Origin of Electric Tissues of Gymnarchus Niloticus. 171 



of the future electroplax, however, a large cell is seen which has only one 

 fibril bundle and four nuclei. It is possible here that this was one cell and 

 that it is growing in size and multiplying its nuclei by amitotic division. 

 As this is the same process which goes on in ordinary muscle cells of this 

 age, it is not surprising to find it going on here, and in older specimens we 

 shall find it the rule. The cytoplasm of all these electric cells is abundant 

 at this stage and is dense staining with the acid dyes. 



As compared with figure 11, plate 3, the next illubtration, figure 7, plate 

 2, is most interesting and is a step of some magnitude in the development 

 of the electroplax. The cytoplasm of such cells as compose this young 

 electroplax is all united into a single mass and the relation of nuclei to fibril 

 bundles is completed. The nuclei are always peripheral and the fibril bun- 

 dles appear to form a single central mass. This is the permanent condition 

 which will obtain throughout the life of the organ, and is also the condition 

 common to some other electroplaxes, as, for instance, Raja and Mormyrus. 



Just how the several fibril bundles become massed as a single bundle is 

 not to be positively stated at this time. The individual bundles can hardly 

 be imagined as moving together through the cytoplasm. It is probable 

 that some of the several bundles as seen in figure 11, plate 3, are lost and 

 absorbed, but it is not probable that all but one are so removed. The central 

 bundle in figure 7, plate 2, looks large enough to be composed of several, 

 such as are seen in figure 11, plate 3. The best explanation is that several 

 remaining bundles are moved toward each other by growth currents in the 

 cytoplasm, or by the absorption of material which lies between them. At 

 the same time, of course, the peripheral cytoplasm is growing in mass and 

 all nuclei tend to remain in this external layer. We shall see later that a 

 very few nuclei are left behind in the central fibi'ous mass in some electro- 

 plaxes. 



In figure 7, plate 2, we see, plainly and indubitably, the first form of 

 the electroplax as found in older fishes. 



The connective tissues which surround the electroplax are becoming 

 more decided in figure 7, plate 2. Also blood-vessels and pigment-cells are 

 oftener seen as in this drawing. One muscle cell, with its myofibrils clotted 

 into several irregular masses and its nucleus in an advanced stage of dis- 

 integration, is seen just between the lower end of the pigment-cell and the 

 electroplax. 



It will be well at this point to examine some of the longitudinal sections 

 of these early stages, in order to make clear several points which can not 

 be so well studied in the transverse views. 



Figure 12, plate 4, is a low magnification picture (X 140) of 6 segments 

 of the caudal part of the body at region D in this 12-day embryo. Only 

 the dorsal part of the body is shown, where a fortunate slant of the section 

 has permitted the knife to pass through both the dorsal and the upper 

 median spindles at the same time. The drawing is an accurate projection 

 from three different sections, so that all parts of each spindle might be 



