THE DYNAMIC FACTOR IN REGENERATION. 2/3 



finds axial relations in the egg polarity, bilaterality, radial 

 symmetry, etc. ; the latter finds the new organs regenerated in 

 definite relations to the old. To some observers the distribution 

 or the stratification of the materials of the egg, has seemed a 

 sufficient basis for the results referred to under the term polarity ; 

 to others it has seemed more probable that there exists in the 

 egg an arrangement or structure that has axial relations from 

 which result not only the depositions of the formed materials 

 but also the nature of the action of the parts. Polarity is from 

 this latter point of view not simply a passive structure, but a re- 

 lation of the parts that directs the shifting series of changes that we 

 call development. At one time one of these views has seemed 

 more probable ; at other times the other. The history of modern 

 experimental embryology and regeneration shows the influence 

 that these views have had on those who have followed the 

 new work. In a general way the two views may be classed as 

 the materialistic or chemical and the dynamic or physical con- 

 ceptions of the developmental process. At present it seems to 

 the writer that the evidence has been steadily pointing to the second 

 of these contrasting views as the more probable. As far as the 

 egg is concerned, the recent experimental work goes to show 

 that the visible inclusions of the protoplasm (yolk, oil and other 

 granules perhaps) are not the fundamental causes of the forma- 

 tive processes, although they may be needed in certain regions to 

 carry out the future development of the structures that there 

 appear. In regard to regeneration it has been evident for some 

 time, that the specification or the differentiation (with its concomitant 

 products), cannot be unreservedly utilized as a basis for an explana- 

 tion of formative processes that take place. For example, if the 

 gross materials or the differentiations of the head end of a plana- 

 rian are the causes of that region being a head, it is inexplicable 

 that when the head is removed it could regenerate a tail. There 

 must be something else behind what we see that is responsible 

 for the change that takes place. These and other considerations 

 lead to the view that there exists a fundamental property of liv- 

 ing matter that is the formative principle of development. On 

 two former occasions, when attempting to analyze the results of 

 regeneration in Tubularia, the author tried to account for the re- 



