on the development of the hypobranchial and laryngeal muscles in ...

146 F. H. Edgeworth
however, had stated (1892) that in an embryo Amphiuma (size not stated)
there were two pits behind the fourth cartilaginous gill-arch, i.e. five bronchial
segments. Marcus (1908) stated that in larvae of Hypogeophis seven gillclefts
are developed videe infra, p. 149), i.e. five bronchial segments.
In Menopoma and Ellipsoglossa seven gill-clefts are developed, the last
two behind the fourth bronchial bar and its muscles. No muscle-plate or
cartilaginous bar is developed in this fifth bronchial segment.
The sum of this evidence suggests that five bronchial bars with related
muscles existed in Amphibian ancestors, but that the fifth disappeared long
ago. Druner videe supra, pp. 129, 130) came to the conclusion that Urodeles
originally possessed a greater number of bronchial arches than four. The
evidence merits discussion. As regards the nerves found by him, it is possible
that they have relation solely to the sixth and seventh gill-clefts. As regards
the muscles, the phenomena of development do not bear out the theory that
the stump of the sixth gill-cleft separates a Transversus ventralis iv from a
Transversus ventralis v. Thus in Menopoma there is only one primordium
developed-that of Transversus iv. This, at first, lies solely in the fourth
bronchial segment, then under the stump of the sixth gill-cleft, then posterior
to it. The variation is due to the fact that, on the partial atrophy of the
sixth gill-cleft, the Transversus iv, the hind end of Subarcualis rectus iv, and
the fourth bronchial bar, migrate into the fifth bronchial segment; and, on
the atrophy of the seventh gill-cleft, still further back. There is a similar
progressive variation of the Transversus iv in relation to the sixth gill-cleft
in Ellipsoglossa, and of Transversus iii in relation to the fifth gill-cleft in
Necturus. The evidence, as regards muscle elements, is thus limited to the
discovery of a Levator arcus v on one side in one larva of Triton. I have not
seen this, however, in the many larvae of Menopoma, Ellipsoglossa, and
Triton crist. I have examined.
It is probable, therefore, that the case described by Druner is an instance
of fluctuation, possibly of atavistic fluctuation, from the usual number of
four bronchial bars and related muscles in Urodela.
In support of these conclusions, it may be added that in Lysorophusa
member of the ancestral Urodeles, from the Pennsylvanian deposits-
Sollas found only four bronchial bars, the first three consisting of ceratobranchial
and epibranchial elements, and the fourth of a train of fragments.
The case of Necturus, with only three branchial bars and related muscles,
probably comes under a different category. All Amphibia, with exception of
Megalobatrachus max. (adult), Necturus and Proteus, have four bronchial bars
and related muscles. Megalobatrachus max. has two branchial bars in the
adult state (Druner), but only the early stages of larval development have
been published hitherto-by de Lange. InNecturus the development, described
above, permits of comparison with Menoporna.
In Menopoma seven gill-clefts are formed, all of which reach the ectoderm.