No. 2.] MORPHOLOGY OF THE MYXINOIDEI. igi 



The central axis of the notochord is marked by a dense 

 white fibrous central core (Figs, i, 2,fc) which extends through- 

 out almost its entire length. The form and size vary some- 

 what in different regions. In the head region the core is 

 tri'radiate in cross-section (Fig. i). In the gill region it is 

 flattened dorso-ventrally, while farther back in the body region 

 it becomes more irregular, and relatively much larger. The 

 central core is made up of dense, coarse fibers, closely packed, 

 and running for the most part longitudinally. The fibers run 

 out among, and are continuous with, the walls of the chordal 

 cells. In the extreme anterior region of the notochord, and in 

 some places near the posterior end, the fibrous core fades out 

 and passes gradually into ordinary chordal tissue. The core 

 contains no traces of nerves or blood vessels. 



Surrounding the chordal tissue we find a cellular sheath 

 having the appearance of an epithelial layer (Fig. 3, ex). In 

 the anterior notochordal region, where the cells of this sheath 

 are larger and more definitely marked, they form a one-celled 

 layer, the cells being somewhat cubical above the notochord 

 and columnar below. Toward the posterior region the layer 

 becomes two or three cells deep, and the individual cells are 

 smaller and flattened. The nuclei are large, and vacuoles are 

 often found in the ends of the cells lying next to the chordal 

 tissue. This layer is directly connected with the chordal cells, 

 and is not separable as a distinct layer. All stages are found 

 between the solid, undifferentiated cells of the external cellu- 

 lar sheath, and the deep chordal cells within walls enclosing 

 large vacuoles. 



The notochordal sheatJi proper (PI. XXII, Figs. 1-4, sh) is 

 a thick, strong investment immediately surrounding the noto- 

 chord. It is fibrous rather than laminated in structure, at 

 least for the most part. The fibers are chiefly circular in 

 direction, but vary somewhat in different regions. They are 

 usually so closely packed as to give a homogeneous appearance 

 to the sheath. No nuclei, nerves or blood vessels are present. 

 In most places the notochordal sheath seems to be made up 

 of different layers, whose fibers vary somewhat in direction. 

 Three layers may usually be distinguished, especially in the 



