ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 65 



buds must therefore be imposed on them from without.^ An even 

 more striking example is the masses produced by the joining up 

 of cells and cell-groups after the tissues of sponges and hydroids 

 have been strained through fine gauze, ground up with sand, or 

 otherwise dissociated. Here, clearly, all traces of the original 

 polarity must have been lost. However, the masses may later 

 develop into miniature sponges with polarity of their own. This 



^i^ 



h 



Experimental imposition of a new primary axis in Corymorpha. In fragments of 

 stem immediately after cutting {a) or after regeneration to form biaxial (c) or 

 single hydranths, immersion in dilute alcohol causes dedifferentiation {b, g). On 

 replacement in sea-water, redifferentiation occurs with a new axis at right angles 

 to the old, with apical region at the centre of the free surface {c, d, h, i). (From 

 Child, Physiological Foundations of Behavior, New York, 1924.) 



polarity must have been induced by external factors.- Similar results 

 have been obtained with hydroids (figs. 27, 132; see also p. 281). 

 Thus, apart from the cogent theoretical reasons advanced by 

 Child, there is abundant evidence, experimental and circum- 

 stantial, for the view that the initial determination of an axis of 

 polarity, or axiation, is due to the action of factors external to the 

 developing organism. 



^ Huxley, 1926; Brien, 1930. 



2 H. V. Wilson, 1907, 191 1 ; Child, 1928 b; Huxley, 1911, 1921 a. 



HEE 5 



