378 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of isolation with reversal of ventrodorsality in the dorsal part, perhaps 

 in consequence of physiological isolation (p. 306). But only by further 

 experiment can we hope to discover what really happens in these cases. 

 It seems evident, however, that ventrodorsality is not fixedly determined 

 in these early stages but can be altered, as was maintained by Boveri 

 (1902). 



INDUCTION OF RECONSTITUTION BY IMPLANTS IN MATURE ANIMALS 



It was shown by Browne (1909) and confirmed by later workers that 

 even small pieces of Hydra from the region about the base of the tentacles, 

 implanted laterally, can persist and determine new apical regions and 

 polarities, largely from host tissue ; that is, the implant is dominant and 

 acts as an inductor. Similar small pieces from other regions of the body 

 are incorporated in the body wall or resorbed; but large pieces may per- 

 sist, reconstitute an apical region, and induce. Pieces of stalk, especially 

 those bearing the foot, tend to persist and grow without modification. 

 It is perhaps of interest in this connection to recall that dye reduction in 

 low oxygen occurs early in the- foot of the fully developed animal and that, 

 if the stalk contracts actively, it reduces more rapidly than the body 

 (p. lOl). 



Autoplastic or homoplastic implantation of very small fragments, even 

 1/4 sectors of very short transverse stem pieces of the hydroid Corymor- 

 pha in small lateral incisions in the stem, may determine new hydranth- 

 stem axes, the graft itself forming only more or less of the apical region of 

 the hydranth and inducing other parts from the host tissue (Fig. 127, 

 A-C). Development of the new axis sometimes occurs when the apical 

 hydranth is present; but its frequency ranges from 35 to 75 per cent 

 higher, according to level of origin and of implantation, when the apical 

 hydranth is removed. Grafts from a distal donor-level, that is, from a 

 higher gradient-level, are more effective in inducing a new axis than those 

 from proximal levels, whether level of implantation is distal, middle, or 

 proximal (Child, 1929a, 1932a). In series of comparable experiments with 

 hosts and grafts and donors of approximately the same length and those 

 of each lot collected at the same time, pieces from a distal donor-level 

 gave a total frequency of new axes at distal, middle, and proximal host- 

 levels of 16.6 per cent with the apical hydranth present and in good con- 

 dition; pieces from a proximal donor-level, only 3 per cent with apical 

 hydranth present. With removal of the apical hydranth at the time of 

 implantation percentages were, respectively, 73.3 and 47.1. In short, the 



