GENERAL INTRODUCTION 19 



edges of the piece come together, and fuse in the same way in which 

 a piece of hydra closes. A new hollow sphere of small size is formed, 

 which then passes through the later stages of development as does the 

 whole normal blastula. 



Still earlier stages of the sea-urchin, or of the starfish, have the 

 power of producing embryos if they are cut into pieces. If the seg- 

 menting egg is separated into a few parts, each part will continue to 

 develop. Even the first two blastomeres or cells will, if separated, 

 produce each a whole embryo (Fig. 11, B). The power of develop- 

 ment of a part does not even end here, for, if the undivided, fertilized 

 egg is cut into pieces, the part that contains the nucleus will segment 

 and produce a whole embryo (Fig. 11, C, upper row). If the egg is 

 cut in two or more pieces before fertilization, and then each part is 

 fertilized, it has been found that not only the nucleated, but even the 

 non-nucleated fragments (if they are entered by a single spermato- 

 zoon) may produce embryos (Fig. 11, C, lower row). 



It may be questioned whether the development of parts of the 

 embryo, or of the egg, into a whole organism can be included in the 

 category of regenerative processes. There are, it is true, certain dif- 

 ferences between these cases and those of adult forms, but as there 

 are many similarities in the two cases, and as the same factors appear 

 in both, we cannot refuse, I think, to consider all the results from a 

 common point of view. 



PHYSIOLOGICAL REGENERATION 



Finally, there are certain normal changes that occur in animals 

 and plants that are not the result of injury to the organism, and these 

 have many points in common with the processes of regeneration. 

 They are generally spoken of as processes of physiological regenera- 

 tion. The annual moulting of the feathers of birds, the periodic loss 

 and growth of the horns of stags, the breaking down of cells in dif- 

 ferent parts of the body after they have been active for a time, and 

 their replacement by new cells, the loss of the peristome in the proto- 

 zoon, stentor, and its renewal by a new peristome, are examples of 

 physiological regeneration. This group of phenomena must also be 

 included under the term " regeneration*" since it is not sharply sepa- 

 rated from that including those cases of regeneration after injury, or 

 loss of a part, and both processes appear to involve the same factors. 



DEFINITION OF TERMS 



The older writers used such terms as " replacement of lost parts," 

 " renewal of organs," and " regeneration " to designate processes 

 similar to those described in the preceding pages. The term regen- 



