f a fish, appear. Then
a flap of skin outside the hyoid arch grows back to cover over the gills;
this is the operculum (op. in Figures 11 and 12, Sheet 22), and it
finally encloses them in a gill chamber, open only by a pore on the
left, which resembles in structure and physiological meaning, but
differs evidently very widely in development, from the amphioxus
atrium. At this time, the lungs are developing as paired hollow
outgrowths on the ventral side of the throat (Figure 12, L.). As the
limbs develop, and the tail dwindles, the gill chamber is obliterated.
The capillary interruptions of the gills on the branchial arches (aortic
arches) are also obliterated. The carotid gland occupies the position
of the first of these in the adult. The front branchial arch here, as in
all higher vertebrata, becomes the carotid arch; the lingual represents
the base of a pre-branchial vessel; the second branchial becomes the
aortic arch. The fourth loses its connection with the dorsal aorta, and
sends a branch to the developing lung, which becomes the pulmonary
artery. The third disappears. A somewhat different account to this is
still found in some text-books of the fate of this third branchial arch.
Balfour would appear to have been of opinion that it gave rise to the
cutaneous artery, and that the third and fourth vessels coalesced to
form the pulmocutaneous, the fourth arch moving forward so as to
arise from the base of the third; and most elementary works follow
him. This opinion was strengthened by the fact that in the higher
types (reptiles, birds, and mammals) no fourth branchial arch was
observed, and the apparent third, becomes the pulmonary. But it has
since been shown that a transitory third arch appears and disappears
in these types.

Section 18. The origin of the renal organ and duct has very
considerable controversial interest.* In Figure 13, Sheet 22, a
diagrammatic cross-section, of an embryo is shown. I. is the
intestine, coe. the coelom, s.c. the spinal cord; n.c. the notochord,
surrounded by n.s., the notochordal sheath, ao. is the dorsal aorta.
In the masses of somatic mesoblast on either side, a longitudinal
canal appears, which, in the torpedo, a fish related to the dog-fish,
and in the rabbit, and possibly in all other cases, is epiblastic in
origin. This is the segmental duct, which persists, apparently, as the
Wolffian duct (W.D.). Ventral to this appears a parallel canal, the
Mullerian duct (M.D.), which