THE OVUM 



247 



present, passes into the uterus and forms part of the waste products of the 

 mucous membrane, and with them is expelled; one of the many instances of 

 the reckless liberality of the natural functions, which are constantly supply- 

 ing redundant matter for the development of new organs or repair of 

 wasted tissue — matter which is often in excess of the demand, or is supplied 

 unconsciously when the conditions are not favourable to its fruitful use. 



THE OVUM 



Changes which occur in the mammalian ovum during its progress to 

 maturity are always going 

 on, from the time of puberty 

 to the end of the productive 

 life of the animal. Its mature 

 state is reached with the oc- 

 currence of sestrum, or heat, 

 and it is to be noticed that 

 during the few days of con- 

 tinuance of this condition 

 there is a marked increase 

 of sexual excitement. The 

 mature ovum or ova are at 

 this time discharged from 

 the Graafian follicle. 



An idea of the form and 

 structure of the mammalian 

 ovum may be gained fi-om 

 an examination of the egg 

 of a bird. The common fowl furnishes the most simple examples, simple 

 because they are prominent objects, easily seen by tlie unaided eye, wdiile 

 the mammalian ovum is a microscopic object, only to be distinguished by 

 the aid of a highly magnifying power. 



In the above illustration (fig. 537) a diagram, with description, 

 exhibits the ovum lying in the Graafian vesicle. 



Physiologists are not agreed as to the successive steps in the formation 

 of the ovum, but it is allowed that the development of the germinal vesicle 

 is precedent to the appearance of the yolk. The germinal spot is to be seen 

 in the germinal vesicle, and presumably the spot is the incipient body round 

 which the vesicle is developed. 



From its origin to its maturation the chief changes which occur in the 

 ovum are those incidental to its growth, and the necessary advance of the 



Fig. 537. — The Ovum lying in the Graafian Vesicle 



a, Stroma or tissue of ovary; i, c, external and internal tunics 

 of Graafian vesicle ; rf, cavity of vesicle ; e, thick tissue of ovum 

 or yolk sac; /, yolk; g, seminal vesicle; A, the spot. 



