RECONSTITUTIONAL PATTERNS IN EXPERIMENT 361 



just beneath the ectoderm may sometimes play a part in locahzing more 

 active regions. In pieces of smaller diameter closure of the cut end pre- 

 ceding development is more rapid and uniform, and a single gradient usu- 

 ally results. 



Multiplication of basal gradients and structures is also frequent (Fig. 

 120, C, F, G, H, I), either at the end of the piece opposite that developing 

 apical parts or elsewhere, even between multiple manubria (Fig. 120, 

 H, I). As previously noted, basal axes are somewhat inhibited axes, and 

 their development is favored by slightly inhibiting or depressing condi- 

 tions. Many of the multipolar forms are mosaics of partial axes (Fig. 1 20, 

 A, B, C, I), that is, each axis, whether manubrium, hydranth, or basal, 

 develops from its apical region as far as its scale of organization, presence 

 of other developing axes, and size of piece permit. These cases give fur- 

 ther evidence that each axis develops basipetally.' 



Physical isolation of the piece from other parts, of course, plays a part 

 in initiating reconstitution in pieces of hydroid bodies; but physiological 

 isolation is also a factor in determining bipolar and many multipolar 

 forms. In long pieces without the original hydranth, dominance of the 

 distal region is insufhcient to prevent development of a dominant region 

 at the proximal end; in short pieces there is little gradient, the two ends 

 are physiologically isolated from each other, begin development at about 

 the same time, and neither can inhibit the other. Appearance of new 

 dominant regions adjoining the level of section and induction of new axes 

 in relation to them, with development of unipolar, bipolar, or multipolar 

 forms, is very generally characteristic of coelenterates.'' 



In some hydroids outgrowth of tissue from the level of section precedes 

 hydranth development, but the outgrowth represents establishment of a 

 new gradient, as is readily demonstrated; and the high region at the tip 

 gradually attains hydranth development if not inhibited, or grows as a 

 stolon if it does not attain hydranth-level in consequence of dominance 

 of other regions or inhibition by external conditions. ■^ 



' For other multipolar forms of Coryniorpha see Child, 1926&; 1927a, b. 



2 Weimer, 1928, 1932, and many earlier papers by various authors on Hydra; numerous 

 papers on many hydroid species; Child, 1933^, and Watanabe, 1937, the scyphozoan Hali- 

 clystus; Gilchrist, 1937c, the scyphistoma of Aurelia; Child, 1903a, b, 1904a, 1908, 1909a, b, 

 the actinians Cerianthus and Harenactis. See also Figs. 14, 15 {B}, 24, 25, in chap. ii. 



3 The algae Acetabularia mediterranea and A . wettsteinii, forms with a polar axis possessing 

 a considerable degree of organization and with whorls of branches, are single uninucleate cells. 

 They are mentioned here because of their remarkable similarity to hydroids in reconstitution. 

 Shorter pieces may even develop as bipolar partial forms (Hammerling, 1934, 1936). Exist- 



