PROVOCATIVE BIOLOGICAL THEORIES 81 



ture, such as a piece of embryonic kidney, is grafted into 

 muscle tissue or skin tissue, and, under the regulatory 

 power of the nerve fibers connected with the central 

 nervous system, with its main seat in the head region, 

 such tissue becomes like that into which it is grafted. In 

 the flatworm, Microstomum, as the animal grows older, 

 the head region loses the dominance it formerly exercised, 

 and a new brain and eyes form about halfway down the 

 body. This, nevertheless, is still under regulatory control 

 of the original head region until it breaks away to become 

 a new individual. The sway of influence of this regula- 

 tory control seems to follow the track of the nervous sys- 

 tem, diminishing with the length of the cords, or distance 

 from the dominant region. The gradation of the influence 

 of the "head" on the ''body" according to distance, Child 

 calls the "metabolic gradient." Therefore, as organisms 

 with a well-defined individuality are always organized 

 about one, or at most a few, broad lines or axes, the sup- 

 position that such metabolic gradients determine the na- 

 ture of the individual has come to be called the Axial 

 Gradient Theory. As a corollary, it is stated that as a 

 central nervous system always forms, if the animal devel- 

 ops far enough, it is to be concluded that the nervous 

 system is the final expression, both in arrangement and 

 in mode of action, of the system of metabolic gradients. 

 Two factors really should be considered in this theory : 

 (1) that the head is a more or less independent unity; 

 and (2) that the control which the head exercises over 



