Vertebrates 



685 



ing, either by treatment with strong sodium 

 chloride sohitions, by stripping the epithe- 

 lium from the ampvitation surface, or by 

 causing a temporary vitamin deficiency. He 

 suggests that the dermal layer of the skin 

 prevents the regenerative processes which 

 lead to the formation of a rapidly growing 

 blastema. 



Gidge and Rose ('44) suggest again that 

 timing of healing is a factor in the normal 

 failure of regeneration in frogs. The dermis 

 redifferentiates before the blastema cells can 

 grow, and the dedifferentiated elements vicar- 

 iously become scar tissue which acts further 

 in inhibiting the regenerative growth. A 

 stripped wound is covered by epidermis but 

 not by dermis. The regenerate consists of 

 cartilage only. When, however, the woimd 

 is covered with larval skin, a distal blastema 

 develops and a regenerate composed of all 

 limb tissues is formed. 



This work forms a promising lead for 

 future work in the puzzling field of anuran 

 limitations for regeneration. In the urodele 

 embryo the ectoderm sometimes inhibits 

 rather than assists in differentiation, but the 

 facts here seem quite different from those 

 needed to explain the regenerative inhibition 

 in anurans. 



The one dominant factor in limb regenera- 

 tion is the import of the nervous system. 

 This has been shown beyond peradventure 

 by Singer ('42-'49) and by Singer and 

 Egloff ('49). There is a basic quantitative 

 minimum of innervation required for limb 

 regeneration. This can come from the motor, 

 sensory or sympathetic nervous system. If 

 this basic minimum is not present limb re- 

 generation will not occur. The limitation of 

 space prevents the discussion which this im- 

 portant work deserves and the reader is re- 

 ferred to the original sources for this 

 material. 



Weissfeiler ('24) and Vallette ('26) 

 worked on the regeneration of the amphib- 

 ian head region. Vallette ('26) found that 

 if the cut is anterior to the nasal capsule, a 

 complete regeneration occurs. If it is pos- 

 terior to it, the wound heals but regenera- 

 tion does not occur. This is true also in the 

 embryo, where May ('27) found that in 

 the early stages the nasal capsule can re- 

 generate but that it is one of the first of the 

 organs to become fixed. 



Weissfeiler found that the forebrain will 

 regenerate if the nasal capsule is not dis- 

 turbed, but not if the capsule is removed. 

 This is an interesting corollary to Burr's 

 ('16) experiment in which, when the em- 



bryonic olfactory placodes are removed, the 

 forebrain fails to develop to normal size. 



Vallette transplanted the jaw to various 

 regions, then cut the jaw and followed the 

 regenerative process. It reconstituted a typ- 

 ical jaw no matter where located. 



Hooker ('25) studied the regeneration of 

 the spinal cord after section in various 

 stages. While the time is progressively longer 

 as the anurans approach metamorphosis. 



B 



Fig. 241. Triton head regeneration. A, The levels 

 of the cuts 1 and 2; B, normal; C, regeneration after 

 cut at level i; D, regeneration after cut at second 

 level. (From A. v. Sziits, '14.) 



the process is complete both morphologically 

 and physiologically. These studies and those 

 of Guyenot and Schotte on the influence 

 of the nervous system will be considered in 

 greater detail under that head. 



ENDOCRINE EFFECTS 



The majority of the workers dealing with 

 the interaction of the endocrine secretions 

 upon the process of regeneration have em- 

 ployed fishes; Grobstein ('42-'48) has con- 

 tributed papers dealing with regeneration of 

 the gonopod under varying conditions. The 

 gonopodium of the male Platypoecilus has 

 a declining regenerative capacity which is 

 due to the inhibition of regeneration of 

 circulating androgen; testosterone propionate 

 changes the female gonopod to form a re- 



