GRADIENTS, FIELDS, AND DETERMINATION 301 



do not represent differentiation, or dedifferentiation and redifferentiation 

 occur. Huxley and De Beer (1923) have described what they regard as 

 dedifferentiation in the hydroid Obelia under inhibiting conditions. The 

 cells of stem pieces of Corymorpha seem to become less differentiated under 

 conditions that obliterate polarity and lead to development of new axes 

 from lateral regions. Cell changes in sea-urchin larvae, regarded as de- 

 differentiation but resulting in degeneration and death rather than re- 

 differentiation, have also been described by Huxley (1922). The paren- 

 chyma cells of planarians and nemerteans are very different in appearance 

 from embryonic cells of the species, but in reaction to section they undergo 

 change in structure and behavior and develop into various organs. These 

 cells, or some of them, are assumed by some to be undifferentiated "forma- 

 tive cells," apparently because of the part they play in regeneration, while 

 others maintain that dedifferentiation takes place.''' Regeneration of the 

 annelid central nervous system from ectoderm involves active prolifera- 

 tion and loss of epithelial character. In the intact animal these cells se- 

 crete cuticle and have a certain structure and form different from that of 

 nerve cells. According to Nuzum and Rand (1924), cells of the pharyngeal 

 epithelium can also give rise to nervous tissue. Pharyngeal and nerve 

 cells certainly appear to be differentiated in different directions. Cells usu- 

 ally regarded as of mesodermal origin, the neoblasts, play a considerable 

 part in regeneration of other annelid organs and are regarded by some 

 authors as embryonic or undifferentiated cells.'^ Faulkner (1932) main- 

 tains that these cells do not come from the coelomic wall, as described 

 by others, but from outside it, that is, from the blastocoel, and that they 

 give rise to germ cells as well as regeneration cells. Sayles (1927) and 

 Weitzmann (1927) regard them as mesodermal cells that have resumed 

 active proliferation. They certainly undergo change in appearance and 

 behavior after activation following section and develop into other organs 

 than the coelomic wall. R. G. Stone also regards them as mesodermal in 

 origin and finds that X-rays inhibit their activation but also inhibit acti- 

 vation of other cells in Tubifex and consequently inhibit regeneration; 

 but this is far from proving that neoblasts are specifically "formative 



'3 Formative cells: Curtis and Schulze, 1924; Curtis and Hickman, 1926; Curtis, 1928; 

 Collings, 1932; Coe, 1934a, b. Dedifferentiation: Nusbaum, 1912; Kenk, 1922; Steinmann, 

 1926; Prielgauskiene, 1933. See also Goetsch, 1929, 1931; Bandier, 1936, and the general 

 papers; Schultz, 1908; Stolte, 1936. Also Curtis, 1940, "The histologic basis of regeneration 

 and reassociation in lower invertebrates," Atner. Nat., 74; Hyman, 1940, "Aspects of regenera- 

 tion in annelids," Amer. Nat., 74. 



'•t See, e.g., Hammerling, 1924a, b; Probst, 1931, 1932. 



