374 PATTERNS AND PROBLEMS OF DEVELOPMENT 



dently the pattern of the new disk of Figure 126, Z), is not determined by 

 the original pattern but has arisen, Hke other bud patterns, from a region 

 of local activation involving both distal and proximal levels of the piece. 

 The tentacle-bearing outgrowths of Figure 126, C, apparently represent 

 less intense activation and less complete dominance; in them mouth and 

 esophagus do not appear. The signiticance of these cases is twofold : they 

 provide further evidence to show how completely independent of the old 

 pattern the new pattern may be, and must be for development of a new 

 polarity; and they agree with other data in indicating that differences in 

 intensity and probably in extent of activation may give different morpho- 

 logical results. They do not differ essentially from cases of reconstitution 

 of new dominant regions but are merely rather striking examples of es- 

 tablishment of new dominance by localized activation, with more or less 

 induction resulting, in Corymorpha, even across a pre-existing pattern 

 (Fig. 125, D). 



Bud formation in Hydra, resulting from local cautery (Tepliakova, 

 1937), and determination of head development in a planarian by local 

 cautery, laceration, and application of certain chemical substances, as 

 reported by Goldsmith (1932, 1933, 1934, 1937, 1940), are similar cases 

 in which the local injury brings about sufhcient activation to initiate de- 

 velopment of a new dominant region and polar pattern. 



QUESTIONS OF PATTERN AND ITS ALTERATIONS IN EARLY EMBRYONIC 

 RECONSTITUTIONS OF ISOLATED PARTS 



Data of early embryonic reconstitutions are discussed in the following 

 chapters. Here it is desired to call attention to certain questions concern- 

 ing effects of section and isolation on egg and embryonic pattern, sug- 

 gested by the reconstitutions of isolated pieces of adult animals. First, 

 it may be noted that in isolated parts of eggs and isolated blastomeres or 

 blastomere groups there is no evidence of regeneration in the strict sense, 

 no outgrowth in relation to a surface of section. Whatever reconstitution 

 occurs is reorganization within the part. What happens to the pattern 

 present in the part when it reconstitutes a whole? We know even less 

 about what happens in these cases than we do concerning reconstitution 

 in an isolated piece of an adult hydroid or planarian, but at present there 

 seems to be no good reason for regarding embryonic and adult reconsti- 

 tution as essentially different. Gradient patterns have been demonstrated 

 in many eggs and embryos. These are unquestionably altered in one way 

 or another in reconstitution, as they are in adult reconstitution. Direc- 



