THE DEVELOPMENT OP THE EMBRYO ITSELF. 135 



difficult to understand that nothing so quickly disorganizes the 

 whole man, physical, meiital, and moral, as sexual excesses, 

 whether by the use of the organs in a natural way, or from 

 masturbation. 



Nature has protected the lower animals by the strong bar- 

 rier of instinct, so that habitual sexual excess is with them an 

 impossibility, since the females do not permit of the approaches 

 of the male except during the rutting period, which occurs 

 only at stated, comparatively distant periods in most of the 

 higher mammals. When man keeps his sexual functions in 

 subjection to his higher nature, they likewise tend to advance 

 his whole development. 



Summary. — Certain changes, commencing with the ripening 

 of ova; followed by their discharge and conveyance into the 

 uterus, accompanied by simultaneous and subsequent modifica- 

 tions of the uterine mucous membrane, constitute, when preg- 

 nancy occurs, an unbroken chain of biological events, though 

 usually described separately for the sake of convenience. 

 When impregnation does not result, there is a retrogression in 

 the uterus (menstruation) and a return to general quiescence 

 in all the reproductive organs. 



Parturition is to be regarded as the climax of a variety of 

 rhythmic occurrences which have been gradually gathering 

 head for a long period. The changes which take place in the 

 placenta of a degenerative character fit it for being cast off, and 

 may render this structure to some extent a foreign body before 

 it and the foetus are finally expelled, so that these changes may 

 constitute one of a number of exciting causes of the increased 

 uterine action of parturition. But it is important to regard the 

 whole of the occurrences of pregnancy as a connected series of 

 processes co-ordinated by the central nervous system so as to 

 accomplish one great end, the development of a new individual. 



The nutrition of the ovum in its earliest stages is efiFected by 

 means in harmony with its nature as an amoeboid organism ; 

 nutrition by the cells of blood-vessels is similar, while that by 

 villi may be compared to what takes place through the agency of 

 similar structures in the alimentary canal of the adult mammal. 



The circulation of the foetus puts it on a par physiologically 

 with the lower vertebrates. Before birth there is a gradual 

 though somewhat rapid preparation, resulting in changes which 

 speedily culminate after birth on the establishment of the per- 

 manent condition of the circulation of extra-uterine life. 



