EXPERIMENTAL ZOOLOGY 187 



shall be engaged in an analysis of the causal relation existing 

 between the several stages, and this can be determined only 

 by experimentation. 



REGENERATION 



As a further example of the work along experimental 

 lines, we may take the present study of the problem of 

 regeneration. Though there is some confusion in usage, most 

 workers use the term regeneration to designate collectively the 

 processes by which an injury resulting from the destruction 

 or removal of a part is made good. The capacity for regenera- 

 tion may be very slight, as in the case of the higher verte- 

 brates where it consists only in the ability to heal wounds, 

 or it may be so great that a small piece cut from an animal 

 will reproduce the whole. To illustrate by a specific case: 

 The fresh water worm known as the planarian has a most 

 striking power of regeneration. If cut in two, each piece 

 may form a new worm, and, even when divided into many 

 transverse segments, by the healing of the cut surfaces and 

 subsequent rearrangement of parts, each bit may become a 

 normal animal. When cut lengthwise, the pieces behave in 

 a similar fashion, unless too great an area of cut surface is 

 exposed. In most instances, the piece forms a new worm 

 having all the characteristics of the original. There are, how- 

 ever, exceptions to this, as in the case of heads formed in the 

 wrong position, a phenomenon known as "heteromorphosis." 

 But generally even a very small piece (one investigator has 

 estimated that a piece only i-25oth of the bulk of the original 

 can form the entire worm) has within it all the factors 

 needed for the formation of the complete organism. 



In the formation of these new animals, as just described, 

 there are two kinds of change ; first, the healing of the wound 

 by the formation of scar tissue ; second, changes in the relative 

 proportions of the piece by which the normal shape is attained. 



