ORIGIN OF AXIATE PATTERN 



55 



becomes the apical pole (Fig. 4) . In these cases it is not 

 the pole through which nutrition enters, but the un- 

 attached pole, which in the medusa is more exposed to 

 external factors and in the sea urchin and Sterna pis to 

 the fluids of the ovary which becomes the apical pole. 

 It seems probable that a differential in oxygen supply 



Figs. 1-3. — Ovarian eggs of jellyfish, Phialidium gregarium; 

 diagrammatic: Fig. i, early growth stages; Fig. 2, later stages; in these 

 stages one surface (the lower in the figure) is separated from the exterior 

 only by thin epithelium and opposite surface is attached and adjoins 

 radial canal; Fig. 3, eggs separated from ovary by teasing, showing 

 region of attachment; shading indicates susceptibility gradient as indi- 

 cated by disintegration and by permanganate. 



and perhaps also in C0 2 concentration are chiefly con- 

 cerned in determining the polarity and the gradient 

 which represents it in these cases. 



In the eggs of at least some higher animals, where oxy- 

 gen as well as nutritive substances reach the egg chiefly 

 or wholly through the blood, the polarity is apparently 

 determined by relation to the blood supply. Bellamy 

 (1919) has shown that in the frog's egg polarity appar- 

 ently develops in definite relation to the vascular 



