GROWTH AND REPRODUCTION 



775 



come into intimate juxtaposition, although, separated by cellular 

 membranes, they do not actually mix. Through the action of these 

 membranes oxygen and nutrient material are supplied by the mother 

 to the foetus, and the waste products of metabolism of the fetus 

 transferred from the fetus to the mother. 



Parturition. After an intra-uterine life of varying duration accord- 

 ing to the species, the foetus is expelled by the process of parturition, 

 or labour. In woman, this occurs at about the end of 280 days. 

 "Labour" is divided into three stages: (1) The first stage, which 

 results in the dilatation of the cervix of the uterus as the result of 

 rhythmical contractions which become more and more frequent; 



cauity of uterus 



dec/dual 'cells 

 syncytium 

 asal. layer of chorion 



mesoblast of chorion 

 decidua reflexa 



decidua seiotina 



cauity of amnion 



decidual cells, 



syncytium 

 basal layer of chorion 



uterine uessel 



embryonic epiblast 

 -archenteron 

 -primitiue coelom 



cauity of uterus 

 woblast of chorion 



FIG. 432. SECTION THROUGH OVUM EMBEDDED ix THE WALL OF THE UTEKUS 

 (F. W. Jones, after Peters and Silcnka, from Keith's "Human Embryology."). 



(2) the second stage, in which the fetal membranes are ruptured and 

 the fetus is expelled, usually head first, from the uterus by means 

 of prolonged, sustained contractions of the uterus occurring at freqi ent 

 intervals; (3) the third stage, in which the after-birth is expelled. 

 The whole process may take thirty hours or more in a primipara a 

 woman who is having her first child. In subsequent births, the process 

 is usually considerably shorter. What factor induces the onset of 



