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THE POPULAR SCIENCE MONTHLY 



place in a 

 epithelium. 



columnar 



erate, and that in regeneration the nervous net forms earlier than the 

 muscles. By taking jellyfishes at the appropriate stage in regeneration, 

 it was found that a stimulus applied to one side of a regenerated area 

 was followed by a muscular response on the other side of this area with- 

 out any observable movement in the area itself. Hence transmission 

 through the regenerated region must have been by nervous means, 

 doubtless by the nervous net. 



In jellyfishes the nervous net will transmit apparently in any direc- 

 tion and in this respect it is in strong contrast with the central nervous 



organs of the higher metazoans, where, especially 

 in the vertebrates, a polarized condition generally 

 prevails. Thus in the spinal nerves of vertebrates, 



fig. 5. nbueomuscd- tt is eas y to send im P ulses through from a dorsal 

 lab Cell (black) in root to a ventral one, but impossible to send them 



in the reverse direction. Apparently the cord 

 contains some structure on its path of conduction 

 that is valve-like and allows impulses to pass in one direction only. 

 Such a condition does not exist in the nervous net of the jellyfishes. 



The neuromuscular organs of the ccelenterates have been considered 

 by so many investigators as the most 

 primitive in the animal kingdom that 

 it is not inappropriate to consider at 

 this place the relations of some of the 

 older views on this subject to those 

 expressed in these articles. 



The discovery by Kleinenberg 

 (1872) of the so-called neuromuscular 

 cells ( Fig. 5 ) in Hydra led this investi- 

 gator to the belief that these cells rep- 

 resented a complete neuromuscular ap- 

 paratus in that each cell-body could be 

 regarded as a receptor and its fibrous condition containing three ceils which 



° . subsequently (lower figure) differ- 



portion as an effector. By growth and entiate into a sense-cell (l), a gan- 



cell division, according to Kleinenberg, g"on-ceii (2), and an epithelial 



separate receptors and effectors would 



be differentiated simultaneously from such single cells. 



The simultaneous differentiation of nervous and muscular elements 

 (Fig. 6) was also accepted by the brothers Hertwig (1878), but in 

 their opinion the two types of tissue did not arise from a common cell 

 as claimed by Kleinenberg, but from separate cells which became simul- 

 taneously differentiated, some to form nerve-cells (sense- and ganglion- 

 cells) and others to form muscle-cells. This view has come to be com- 

 monly accepted by the majority of investigators. 



The independent origin of the nervous system and its secondary 



Fig. 6. Differentiation of Neu- 

 romuscular Constituents from an 

 Indifferent Epithelium. The up- 

 per figure represents an indifferent 



