PHYSIOLOGICAL GRADIENTS. 175 



many processes of agamic reproduction, e.g., in flatworms and 

 annelids and may of course also be inherited in many eggs. On 

 the other hand, it may conceivably be determined in some eggs 

 by ovarian conditions, by conditions connected with maturation 

 or with fertilization, or perhaps even by conditions arising later 

 in development. Moreover, many bilateral forms develop char- 

 acteristic asymmetries during the course of development, e.g., 

 the asymmetry of gasteropod mollusks and the visceral asym- 

 metry of vertebrates, or a well-developed bilateral symmetry 

 may give rise in metamorphosis to a radial-bilateral pattern of 

 very different kind, as in certain echinoderm groups. Our 

 knowledge concerning the physiological aspects of the origin of 

 symmetry in animals is still very fragmentary, but the earliest 

 indications of the presence of a particular symmetry 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 ex- 

 posure 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 reproduction, i.e., be in- 

 herited in a particular case, symmetry like polarity must in the 

 final analysis arise through differential exposure to the action of 

 external factors. On the other hand, even if we grant that the 

 differential exposure of the egg of the medusa, the sea urchin, 

 Stcrnaspis, etc. (see p. 173), 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 dif- 

 ferential exposure of the egg is not fortuitous, but is determined 

 by the hereditary mechanism of the organism. The epithelial 

 arrangement of eggs in the medusa gonad, the position of the sea 

 urchin egg in the ovary, the development of the peduncle and 

 the vascular loop in Sternaspis, and the circulatory pattern in 

 the chorion of the frog's egg are all features of the hereditary 

 mechanism of the organism concerned. Even in such cases then, 

 as well as in cases where the axiate pattern persists through 

 reproduction, the hereditary mechanism is concerned in the origin 

 of the axiate pattern of the new individual. Moreover, even in 

 the case of Fucns where the determination of polarity by the 



