MEANS AND METHODS OF REPRODUCTION 



163 



vessels of embryonic origin pass from the embryo to the placenta. 

 In some mammals, the pig for example, there is at birth a clean 

 separation of the embryonic from the uterine portion of the 

 placenta; in other mammals, as in Man, some of the uterine 

 portion is shed along with the entire embryonic portion, the 

 whole being known as the afterbirth. The uterine tissues, lost at 

 birth or parturition, are soon restored by the uterus. The 



uv 



Fig. 108. — -Diagram of gravid human uterus, anterior wall removed, a, amni- 

 otic cavity, filled with fluid in which the foetus (late embryo) rests; ft, Fallopian 

 tube (oviduct); u, uterus; uc, umbilical cord, containing foetal arteries and veins 

 through which blood circulates to and from the placenta, pumped by the foetal 

 heart; uv, uterine vessels, carrying maternal blood to and from the placenta; v, 

 vagina. The shaded portions of the uterus are shed at birth. 



young are expelled by the contractions of the muscular walls of 

 the uterus. In Man, the navel is the place on the abdomen where 

 the umbilical cord was attached during intrauterine development 

 (Fig. 108). 



General. — The organs of reproduction, the ovary and the 

 testis, as has been said before, are those producing germ cells, 

 ova and spermatozoa, which, when united, are capable of repro- 

 ducing a new animal. Both egg and sperm are merely detached 

 portions of the parent body, possessing the capacity, when 

 brought together under proper conditions, of developing a new 

 individual. In parthenogenesis the ovum alone can do this. 



