Page:Popular Science Monthly Volume 84.djvu/127

Rh 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 Rakowitza.

The evolution of nerve and muscle, so far as this problem can be attacked in such lowly form as the sea-anemone and other cœlenterates, is a question about which there has been much difference of opinion. As early as 1872 Kleinenberg showed that in the fresh-water coelenterate, 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 neuromuscular cells of Kleinenberg were in reality simply epithelio-muscle cells and were without nervous significance. These investigators, in opposition 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 paragraph. 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