THE NERVOUS SYSTEM 123 



to represent this step in the evolution of the nervous system. This 

 general view of the origin of the central nervous organs was advanced 

 as early as 1886 by Kleinenberg and was reaffirmed ten years later by 

 Eakowitza. 



The evolution of nerve and muscle, so far as this problem can be 

 attacked in such lowly form as the sea-anemone and other ccelenterates, 

 is a question about which there has been much difference of opinion. 

 As early as 1872 Kleinenberg showed that in the fresh- water coelenter- 

 ate, Hydra, there were certain peculiar T-shaped cells that he called 

 neuromuscular cells and that he believed to represent both nerve and 

 muscle. In these cells the arm of the T reached the surface of the 

 animal and was thought by Kleinenberg to act as a nervous receptor; 

 the cross-piece being contractile was known to be muscle. Kleinenberg 

 assumed that the division of such cells and the differentiation of their 

 parts were the processes which gave rise to the nervous and muscular 

 tissues of the higher animal. In 1879 the Hertwigs in their account 

 of the structure of sea-anemones showed that the so-called neuromuscu- 

 lar cells of Kleinenberg were in reality simply epithelio-muscle cells 

 and were without nervous significance. These investigators, in opposi- 

 tion to Kleinenberg, advanced the view that nerve and muscle, though 

 simultaneously differentiated, were derived from different groups of 

 cells. According to both Kleinenberg and the Hertwigs nerve and 

 muscle were simultaneously evolved, but Kleinenberg maintained that 

 these tissues came from a single form of cell, the Hertwigs that they 

 arose from separate kinds of cells. 



My own studies on the origin of nerve and muscle have led to 

 rather different conclusions from those summarized in the last para- 

 graph. In studying the reactions of one of our common sponges, 

 Stylotella, I was impressed with the extreme slowness with which the 

 animal responded to a stimulus. The oscula of this sponge can be made 

 to close by the application of several kinds of stimuli. The closure of 

 these openings is accomplished by the contraction of the ring of 

 muscular tissue surrounding them. This response occurs some minutes 

 after the stimulus has been applied, a condition in strong contrast 

 with the quick reactions of such animals as sea-anemones. These forms 

 respond to most stimuli within a second or so, the sponges only within 

 minutes. Moreover, in sponges transmission from the place where 

 the stimulus is applied to the responding muscle is possible only over 

 very short distances and is carried on at a very slow rate. Transmission 

 in Stylotella resembles very closely the kind of transmission seen in 

 ciliated epithelium. The successive beat of the cilia is dependent upon 

 an impulse which progresses from cell to cell in the epithelium at a 

 relatively slow rate and is neither purely mechanical nor nervous in its 

 method of propagation. It probably represents a primitive form of 



