66 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



Once the axis of polarity has been determined in an egg, it often 

 becomes manifested by a stratified and graded distribution of egg- 

 contents, some of which may be visibly distinct, such as pigment, 

 fat, yolk, etc. (e.g. Arbacia). It is to be noted, however, that this 

 stratification of materials is only an effect and not a cause of polarity. 



iirS'' 



te 



-02? 



X' 



/^l 



^ 



Fig. 27 



Differentiation after dissociation in the hydroid Pennaria. The dissociated cells 

 united to form rounded reconstitution-masses which surround themselves with 

 perisarcs {op) and later form outgrowths which give rise to stolon-like structures 

 {x) and normal hydranths. (After H. V. Wilson, from Gray, Expermiejital 

 Cytology, Cambridge, 193 1.) 



A completely new restratification can be induced in any direction 

 by means of the centrifuge, but development continues to be 

 governed by the original axis of polarity.^ It is probable in these 

 cases (Arbacia) that the polar organisation of the egg, once it is 

 determined, resides in the cortex and an invisible internal frame- 

 work of more viscous cytoplasm which resists the displacing action 



^ Morgan and Spooner, 1909. 



