408 E. W. MACBRIDE. 
possible to conceive of as originating from epithelium. Here 
again we are astonished by Professor Spengel’s strange ideas 
about Vertebrate ontogeny. In Amphioxus the notochord 
is a mid-dorsal strip of hypoblastic epithelium con- 
stricted off by the meeting of lateral grooves, exactly 
in the same manner as the hinder part of the noto- 
chord is formed in Balanoglossus. Further, accord- 
ing to Balfour,' the first changes which occur in it are 
the production of vacuoles in the cells and the forma- 
tion ofacuticular sheath, exactly comparable to the 
thickened “border membrane” (Grenzmembran) in 
Balanoglossus. Spengel says that there is no evidence of 
the formation of skeletal products from this membrane in 
Vertebrata. The answer to this is that the greater part of the 
thick notochordal sheath in Amia is formed from the thickening 
of this membrane, and that, indeed, it is only in virtue of 
possessing such a firm membrane that a mass of vacuolated 
cells can function as a supporting structure at all. The later 
changes which occur in the notochord of Amphioxus, viz. the 
interposition of discs of cuticular substance between the cells, 
are not without parallel in some species of Enteropneusta. In 
Balanoglossus Kupfferi the lumen of the neck is occluded 
and its tissue broken into pieces by invading masses of cuticular 
substance. We can, I think, go even further, and maintain 
that in the chondroid tissue we have the first trace of 
amesoblastic sheath. Spengel maintains that this tissue is 
fundamentally different from cartilage, since in the latter the 
cells are derived from a solid blastema, whereas in the former 
we have a number of solid cellular outgrowths from the 
celomic wall. This appears to me to be a difference of detail 
rather than of principle, more especially as Hatschek! has 
shown that the solid sclerotome of the Elasmobranch embryo 
is represented by a hollow diverticulum in the larva of Am- 
phioxus. Figs. la, 6, and ec in Pl. 30 are designed to show 
that the arrangement found in Balanoglossus is to some extent 
1 «Text-book of Comparative Embryology,’ vol. ii. 
? Hatschek, “ Ueber den Schichtenbau von Amphioxus,” ‘ Anat, Anz.,’ 1888. 
