1 68 Papers from the Marine Biological Laboratory at Tortugas. 



part of the myotome with its strongly developed outer layer {o.f.). The 

 vertebral fibers mentioned above are not visible, but their position in a few 

 following sections is indicated by a line one can imagine to be drawn 

 between the points marked with the figure . It has been considered 

 unnecessary to figure the myofibrils longitudinally under large magni- 

 fication. The cross-striation is very plainly visible and is the same in all 

 parts. Each of several regions of this embryo was examined and in all its 

 parts were the same conditions found. We may sum up by stating that the 

 embryo of 9 days age shows no electric tissue and but a very weak indication 

 of the development of such a tissue, all of the myotome cells being decidedly 

 of the muscular type. Our only evidence is drawn from future stages, as 

 to the position of such electric tissues and from the otherwise unexplained 

 precocity of the fibers in our 9-day embryo, in that same position. 



STUDIES OF AN EMBRYO TWELVE DAYS OF AGE. 



This animal (text-fig. 6) showed a considerable increase in size, not so 

 much in length as in thickness. The posterior part of the body of this 

 specimen was divided into the following pieces: First, from a short dis- 

 tance behind the anus to 14 

 vertebral segments further 

 caudad. Two-and-one-half 

 segments were cut ofif of this 

 by serial, transverse sections 

 from the posterior end (re- 

 gion ^ i) and the remaind- 

 er was cut into vertical, lon- 

 gitudinal sections from right 

 to left (region A). Second, 

 15 segments more were re- 

 moved and 3 segments of 

 the posterior end were again 

 cut as serial, transverse sections (region 5i) and the remaining 12 segments 

 were cut serially in horizontal, longitudinal sections (region B). Third, 14 

 segments were again cut off and the posterior 3 segments were cut trans- 

 versely (region Ci), while the remainder was cut in vertical, longitudinal sec- 

 tions (region C). The next 18 vertebral segments formed a piece that 

 was cut in vertical, longitudinal sections (region D), without any transverse 

 sections being made, and this left a small bit of somewhat curled tail-tip, 

 which was so young in development that its segments could not be easily 

 counted. This is cut transversely in series (region E). 



Study of this embryo may best begin by examining a transverse section 

 through the posterior part of the body (fig. 5, plate 2) to see what has 

 become of the inner parts of the myotomes. Looking first for the point 

 L.M. as seen before in figure i, plate i (marked with O), we can see at once 

 that it is present as a compact mass of muscle-like tissue now clearly 



Fig. 6. Diagram of body of an embryo of Gymnarchui 12 days 

 old. Lines and letters indicate the regions studied. For ex- 

 planations see text. (Copied from the same source as fig. 5.) 

 X about 3-2S. 



