ERUPTION OF THE PERMANENT TEETH 139 



From these diagrams it will be seen that there are three general 

 types of growth in the white rat as demonstrated by Hatai and 

 Donaldson: (1) represented by the brain, which is "characterized 

 by a very rapid growth in weight at an early period and after this 

 period the rate of growth is much reduced;" (2) represented by the 

 extremities, spleen, thymus and thyroid which are characterized 

 by a relatively rapid rise at an early period followed by a straight 

 line at an angle from the base line always much greater than that 

 of type 1; (3) represented by the sex glands, which have an 

 irregular growth, first slow, then rapid, then slow again. 



In any consideration of development it must be remembered 

 that general laws should apply to all forms of mammals, at least, 

 yet inasmuch as certain parts may be pathological (the brain of 

 man is a pathological organ in relation to other forms and to evo- 

 lution) any laws made for one form may not apply to another. 



The first part of the white rat to develop in prenatal growth 

 is the trunk, including the heart, spinal cord and somites, which 

 form 65 per cent of the total body weight in the first month of 

 intrauterine life. Next the head develops, including the brain, 

 skull, eyeballs and face, which forms 45 per cent of the total body 

 volmne at 2 months. The extremities maintain a uniform de- 

 velopment throughout the prenatal period after their initial 

 rise in the second month, and so do the spleen, thymus and thy- 

 roid, although the relative growth of the last three is small com- 

 pared to the relative growth of the extremities. The individual 

 organs have a maximum relative volume during the prenatal 

 period of growth as follows: heart, first month; brain, second 

 month; liver, third month; lungs, fourth month; and kidneys, 

 seventh month. 



The head in man is reciprocal in its growth to the trunk, and 

 the trunk and the parts of the extremities are reciprocal to each 

 other and to the extremities, as demonstrated by Godin (23, 24) 

 and Pfitzner (45, 46), therefore these parts may be called com- 

 plementary structures. The development of the heart is coin- 

 cident with the development of the trunk, and the development 

 of the head is coincident with the development of the brain, 

 therefore the heart and brain may be called complementary 



