I90 AYERS AND JACKSON. [Vol. XVII. 



Endoskeleton. 



The endoskeleton, excepting the notochord, is mainly mem- 

 branous. Here and there it is reinforced by cartilaginous 

 rods, bars, rings, and plates, which lie within the membranous 

 skeleton, and may be regarded simply as chondrifications along 

 certain lines of stress. No bone or calcified tissue of any 

 kind is present anywhere in the skeleton. 



The notochord (PI. XXII, Figs, i, 2, 4-6 ; PI. XXIII, Fig. 

 14, nt) is an elastic cylindrical rod extending nearly the entire 

 length of the body (14-26 inches). It lies in the median 

 dorsal line, deeply imbedded between the lateral trunk mus- 

 cles of the right and left sides. Anteriorly it tapers to a 

 point and ends between the parachordal cartilages. Posteri- 

 orly its termination lies in a groove upon the dorsal surface 

 of the median ventral cartilage, and its end lies a short dis- 

 tance in front of the posterior end of the spinal cord. The 

 shape of the notochord in cross-section varies somewhat in 

 different regions. Though approximately circular, it is usually 

 somewhat flattened or slightly concave above, where it is in 

 contact with the neural tube. In the extreme anterior region 

 it is laterally compressed and somewhat triangular in cross- 

 section. 



The notocJiordal tissue (PI. XXII, Fig. 3, nt) is made up 

 of cells containing very large vacuoles l(vc) filled with a clear, 

 homogeneous liquid, around which the protoplasm forms an 

 extremely thin layer. The chordal tissue has a honeycomb 

 structure which produces the characteristic reticular appear- 

 ance seen in sections. The cells are somewhat flattened, with 

 angular walls, and have their long axes placed radially, as 

 shown in Figs, i and 2 of PL XXII. The nuclei are prom- 

 inent, and generally found lying in the protoplasmic layer 

 toward the ventral side. In some cases, however, especially 

 toward the outer surface of the notochord, nuclei are found 

 near the center of the cells, and apparently connected with 

 the cell wall by protoplasmic strands. The striking resem- 

 blance between chordal tissue and certain vegetable tissues 

 has often been noted., 



