866 A MANUAL OF PHYSIOLOGY 



The ovum also begins as a typical cell with nucleus (germinal 

 vesicle) and nucleolus (germinal spot), and it forms, by its repeated 

 subdivision, all the cells of the foetal body. But, except in some 

 (parthenogenetic) forms, it never awakens to this reproductive activity 

 till fecundation has occurred ; and fecundation essentially consists in 

 the union of the male with the female element, or rather in the union 

 of the male and female nucleus. 



From time to time a Graafian follicle, over-distended by its liquor 

 folliculi, bursts on the surface of the ovary and discharges an ovum. 

 The frayed end of the Fallopian tube, rising up finger-like from the 

 dilatation of its bloodvessels, grasps the ovum, and it is passed 

 slowly along the tube by the downward-lashing cilia which line it. 

 If not impregnated, it soon perishes amid the secretions of the 

 uterus how soon has been matter of discussion, and can hardly be 

 considered as settled. If, however, impregnation occurs, the ovum 

 becomes fixed in one of the crypts or pouches of the uterine mucous 

 membrane (decidua serotina\ which grows round it as the decidua 

 reflexa. 



Menstruation. In the mature female, from puberty, the age at 

 which the reproductive power begins (thirteenth to fifteenth year), 

 on till the time of the menopause (fortieth to fiftieth year), at which it 

 ceases, an ovum or it may be in some cases more than one is dis- 

 charged at regular intervals of about four weeks. This discharge is 

 accompanied by certain constitutional symptoms and local signs that 

 last for a variable number of days. The genital organs are congested, 

 and a quantity of blood, which varies in different individuals, but is 

 usually from 100 to 200 grammes that is to say, T V n to A** 1 ^ tne 

 whole of the blood in the body is shed. At the same time, the 

 whole or a portion of the mucous membrane of the uterus is cast off. 



As to the physiological meaning of this menstruation, as it is 

 called, opinion is divided. Two chief theories have been proposed to 

 account for it, both of which agree in considering the phenomenon to 

 be connected with a preparation of the uterus for the reception of the 

 ovum. But according to the theory of Pfliiger the mucous membrane 

 is stripped off (by a process analogous to the ' freshening ' or paring 

 of the indurated edges of a wound by the surgeon, in order that 

 union may occur when they are brought together) on the chance, so to 

 speak, that an impregnated ovum may arrive. On the alternative 

 theory, this change takes place because the ovum has not been 

 impregnated, and the bed prepared for it is therefore not required 

 (Reichert, Williams, etc.). 



Development of the Ovum. Before fecundation, and apparently 

 as a preparation for it, the ovum is the seat of remarkable changes, 

 which have been most fully studied in the eggs of certain invertebrate 

 animals. A spindle-shaped structure appears stretching between the 

 nucleus and the surface of the ovum ; at its outer end a small round 

 body, the first polar body, rises up from the surface of the egg as if it 

 were being squeezed out of it, and is finally extruded. In most cases 

 the process is repeated; a new spindle forms and a second polar 

 body or directive corpuscle is cast out. As to the significance of 



