ORIGIN AND DEVELOPMENT 51 



Thus, as an example, the framework of the 

 liver is formed by connective tissue from the 

 mesoderm, while the hepatic cells are derived 

 from the cells of the endoderm and have the 

 same origin as the cells lining the alimentary 

 canal. To bring those two structures together 

 to form a liver implies growth from different 

 points and at the proper time. 



23. Each tissue and organ has its heredi- 

 tary peculiarities. These are often obvious in 

 the features, the colour of the eyes and hair, 

 and the stature ; they are seen also in many of 

 those mental peculiarities that contribute to 

 the making of character and individuality ; 

 but they are not so evident in the arrange- 

 ments of individual organs. There can be 

 little doubt, however, that hereditary char- 

 acters may affect all the tissues and organs 

 of the body. They are not merely superficial. 

 All of these remarkable phenomena that lie 

 at the beginning of life are physiological 

 processes ; but science has not yet been able 

 to trace them from the physiological stand- 

 point. Interest in them, at present, is mainly 

 morphological, that is as regards the laws that 



