710 The American Naturalist. [August, 
Against'the first view it is to be urged that the tubular stage ot 
the notochord does not appear actually, but is assumed for lower 
vertebrates, because the chorda appears in them as a groove and 
as afterwards separated off, and although it is then a solid rod of 
cells it has been considered to represent an epithelial tube ; more- 
over, this stage occurs after the notochord is permanently sepa- 
rated from the entoderm, and finally the whole of the rod of cells 
(or walls of the chordal tube) participates in the formation of the 
notochord. The mammalian “chorda-canal” is a true tube, 
which the notochord is not; it fuses with the entoderm, and the 
true notochord is separated off subsequently, only the dorsal 
part of its walls produces notochordal cells. These characteris- 
tics are, on the other hand, precisely those which belong to that 
hinder portion of the archenteron which we call in other verte- 
brates the blastoporic canal ; this canal produces the notochordal 
cells from its dorsal wall; it passes through a mass of cells, and 
lies in Sauropsida (the vertebrates nearest the mammals) above 
the archenteric cavity proper. Against the second view—E. Van 
Beneden’s—that the “ chorda-canal”’ is the archenteron, it is to be 
remarked that the archenteron is bounded above by entodermal 
cells and below by the entodermal yolk, and the representative of 
the yolk is not to be found in the lower wall of the “ chorda- 
canal.” Hence it seems to me clear that Minot’s view is the only 
defensible one, for the chorda-canal agrees in its essential features 
with the blastoporic canal of vertebrates, and only with that 
canal. 
If this homology is correct then the canal must lead into the 
archenteron; hence the large space within the inner layer must 
be homologous with the archenteron, because the chorda-canal. 
opens into it. This leaves us still entirely in the dark as to how 
the development of the mammalian entodermal canal is to be 
homologized with its development in other vertebrates. 
According to the view I have advocated, the blastoporic canal 
of mammals is peculiar in persisting for a long time as a separate 
canal above the archenteron, and then losing its lower wall along 
_ a considerable stretch at once; in other vertebrates it loses in front 
as fast as it grows behind, so that it is always short. 
