Vertebrates 



693 



another link from a different angle to the 

 chain of circumstances which tie so many 

 ideas with the embryonic nature of the blas- 

 tema. The eye is being used as a test for 

 the reaction of the new forming tissvie. 

 Schotte adopts the thesis that the fate of a 

 regenerate depends upon the inductive action 

 of the organ used which is based on Weiss' 

 ('26a) experiment in which the tail blastema 

 was substituted for a leg (Guyenot and 

 Schotte, '27). Schotte and Hummel ('39) 

 used tissues from larvae throughout one series 

 of experiments, working with the limb blas- 

 tema. They made four types of transplant: 

 (1) ectodermal cap, (2) cap plus mesen- 

 chyme, (3) mesenchyme separated from cap, 

 and (4) regenerating mesenchyme attached 

 to old tissues of the limb. They used regen- 

 erates not older than 5 to 6 days in order 

 to have indifferent tissues. The blastemal 

 tissue was placed in the left eye of the host 

 after removal of the lens, while the right 

 eye served as the control after its lens had 

 been removed. 



Regeneration may occur from fragments 

 of lens epithelium left behind and here it 

 is exceedingly rapid. In their control series 

 regeneration occurred in one case in two 

 days, in another in three, both of which 

 they attributed to lens epithelium left be- 

 hind. Regeneration changes as modifications 

 of the transplant occurred in 69 per cent of 

 the xenoplastic cases and in 33 per cent of 

 the homoplastic. Lens formation in the total 

 series was 23 per cent, with 46 per cent 

 occurring in the xenoplastic series. These 

 figures Schotte and Hummel were at a loss 

 to explain and they relied upon a statistical 

 study which was to some extent vitiated by 

 their footnote (4), in which they stated 

 that the errors decreased as experimentation 

 proceeded, which indicates that both criteria 

 and observations of completeness of opera- 

 tive removal were inadequate at the time 

 of the first operation. 



The confusing numerical differences be- 

 tween the homoplastic and xenoplastic com- 

 binations seem quite significant and can be 

 interpreted in the light of Wachs' ('14) 

 experiments. The xenoplastic graft is much 

 more frequently resorbed and in the case of 

 some anuran species we know definitely that 

 resorption takes place with extreme rapidity. 

 Let us postulate for the sake of argument 

 that the degenerating blastema liberates sub- 

 stances into the eye which may stimulate 

 lens development — the chemical factor. 

 These substances are not liberated while 

 the blastema is intact. Further, xenoplastic 



tissue during degeneration sometimes forms 

 epithelial pearls which might easily simulate 

 the histological picture of early lens forma- 

 tion. Schotte and Hummel may be right, 

 but the persistence of qualifying footnotes 

 throughout their discussion indicates that 

 all possible factors have not adequately been 

 weighed. 



Their best case does not depend upon the 

 statistical interpretation but upon structural 

 organization — blood and notochordal ele- 

 ments were found in one case (tail blas- 

 tema) surrounded by capsular fibers. Either 

 the blastema was being transformed, or else 

 the lens capsule was proliferating so fast 

 that it circled the resorbing blastema before 

 complete internal disintegration occurred. 



The conclusion of Schotte and Hummel 

 that the regenerating tissues of urodeles and 

 anurans are totipotent, in the sense that they 

 are capable of differentiations which are 

 normally observed only in embryonic tissues, 

 will certainly have to await acceptance sub- 

 ject to a more critical experimental analysis. 



The original work on the eye by Colucci 

 (1891) was to determine how much regen- 

 erative capacity the eye would show. The 

 whole eye will regenerate provided some 

 of it has been left behind. Regeneration may 

 not occur if only the choroid is left behind. 

 Fuchs ('24) found that tadpoles could re- 

 generate the whole eye from the optic stalk. 



Reyer ('48) has brought together all the 

 work vipon the embryos and larvae of Tri- 

 turus, studying carefully the regeneration 

 of the lens from the dorsal iris in five age 

 groups. The process in every case is similar 

 although varying in minor details, and swell- 

 ing and depigmentation occur in the older 

 animals. The reader is referred to Reyer 

 ('54) for a complete review and discussion 

 of the lens problem and its general relation- 

 ship to definitive potentiality. 



CONCLUSION 



The present discussion of regeneration 

 aims merely to point out a few of the high 

 points of what has been done as well as to 

 point to a few of the things which need to 

 be done. Since the work has been centered 

 about the limb, eye and tail these have re- 

 ceived the greatest amount of attention. The 

 regenerative capacities of internal organs 

 have been scantily treated, although the 

 recent literature on liver regeneration shows 

 that many of the fundamental potentialities 

 of organ-forming tissues still remain for 

 discovery. The fragmentary work which has 



