ORGANIC EVOLUTION 



217 



vessels of the mother. This becomes the source of food 

 supply for the embryo during its long prenatal existence. 

 In the corresponding villi of the membranes of the embryo 

 copious capillary blood vessels are developed as a part of the 

 embryonic circulation: these are its food taking organs. 

 The process of nutrition is one of exchange of blood content 

 between mother and offspring by diffusion through the thin 

 walls of the villi. It is quite comparable to the exchange of 

 gases which takes place in the gills or lungs. Both food (in 

 solution) and oxygen are withdrawn from the passing cur- 

 rents of the mother's blood, and into the same currents are 

 discharged the carbon dioxide and all other waste from the 

 body of the embryo until its birth. 



The body of the embryo immersed within the amniotic 

 sac, closes to a narrow opening (the umbilicus) on the ventral 

 side of the abdomen and the closure elongates into a long 

 stalk-like umbilical cord through whose vessels nourishment 

 is drawn from the placenta until embryonic growth is ended. 

 The embryo then hangs on the cord, like a ripened fruit upon 

 its stalk. At birth the stalk is severed, and the feeding 

 organs of the embryo, full formed and functional, are called 

 into action. 



Such are the means by which the maximum of provision 

 for development of young is attained in mammals, and to 

 these there is added the development of milk from the 

 mammary glands as a further food supply for infancy. 

 What a vast difference exists in bodily equipment between 

 a new born mammal and the microscopic gastrula of a 

 lancelet ! 



The life process in the salamander and in other verte- 

 brates, is not very different from that in the worm. Indeed , 

 it is much the same in its essentials in all animals, the differ- 

 ences occurring in the ways and means whereby these 

 are accomplished. The essential processes are compre- 



