38o PATTERNS AND PROBLEMS OF DEVELOPMENT 



dranth may develop; and at proximal host-levels 10-25 per cent of grafts 

 from proximal donor-levels induce development of basal ends but can 

 also induce hydranth-stem axes. Since the grafts are not oriented when 

 implanted, but the new axis develops approximately at right angles to the 

 host axis, it appears highly probable that the original polarity of the graft 

 plays no essential part in determination. Apparently the graft and per- 

 haps host tissue about it act primarily as a group of active cells and, as 

 such, determine a new dominance and gradient extending into the host 

 tissues. Whether a complete hydranth with stem, proximal tentacles, and 

 stem, stem only, or basal end develops apparently depends on degree of 

 activation of the cells of the graft and surrounding tissue, as determined 

 by level of origin of graft, level of implantation, and inhibiting effect of 

 host dominance. Occasional cases of duplication of hydranth occur in 

 which the graft forms the apical part of one hydranth, the other being 

 formed entirely from host tissue. One case among several hundred has 

 been observed in which the graft lay between two new hydranths and 

 took no part in the formation of either. These duphcations are almost 

 completely limited to distal stem-levels, where new axes are determined 

 by simple partial section without implant more frequently than elsewhere 

 and are apparently cases in which sufficient activation of host tissues oc- 

 curred to determine one, or even two, new axes. In the latter single 

 case, the graft, though not inducing, apparently served as an obstacle to 

 closure of the wound and so brought about more activation and growth 

 of host tissues. 



Fully developed hydranths and apical parts of hydranths, when iso- 

 lated, do not reconstitute proximal parts; but even the distal half of a 

 fully developed manubrium, implanted laterally, can induce the proximal 

 parts of a hydranth and a stem from host tissues (Fig. 127, £). Implanted 

 transverse pieces from the middle of fully developed manubria develop 

 hypostome from the distal end, or sometimes from both ends; and both 

 unipolar and bipolar hypostomes may induce a single proximal hydranth 

 region and stem (see also Beadle and Booth, 1938). 



In many animals with a new axis developing laterally from graft or in- 

 cision, the reconstituting host hydranth is more or less inhibited, and the 

 host stem distal to the new axis becomes slender, as in Figure 127, E. 

 After it has developed, the new axis can completely inhibit development 

 of a host hydranth at more distal levels within a certain distance and can 

 even induce development of a base from both distal and proximal ends 

 of a piece of host stem isolated with it (Fig. 127, /^) ; or, if it is less effec- 



