120 



COMPARATIVE PHYSIOLOGY. 



As a matter of fact, the pig, sheep, goat, cat, dog, and certain 

 other animals may conceive when less than one year old ; and the 

 cow and the mare when under two years. 



At such periods these animals are not of course mature and 

 should not he bred. 



OVULATION. 



In all vertebrates, at periods recurring with great regularity, 

 the generative organs of the female manifest unusual activity. 

 This is characterized by increased vascularity of the ovary and 

 adjacent parts ; with other changes dependent on this, and that 

 heightened nerve influence which, in the vertebrate, seems to be 

 inseparable from all important functional changes. Ovulation 

 is the maturation and discharge of ova from the G-raafian folli- 

 cles. The latter, reaching the exterior zone of the ovary, be- 

 coming distended and thinned, burst externally and thus free 

 the ovum. The follicles being very vascular at this period, 

 blood escapes, owing to this rupture, into the emptied capsule 

 and clots ; and as a result of organization and 

 subsequent degeneration undergoes a certain 

 series of changes dependent on the condition 

 of the ovary and related organs, which varies 

 according as the ovum has been fertilized or 

 not. When fertilization occurs the Graafian 

 follicle undergoes changes of a more marked 

 and lasting character, becoming a true corpus 

 luteum of pregnancy. 



The number of G-raafian follicles that ma- 

 ture and the number of ripe ova that escape at 

 about the same period varies, of course, with 

 the species and the individual, and is not al- 

 ways the same in the latter. 



In species that usually bear several young 

 at a birth a corresponding number of ova must 

 be ripened and fertilized at about the same 

 time ; while the reverse holds for those that 

 usually give birth to but one. 



The ovum in the fowl is fertilized in the 

 upper part of the oviduct; in the mammal 

 mostly in this region also, as is shown by the 

 site of the embryos in those groups of animals with a two- 

 homed uterus, and the occasional occurrence of tubal pregnan- 



Fia. 131.— Ovary of 

 rabbit at period 

 of oestrum, show- 

 ing various stages 

 of extrusion of 

 ova. (Chauvean.) 



