160 Papers from the Department of Marine Biology. 



There is no change in either the innervation or the circulation of this 

 embryo. 



The electroplaxes have now become very narrow and elongate and 

 are arranged in parallel rows, strictly parallel except at the extreme 

 edges or where a muscle passes through. Ventrally the surface is 

 completely broken up into papillae by the vacuoles, a few of which still 

 persist, forming bridges across from one to the other. The electric 

 layer has completely lost its vacuoles, but no black rods can be seen. 

 The striations of the nutritive layer have taken on an entirely new 

 appearance, differing but little from those in the adult. They no 

 longer pass straight across the cell, but are broken up into a large 

 number of small, curving striations, running in all directions and over 

 and under each other in a manner which gives the effect of a maze. 

 They are divided into groups, however, each group being composed of a 

 few very definite, sharp lines, strictly parallel to each other, but with 

 no relation to the lines of any other group. The lines of one group are 

 all in one curved plane. They may lie over or under another group, 

 but not between the lines of another group. The optical effect of these 

 lines is like that of normal muscle striations, there being a dark area 

 close to a light area (plate vi, fig. 3, adult). 



It can be seen in section that the curious infoldings and overlapping^ 

 of electroplaxes are beginning to form. Some of the electroplaxes run 

 along parallel to the rest for some distance and then drop down per- 

 pendicularly one, two, or three rows, when they turn and continue in a 

 straight line between two other electroplaxes. The electroplaxes be- 

 tween which they pass come up close to them so as to fill in the spaces, 

 but whether they have been cut in two by the dropping of the electro- 

 plax or whether it has merely passed between two shorter electro- 

 plaxes could not be determined. This peculiar phenomenon has not 

 been described for the electric organs of any other fish, but a possible 

 explanation for this lies in the fact that the electroplaxes of Astro- 

 scopus must turn as they grow in order to properly orient themselves 

 with reference to each other and to the body-surface, so that it is not 

 surprising to find that some of them have become twisted in this par- 

 ticular manner and remain so. In all the other electric fishes which 

 have been studied up to the present time, with the exception of Malop- 

 terurus, the electric tissues have been derived from muscles which are 

 so oriented from the start that the electroplaxes come off from them in 

 the exact position best suited for their purpose. They have, therefore, 

 only to increase in length and to multiply then* nuclei in order to form 

 the regular parallel rows which are invariably found in the adult organ. 



It is not mere chance, however, that the electric organs are formed of 

 parallel electroplaxes any more than it is by chance that the electro- 

 plaxes are long, narrow cells with a definite polarization. In order to 

 get an electric current the cells must be parallel, they must be polarized, 



