ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 63 



or sea-water, may be regarded as containing more oxygen than the 

 tissues of the ovary. But in other forms, such as vertebrates, not 

 only does the coeiomic fluid in such small enclosed spaces as the 

 ovarian cavity lose its respiratory function, but the ovary itself is 

 well supplied with blood-vessels, and there is therefore reason to 

 believe that the oxygen-supply of the tissues of the ovary is greater 

 than that of the fluid surrounding the ovary. It is consequently 

 of great interest to find that in birds the exposed side of the 

 oocyte becomes the vegetative pole of the egg, while the attached 

 side becomes the animal pole.^ The same is true in Amphioxus, 

 but here the attached side of the egg is turned towards the 

 secondary ovarian cavity which is close to the atrial cavity, from 

 which oxygen is probably derived. (For the frog, see p. 35.) 



Asexual reproduction and regeneration phenomena also provide 

 a number of examples in which polarity is induced from the outside, 

 and such cases are, from the standpoint of general theory, as im- 

 portant as those concerning development from an egg. An axis 

 of polarity can be experimentally induced in regenerating frag- 

 ments of the Hydroid polyps, Obelia and Corymorpha. These 

 organisms are built on a radially symmetrical plan, with an axis 

 passing down from the oral end of the polyp along the stem. If a 

 piece of Obelia stem be isolated, it normally retains its polarity, as 

 shown by its regenerating a polyp at the original distal end earlier 

 than at the proximal end. But if such cut pieces are subjected to 

 the passage of an electric current of a certain strength through the 

 water in which they are lying, it is found that regardless of the 

 original polarity of the pieces, polyps are regenerated only at that 

 end which points towards the anode : while stems (or stolons) may 

 be formed from the end which is directed towards the kathode. This 

 shows that the original polarity can be overridden by external stimuli 

 such as an experimentally controlled electric current^ (fig. 25). 



A piece of the stem of Corymorpha regenerating in normal sea- 

 water likewise retains its polarity, and regenerates a polyp at its 

 originally distal end. But if such a piece is placed in water contain- 

 ing weak poisons, it dedifl'erentiates and loses its form, becoming 

 converted into a banana-shaped mass lying on the bottom of the 

 vessel. Replacement in clean water will lead to regeneration of a 



^ Conklin, 1932. ^ Lund, 1921, 1923 A, 1924. 



