In certain mammals ovulation has been shown to be dependent upon 

 the stimulus induced by copulation, as, for example, the ferret, mink, rabbit, 

 cat, shrew, etc. The stimulus, carried through the nervous system, affects in 

 some way the anterior lobe of the pituitary gland which then produces in- 

 creased amounts of LH in addition to FSH. These females experience estrus 

 spontaneously, but later follicle growth and egg discharge are dependent upon 

 the added stimulation afforded by copulation. 



The element of nervous stimulation has a fundamental relationship to the 

 ovulatory phenomena in the vertebrates. Dependent ovulation occurs in cer- 

 tain birds, such as the pigeon, where mating provides a psychic or nervous 

 stimulation which effects ovulation. The presence of two eggs in the nest tends 

 to suppress ovulation. The removal of these eggs will arouse the ovulatory 

 procedures. However, the pigeon may sometimes lay eggs without the presence 

 of a male. In wild birds in general, the mating reaction is linked to the 

 stimulus for egg laying. The hen, on the other hand, is not dependent upon 

 copulation, but in many of the domestic varieties the presence of a number 

 of eggs in the nest appears to suppress egg laying. In the lower vertebrates 

 nervous stimuli also appear to have an influence upon ovulation. The mating 

 antics of many fish and amphibia may be connected with ovulatory phenomena. 



10. Egg Viability after Discharge from the Ovary 



The length of time that the egg may survive and retain its capacity for 

 fertilization after leaving the ovary depends upon the nature of the egg and 

 its membrane and the surrounding environment. In the urochordate, Styela, 

 the egg may remain for 3 to 4 hours after it is discharged into the sea water 

 and still be capable of fertilization. In the elasmobranch fishes, reptiles, and 

 birds the conditions of the oviduct are such that fertilization must take place 

 in the upper part of the oviduct within a few seconds or minutes after the 

 egg reaches the infundibular portion. In Fundulus heteroclitus and possibly 

 many other teleost fishes, the egg must be fertilized within 15 to 20 minutes 

 after spawning. In the frog, the egg passes to the uterus at the lower end of 

 the oviduct shortly after it leaves the ovary. Under ordinary reproductive tem- 



