224 THE EVOLUTION OF SEX. 



nutritive sheath or follicle around. In mammals, each follicle forms a cavity 

 containing a fluid. Into this the ovum, surrounded by a mass of follicle-cells, 

 projects. When mature, the follicle with its contained ovum has attained a 

 superficial position. By the bursting of the ripe follicle the ovum is expelled, 

 and passes into the approximated and ciliated upper end of the oviduct or 

 Fallopian tube. The rupture of blood-vessels in the substance of the ovary 

 fills up the Graafian follicle with blood. The white corpuscles form a frame- 

 work resembling connective tissue, in which the solids and corpuscles of the 

 blood-serum, with coloring matter derived from the haemoglobin of the latter, 

 are retained. The whole constitutes the "corpus luteum," which, should 

 pregnancy occur, may persist and undergo further retrogressive changes, or 

 otherwise gradually disappear. 



As to the direct causes of this process of ovulation there is some difference 

 of opinion. The congestion of the blood-vessels of the ovary, its own internal 

 turgidity, a slight contractility of its stroma, have been regarded as determining 

 factors. The process seems, however, rather to depend upon the growth and 

 turgescence of the individual follicle. The question of the relation of ovulation 

 to the process of copulation in the higher animals has also been much dis- 

 cussed. Though we certainly know that ovulation is of regular occurrence 

 whether fecundation takes place or not, it seems that m many cases copulation 

 is speedily followed by the liberation of an ovum ; nor is it difficult to see how 

 the profound nervous and circulatory excitement associated with the former 

 process might accelerate the bursting of a follicle. Leopold has conclusively 

 shown, however, that ovulation may also long precede impregnation. 



Since the oviduct, unlike its male counterpart, is not, in the vast majority of' 

 vertebrates, continuous with its associated organ, it is often difficult to see how 

 the ova once liberated into the body-cavity find their way safely into the small 

 opening of the duct. In the frog, however, tracts of the peritoneal epithelium 

 becomes ciliated, so propelling the ova in the right direction. In reptiles, 

 birds, and mammals the open end of the oviduct is widened, fringed, and 

 ciliated, and lies close to or even touching the ovary ; muscular fibers too are 

 present, and more or less active movements of this cilated end over the ovarian 

 surface have been alleged to occur. The oviduct once reached, the down- 

 ward progress of the ovum is insured by the cilia of the epithelial lining, and 

 probably also by peristaltic movements of its muscular coat. 



There is no doubt that the advent of sexual maturity varies with 

 enviromental conditions of climate, food, and the like. Broadly 

 speaking, sexuality becomes pronounced as growth ceases. Especially 

 in higher organisms, a distinction must obviously be drawn between 

 the period at which it is possible for males and females to unite in 

 fertile sexual union, and the period at which such union will naturally 

 occur or will result in the fittest offspring. In the lower animals, where 

 the individual life is usually shorter, sexual maturity is more rapidly 

 attained, though we find cases such as that of the fluke (Poivstomuni) 

 so commonly present in the bladder of the frog, where maturity of 

 the reproductive organs does not occur for several (three) years, and 

 maturity of growth for some years afterwards. In cestode parasites, 

 the bladder-worm stage remains indefinitely asexual, until in fact the 



