ORIGIN OF AXIATE PATTERN 59 



dition between the more exposed and probably more 

 stimulated growing borders of the somatic plate, the 

 blastoderm or the blastopore lips and other adjoin- 

 ing regions. Moreover, many bilateral forms develop 

 characteristic asymmetries during the course of devel- 

 opment, e.g., the asymmetry of gasteropod mollusks 

 and the visceral asymmetry of vertebrates, or a well- 

 developed bilateral symmetry may give rise in metamor- 

 phosis to radial-bilateral pattern of very different kind, 

 as in certain echinoderm groups. Our knowledge con- 

 cerning the physiological aspects of the origin of sym- 

 metry in animals is still very fragmentary, but the 

 earliest indications of the presence of a particular sym- 

 metry pattern are gradients in physiological condition, 

 which, so far as the evidence goes, are similar to the polar 

 gradients, and in plants we see the different symmetry 

 patterns arising through differential exposure to the 

 action of external factors. In the light of all the facts 

 we are justified in concluding that even though a 

 particular symmetry pattern may persist through repro- 

 duction, i.e., be inherited in a particular case, sym- 

 metry like polarity must in the final analysis arise 

 through differential exposure to the action of external 

 factors. 



Even if we grant that the differential exposure of 

 the egg of the medusa, the sea urchin, Sternaspis, etc. 

 (see p. 54), determines the polarity and that symmetry 

 may also be determined by relation to environment, it is 

 evident that, except in some of the simpler organisms, 

 the differential exposure of the egg is not fortuitous, 

 but is determined by the hereditary mechanism of the 

 organism. The epithelial arrangement of eggs in the 



