242 THE EVOLUTION OF SEX. 



Strengthening of bones and muscles, and the profound psychical 

 changes which accompany the whole series of processes, are also 

 familiar. 



In higher vertebrates, the sexual maturity of the female is 

 marked by a cellular activity within the ovary, not less remark- 

 able than that in the testes. Associated therewith are minor 

 but often very important characteristics, such as the increased 

 mammary development in mammalia. In some of the lower 

 animals, such as certain marine annelids, the ova become so 

 numerous that their disruption or liberation is in great part a 

 mechanical necessity. The same might be said of fishes, 

 reptiles, and birds. At the same time the enlargement and 

 escape of the ova are doubtless expressions of a normal cellular 

 rhythm, of which hints are given in the frequent passage from 

 an amoeboid to an encysted phase, in the occasional relapse to 

 the former, and in the fatty degeneration or death of ova which 

 have not accomplished their destiny. 



The primitive ova of vertebrates lie in clusters in the substance or 

 stroma of the organ, and are produced from the essential germinal 

 epithelium. Only a minority, however, grow into genuine ova ; others, of 

 smaller size, form a nutritive sheath or follicle around them. In mammals, 

 each follicle forms a cavity containing a fluid. Into this the ovum, sur- 

 rounded 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 framework resembling connective 

 tissue, in which the solids and corpuscles of the blood serum, with colour- 

 ing matter derived from the hemoglobin 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 discussed. Though we certainly know 

 that ovulation is of regular occurrence whether fecundation takes place or 

 not, it seems that in 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 



