320 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ing corals; lateral zooids bud from about its base and are dorsiventral in 

 the direction of the branch axis, or, perhaps more correctly, in the direc- 

 tion of the radius of the apical zooid in which the bud appears. At a 

 certain distance from the dominant apical zooid a lateral zooid may trans- 

 form into a radial apical zooid and become the apex of a new branch; or 

 if the apical zooid is removed, one or more of the uppermost lateral zooids 

 may become an apical zooid. The resemblance to the dominance of the 

 stem tip over lateral buds in plants is evident, but the mechanism of domi- 

 nance is undoubtedly different. In certain other coral species the zooids 

 are equivalent; and in some an apical dominant zooid may be present in 

 favorable, and all zooids equivalent in unfavorable, environment. 



It is suggested that the nerve net is the chief factor in dominance in 

 hydroids and other coelenterates; but whether the limited range of domi- 

 nance in these forms depends on transmission with a decrement in the net 

 or on the relation between rate of increase in length of the body and of 

 differentiation of the net, the experimental data do not show. If the nerve 

 net shares in the gradient difference along the stem of such a form as 

 Tiibularia, for example, there may be a transmission decrement from 

 higher to lower levels. The time required for attainment of dominance 

 by a new hydranth in reconstitution suggests that reorganization of the 

 nerve net may be concerned. 



A different conclusion has been reached by Barth (1938a). He finds 

 that an oil drop introduced into the coenosarcal cavity of the Tuhularia 

 stem blocks dominance and concludes that the factor determining domi- 

 nance is transported in the circulation in the gastrovascular cavity of the 

 stem. If this is the case, the factor must be inactivated or disappear in 

 the course of transport, for the range of dominance is limited, and under 

 given conditions the limit is at a rather definite distance from the domi- 

 nant hydranth. However, since Tuhularia is highly susceptible to decrease 

 in oxygen (Barth, 1938ft) and the perisarc is not highly permeable to 

 oxygen, it seems possible that the oil drop may decrease oxygen supply 

 sufficiently in the coenosarc in contact with it to block transmission 

 through the nerve net. Relations of dominance and physiological isola- 

 tion are essentially similar in Corymorpha and Tuhularia, but in Cory- 

 morpha a large number of small longitudinal canals lying just beneath 

 the ectoderm represents the gastrovascular cavity of the stem. Regular 

 or uniform circulation in these appears impossible because the naked stem 

 contracts, extends, and bends almost continuously, even in isolated 

 pieces; but localization, length, and time of appearance of hydranth pri- 



