340 PATTERNS AND PROBLEMS OF DEVELOPMENT 



evident that the old nervous system is not an essential factor in establish- 

 ment of a new dominant region in many species of hydroids, planarians, 

 nemerteans, and annelids. 



In certain triclads (e.g., Dendrocoelidae) and in various nemerteans 

 and annelids head regeneration occurs only anterior to a certain body- 

 level, characteristic for the species. So far as data are available, those 

 rhabdocoels that do not undergo fission, poly dads, and some nemerteans 

 and annehds do not reconstitute cephalic ganglia unless some portion of 

 the original gangHa remains; that is, determination of an entirely new 

 dominant region apparently does not occur.^ We do not know whether, 

 or to what extent, nonnervous cells are concerned in regeneration of 

 cephalic gangHa in these cases, but it is evident that only cells of the more 

 anterior body-levels, whether nerve cells or others, are able to give rise 

 to ganglia under known experimental conditions. This inability to re- 

 generate ganglia and head does not mean that no cells capable of regenera- 

 tion are present, for complete posterior regeneration occurs at levels 

 incapable of head formation. It may be suggested that rate or intensity 

 of activation following section at these levels is not sufficient to render the 

 cells independent of other parts, and so to make possible development of 

 new ganglia and head, but is sufficient to permit development of posterior 

 ends under the dominance of the old nervous system. 



The hypothesis that absence of head regeneration posterior to certain 

 body-levels is due to decrease in number of undifferentiated "formative 

 cells" from anterior to posterior levels fails to account for the rapid and 

 extensive posterior regeneration at those same levels.^ 



Presence of the radial nerve at the cut surface, even if it is separated 

 by section farther proximally from the rest of the nervous system, is es- 

 sential for regeneration of the ophiurid arm, according to MorguHs (191 2). 

 Schapiro (19 14) finds that regeneration of the distal part of the starfish 

 arm is possible even when a rectangular piece, including both dorsal and 

 ventral body wall, is removed at a more proximal level of the arm, leaving 

 only lateral connection with other parts. In this experiment a piece of 



'' For examples see the following: F. R. Lillic, 1901, a dendrocoelid triclad; Child, 1904ft, 

 19106; L. V. Morgan, 1905, the polyclad Leptoplana; Coe, 1932, a nemertean; T. H. Morgan, 

 1897, 19026; Hyman, 1916a; Sayles, 1936, annelids. Various other papers give similar data, 

 and unpublished data on a number of rhabdocoel species (Child) show absence of head re- 

 generation in absence of ganglia. 



' For this hypothesis see Curtis and Schulze, 1924; Curtis and Hickman, 1926; Curtis, 1928. 

 Steinmann, 1926, describes extensive dediflerentiation in planarian reconstitution, and 

 Bandier, 1936, is unable to distinguish "formative cells" from other parenchyma cells in the 

 reconstitution of land planarians. 



