.s. A. Grimes, from National Audubon Society 



MOTHER BROWN THRASHER FEEDING HER FAMILY 



Among most species of birds the new individual depends upon the parents for the 

 heat that is essential for the development of the embryo; and the newly hatched birds 

 depend entirely upon the parents for food and protection during the first few weeks 



intestines (see page 171); but whereas the villi of the digestive system 

 extend freely into the food cavity, those of the placenta are embedded 

 among blood vessels of the uterus. The blood stream of the embryo is sup- 

 plied with nourishment from the blood stream of the parent, but the two 

 streams never mingle: they are separated by the two distinct sets of capil- 

 laries and by lymph spaces. 



The embryo is thus in every sense a "parasite", living within the larger 

 organism. Diffusion is constantly taking place between the blood vessels of 

 the uterus and the blood vessels of the placenta, which are parts of the 

 embryo's circulatory system. In this diffusion there is an exchange of dis- 

 solved food and of dissolved urea and other waste substances resulting from 

 the metabolism of the embryo. There is also an exchange of dissolved oxygen 

 and carbon dioxide. The embryo thus depends upon the mother not alone 

 for its digested food supply, but for its respiration and excretion too — as 

 well as for mechanical protection and a constant temperature. 



The capillaries of the placenta are connected with the blood system of 

 the embryo by way of arteries and veins running through a flexible "umbilical 



424 



