4i6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ference. In short pieces lying undisturbed on one side with httle move- 

 ment, a hydranth frequently develops from the upper, and a base from 

 the lower, side of one or both ends (Fig. 141, D). Here the new gradient 

 and axiate pattern are determined across one or both ends of the piece 

 by the differential between upper and lower surfaces. 



In all these experiments with contact-free-surface differential it is pos- 

 sible that accumulation of CO2 at the surface in contact, as well as low 

 oxygen, is concerned in determining the developmental pattern. Increase 

 in hydrogen-ion concentration in consequence of CO2 accumulation is 

 probably not sufficient to have any appreciable effect. In the light of the 

 other experiments it appears highly probable that the oxygen differential 

 is the chief, if not the only, determining factor. 



Exposure to various inhibiting agents — ethyl ether, ethyl alcohol, ethyl 

 urethane, chloretone, and HCl-sea water (CO2?) — preceding or following 

 section or both, results, after return to water, in great increase in fre- 

 quency of determination of new axes by the differential between upper 

 and lower sides and in frequency of multiple polarities from a piece 

 (Child, 19270, h). Little or no movement occurs in such pieces after return 

 to water until development is more or less advanced, and with sufficient 

 exposure to the agent they often become more or less flattened on the 

 glass or lose their characteristic structure to some extent. In Figure 142, 

 A and B, traces of the longitudinal entodermal canals remain and make 

 it evident that the new polarities are at right angles to the old. In ^ a 

 hydranth develops from the upper, a base from the lower, side of each 

 end. In Figure 142 {B, early, and C, later, stage of the same individual) 

 the hydranth develops from the upper, the base from the lower, side, 

 and reconstitution from the ends does not take place. In D the piece lost 

 its structure and became an almost hemispherical mass, and the new 

 axes arose as buds from its upper free surface, the lower side becoming a 

 base and secreting perisarc. In E both hydranths and bases develop from 

 the upper surface, and a large basal area from the lower. The positions 

 of the two bases on the upper side suggest that they are determined by 

 the dominance of some or all of the hydranths. The larger base develops 

 between the larger hydranths, much as in many bipolar forms, and the 

 smaller appears below the small hydranth. 



The new multiple hydranth-stem axes developing after decrease or 

 obliteration of the original gradient by differential inhibition are ad- 

 ventitious as regards position, that is, their localization has no definite 

 or constant relation to a pre-existing pattern. Their general localization 



