V GENERAL REFLECTIONS AND CONCLUSIONS 281 



Ascidian Cynthia, for example), or by the arrangement of the 

 blastomeres (as by the large posterior cell of Annelids and 

 Mollusca), and this point, together with the axis, determines 

 a plane about which the ovum is bilaterally symmetrical. The 

 axis and the plane of symmetry of the egg are definitely related 

 to the axis and symmetry of the embryo, the substances to its 

 primary organs. 



Further, in very many, though not in all, instances the parts 

 of the ovum blastomeres or egg-fragments are totipotent; 

 and the same is true of the parts of elementary organs like the 

 archenteron of Echinoderms or the optic vesicle of Amphibia. 

 The totipotence is, however, sooner or later lost, and this limita- 



FIG. 165. Diagram to illustrate Driesch's conception of the minute 

 structure of the (Echinid) ovum. It is supposed to be composed of 

 particles all similarly polarized and oriented to the whole. In A one 

 meridional half is shown. In B this has become spherical and the parts 

 have been disturbed. In C they have regained the original orientation. 

 (From Korschelt and Heider.) 



tion is apparently due to the way in which the substances are 

 distributed in the ovum, an explanation which seems to be 

 accepted by Driesch for most cases. But in accounting for the 

 phenomena in the Echinoderm egg, the form with which he 

 himself has chiefly experimented, he urges a different hypothesis. 

 Here he conceives of the egg as composed of like particles, each of 

 which is polarized and oriented in the same manner as the egg 

 itself (Fig. 165), and consequently the only limitation to totipo- 

 tence is due to size ; any isolated part that is not too small can 

 develop into a whole as soon as its polarized particles have re- 

 assumed a similar orientation. Again, it is stated that the 

 blastomeres can be disarranged to any extent without interfering 



