Section XIII 

 REGENERATION 



CHAPTER 1 



Invertebrates 



L. G. BARTH 



The process of regeneration presents many 

 of the problems of embryonic development. 

 In some instances the end product of the two 

 processes is exactly the same although the 

 form changes are vastly different. For ex- 

 ample, a young ascidian may arise by de- 

 velopment of the fertilized egg, through 

 metamorphosis or by a process of regenera- 

 tion from a part of the adult ascidian. Sim- 

 ilarly a sponge may differentiate from a mass 

 of cells that resulted from an aggregation of 

 the dissociated cells of an adult sponge, or 

 from the cleavage and development of a sin- 

 gle egg cell. Certain differences between the 

 two processes are worth emphasis. On the 

 whole, regeneration entails relatively less cell 

 division than embryonic development. Indeed 

 in some species little or no cell division has 

 been reported. Another difference is in the 

 relative amount of growth. The egg increases 

 in size and yolk is converted into protoplasm 

 during embryonic development. In the case 

 of regeneration the protoplasm is already 

 synthesized. The regeneration of a hydranth 

 in Tubularia involves little or no growth, 

 since the regenerate differentiates directly 

 from the cells of the stem. Thus regeneration 

 offers a means of dissociating the processes 

 of cell division and growth from that of dif- 

 ferentiation. 



Regeneration also invites a study of the 

 reversibility of differentiation. During em- 

 bryonic development a progressive change 

 from vmdifferentiated to differentiated cells 

 is observed. Accompanying differentiation a 

 restriction in the number of potencies of any 

 particular cell is found. In the adult most if 

 not all cells are in the differentiated state. 

 Therefore when regeneration occurs, some 



cells must undergo dedifferentiation or else 

 the new parts must come from reserve cells 

 which retain the potencies of embryonic 

 cells. In the highly organized but very labile 

 adult ascidian, cutting experiments have 

 demonstrated that no single organ is neces- 

 sary for the regeneration of a complete in- 

 dividual (Driesch, '06). And thus we may 

 ask from what cells do the nerve ganglia 

 arise when the abdominal region regen- 

 erates? 



Whatever the differences, the basic prob- 

 lems of embryology and regeneration remain 

 the same and in this treatment we will use 

 the same organization of subject matter as 

 in the analysis of embryonic development. 

 The problems under discussion are: 



1. Polarity of regenerates 



2. The stimulus for regeneration 



3. Potencies of cells 



4. Correlative differentiation and induc- 

 tion 



5. External environment 



6. Metabolism of regenerates. 



THE POLARITY PROBLEM IN 

 REGENERATION 



Just as a developing egg possesses an in- 

 itial polarity, so too a regenerating fragment 

 of an adult animal may exhibit a polarity. 

 The initial polarity of an egg is an animal- 

 vegetal polarity while that of the regenerate 

 may be an anteropostei'ior polarity or an 

 apicobasal polarity or a distoproximal polar- 

 ity. Fragments of Dugesia regularly form 

 heads at the anterior cut surface of each 

 fragment and tails from the posterior cut 

 surface. In this form the anteroposterior 



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