56 



THE ORIGIN OF THE NERVOUS SYSTEM 



supply, the apical pigmented pole arising on the arterial, 

 the basal unpigmented pole on the venous side (Fig. 5). 

 In these cases the oxygen supply or the conditions 

 determining rate of respiratory exchange are apparently 



4 5 



Fig. 4. — Growing oocyte of Skruaspis scutata (from Child, 1915&). 



Fig. 5. — Semi-diagrammatic view of ovarian egg of frog, showing 

 distribution of arteries and veins in relation to pigmentation and polar- 

 ity: a, artery, v, vein, p, pedicle (from Bellamy, 1919). 



the chief factor in determining polarity, the region of 

 most rapid exchange becoming the apical pole. 



THE ORIGIN OF S\ MM I TRY 



Symmetry in animals, like polarity, is primarily 

 indicated by gradations in physiological condition, and 

 although a particular kind of protoplasm may give rise 

 normally to a radial or a bilateral animal, experiment 



