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Embryological

electric, organs, fishes, electroplax and parallel

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EMBRYOLOGICAL.

In 1877 Babuchin (4), in 1888 Ewart (49), and in 1894 Engelman (39) worked out the origin of the electric organs of Torpedo and the Mormyricke, then known as the pseudoelectric fishes, showing that, in general, electric organs have arisen by the modification of certain striated muscle-cells. In Torpedo and the RajidEe each electroplax arises from a single cell, but Babuchin has shown (2, 3) that in the Aiormyrithe a syncytium of cells from the fibers of the sacrolumbalis muscles may go into the formation of one electroplax. Dahlgren has shown that each electroplax probably arises from the union of all the muscle-fibers in one myotome. It was in the midst of these researches that Babuchin (2) came to the conclusion that, inasmuch as the origin, structure, and functional activity of the pseudoelectric organs, although diminutive and less highly differentiated than those of Torpedo and Gymnotus, nevertheless are fundamentally the same; therefore, "Es existieren keine pseudoelektrischen organe, es gibt nur grosse und starke, kleine und schwache elektrische organe." The term pseudo electrical organs has since been definitely abandoned and the Mor myridse and the Rajidle are known as the weak electric fishes.

In 1880 Fritsch (44) described in detail the electric organs of Malop terurus and startled investigators by attempting to show that the elec troplaxes were neither arranged in parallel series, as in all other electric fishes, nor derived from muscle-cells. He tried to prove that the organs in this fish are developed from certain gland-cells in the skin, an observation which aroused much discussion and which never has been substantiated. No investigator has been able, on the other hand, to prove the origin of the organs from any of the muscle-cells, so the mat ter remains an open question to this day, with a possibility of thus bringing Malopterurus into line with the other electric fishes. The peculiar structure of the electroplaxes, however, and the inability to harmonize the direction of the current with Pacini's law are facts which can not be denied, and Malopterurus stands as the great excep tion to the general rules for electric fishes. At any rate, Malopterurus has not sacrificed any of its motor muscles to the formation of the organs and its movements are not in any way hampered, while the electric coat, which completely surrounds the body, forms an effective protection as well as a means of easily capturing its food. Gymnotus and Torpedo, on the other hand, move so slowly that they have to numb their prey at some distance and then follow them up at their leisure. It is doubtful whether the weak electric fishes use their electric organs for either protection or for capturing food, although it is possible that they may capture in this way small crustacea, and other minute inver tebrates.

Recently Professor Dahlgren has become interested in certain Amer ican species of electric fishes. He has published several papers, one on

the anatomy and muscular origin of the electric organ of Gymnarchus (32), an African form distantly related to the Mormyridte, and several on the gross anatomy and habits of the star-gazer, Astroscopus (53, 34, 36), a marine teleost of the toad-fish group, a group hitherto unrepre sented among the electric fishes. In 1906 Dahlgren (36) and Silvester published an account of the adult Astroscopus. The authors had been interested in reports from Charles H. Gilbert and J. A. Henshall (Jordan 64, es), who reported having felt shocks from the two species Astroscopus guttatus and Astroscopus y-grcecum. Fishermen who were interviewed said that they had always known of the numbing power of these fishes and had often received shocks from stepping on them as they lay buried in the sand.

The organs of Astroscopus form two irregular, vertical columns, one just behind and somewhat under each eye, the muscles of which are embedded in the organ. Each organ is roughly oval in section and is composed of a number of parallel plates separated by electric connec tive tissue and made up of about 20 separate electroplaxes lying side by side. The number of plates in each organ does not exceed 200, but they are very thin and their constituent electroplaxes are deeply indented on the edges. The electroplax bends on itself so as to over lap on its own body at some points of considerable area, and so as to make it necessary for the overlapping portion to find room in the next layer in which to secure nerve and blood supply. From 3 to 5 of the larger electroplaxes form the central area of the layer and from 8 to 12 smaller ones are arranged around it and fill in the outline. The electroplax is placed in the organ with the electric layer upward and the nerve supply approaching it from above. The blood supply comes into contact with it on the lower or nutritive surface, which is evaginated into a number of papillte that occupy more than two-thirds of the thickness of the electroplax. The nuclei of these papilla? are oval and differ slightly both in size and shape from those of the electric layer, which are evenly arranged in a series. The striation of this layer is remarkable and quite as definite and conspicuous as in the weakest of the Rajidte (plate vt, fig. 3). It consists of fine, sharp lines spaced evenly and parallel to one another, but curved in many directions. In section one appears to be looking on various surfaces, the striations on one surface being parallel to each other but crossing above and below other sets of similar parallel striations. Dahlgren believes these lines to represent the edges of an equal number of curved and parallel sur faces, seen in actual or optical section. There is no variation in the distance between the lines, however, and no oblique or surface views of the striations appear (Dahlgren 36).

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