302 PATTERNS AND PROBLEMS OF DEVELOPMENT 



cells. '"5 According to Probst (1932), the annelid Owenia regenerates with- 

 out neoblasts. In short, neoblasts are apparently not necessary for an- 

 nelid regeneration, and most of the evidence indicates that when they 

 are present they are activated and dedifferentiated cells from the coelomic 



wall. 



A similar difference of opinion exists concerning reconstitutional de- 

 velopment of ascidians, either from winter buds or cell masses remaining 

 after degeneration of zooids or directly from pieces of zooids or stolons. 

 However, even though many cells degenerate and die when reduction of 

 zooids or isolated pieces occurs, the chief reason for regarding the cells 

 that remain and give rise to new individuals as undifferentiated seems to 

 be the fact that they do remain and develop. In these cells form, appear- 

 ance, activity, and relation to other cells undergo change in connection 

 with the new development, apparently a regression to a less differentiated 

 condition and a new progressive differentiation.'^ 



Butler and, later, Thornton have presented very definite evidence of 

 dedifferentiation in regeneration of the amphibian limb. Thornton main- 

 tains that the regenerating tissue, the blastema, originated by the dediffer- 

 entiation of cells of the limb stump.'' That dedifferentiation is involved 

 in lens regeneration from the dorsal margin of the iris in adult amphibia 

 appears beyond question. The iris cells lose their pigment, resume divi- 

 sion and growth, and differentiate into a lens (p. 396). Retinal cells may 

 also give rise to cells resembling lens cells. Changes suggesting dedifferen- 

 tiation have been observed in various explanted tissues by many investi- 

 gators, but some maintain that it never occurs. In cultivation of embry- 

 onic rat tissue on the chorioallantois of the chick there seems to be con- 

 siderable dedifferentiation (Nicholas and Rudnick, 1933). Many other 

 cases of apparent dedifferentiation might be cited. 



General discussions of the subject show the same difference of opinion 

 as special papers. Schultz (1908) and Nusbaum (191 2) maintained that 



'5 R. G. Stone, 1932, "The effects of X-rays on regeneration in Tiihifcx lubife.w" Jour. 

 Morphol., 53; 1933, "The effects of X-rays on anterior regeneration in Tubifex tubifex," ibid., 

 54. See also L. H. Hyman, 1940, ".\spects of regeneration in annelids," Amer. .\at., 74. 



'^Schultz, 1907, 1908; Huxley, 1921a, 1926; Spek, 1927. 



•7 E. G. Butler, 1933, "The effects of X-radiation on the regeneration of the fore limb of 

 Amblystoma larvae," Jour. Exp. Zool., 65; 1935, "Studies on limb regeneration in X-rayed 

 Amblystoma larvae," Anat. Rec, 62. See also C. S. Thornton, 1938a, "The histogenesis of 

 muscle in the regenerating fore limb of larval Amblystoma punctatitm," Jour. Morphol., 62; 

 19386, "The histogenesis of the regenerating fore limb of larval Amblystoma after exarticula- 

 tion of the humerus," ibid. 



