CENTRAL ORGANIZATION OF VISION 511 



can diffuse through or can be trapped in gelatine and thus can be transferred to 

 another plant, therein to produce the typical response. The exjoeriments of Ricca 

 (1916) on the highly irritable Mimosa pudica, or of Mangold (1923) on insectivorous 

 plants, bring out the same point ; although the later investigations of Bose and Das 

 (1925), Bose (1926-28) and Molisch (1929) would seem to indicate that in these very 

 highly specialized forms many of the characteristics of nervous activity may be closely 

 simulated. The difference, however, between the primitive response to light in plants 

 and animals is merely a difference of method ; the reaction is fundamentally the same, 

 the transformation of a photochemical change into a motorial response. 



THE NERVOUS CONTROL 



Although hormonal control persists in animals, particularly in the 

 regulation of their basic activities, their movements and responses to external 

 stimuli are active rather than jDassive ; the explosive response fired by 

 " trigger-action " gives them mobility. Even in the most primitive animals 

 the energy provided by the photochemical reaction contributes to the 

 chemical activation of neighbouring molecules, thus kindling a chain of 

 chemical changes by means of which a phase of excitation is propagated 

 through the protoplasm from the site of stimulation to the site wherein 

 the response is produced. It is interesting that in organisms as lowly as 

 Protozoa, differentiated fibrils are evident formed by basal granules arranged 

 in longitudinal rows within the single cell, one at least of the functions of 

 which is to coordinate the movements of the cilia (Neresheimer, 1903 ; Gelei, 

 1935). The evidence is convincing that some of these are paths for the 

 propagation of stimuli, since microdissection experiments have showTi that 

 when they are cut the rhythm of the movements of cilia is disrupted (Taylor, 

 1920-41 ; MacDougall, 1928 ; MacLennan, 1935). There is also evidence 

 that in colonial Protozoa, conduction can in this way proceed from cell to 

 cell by intercellular fibrils (Taylor, 1941) (Fig. 666). In these forms this 

 phenomenon is too rapid to be due to diffusion and too slow to have an 

 electrical basis, and it is probable that these fibrils result from the preserva- 

 tion through natural selection of chance molecular patterns in the protoplasm 

 which favour the relay of a train of chemical reactions, and that from these 

 strand-like plastids nervous tissue, with its specialization as a conductor, 

 had its origin (Bovie, 1926). 



Once an effective intracellular means of conduction has been established, 

 the obvious method of advance is for part of a cell to stretch and become 

 specialized. In this way certain of the surface cells which, because of their 

 exposed position receive stimuli from the environment, send long processes 

 inwards conveying the message of their stimulation to neighbouring parts 

 of the organism. Eventually, stretching many times their own breadth, 

 they leave the surface layer and, abandoning sensory reception, specialize 

 in conducting the excitations of other cells so that fuially a network of 

 conducting paths is laid down underneath the integument and the entire 



