Origin of Electric Tissues of Gymnarchus Niloticus. 177 



Another and more simple step in development is indicated in figure 19, 

 plate 6, which was taken from an electroplax in a dorsal spindle of this 

 embryo at the region E (see text-fig. 7). 



Two points of interest will be spoken of in connection with this stage. 

 The shape has changed as follows: The middle part has both actually and 

 comparatively widened over the breadth shown by the middle part of the 

 electroplax seen in figure 18, plate 6, and this increased width is due to a 

 broadening of the fibrous core alone, the outer nucleated layer remaining 

 the same. 



Accompanying this widening is also an actual shortening of the structure. 

 Thus, part of the increased bulk of the middle is due to an absorption of the 

 two ends. Apparently the anterior end suffers the greater amount of 

 absorption, for it is usually shorter and thinner than the posterior. In 

 this connection we must remember the previous vacuolization of this 

 anterior end as seen in figure 18, plate 6. It would appear that the vacuoli- 

 zation was part of an absorption, or rather of an atrophic process. Con- 

 siderable traces of it remain in figure 19, plate 6, and it is also still noticeable 

 that the anterior end does not stain deeply. The posterior end retains its 

 length and vigor to a greater degree. It seems to become considerably 

 narrowed, however. 



The form of the electroplax also begins to show a marked change through 

 the development of spurs, branches, or papillse which begin to protrude from 

 both its ends, particularly from its posterior end. Some indication of this 

 was visible already in figure 18, plate 6, as small lumps or "shoulders" that 

 marked the organ roughly into three parts anterior, middle, and posterior 

 thirds. These growing spurs begin to give the middle third a distinct 

 truncate or cylindrical form. The nerve fibers also attach themselves to 

 the sides of the growing papillae, particularly to their bases. The papillae 

 show an inclination to grow out of the sides of the posterior third. 



We will now pass to the last and oldest stage of the electroplax which 

 can be found in the embryos which Budgett collected in Africa. This is 

 found in the E region of the 42-day embryo or larva of Gymnarchus and is 

 represented in longitudinal sections under a low magnification by figure 23, 

 plate 9, which shows parts, or the whole, of about 1 1 electroplaxes lying in 

 place in the ventral, lower middle, and upper middle electric spindles of 

 this region. Figure 21, plate 8, also shows a transverse section just cephalad 

 of this point, in the D region, where cross-sections of the 8 electric spindles 

 reveal partial or complete transverse sections of 7 electroplaxes, but only 

 the electric connective tissue which fills the tube at this point, in the eighth. 



Figure 20, plate 7, represents a longitudinal section of one of the electro-' 

 plaxes shown in figure 23, plate 9, and will serve as a basis for our last study 

 of this series. 



The form has continued to follow the development which was indicated in 

 figure 19, plate 6. The total length of the structure has shortened somewhat 

 more, while its width in the middle part has increased twofold. These 



12 



