284 ELEMENTS OF BIOLOGY 



animal organization, namely, the principle of dominance and sub- 

 ordination. More specifically, anterior regions, or certain of them 

 at least, are leading elements that in some way control and influence 

 the developmental fate of adjacent and more posterior regions. 



In recent years careful studies of the nature of the diflferences 

 between such dominant regions and subordinate parts have shown 

 that the dominant regions are more active physiologically; they un- 

 dergo dififerentiation sooner, they grow more rapidly, and are first 

 to be poisoned by powerful toxic substances. In the comparatively 

 few cases where it has been possible to make the test, it has been 

 found that dominant regions consume oxygen more rapidly than 

 do subordinate regions; furthermore, when connected through a 

 sensitive galvanometer, it is found that a dominant region bears a 

 positive electrical charge in relation to the charge of the subordinate 

 region, very probably because of its more rapid oxidative metab- 

 olism. A sufficient number of facts are not yet at hand to permit us 

 to conclude definitely that the control of form and organization is 

 efiFected by the electrical relations between parts, but the facts sug- 

 gest the possibility that this may be the basis of the mechanism. 

 As regards the development of form in the human body, we have 

 at hand only a few general facts, largely relating to the more rapid 

 growth and development of the anterior end and demonstrating 

 that the law of antero-posterior development applies. In the absence 

 of other evidence it would not be sound science to assume without 

 qualification that the mechanism that causes the human body to 

 assume antero-posterior organization and bilateral symmetry is elec- 

 trical in nature; the possibility is suggested, however, by the facts 

 known concerning the nature of dominance in simple forms. 



As organ systems in the embryo develop, the form characteristic 

 of that particular animal begins to appear. The process is gradual 

 and during the early period of development all embryos within the 

 phylum look very much alike. Thus a human embryo strongly 

 resembles that of a bird or of a fish during its early stages; pres- 



