42 PATTERNS AND PROBLEMS OF DEVELOPMENT 



showed that length of piece became significant only in shorter pieces than 

 those used here. Short pieces from the esophageal region often show bi- 

 polar reconstitution with development at the original oral end more rapid 

 than at the aboral (Fig. 15, -B). 



Some studies of coelenterate reconstitution have been chiefly concerned 

 with other points and give no information concerning differences in rela- 

 tion to body-level. Moreover, the original gradient differences in some of 

 the more sensitive branching hydroids apparently decrease or even disap- 

 pear under laboratory conditions, particularly if the animals are kept in 

 standing water; hydranths degenerate and stolons develop in place of hy- 

 dranths over the whole stock. 



Although ctenophores are not now regarded as coelenterates, it may be 

 noted here that no evidence of gradient difference has been observed in the 

 reconstitution of Mnemiopsis (Coonfield and Goldin, 1937), although 

 other evidences of an apical-oral gradient have been found (pp. 106, 327). 



BODY-LEVEL AND RECONSTITUTION IN TURBELLARIA 



Among the turbellaria the triclads are the most interesting forms in this 

 connection because of the great capacity for reconstitution of certain 

 species of the group and various limitations of this capacity in other spe- 

 cies, because reconstitution in the group has been studied by many in- 

 vestigators, and because the gradient pattern is clearly evident in it." 



The described American species of Dugesia undergo fission at definite 

 body-levels posterior to the mouth. The act of fission, which consists of an 

 independent motor reaction of the posterior zooid region, is not usually 

 preceded by visible morphogenesis but is a transverse rupture followed by 

 reconstitution. This rupture, however, occurs at a definite body-level, and 

 various lines of physiological evidence indicate that in animals above a 



' Recent taxonomic studies of American planarians by Hyman (1931) and Kenk (193511) 

 have resulted in extensive change in names of genera and species. According to Hyman, 

 the genus Planaria does not occur in the United States, and the genus name Euplanaria re- 

 placed it for American species. However, it has recently been discovered by Hyman (1939) 

 that the genus name Dugesia has priority over Euplanaria. This is an unfortunate discovery, 

 for the names Planaria and Euplanaria are good descriptive designations and the anglicized 

 "planarian" has definite meaning. Now American forms of the genus become Dugesia, while 

 the European Planaria remains unchanged. The species distinguished by Hyman (1931) as 

 Euplanaria maculata and E. novangliae, are, according to Kenk, a single species; and for this 

 species the name Dugesia tigrina has priority, according to Hyman and Kenk. This species is 

 regarded as including the form named P. lata by Sivickis (1923); see also Watanabe, igssb- 

 Planaria simplicissima (Curtis, 1900) becomes Curtisiaforemanii; P. velaia becomes Fonticola 

 velala; and the American dendrocoelid, often called Dendrocoelum lacteum, is Procotyla fluviatilis 

 (Hyman, 1931). The revised nomenclature is used in the following pages. 



