1 66 Papers from the Marine Biological Laboratory at Tortugas. 



text-fig. 5) to be sectioned as follows: First, from in front of the anus to a 

 point about 26 vertebral segments posteriorly. The last three segments 

 of this portion were sectioned transversely and serially (region Ai) while 

 the remaining anterior part was sectioned vertically and longitudinally 

 (region A). Another portion of 19 vertebral segments further caudad was 

 removed and its posterior two segments sectioned transversely (regional), 

 while the anterior part was cut as before in vertical, longitudinal sections 



Fig. S. — Outline of body of an embryo of Gymnarchus nine days old. Transverse 

 lines and letters indicate parts sectioned for study. For explanations see 

 text. (Copied from Assheton in "The Work of John Samuel Budgett.") X 5. 



(region B). A third part, of i8 more vertebral segments, was treated in 

 the same way, except that no transverse sections were taken and the entire 

 piece was cut vertically and longitudinally (region C), while the remaining 

 portion or tail-tip was cut serially in transverse sections and forms a series 

 (region D). 



Figure i, plate i, shows a transverse section through the body a rather 

 short distance from the extremity of the tail or at a point where we are sure 

 that the electric organ will be well developed a little later in life. It may 

 be thought that such a section could be taken for study to better advantage 

 in a more anterior position on account of the earlier anterior development 

 just discussed, but it must be taken into account that the tail segments are 

 being added and are still growing rapidly at this age, that they are very 

 short and very crowded, and therefore the location of this section is, in 

 reality, fairly well forward in the future electric spindles. Conditions were 

 much the same in region C. 



This section shows a good development of muscle fibers as indicated 

 by the shaded area. As is usual in vertebrates, the most advanced stages 

 of muscle-cell development are to be seen at the lateral periphery of the 

 myotome. Here, at the point indicated in figure i , plate i , by the dotted line, 

 a layer of the outer muscle cells, two or three deep, has acquired an average 

 of about 26 myofibrils (an average of 14 counts). These are grouped in one 

 (fig. 2, plate I, B and C) or sometimes two bundles in the cell (fig. 2, plate i, 

 A) and their correct spacing and their thickness and staining power indicate 

 muscle cells of normal development and good functional activity. The 

 remaining and inner muscle cells of the myotome, forming its larger bulk, 

 show, as one examines them successively farther inward (toward the median 

 line), a series of earlier stages, until at many points on the inner edge of the 



