HISTOGENESIS OF ELECTRIC TISSUES 113 



ronarce is similar to an electroplax of Raja which has become wider 

 and very much thinner, so thin that the striated layer is obliterated and 

 the others reduced to a minimum. 



LITERATURE 



ENGELMANN. " Die Blattschicht, etc., der gew. Rochen," Arch. f. PhysioL, Pfluger, Band 



LVII, t. 2, S. 149. 

 BALLOWITZ. "Uber den feineren Bau des Elektrischen Organs des Gewohnlichen Rochen," 



Arch. f. mik. Anat., Band XLII, 1892. 



ONTOGENETIC DEVELOPMENT OF THE ELECTROPLAX IN ELAS- 

 MOBRANCH FISHES 



A study of the developing electric organ in a skate is most illuminat- 

 ing as to the real significance of this tissue. It has been worked out in 

 a common form of skate, Raja batis, by Dr. 

 J. C. Ewart, and the following description is 

 drawn from that paper and the paper by 

 Englemann. 



The young embryo of this skate has no 

 indication of any electric tissue. The place 

 that will be occupied by this organ, a little 

 later in the development, is filled by the 

 young muscle fibers that differ in no 

 visible way from those about them (Fig. 

 107, A). 



In an embryo of about 7 cm. the first 

 appearance of the development of electric 

 tissue is a swelling of the anterior ends of 

 the muscle fibers in the centers of the future 

 electric spindles (Fig. 107, B). The nuclei 

 have increased in number in this enlarged 

 part of the fiber, and some of them appar- 

 ently have migrated from among the muscle 

 fibrils and come to lie in the undifferentiated 

 cytoplasm between the fibril-bundle and the 

 sarcolemma (by sarcolemma in this case is 

 meant a cell-membrane which is somewhat 

 more evident in young muscle cells and in 

 some electroplaxes than in most older 

 muscle fibers). These changes first occur in 

 the fibers that occupy the central axis of the future spindle-shaped organ 

 and then in successive outer shells of the fibers until the organ is com- 

 pleted. All future changes occur in the same order. 



FIG. 107. A-D. Four stages in 

 the development of an electro- 

 plax from a muscle-like electro- 

 blast in Raja batis. A is in all 

 respects like a muscle fiber; B 

 shows an enlargement of the an- 

 terior end; C and D show the 

 steps which practically complete 

 the process. (After J. EWART.) 



