THE LIVING ORGANISM 231 



undergoing purification in structures called 

 lymphatic glands, is returned to the venous 

 system by the main lympathic ducts, the 

 whole forming the lymphatic system. 



Early even in the development of the 

 embryo animal, and long before birth, those 

 cells in male or female which are to serve the 

 purpose of carrying on the tide of life into 

 the next generation, by uniting with cells 

 from an individual of the opposite sex, are 

 segregated off into separate organs. During 

 the growth of the young individual these are 

 active all the time. It is a mistake to think 

 they have only become active at puberty. 

 They only become capable of producing 

 active sexual cells at puberty, but long 

 before that period they have been actively 

 exercising secret chemical functions in the 

 body, by means of what are known as internal 

 secretions, which are poured into the blood and, 

 being carried to other parts, stimulate these 

 to develop in definite ways. These activities 

 become enhanced as puberty approaches, 

 and such internal chemical secretions induce 

 and control those secondary sexual changes 

 which outwardly differentiate in increasing 

 degree male and female at this age. 



The detailed study of the work of these 

 various co-ordinated systems is the domain 



