674 PATTERNS AND PROBLEMS OF DEVELOPMENT 



The recent extended consideration of the problems of symmetry and 

 asymmetry by W. Ludwig (1932) and the data presented by Schleip (1929) 

 show the various types of these patterns and the manner in which they 

 become evident in development, but the attempts of these and other 

 authors to throw light on questions of origin and nature of these features 

 of developmental pattern, while suggestive as far as they go, are chiefly 

 significant in indicating our lack of definite information. If polar patterns 

 are determined by factors in ovarian environment, the question at once 

 arises whether there are factors in the ovarian environment of the oocyte 

 that might determine symmetry and asymmetry. 



RADIAL SYMMETRIES 



Radial symmetry may be primarily nothing more than absence of any 

 pattern except surface-interior about the polar axis. It is apparently no 

 more than this in many blastulae, sponge larvae, hydroid planulae, post- 

 tentaculate regions of many hydroids, and many cylindrical organs. 

 Often, however, differences appear in radii of the same body-level as de- 

 velopment progresses. In coelenterates, for example, certain radii may 

 become tentacle radii; in starfishes, arm axes. Some radial patterns de- 

 velop from patterns not primarily radial, and these from still earlier pat- 

 terns, apparently completely radial. The radii about the polar axis of the 

 Corymorpha planula are apparently alike ; but the succession of tentacles 

 differs in different individuals, as H. B. Torrey (1907) has pointed out and 

 the writer has also observed. Tentacle development may begin with ap- 

 pearance of a single tentacle at some point of the periphery of the planula, 

 a second developing later on the opposite side, still later two others, op- 

 posite and midway between the first and second. In other individuals two 

 opposite tentacles or three equidistant or occasionally four equidistant 

 develop simultaneously, and in still others the first two tentacles may be 

 not opposite but both in the same half of the periphery. Moreover, the 

 order of appearance of distal and proximal tentacles may differ in a single 

 individual. Evidently the final radial tentacle pattern results from de- 

 velopment of tentacles in the spaces between other tentacles at a certain 

 body-level. The individual differences in tentacle order perhaps result 

 from the fact that the Corymorpha planula does not swim but in earlier 

 stages lies with one side in contact with the substrate and only gradually 

 erects its apical end about the time that tentacles develop. Tentacle de- 

 velopment may be slightly retarded on the side on contact, as it is in re- 

 constitution of pieces; and different orders may result according as erec- 



