238 THE ORIGIN OF THE NERVOUS SYSTEM 



Kleinenberg (1872), for example, held that the 

 primitive condition with receptor, conductor, and 

 effector within the limits of a single cell is found in the 

 neuro-muscle cell of the Hydra ectoderm (Fig. 68). 

 The outer exposed surface of the cell is the receptor, 

 the elongated contractile base the effector, and the 

 intermediate region the conductor. The effect of 

 excitation in this unicellular arc is motor in character, a 



simple contraction of the basal 

 effector portion of the cell. Ac- 

 cording to this conception, re- 

 ceptor, conductor, and effector 

 develop in functional rela- 

 tion, first within a single cell, 

 and only later in the course 

 of evolution become sepa- 

 rated into different cells. The 

 brothers Hertwig (1878) agree 

 with Kleinenberg as regards 

 the simultaneous development of nerve and muscle, but 

 their studies on the medusae lead them to believe that 

 receptor, conductor, and effector arise as separate cells 

 (Fig. 69). Claus (1878) and Chun (1880) maintained 

 that the nervous system arises independently of muscles 

 and only secondarily becomes connected with them, while 

 Samassa (1892) and Schaeppi (1904) hold that a nervous 

 system without effectors is difficult to conceive. More 

 recently Parker (1909, 1910a, 19106, 191 1, 1914, 191 je, 

 1919) concludes from his study of sponges that muscles 

 appear first as independent effectors and in connection 

 with them the first true nervous organs in the form of 

 receptors, sensory cells, develop, a condition very similar 



Fig. 68. — An epithelio-muscle 

 cell ("neuro-muscle cell") of 

 Hydra, the contractile basal 

 portion indicated by shading. 



