366 SECTIONAL TRANSACTIONS.— D. 



Dr. J. Needham. — Chemical interpretation of fields (11.30). 



Although the time has hardly yet come when it is possible to make 

 much progress in the investigation of the chemical basis of morphogenetic 

 fields, some preliminary thinking is not without interest. That three- 

 dimensional co-ordinate systems have an objective existence in the form of 

 gradient nets within developing embryos now hardly admits of doubt, but 

 the evidence that these gradients are of metabolic rate is not convincing. 

 Diffusion gradients of active substances, that is to say, chemical molecules 

 which exert some orienting action upon protein molecules, are conceptually 

 more helpful. It should, however, be remembered that gradients in a mor- 

 phological field may also be gradients of varying degrees of randomness in 

 molecular orientation, the active centre being itself stationary. We need 

 to know a great deal more about the shapes and coherence properties of 

 protein molecules, and about the relation between histological structure 

 and protein fibre arrangement. Finally the morphogenetic field must be 

 constructed of qualitatively, not merely quantitatively, varying differences 

 along its axes. 



Prof. E. G. Conklin. — Summary of the Discussion (12.0). 



Afternoon. 

 Mr. G. E. H. Foxon. — Orientation to gravity in Crustacea (2.15). 



Orientation to gravity is of importance to various groups of Crustacea for 

 different reasons. In the higher Crustacea a definite organ of balance 

 (statocyst) is commonly found, but in certain cases, particularly in the lower 

 Crustacea and the larvae of the higher Crustacea no such organ is present. 

 In such cases orientation to light is usually so dominating a factor that in 

 the presence of light response to gravity disappears ; if, however, such 

 creatures are observed in red light (to which they do not appear to respond) 

 orientation to gravity is seen to exist. 



Experiments have shown that it is possible to make a series covering the 

 various methods of orientation to gravity thus : 



(1) Orientation to gravity entirely mechanical : example Chirocephalus . 



(2) Orientation to gravity mechanical but depends on the normal activi- 

 ties of the animal being maintained : example Daphnia. 



(3) Orientation to gravity the result of muscular action, but no special 

 sense organ present : a example the larvee of many Brachyura. 



(4) Orientation to gravity the result of muscular action, a special sense 

 organ present : example adult Brachyura. 



Mr. A. G. Lowndes. — The term ' gnathobase ' (2.45). 



In Lankester's classical essay (1881) on the appendages of Apus he defines 

 the proximal endite of the trunk limb as a ' gnathobase,' stating that ' it is 

 a jaw process ' and ' clearly has the function of assisting, by means of apposi- 

 tion to its fellow of the opposite side, in seizing and moving particles which 

 may be introduced into the mouth.' 



Lankester's use of the term has been severely criticised on three separate 

 occasions by Prof. Graham Cannon, 1927, 1928 and 1933, who together 

 with Dr. Manton (1927) pointed out that in the Anostraca the proximal setae 

 of the basal endite are covered over medially by setae from the limb in front 

 so that they cannot transport food towards the mouth and certainly do not 

 work in apposition to their fellows on the opposite side. 



