Vertebrates 



677 



structures, the capacity for regeneration 

 again appears. The nervous system which 

 regenerates in the early shield stages fails 

 to regenerate in later formative stages. 



AMPHIBIA 



Regenerative processes have been studied 

 in amphibians over a very long period. Spall- 

 anzani (1768) recorded many definite ex- 

 periments in the Prodromo, and the lit- 

 erature of the nineteenth century is full of 

 allusions to naturally occurring forms which 

 came to notice because of the abnormal- 

 ity which appeared either during or after 

 the regenerative process. These forms which 

 appear with a fair frequency in nature often 

 are exceedingly difficult to interpret and in 

 some cases impossible to duplicate in the 

 laboratory. Hellmich ('30), for example, 

 figured a salamander with a single but 

 huge limb near the dorsal midline at the 

 general limb level. In this particular location 

 an embryonic transplant would have to be 

 made early in the embryo and probably 

 would resorb in a great proportion of such 

 cases. 



It is impossible to interpret the abnormali- 

 ties found in nature except hypothetically. 

 Bateson (1894) gave a rough classification of 

 abnormalities, as did Przibram ('26) in his 

 later catalogue. Tornier ('06) showed that 

 all the forms produced could be called ex- 

 amples of the multiplicities occurring after 

 regeneration by a single but drastic experi- 

 ment. With a single cut through a tadpole 

 extending through the forming hind limbs, 

 Tornier secured animals with 6 to 8 mul- 

 tiple limbs. 



The situation is considerably clouded in 

 amphibians, owing to the life histories of 

 various forms. Not all amphibians have an 

 equal power for regeneration at all stages 

 in their history; the anurans normally are 

 restricted in regenerative capacity to larval 

 stages. The length of the embryonic and 

 larval stages varies tremendously; in some 

 of the tropical forms metamorphosis is al- 

 most part of the embryonic period; in others, 

 as in the bullfrog, two full years may lapse 

 before metamorphosis is complete. These 

 variabilities when correlated with the capac- 

 ity to regenerate offer little in the way of a 

 constant around which we can collect the 

 varied miscellany of events. Kammerer ('05) 

 discussed an absolute age and claimed that 

 this, rather than the stage, determines re- 

 generative capacity. We find this same 

 general idea in general growth studies when 



the necessity for evolving a base level has 

 given rise to all sorts of postulated constants. 



Embryonic regeneration constitutes dan- 

 gerous ground, for it is difficult to speak of 

 regeneration in a structure which is going 

 through its formative stages. Nevertheless, 

 the term has been used frequently and the 

 issue was clearly raised when the Roux- 

 Driesch controversy on capacity of the half 

 egg was occupying the center of biological 

 attention. The circumstances of this situa- 

 tion are well known. Saiffice it to state that 

 Roux's idea of the egg as a mosaic was found- 

 ed upon the half embryo developing from 

 a single blastomere. Schultze (1894) turned 

 the egg after killing one blastomere and 

 secured a small but complete embryo. When 

 Roux restudied the question he was faced 

 with a negation of the mosaic idea, but in- 

 stead of retracting he called the process by 

 which more than a half embryo is formed 

 post-regeneration. Now it is well known 

 that the amphibian egg is capable of a fairly 

 complete internal reorganization; it has been 

 proved most conclusively that its capacity 

 to continue development and ultimately 

 produce a complete and normal embryo 

 can hardly be blocked by any experimental 

 contingency. In the face of these facts, it 

 might be better to assume that the reconsti- 

 tution is a form of regulation which is pos- 

 sessed to an imusual degree by the em- 

 bryo. 



This is shown clearly in some of the 

 limb experiments (Harrison, '18). In some 

 cases of incomplete duplicity the manus de- 

 veloping after rotation of the embryonic 

 limb disc may be represented only by a 

 small spear of tissue. If this be cut after it 

 has developed to the late embryonic or early 

 larval stages, the regenerating appendage 

 frequently is much more complete than that 

 originally secured and will show sufficient 

 morphological criteria for an accurate diag- 

 nosis of the asymmetry. Swett ('24) also 

 used this method in studying the process of 

 reduplication. In both these cases the in- 

 vestigator is using regenerative processes to 

 explain embryonic reactions during the proc- 

 ess of reorganization of the limb. These 

 processes differ only in degree from the 

 original formation of the end organ. The 

 embryonic structure is drawn from a wide 

 embryonic field, while the regenerated struc- 

 ture is derived from a much smaller and 

 more localized field. The problem of the field 

 of regenerative organization has been ably 

 discussed by Weiss ('26a) and Guyenot ('29). 

 The essential point which the field concept 



