54 THE ORIGIN OF THE NERVOUS SYSTEM 



Child, 1915c, 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 experi- 

 mental conditions. 



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 differential exposure. In various hydromedusae, 

 for example, the growing 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 (Figs. 1-2). When portions of the ovaries are 

 slightly teased to separate the eggs it is found that the 

 free end of the egg, the end nearest the exterior, repre- 

 sents the high region, and the attached end, the low region 

 of a gradient in susceptibility and permanganate reduc- 

 tion (Fig. 3). A similar gradient appears in develop- 

 mental stages, and while the absence of good landmarks 

 makes it impossible to demonstrate that the later 

 gradient is identical 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 Sternaspis an aberrant 

 annelid, a peduncle containing a vascular loop, develops 

 in connection with each growing oogonium, but my 

 <>1 nervations indicate that the free pole, not the attached 

 pole of the egg, into which the vascular loop enters, 



