ORIGINS OF EMBRYONIC PATTERNS 675 



tion of the hydranth region occurs earlier or later in relation to tentacle 

 development. Quite apart from this possible differential, there is evi- 

 dence of a spatial relation in the tentacle order. If one tentacle precedes 

 others, the second tends to develop as far as possible from it. If two ap- 

 pear simultaneously, they are almost always opposite, or almost opposite; 

 and if three or four appear simultaneously, they are approximately equi- 

 distant. The observations suggest that a developing tentacle is dominant 

 over a certain distance and that, about the periphery of the planula, not 

 more than three or occasionally four tentacles can develop. Only as this 

 periphery increases in size do more tentacles appear in the spaces be- 

 tween those already present. Evidently in this and various other hydroids 

 the radial symmetry of the mature hydranth is not predetermined but de- 

 velops gradually, often through an asymmetrical or a "dorsiventral" 

 stage, probably determined by external conditions, from a pattern in 

 which all radii at a given body-level are ahke. In general the number of 

 tentacles which can develop simultaneously about the periphery of a 

 coelenterate larva probably depends on the range of tentacle dominance 

 in relation to size of the periphery. Two different quadriradial patterns, 

 the perradial and adradial, appear in scyphomedusae ; and a third, multi- 

 radial tentacle pattern, is present in some. In certain actinians two oppo- 

 site radii become difTerent from others in mesenterial and esophageal pat- 

 tern, but the tentacle pattern remains multiradial. The apparently multi- 

 radial symmetry of tentacles and mesenteries in certain others — for ex- 

 ample, Cerianthus — results from a sort of ventrodorsality ; that is, ten- 

 tacles and mesenteries develop successively from one region of the pe- 

 riphery. In short, radially symmetrical patterns may result from radially 

 asymmetrical or may become asymmetrical; or certain regions may be 

 radial, others asymmetrical, ventrodorsal, or bilateral. How patterns like 

 the four radii of the scyphozoa or the biradial and ventrodorsal mesen- 

 terial and esophageal patterns of anthozoa are localized about the polar 

 axis we do not know at present. 



VENTRODORSALITY AND DORSIVENTRALITY 



In many animals ventrodorsal or dorsiventral pattern appears first dur- 

 ing the course of development as a graded differential in condition vertical 

 to the polar gradient. Bilaterality is, at least in most forms, merely an 

 expression of this apparently secondary pattern. Bilaterality, as well as 

 polarity, is determined in the ovary, according to E. B. Wilson (1925, 

 pp. 1021-25), but he does not suggest how it originates. If it is so deter- 



