28O REGENERATION 



a mixture of these substances), yet we have no reason to suppose that 

 the organism is anything more than the expression of its physical and 

 chemical structure. The vital phenomena are different from the 

 non-vital phenomena only in so far as the structure of the organism 

 is different from the structure of any other group of substances. 



Nageli has stated that each part acts as though it knew what the 

 other parts are doing. His idea of the idioplasm involves a con- 

 ception of the organism as a whole and not simply the sum total 

 of a number of parts. Hertwig, who maintained at one time that 

 the development of the embryo is the resultant of the action of the 

 cells on each other, admits in his work on Die Zelle nnd die 

 Gewebe that while this is in part true, yet on the other hand the 

 whole also exerts an influence on its parts. Driesch, who hypotheti- 

 cally suggested at one time that the nuclei act as centres of con- 

 trol of the cell by means of enzymes, has later adopted a widely 

 different view. Whitman has made a strong argument to the effect 

 that the cell theory is too narrow a standpoint from which to 

 treat the organism, and on several occasions I have urged that the 

 organism is not the sum total of the action and interaction of its 

 cells, but has a structure of its own independent of that of the cells. 



This discussion will suffice to show some of the opinions that have 

 been held as to the nature of the organization of the organism. Let 

 us next ask what properties we may ascribe to it. 



It has been found that certain polar, or rather dimensional, rela- 

 tions are characteristic of the organization. The term "polarity" ex- 

 presses this in a limited way, but refers only to one line having 

 two directions, while we now know that the dimensional properties 

 relate to the three dimensions of space, and for this idea we might 

 make use of the term heterotropy. Thus we find that a piece of 

 a bilateral animal regenerates a new anterior end from the part that 

 lay nearer the anterior end of the original animal, a new right side 

 from the part that was nearest the original right side, and a new 

 dorsal part from the region that lay near the original dorsal 

 part, etc. 



The polarity of a part can be changed in certain forms, as in 

 tubularia, by exposing the posterior cut-end to the external factors 

 that bring about the formation of a hydranth, or, as in hydra, by 

 grafting in a reversed direction a smaller piece on a larger one. In 

 Planaria lugubris and in the earthworm the polarity of the new 

 tissue may be reversed, as compared with that of the part from which 

 it develops, if the new part arises from certain regions of the body. 

 A curious instance of the effect of the polarity is shown by the regen- 

 eration from an oblique surface in planarians. The new head arises 

 from the more anterior part of the new material, rather than from the 



