198 THE SCIENCE OF BIOLOGY 



closing years of the nineteenth century, when descriptive 

 embryology was still a dominant form of investigation, is the 

 fact that the study of embryology has become largely ex- 

 perimental. The sequence of stages has been well enough 

 established to make the more subtle causes of development 

 the subject of investigation. 



Regeneration : The term regeneration is used to desig- 

 nate the process by which an animal or plant repairs the 

 losses resulting from destruction or removal of parts. 

 Capacity for regeneration may be small, as in the higher 

 vertebrates which possess only the ability to heal wounds, or 

 it may be so great that a small piece cut from the organism 

 will reproduce the whole. The fresh-water worms known as 

 planarians possess astonishing powers of regeneration. If 

 the adult worm be cut in two transversely (Fig. 26), each 

 piece becomes a complete animal. When divided length- 

 wise, the pieces behave in a similar manner, failing to 

 regenerate only when too great an area of cut-surface is ex- 

 posed to bacterial infection or from other untoward circum- 

 stances. In whatever way the piece may be removed, it 

 tends to form a new individual having the characteristics of the 

 original body, although there are some exceptions, as when 

 heads or tails are formed in the wrong position (Fig. 26 D 1 ) 

 a phenomenon which is termed heteromorphosis. Even a 

 very small piece (one investigator has estimated that a 

 piece only 1/250 of the bulk of the original can form the 

 entire worm) is able to heal its wounded surfaces and so to 

 change its shape and proportions that growth alone is 

 necessary for the production of a normal individual. 



In the formation of these new individuals by regeneration, 

 we observe, in addition to a healing of the wound, a change 

 in the relative proportions of the piece by which the normal 

 shape is regained. This latter phenomenon is termed 

 regulation. After it has occurred, the new individual merely 

 grows to the original size. Another phenomenon is the 

 polarity or determination of the axes of symmetry. No 



