REPRODUCTION 99 



placenta. Through this an exchange of nutriment 

 and gases takes place between mother and child. 

 The embryo does not lie free in the uterine cavity, 

 but is enclosed in a bag of membranes (a sort of 

 bladder), which is filled with water in order to allow 

 the embryo to move about freely. The embryo is not 

 drowned in this water, because it does not breathe 

 air at all, but breathes, as it were, through the um- 

 bilical cord, receiving, as just pointed out, the neces- 



TTT 



FIG. 40. THREE EARLY SUCCESSIVE STAGES OF THE HUMAN EMBRYO. 



(Prom " Darwin and After Darwin," by G. J. Romanes. By permission of 

 Messrs. Longmans, Green & Co.) 



sary gases from the mother. The embryo during its 

 growth passes through various changes. If by acci- 

 dent birth should occur before the sixth month, we 

 call it abortion or miscarriage, as the child cannot 

 live. When the sixth month is reached it is capable 

 of independent life, and is now called a foetus. Such 

 a child, if prematurely born, is however very difficult 

 to rear. The older and more developed the foetus, the 

 greater its chances of life if it should happen to be 



