January 25th, ipi6.] PROCEEDINGS. xxvii 



Professor William H. Lang, M.B., D.Sc, F.R.S., considered, 

 in the light of the principles contained in the President's 

 Address, some cases of dorsiventral symmetry of cryptogamic 

 plants. It seemed difficult to apply these principles to the 

 plants, where symmetry often appeared to rest on deeper and 

 still unexplained differences of the germ cells, There was no 



reason to believe that specific differences were less readily 



drawn in radial than in dorsiventral plants. 



Mr. D. Thoday, M.A., suggested that some degree of 

 plasticity, expressed in a certain range of variability, might be 

 explicable on the analogy of alio tropic forms of crystals, without 

 assuming any more fundamental change of "specific substance." 

 The known properties of fibres and other plant structures 

 demanded a definite arrangement of the particles composing 

 them, so that they could hardly be termed amorphous ; at the 

 same time their chemical complexity suggested the possibility 

 of a greater variety of modes of arrangement, and perhaps also 

 less definiteness and stability, than were exhibited by the 

 crystalline forms of relatively simple substance. Such a mode 

 of arrangement in substances formed by the protoplasm might 

 be a reflection of a corresponding structural arrangement in the 

 protoplasm ; and this might be in some degree hereditary, 

 though more susceptible to environmental influences than the 

 chemical composition of the " specific substance." 



The President in reply said that he was prepared to 

 admit that in the evolution of the animal kingdom as a whole 

 the sedentary habit came first and the radial symmetry followed, 

 but he joined issue with Dr. Tattersall as regards the relative 

 variability of radially symmetrical and of bilaterally symmetrical 

 animals. 



It is quite true that in some Coelenterata the radial sym- 

 metry is remarkably constant, as for example the octoradial 

 symmetry of the Alcyonaria, but when variation does occur, as 

 for example in Medusae and Sea-anemones, it affects important 

 vital organs such as the stomach, gonads and sense organs to a 



