678 



Regeneration 



has added is the postulation of a directive 

 force orienting particles in a field. 



In the urodeles the speed of regeneration 

 is sometimes much faster in the larvae than 

 in adults. In older larvae and in neotenous 

 forms there is sometimes a noticeable failure 

 in the capacity of the organism to regenerate 

 after minor operations, e.g., a slight V- 

 shaped piece of tissue may be removed from 

 the dorsal fin extending to the upper surface 



Fig. 234. Regeneration of the salamander ex- 

 tremity {Triturus taeniatus). A, a-h, Varying stages 

 in the regeneration of the forelimb; B, differentia- 

 tion of skeletal parts in the right hind leg; C, 

 further advanced differentiation in the left forelimb. 

 (From Fraisse, 1885.) 



of the myotome. The edges heal but the 

 tissue, which is of a rather nondescript and 

 unorganized type, may not be replaced. In 

 fishes when the fibrillae alone are removed 

 from the dorsal fin, there is no reconstitu- 

 tion. 



An entirely different situation occurs in 

 the anvirans where normally the limbs re- 

 generate only during larval stages. The 

 exceptions will be considered below. Bar- 

 furth (1895) worked on the limb problem 

 in Rana fusca. When the limb was ampu- 

 tated while still in the condition of a small 

 bud (no real limb), regeneration occurred 

 and was complete in detail. When an ampu- 

 tation was performed upon a paddle-shaped 

 appendage when the digits were just begin- 

 ning to show, imperfectly formed appendages 

 were secured, the deficiency being marked 

 in the foot. After the stage where digitation 

 is complete and the knee bud is marked, 

 an amputation provokes little or at most 

 imperfect regeneration. These results were 



repeated by Byrnes ('04) upon the forelimb 

 with nearly identical results. 



Guyenot ('27) stvidied the decrease in 

 capacity to regenerate. The capacity to re- 

 generate lasts longer in the tail than in 

 the limb. If a limb is heteroplastically 

 grafted from a toad to a salamander it fails 

 to regenerate in its new location. If, however, 

 a tail is similarly grafted, it will regenerate. 

 The failure of the limb is due to something 

 intrinsic in the limb tissue and is not due 

 to the internal medium. The transplanted 

 urodele tail will regenerate even though 

 retrogressing during metamorphosis. 



Harrison's ('21) thorough analysis of the 

 problem of asymmetry with the embryonic 

 forelimb has been adequately supplemented 

 by Weiss' ('26b) results with larval and 

 adult limbs. Weiss cut the limb so that he 

 got two definite surfaces, from each of which 

 a complete limb developed. The regenerative 

 blastema is a harmonious equipotential sys- 

 tem in the Driesch sense. It is possible, in 

 the light of Holtfreter's ('47) experiments, 

 that the blastema may serve as an organiz- 

 ing center and that the results obtained in 

 some transplantation experiments may be 

 secured from host tissue by induction. This 

 is particularly true in early larval stages. 

 The active regeneration capacity of newts 

 and salamanders is almost unlimited, par- 

 ticularly with respect to the limb. This will 

 regenerate at any time and at any level 

 (Fritsch, '11). The limb and its girdle can 

 be removed and regeneration of the limb 

 will take place. In other words, no matter 

 how complete the removal, the limb is re- 

 constituted and is a complete functional 

 limb. 



Braus ('09) showed the independence of 

 limb formation in the embryo from skeletal 

 parts. Wendelstadt ('04) thought, on the 

 basis of his experiments, that the presence 

 of the skeleton was essential for regeneration. 

 A part of the bone when removed was re- 

 generated, but when the whole structure 

 was carefully disarticulated no regeneration 

 occurred. Strasser (1879), Goette (1879) and 

 later Morrill ('18) had shown that when the 

 limb was sectioned, perichondrium and peri- 

 ostium became very active and made up a 

 substantial part of the blastema. Weiss ('25a) 

 repeated and extended Wendelstadt's experi- 

 ments and showed that removal of the skele- 

 ton did not prevent the appearance of skele- 

 ton in the newly regenerating limb, but in 

 confirmation of Wendelstadt there was no 

 regeneration of a completely extirpated skele- 

 tal element in its old bed. Only if a remnant 



