REPRODUCTION 573 



check the flow, and this is always liable to be followed by serious 

 consequences. A sudden chill often has the same effect; hence a 

 menstruating woman ought always to be warmly clad, and take 

 more than usual care to avoid draughts or getting wet. At these 

 periods, also, the uterus is enlarged and heavy, and being (as may 

 be seen in Fig. 152) but slightly supported, and that near its lower 

 end, it is especially apt to be displaced or distorted; it may tilt 

 forwards or sideways (versions of the uterus), or be bent where the 

 neck and body of the organ meet (flexion) . Hence violent exercise 

 at this time should be avoided, though there is no reason why a 

 properly clad woman should not take her usual daily walk. 



The absence of the menstrual flow (amenorrhea) is normal dur- 

 ing pregnancy and while suckling; and in some rare cases it never 

 occurs throughout life, even in healthy women capable of child- 

 bearing. Usually, however, the non-appearance of the menses at 

 the proper periods is a serious symptom, and one which calls for 

 prompt measures. In all such cases it cannot be too strongly im- 

 pressed upon women that the most dangerous thing to do is to take 

 drugs tending to induce the discharge, except under skilled ad- 

 vice; to excite the flow, in many cases, as for example occlusion of 

 the os uteri, or in general debility (when its absence is a conserva- 

 tive effort of the system), may have the most disastrous results. 



Fertilization. As the ovum descends the Fallopian tube the 

 changes of menstruation are taking place in the uterus. Fertiliza- 

 tion usually takes place in a Fallopian tube. The spermatozoa are 

 carried along partly, perhaps, by the contractions of the muscular 

 walls of the female cavities, but mainly by their own activity. 

 Occasionally the ovum is fertilized before reaching the Fallopian 

 tube and fails to enter it, giving rise to an extra-uterine pregnancy. 



The actual process of the fertilization of the ovum has only been 

 observed in the lower animals, but there is no doubt that the phe- 

 nomena are the same in all essentials in all cases. Some of the 

 spermatozoa penetrate the zona radiata and one of them enters the 

 ovum. After the entrance of a single spermatozoan a membrane 

 forms inside the zona radiata (Fig. 156) which prevents others 

 from entering. The head of the spermatozoan, which is chiefly 

 chromatin, becomes separated from the tail; the latter disappears, 

 probably by absorption into the substance of the ovum. The head, 

 now known as the male pronucleus, approaches the chromatin mass 



