Chapter IV 



THE ORIGIN OF POLARITY, SYMMETRY, 

 AND ASYMMETRY 



§1 



It has been seen that when the amphibian egg is laid, all that can 

 be said about its future development is that the anterior end of the 

 animal will be formed near the animal pole, and the posterior end 

 near the vegetative pole. In all animals above the Protozoa, the 

 primary differentiation during their development is this axiation, 

 as Child calls the determination of the axis of polarity. 



It is of great importance to realise that the factors invoked in 

 order to explain the determination of polarity are external to the 

 egg. In the sea-weed Fucus, it is found that the determination of 

 polarity is normally due to the direction of incident light. ^ But it 

 has been shown experimentally that the application of an electric 

 current is also capable of inducing the determination of the axis of 

 polarity in the egg of Fucus, in any direction, at will.^ Further, it 

 is found that when Fucus eggs are placed in groups very close to one 

 another, each egg develops a polarity in such a way that its apical 

 point faces away from the group. ^ Here it seems that a chemical 

 factor is responsible, for the COa-tension will be higher and the 

 oxgen-tension lower in the midst of the eggs in the group than in 

 the surrounding fluid (fig. 23). 



One of the agencies capable of inducing polarity in the egg of 

 Fucus thus appears to be differential exposure to oxygen, and the 

 same is true of many animals. In the sea-urchin the oocyte develops 

 with one pole attached to the wall of the ovary, and the other 

 pole projecting freely into the cavity, and exposed to the ovarian 

 fluid and nutritive wandering cells. It appears that the centre of 

 this portion, where physiological exchange with the immediate en- 

 vironment is most active, will become the animal pole of the egg.^ 



Similar cases, where the attached and free surfaces of the 



^ Hurd, 1920; Whitaker, 1931. ^ Lund, 1923 b. 



^ Jenkinson, 1911; Lindahl, 1932. 



