50 PRINCIPLES OF PHYSIOLOGY 



as to ensure constant growth, while its sluggish 

 life implies the production of a minimum of 

 waste products. Thus growth goes on steadily 

 and with astonishing rapidity. Tissue after 

 tissue and organ after organ are formed, not in 

 a definite order as regards time but contem- 

 poraneously, as if some kind of directive agency 

 were at work. There are even examples of 

 something like foreknowledge in the building 

 up of the fcetus. Stores of glycogen are sup- 

 plied for the nutrition of embryonic tissues. 

 Iron is collected in the body of the foetus, 

 from the mother's blood, so that an abundance 

 of this metal, all important for the develop- 

 ment of red blood corpuscles, is found in the 

 newly-born when in the new condition of exist- 

 ence it is nourished by milk, which contains 

 only a small supply of iron. Iron is needed, 

 but as the milk does not contain enough of iron 

 for the wants of the organism during lactation, 

 the child utilizes the iron that has been already 

 stored. In development, too, one of the most 

 remarkable phenomena is the formation of 

 organs in most of which tissues take part that 

 are supplied by different layers of the embryo. 



