318 A. W. BELLAMY. 



increase in susceptibility during the early stages of development, 

 and to provide for this and to carry the effects of differential 

 inhibition somewhat farther than is possible where the eggs 

 remain in the same concentration throughout the experiment, a 

 number of experiments were done where the solution was gradu- 

 ally diluted as development proceeded. 



In the experiments on recovery, the eggs were removed from 

 the solutions and washed in several changes of water to remove 

 any of the chemical remaining in the jelly. 



III. THE ORIGIN OF POLARITY. 



i. Polarity in Other Organisms. There is abundant evidence 

 to show that for many organisms and especially among the 

 plants, polarity and symmetry arise in response to external con- 

 ditions. Polarity in the egg of the alga Fucus, and in the spore 

 of Equisetum is usually determined by the direction of incident 

 light. In some algae, polarity and symmetry are directly under 

 the influence of light, even in the vegetative thalli, and are 

 reversible or modifiable by change in light relations. In various 

 liverworts and free prothallia, and even in certain phanerogams, 

 light is an important factor in determining dorso-ventrality in 

 branches. 



The polarity of a number of plant and animal eggs bears a 

 definite relation to their manner of attachment to the parent 

 body during growth stages. In the phanerogams, for example, 

 the free end of the egg becomes the apical, the attached end 

 the basal end of the plant axis, and the same relation holds for 

 a number of the lower animals. 



Likewise a polar axis of symmetry once established may be 

 obliterated experimentally and a new one induced by conditions 

 external to the organism. This has been done by H. V. Wilson 

 (1907, 1911) in certain sponges and hydroids, work which was 

 confirmed by Hargitt (1915). In experimentally produced bi- 

 axial forms in planaria, hydroids and annelids, a new axis arises 

 1 80 from the old one. In Corymorpha, e.g., Child showed that 

 when pieces of the stem are placed in 2 per cent, to 2 1/2 per 

 cent, alcohol in sea water, "in the course of a few days the pieces 

 become shorter and more rounded, decrease in size, and lose the 



