35© PATTERNS AND PROBLEMS OF DEVELOPMENT 



section, and still others have recorded occurrence of amitosis in the new 

 tissue/^ Those who hold that cell migration is the chief or only factor in 

 planarian reconstitution apparently believe that some factor brings about 

 migration not only of cells but of whole organs, such as the pharynx, until 

 normal form and proportions are approached or attained (morphallaxis, 

 Morgan) ; but the fact that these changes in form and proportion are much 

 less rapid and less complete when the animals are not fed than in those 

 well fed as soon as they are able to feed after reconstitution suggests that 

 proliferation and differential growth are concerned. Growth of cells certain- 

 ly takes place in the new tissue of Dugesia. In earlier stages of head and 

 tail regeneration nuclei are much closer together, and there is much less 

 cytoplasm than in fully developed parts. Whatever the role of cell migra- 

 tion in planarian reconstitution, other factors are certainly concerned in 

 determining the differences in size of head and scale of organization in 

 relation to body-level and length of piece. In pieces of equal length rate 

 of head regeneration and length of prepharyngeal region decrease pos- 

 teriorly to the fission zone (Watanabe, 1935a; Rulon, 1936a), or in species 

 without posterior zooid, to the posterior end or as far posteriorly as head 

 regeneration occurs. In pieces below a certain length, which differs with 

 body-level, head regeneration is partly or completely inhibited by a stimu- 

 lation from the posterior cut surface and the activation of cells following 

 section; delay of posterior section decreases or abolishes this inhibition 

 (pp. 181-83). Nutritive condition of the animal is also concerned; in 

 pieces below certain lengths head regeneration is more inhibited in starved 

 than in well-fed animals (Child, 1920a). 



Form and proportions of "normal" well-fed animals apparently repre- 

 sent approach to, or attainment of, an equilibrium of supply and demand 

 of material at the different physiological levels of the pattern. Reconsti- 

 tuting pieces differ at first from this norm according to length of piece 

 and level of origin, and the relative requirements of parts also differ, so 

 that some increase in size more rapidly than others and more rapidly in 



'■t As regards cell migration, see Stevens, 1907; Steinmann, 1926; Bandier, 1936, and cita- 

 tions by these authors. Bandier finds no evidence of cell division of any kind in reconstitution 

 of a land planarian. Flexner (1898) observed mitoses following section in a planarian. In 

 pieces of Dugesia dorotocephala numerous mitoses appear during the first day or two following 

 section, both adjoining level of section and in the region in which the new pharynx develops, 

 and scattered mitoses appear in other parts of the piece (Child, unpublished). Bardeen (1902) 

 and P. Lang (191 2, 1913a, b) believe that amitosis occurs, but Steinmann and Bandier find no 

 amitosis. Murray (1927) observed amitosis in living parenchyma cells of Dugesia in tissue 

 culture and was able to follow the stages of division from beginning to complete cytoplasmic 

 separation. The writer has also observed these divisions in Murray's cultures. 



