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ROBERT BENNETT BEAN 



Diagram 2 Postnatal growth in the white rat 



structures. The development of the heart is also reciprocal to 

 that of the lungs. The heart develops in time before the de- 

 velopment of the brain and the lungs, but it is complementary 

 to both. Likewise the liver may be reciprocal to the lungs, 

 heart and brain in its development, and so each may be recip- 

 rocal to the other, but if we examine adjacent organs in their 

 development, such as the head and trunk, the heart and lungs, 

 and the liver and intestine, as well also as the upper and lower 

 teeth, their reciprocal development makes them logical comple- 

 ments of each other. 



Diagram 3 Periods of growth in stature and eruption of the teeth 



The postnatal development of the structures of the white rat as 

 given by Jackson may be grouped into four periods, and I have 

 roughly approximated these periods for man. The brain and 

 lungs develop most rapidly soon after birth, the heart and kid- 

 neys a little later, followed by the development of the stomach, 

 intestine and liver. The sex glands develop irregularly but their 

 most rapid period of development immediately precedes puberty. 



I have roughly approximated the periods of most rapid growth 

 in stature and the most rapid development of the teeth after 

 birth, and have placed them in diagram 3. This is a tentative 

 scheme and awaits further observations for confirmation. 



The first period of postnatal growth is the most rapid of all, 

 and is associated with the development of the trunk and extremi- 

 ties. This is followed by the eruption of the temporary teeth, 

 associated with the rapid development of the brain. This re- 



