PHYSIOLOGICAL GRADIENTS. 173 



, Figs. 38, 39). The localization of the new axis in such cases 

 appears to be largely a matter of slight fortuitous differences in 

 activity in different cells or cell groups in consequence of which 

 certain cells or groups react more rapidly than others to the ex- 

 perimental conditions. 



The Annual Egg. As regards the animal egg, the evidence is 

 very incomplete, but indicates that in at least many forms polarity 

 is determined during the growth period of each egg by differ- 

 ential exposure. In various hydromedusse for example the grow- 

 ing oogonia constitute a columnar epithelium, one end of each 

 cell being separated from the exterior only by a layer of very 

 thin flattened cells, while the opposite end is attached and adjoins 

 the radial canal. When portions of the medusa ovaries are 

 slightly teased to separate the eggs it is found that the free end 

 of the egg, the end nearest the exterior, represents the high region, 

 the attached end the low region of a gradient in susceptibility 

 and permanganate reduction. A similar gradient appears in de- 

 velopmental stages and while the absence of good landmarks 

 makes it impossible to demonstrate that the later gradient is iden- 

 tical with the earlier, there can be little doubt that it is. In the 

 sea urchin the oogonium is attached to the wall of the ovary at 

 one small region of its circumference and here also, as Boveri 

 has shown, the free pole becomes the apical pole and represents 

 the high end of a gradient. In these cases it is not the pole 

 through which nutrition enters, but the unattached pole which 

 in the medusa is more exposed to external factors and in the sea 

 urchin to the fluids of the ovary which becomes the apical pole. 

 It seems probable that a differential in oxygen supply and per- 

 haps also in CO 2 concentration are chiefly concerned in deter- 

 mining the polarity and the gradient which represents it in these 

 cases. Even in the worm Sternaspis, in which a peduncle con- 

 taining a vascular loop develops in connection with each growing 

 oogonium, the attached pole of the egg, into which the vascular 

 loop enters becomes the basal pole. 



In the eggs of the higher animals where oxygen as well as 

 nutritive substances reach the egg chiefly or wholly through the 

 blood, the polarity may apparently be determined by relation to 

 the blood supply. Bellamy ('19) has shown that in the frog's egg 



