FORCE AND MOTION 41 



which the food, one eats is converted into blood and 

 passes into the arteries and veins. 



The embryo at first, has no heart, arteries, nor 

 veins. After its body has developed and grown to a 

 certain extent, the mother's heart and arteries carry 

 arterial blood to it through the "umbilical vein.' 

 This blood finally reaches the heart of the embryo, 

 and is carried by its heart and arteries to every part 

 of its body, then returned through "two umbilical 

 arteries" and the placenta to the veins of the mother. 

 In this way, the embryo has a sort of circulation of its 

 own. But it appears to have no independent circula- 

 tion during the first three or four months of its life; 

 and the blood which circulates through it must be 

 aerated or oxygenated in the mother's lungs. 



We may say, in general terms, that the mother's 

 heart and arteries exert all the force and produce all 

 the motion which build up the embryo. It is true that 

 the work of her heart and arteries in supplemented, 

 after a time, by that of the heart and arteries of the 

 embryo but the latter work is a small part of the 

 whole. 



The water in a stream runs from its head to its 

 mouth because the latter is nearer to the center of the 

 earth than the former. In other words, the water in 

 every stream is carried forward by the force of 

 gravitation. The water in a steam carries silt (mud, 

 fine earth, etc.) which is deposited along its course and 

 at its mouth. As already stated, the mother's blood 



/ / 



is carried to the embryo body by the force of her heart 

 and arteries. Her blood conveys to the embryo, the 

 materials of which it is built lip, as the water in a 



