752 MR FRANK J. COLE 
nuclei in the elastica externa, and in Bdellostoma AyERS and Jackson describe parts 
of it as exhibiting a “‘ distinctly fibrous structure.” This is also the case in Myxime in 
the isolated elastica, as shown by v. Exner, the fibres being circular and spindle-shaped 
and closely packed without any spaces. AYERS and Jackson state that no part of the 
chorda is a cuticular product—a conclusion previously emphasised by v. EBNER. 
There is no elastica interna. 
Notochordal Sheath (nt. sh.**).—This consists typically, but not everywhere, of — 
three perfectly distinct sheets—the external, middle, and anternal layers of the noto- 
chordal sheath. According to G. Rerzius, all three agree in structure and also exhibit 
the same micro-chemical reactions, and constitute the true chordal sheath traversed by 
perforating tubules (?), representing one sheath only, as first pointed out by KOLLIKER. 
As shown by v. Eswnrr, all three layers consist of non-cellular fibrillee coursing 
transversely in large undulating curves round the chorda. The curves in the three layers 
do not correspond, or rather those of the middle layer do not correspond with the 
other two, thus emphasising the boundaries between the layers. The bends of the 
curves always corresponding, no matter how they may be directed, linear effects are 
produced in the isolated sheath, and in this way we may distinguish a dorsal, a ventral, 
and a paired lateral longitudinal line. Of the three layers, the external (nt. sh.) is 
usually as wide as the other two together, whilst the internal layer (nt. sh.*) is always 
the weakest. v. EpnER has shown that in the tail, where one of the layers is suppressed, 
it is the middle one (wt. sh.”). Stained with methyl-blue-eosin the external and 
internal layers stain a faint pale-blue, whilst the middle layer is sharply contrasted 
in pink. 
Chordal Epithelium (ch. ep.).—This is very greatly reduced, and in this respect may 
be compared with Bdellostuma (Ayurs and Jackson) and Petromyzon (v. Epnmr). 
It consists of a very thin layer of granular protoplasm applied to the internal layer of 
the notochordal sheath, and which is raised up at intervals into small heaps, each 
lodging a nucleus. There is no observable division into cells, apart from the heaping 
arrangement, nor is there more than one layer of the nuclei, in which I[ confirm 
G. Rerzrus and v. Hepner. The chordal epithelium is connected with the chordal cells, 
or rather the walls of the latter are opposed to the epithelium. In the small 6°5 em. 
and 10 cm. Hags the heaping arrangement is wanting, and the nuclei, as one would 
expect, are very close together—+.e. a typical epithelium exists. The chordal epithelium 
cannot, of course, be morphologically separated from the chordal cells; both form one 
coherent tissue. This is more evident in Bdellostoma, according to AyrRs and 
JACKSON, where the epithelium may be two or three layers deep, and transitional cells 
are found connecting it with the vacuolated chordal cells. . 
Chordal Cells (ch. c.).—The entire mass of the so-called chordal “jelly ” consists of 
vacuolated nucleated cells. The size of these cells varies in different individuals and 
at different regions of the chorda, but generally there is a narrow zone of very small 
cells associated with the chordal epithelium, and they then rapidly increase in size 
