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THE AMERICAN NATURALIST [Vol. XLII 



are clustered at its anterior end not because the brain is 

 there but because this is the end of the animal most likely 

 to receive stimuli, and the brain is at this end because it 

 has developed from this sensory equipment. The brain, 

 in other words, is the appendage of the sense organs. 



In tracing thus the growth of the three elements of the 

 neuro-muscular mechanism, the muscle, sense organ, and 

 brain, I have endeavored to keep before you their rela- 

 tions to those primitive organs, the ectodermic and ento- 

 dermic cell layers, and to make clear to you how these 

 cell layers come to be part and parcel of this growth. This 

 subject might have been illustrated by any other set of 

 organs than those concerned with the neuro-muscular 

 mechanism; thus it is well known that the skeleton has 

 been differentiated chiefly under the influence of the 

 muscles, and that the digestive apparatus is as intimately 

 associated with the differentiation of the circulatory or- 

 gans as these are with the respiratory and excretory sys- 

 tems. But to discuss such relations even in a brief way 

 would trespass too much on our present time, and I must 

 therefore pass them by. Suffice it to say that in the main 

 these relations constitute that province of zoology called 

 by Goethe morphology, which includes the fundamental 

 aspects of the form of adult and developing animals and 

 which has been a field that for a century past has elicited 

 the keenest interest from some of the most profound 

 masters of zoology. 



No one who has become deeply interested in morpho- 

 logical problems can have proceeded far without fre- 

 quently meeting questions of a physiological nature. To 

 answer these the simple observational methods of the 

 past are insufficient, and it is necessary to resort to ex- 

 perimental procedure such as has been for a long time the 

 practice of chemists and physicists. From this stand- 

 point one enters what may be regarded as the most recent 

 field of zoological research. Since the elementary sub- 

 stances of the animal body are the same as those of the 

 inorganic world and since the stream of energy flowing 



