SOME PROBLEMS OF REGENERATION. 195 



them into line with the results of embryology because of an 

 assumed necessity of the so-called germ-layer theory that ecto- 

 derm, endoderm, and mesoderm always give rise to the same 

 layers and organs. Recent years have witnessed the decline 

 of the germ-layer theory, and numerous discoveries in budding 

 and dividing animals as well as in regeneration of new parts 

 have shown that the germ-layers do not in some cases play the 

 part assigned to them. The overthrow of the germ-layer dogma 

 has done much to prepare the way for a broader conception of 

 development and cell-differentiation. If I mistake not, there is 

 a tendency at present, that is slowly gaining ground, to give 

 up as unprofitable the interpretation of regenerative and even 

 embryological phenomena in terms of speculative phylogeny. 



A large body of information has now been gathered in regard 

 to the power of regeneration in the plant and animal kingdoms, 

 and the time has come, I believe, when we can profitably cease 

 from indiscriminate mutilations, and when, after an examination 

 of the known facts, we can ask ourselves what are the real 

 problems that are presented to us in the regeneration of living 

 things. We can then hope to direct our experiments to some 

 definite purpose toward the solution, if solution be possible, 

 of the main problem. It must be admitted that well-directed 

 attempts often prove futile, while accidental discoveries are 

 sometimes of the greatest importance. Nevertheless, the solu- 

 tion of one well-propounded problem may fully repay the 

 investigator for a dozen failures. The failures are due, no 

 doubt, not to the capriciousness of nature, but to our inability 

 to grasp all of the conditions within and without the animal or 

 plant, and also in no small part to our failure to appreciate what 

 questions we can expect to be answered and what not ; for 

 lacking, as we do, all fundamental knowledge of the phenom- 

 ena that we call life, it is not surprising that we find ourselves 

 so often foiled. 



My present object is to examine some of the problems of 

 regeneration that are at present before us, and then to see if 

 we can get any clearer idea of the main problem of regenera- 

 tion. Under the following headings I have arranged as many 

 problems. 



