LECTURE XXI 



REGENERATION (continued) 



Phyletic origin of the regenerative capacity — Tlie liberating stimuli of regeneration 

 — Production of extra heads and tails in Planarians (Voigt) — Regeneration in the 

 Starfish— Atavistic regeneration in Insects and Crustaceans — Progressive regeneration 

 — Regeneration has its roots in the differentiation of organisms — The nuclear substance 

 of unicellular organisms is the first organ for regeneration — The ultimate roots of 

 regeneration. 



In the previous lecture we have considered many different forms 

 of regeneration, and have recognized them as adaptive phenomena ; 

 we have now to inquire how such regeneration- adaptations have 

 arisen, and this is a very difficult question even in general, while in 

 particular cases it is often quite unanswerable at present. In regard 

 to the case last discussed, the regeneration of the lens in the eye of 

 Triton, our hypotheses would require to reach back to the time of the 

 primitive vertebrates with an unpaired eye, for the lens of the paired 

 vertebrate eye, from Mammals down to the lowest Fishes, does not 

 arise in embryonic development from the retinal cells, but alwaj^s 

 from the corneal epithelium, as the elaborate researches of Rabl have 

 recently shown. It is true that the unpaired parietal eye of some 

 reptiles forms its lens from the cells of the retinal layer, but it would 

 be difficult to demonstrate the possibilit}^ of a genetic connexion 

 between it and paired eyes, and in the meantime we must refrain 

 from elaborating a hj^pothesis as to the origin of the marvellous 

 faculty the retinal cells possess of transforming themselves into 

 lens-fibres. 



But it is easier to form some sort of picture of the origin and 

 adaptation of the faculty of regeneration in general. 



We saw that the power of regenerating a part can be localized, 

 and that it does not belong to all the cells of the body, but only to 

 some of them, and we have to ask how and by what steps it has been 

 imparted to these. The faculty depends on the possession of a 

 regeneration-primordium (Anlage), and this again, in our mode of 

 expression, consists of a definite complex of determinants, and as 

 determinants are the products of an evolution, and thus are vital 

 units which have arisen historically, tliey can nowhere suddenly 



