216 THE AMERICAN NATURALIST [Vol. XLIII 



however, probably be a mistake to regard the production 

 of these hormones as limited to a few organs such as the 

 thyroids, suprarenal bodies, etc. The most recent work 

 in this field points to the conclusion that all active organs 

 of the body, nerve centers, muscles, glands, etc., produce 

 hormones which in the blood probably exert extensive in- 

 fluences on the parts with which they come in contact, and 

 examples of this kind are being rapidly discovered. It 

 was formerly supposed that the secretion of the pan- 

 creatic juice, which is poured into the small intestine 

 when the partly digested food from the stomach reaches 

 that organ, was dependent upon a nervous signal given to 

 the pancreas from the intestine, but it is now well estab- 

 lished, through the brilliant work of Bayliss and Starling, 

 that the action of the acid food on the walls of the in- 

 testine produces a hormone, called secretin, which when 

 carried in the blood to the pancreas will cause that organ 

 to secrete. The evidence of this lies in the fact that when 

 a small amount of secretin is injected directly into the 

 blood stream of a mammal, the • pancreas, whose nerve 

 supply may have been cut off, will begin to secrete with- 

 out the presence of food in the intestine. Still more 

 remarkable is the correlation between the mammary 

 glands and the embryo in mammals. It is well known 

 that as the time for the birth of a mammal approaches, 

 the mammary glands of the parent grow in- size and struc- 

 tural changes appear preparatory to the secretion of milk. 

 This correlation between the growth of the embryo and 

 the growth of the mammary glands can not depend upon 

 nervous coordination, for the nerves of the embryo have 

 no connection with those of the maternal body. The 

 correlation depends upon a substance, a hormone, pro- 

 duced in the body of the embryo and transmitted to the 

 blood of the mother, whereupon it so influences the mam- 

 mary glands as to start their growth. The evidence for 

 this lies in the fact that if the extracted juice of a rabbit 

 embryo is injected periodically into the circulation of 

 a virgin female rabbit, her mammary glands can be in- 



