THE AMERICAN NATURALIST [Vol. XLIV 



still in a very young stage, a mere myo-blast possessing 

 but one nucleus, at the time it begins to change. It is the 

 end furthest from the electric surface or negative pole 

 of the future electroplax that begins to enlarge first 

 into a club-shaped form and then into a disc which 

 continues to expand laterally and grow thinner until it 

 becomes an exceedingly thin plate. The nucleus of the 

 young myoblast or electroblast divides by amitosis into 

 many nuclei, which are distributed through the plate and 

 the nerve supply is large and ends in tiny discs that cover 

 the whole upper, negative or " electric " surface. 



In Gymnarchus, the only teleost whose histogenesis is 

 known, the electroplaxes of the adult fish are arranged in 

 eight long cylindrical masses, four on each side, and em- 

 bedded in the muscle of the tail. Each cylinder consists 

 of a row of the thick electroplaxes spaced apart by about 

 their own length of the jelly-like " electric connective 

 tissue" usually found in these organs. Each electroplax 

 confonns to the outline of the cylinder laterally and is 

 bounded anteriorly and posteriorly by surfaces from 

 which a few short blunt papilla? emerge. The electro- 

 plax consists of a central core of a fibrillar nature, the 

 fibrils being arranged in the form of approximate layers 

 and giving the appearance of a transverse striation of this 

 region. As will be seen below, this striation does not 

 represent the striation of the voluntary muscle of verte- 

 brates. 



This core is covered by a layer of undifferentiated cyto- 

 plasm of moderate thickness which contains the numerous 

 nuclei of the syncytium. The nuclei are arranged in a 

 single layer, but, since the central core does not extend 

 into the majority of the papilla?, the nuclei form a central 

 mass in these appendages. 



The nerve supply consists of several large medullated 

 fibers which approach each electroplax from the rear and 

 are easily seen to end on its posterior surface. Each of 

 the naked ultimate branches terminates in a knobbed end 

 plate which is imbedded in the substance of the outer 



