Origin of Electric Tissues of Gymnarchus Niloticus. 



175 



considerable size and one in which the oldest tissues are almost the same, 

 for an understanding of the adult structure as those of a fully grown fish 

 would be. Text-figure 7 gives an outline of this specimen, which was about 

 63 mm. long and whose tail part was cut off and divided and sectioned as 

 follows: Beginning at about the level of the anus, a portion composed of 6 

 vertebral segments was sectioned in a vertical and longitudinal direction 

 (region A) ; then passing caudad, the next 2 segments were cut transversely 

 (region ^1); then the next 12 were cut longitudinally and horizontally 

 (region C) ; then the next 6 were cut trans versety (region Ci) ; then 13 others 

 were cut longitudinally and vertically (region D); then 7 more were cut 

 transversely (region Z>i) ; then the next 14 were cut longitudinally and 

 approximately vertically (region E), while the tip of the tail, composed of 

 some 12 or more segments, was sectioned transversely (region jEi). 



In the embryo of 12 days the youngest developmental stages of the 

 electric tissue were found in the most distal portion of the tail, which at that 

 time had but recently been extended by growth from the body and was still 

 in process of extension. The oldest and consequently the most-developed 

 electroplaxes were to be found in the anterior or cephalic end of the spindles. 



In the present embryo, or larva, of 42 days, the conditions are reversed, 

 and the electroplaxes in the extremity of the tail have passed those farther 

 up in the body in their differentiation, and have reached a much greater 

 and more complete development. Accordingly we will select for study one 

 of the most anterior and least developed examples and compare it with that 

 electroplax last studied, which is represented by figure 17, plate 6. 



Figure 18, plate 6, is drawn from one of the ventral electroplaxes of the 

 region C, as shown in text-figure 7. This position makes it fairly well forward 

 in the spindle, although portion B would have shown slightly younger stages. 



Fig. 7. — Diagram of body of an embn'o, or larva, of Gymnarchus, about 42 days old. 



Lines and letters indicate regions studied. For explanations see text. 

 (Copied from the same source as text-fig. 3.) X about i and 1.$. 



The fibril core will first attract our attention. The first noticeable 

 feature is that this core is growing in mass and volume all through the 

 electroplax, but also far faster in its center than at either end. Throughout 

 its course it has assumed a unified appearance which shows no trace of the 

 several fibril bundles which have gone to compose it. In the narrower ends 

 the mass is straightest and its component fibers appear most parallel, being 

 but slightly wavy. As we follow them from either end towards the middle 

 it can be seen that their course becomes more and more wave-like, until, 

 in the middle, they have been thrown into decided folds. Their actual 



