176 Papers from the Marine Biological Laboratory at Tortugas. 



course can be somewhat better followed with thicker sections and deep 

 eosin staining than with preparations of the usual kind. 



As to the fibrils themselves, they have lost the transverse striation 

 altogether. By this I mean that it is no longer stainable. It seems that, 

 in the straight ends, with a good immersion lens one can see traces of this 

 striation, left as a thickening of the fibrils, at points that may represent 

 the previous position of the anisotropic M spindles. 



Whether the increase in the mass of the fibrillar core, which goes on as 

 the electroplax grows, is due to the thickening of the individual fibrils or to 

 the laying down of more and new fibrils or to the deposit of an interfibrillar 

 substance between them, was not decided. The first two conditions seemed 

 the most probable, because of the apparent absence of much interfibrillar 

 substance in the oldest and largest electroplaxes. 



The cytoplasm of the outer layer begins to be of interest at this stage. 

 Its most particular point of interest lies in the fact that it is decidedly 

 different in structure at its anterior end from its posterior end. This is 

 shown in its staining capacity as well as in its actual structure. In the 

 specimens stained in iron haematoxylin and eosin the cytoplasm at the 

 posterior end stains deeper than that at the anterior end, with both dyes. 

 It can also be seen to be granular in structure a sort of general granulation 

 with a few larger granules of a substance which stains somewhat like chro- 

 matin. In particular, its cytoplasm is darker than the fibrillar core at this 

 posterior end. 



As the cytoplasmic layer is examined in an anterior direction, it is seen 

 to stain lighter and at the same time to contain an occasional vacuole. This 

 condition increases until, at the anterior end, about one-fifth of the length 

 of the entire structure is covered with a cytoplasm which is so much vacuo- 

 lated that it appears as a delicate reticulum in the meshes of which the 

 nuclei lie. The fibril core extends out to the end or almost to the end, and 

 it can be seen that it is darker in color and denser than the cytoplasm 

 covering it. Some of the finer, granular material is found out as far as the 

 reticulated tip. 



A new element of interest begins to become apparent in this stage, and 

 that is the point of attachment of the nerve. The strong medullated 

 fibers come from the cord, pass through the spinal ganglion, and enter the 

 connective-tissue "tube" of the spindle at the level of the posterior third of 

 each electroplax. The fibers wind and turn considerably, in this vicinity, 

 and are finally applied to the surface of the electroplax over an area which 

 may be described in this specimen as its second sixth part from the posterior 

 end. Thus the extreme posterior end does not receive any nerve-endings 

 at this period. 



The cytoplasm shows a number of indentations where the nerve is ap- 

 plied and the axis-cylinders of the nerve can be traced into these spaces, 

 which they apparently fill with a club-shaped nerve-ending. This ending 

 can not be satisfactorily described until some of the special nerve methods 

 can be used to elucidate it. 



