62 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



oocytes are exposed to different conditions, would appear to 

 account for the polarity of the unfertilised egg in many other forms, 

 e.g. Chaetopterus,^ Sternaspis,^ Cerebratulus,^ and Cyclas,^ where 



Fig. 24 



The primary gradient in oocytes, a, In the Annehd Sternaspi^ the oocyte is 

 attached by a narrow peduncle containing a vascular loop, and the nucleus is at 

 the opposite end, which protrudes into the ovarian cavity. The attached end 

 becomes the vegetative pole, the free end the animal pole, b, Gradient in 

 amount of yolk and size of yolk-spheres in the oocyte of the frog; p, pedicle of 

 attachment. (From Child, Physiological Foimdations of Behavior, New York, 1924.) 



the exposure is to the ovarian fluid, and the Coelenterate Phiali- 

 dium,^ where the exposure is to the surrounding sea-water. In all 

 the above-mentioned cases the surrounding fluid, be it ovarian 



C. B. Wilson, 1900. 



54- 



^ Lillie, 1906. 



* Stauffacher, 1894. 



- Child, 1915 B, p. 341. ^ 



^ Child, 1921 B, p. 



