42 BIOLOGICAL LECTURES. 



may be caused to arise from a single stump, provided the re- 

 growth of the toe be so interfered with as to compel regenera- 

 tion from two terminal regenerative surfaces instead of one. 

 This must follow from the law demonstrated by Barfurth's ex- 

 periments, namely, that the regeneration of an organ tends to 

 occur uniformly over and in a direction normal to the regener- 

 ating surface. In this way it is possible to mechanically deter- 

 mine the direction in which a regenerated part shall be repro- 

 duced by merely making changes in the angular relations of 

 the plane of the regenerating surface to that of the axis of 



.-•'' 3' / 



Fig. 2. 



the body, as indicated by the diagram in Fig. 2 of the regener- 

 ated tail of a tadpole. Here the line I indicates the plane 

 along which the tail has been removed, upon which regenera- 

 tion will restore the tail straight backward to the dotted area a. 

 If the plane of section is along the line II the tail will regener- 

 ate upward so as to be restored over the area indicated by the 

 dotted line enclosing b. If the plane of section of the tail be 

 along III the tail will be regenerated downward to the dotted 

 line enclosing the area c. It is therefore evident that Bar- 

 furth's law determines the inclination of the axis of the regen- 

 erated part to the body-axis, through the different conditions 

 of surface-tension that must be set up over regenerating sur- 

 faces, whenever the inclination of these to the axis of the 

 whole organism is changed. 



New equilibria of surface-tension established reciprocally be- 

 tween the cohering but independently developing segments of 

 the oosperm of the sea-urchin, that have been imperfectly sepa- 



