240 THE ORIGIN OF THE NERVOUS SYSTEM 



such conditions the development of a definite, persistent 

 receptor-conductor-effector structure is of course impos- 

 sible. But where the gradients persist, whether from the 

 surface inward or as axial gradients of the body as a 

 whole, they afford a physiological basis for the develop- 

 ment of a structural nervous system. In a multicellular 

 organism, such as Hydra, where the body cells form 

 coherent epithelial layers and only certain portions of 

 the cell surface are directly exposed to the extra-organic 

 environment the conditions exist for the development of 

 an excitation gradient and a physiological axis in each 

 ectoderm cell with the exposed surface as receptor, and 

 this pattern may coexist with a general axiate pattern of 

 the whole body. The visible differentiation of an effector 

 region, e.g., a contractile portion, in such a cell may occur 

 before anything recognizable as nervous structure is 

 visible, for reception and transmission of excitation 

 apparently do not bring about modification of proto- 

 plasmic structure as rapidly or as extensively as the 

 development of contractility. In short, the neuro- 

 muscle cell of Kleinenberg is exactly the sort of system 

 we might expect to find in a cell layer w r ith definite 

 surface relations to the exterior. It represents simply 

 one possibility in the origin of a receptor-conductor- 

 effector system, and the grounds for doubting its exist- 

 ence are morphological rather than physiological. 



Parker's conclusion that the myocytes of sponges 

 are independent effectors is doubtless morphologically 

 correct, but physiologically either some portions of these 

 myocytes themselves function as receptors and con- 

 ductors or else other cells perform these functions. 

 The presence or absence of anything recognizable as 



