Vertebrates 



679 



of a skeletal element is left behind will this 

 regenerate in its own right. Bischler ('26) 

 showed that Wendelstadt's results were 

 faulty, for when repeated in a large variety 

 of experiments, she secured regeneration of 

 both the free extremity and the skeleton. 



Milojevic ('24) thought he had determined 

 a polarization of the materials in the blaste- 

 ma. By interchanging the cap of fore and 

 hind limb regenerating blastemas he found 

 that during the first ten days the basal part 



nutritional conditions, the interaction of 

 other systems with their effects together with 

 degenerative phenomena and capacities of 

 regrowth. In this chaotic milieu the inves- 

 tigator is always trying to find order and 

 certainly reaches a point when his own 

 observations are coalesced into a subjective 

 unit which appears satisfactory. So far, our 

 ideas of the formation of the blastema can 

 be outlined in much oversimplified form 

 vmder the following headings: 



DAYS OF REGENERATION 



Fig. 235. Changes in the epidermal mitotic index {E.M.I.) and mesenchymal epidermal ratio {M.E.R.) 

 during a 28-day period of regeneration. (From Manner, '53.) 



influenced the distal part no matter what 

 the orientation of the graft. After this indif- 

 ferent period the distal part develops accord- 

 ing to its origin with respect to both its 

 axes and the form of the appendage. This 

 work may be correct but the criteria used 

 are not as reliable as Milojevic thought 

 them to be. In the early regenerates it is 

 very difficult to differentiate the fore from 

 the hind limb and the bones themselves are 

 too similar in form to permit an accurate 

 interpretation. 



The origin of the blastema has proved a 

 fascinating enigma. So far there has not 

 been a single absolutely critical experiment. 

 Every worker in the field of limb regenera- 

 tion has honestly tried to come to grips with 

 this question, which is an exceedingly im- 

 portant one since it involves tissue reactions, 



1. Regeneration by extension, in which 

 after the healing of the wound and the de- 

 differentiation of tissues the regeneration 

 blastema is formed and then becomes or- 

 ganized with the parts remaining, each 

 giving rise to like tissues and the structures 

 which grow to normal limits in replacement 

 of the missing parts. This type of regenera- 

 tion seems to occur in the tail but is not 

 clear-cut in the limb. 



2. Blastema formation, by the invasion of 

 the wovmd area by new elements, generally 

 from the blood stream, which act in the 

 organization of the blastema and join with 

 the cells there in the new growth process: 

 hematogenic origin. 



3. The participation of the epidermis, 

 through dedifferentiation and a direct amal- 

 gamation of the epidermal elements into 



