THE REFLEX ARC 241 



nerve cells or fibers is physiologically of secondary 

 importance. As a physiological entity the receptor- 

 conductor-effector system is present in the sponges, 

 either in single cells or in cell series, and if the myocytes 

 react directly, without relation to other cells, they 

 themselves must be physiologically more or less like 

 neuro-muscle cells. Physiologically speaking, a pure 

 effector is impossible, for even the most highly specialized 

 effector must be capable of receiving stimuli from with- 

 out. As a matter of fact every part of the reflex arc 

 like the primitive excitation gradient functions as 

 receptor, conductor, and effector, at least in one direction. 

 Nevertheless the different regions of the arc are in the 

 broad sense predominantly receptors, conductors, and 

 effectors, as their differentiation indicates. In this 

 sense it is undoubtedly true, as Parker maintains, that 

 effectors appear, i.e., become morphologically distin- 

 guishable earlier than nervous structures. The condi- 

 tion in the medusae, as described by the Hertwigs, in 

 which receptor, conductor, and effector are different 

 cells, does not by any means exclude the possibility of 

 the existence of the neuro-muscle cell, i.e., the system 

 in unicellular form, even in the same individual. In 

 Figure 69, for example, where the system as conceived 

 by the Hertwigs is shown, it is certainly possible that 

 the contractile basal portion of the cell m may be 

 activated not only by the receptor-conductor system 

 represented by the other two cells, s and n, but by 

 excitation at its own receptor surface and transmission 

 in its own protoplasm. Such a cell may be a neuro- 

 muscle cell and at the same time its contractile portion 

 may be the effector of one or several other arcs. 



