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

154 F. H. Edgeworth
Urodela which no longer have a metamorphosis. Menopoma would be similar
if development ceased at the stage of 24 mm.
Gdppert (vide p. 132) held that the form ofthe arytenoids, and the attachment
of the Laryngei to them, in Necturus and Proteus represents the primitive
condition. But the developmental phenomena appear to negative such a view
and to show that, as DrUner thought, the condition is secondary to a more
primitive one, in which, as e.g. Triton, the arytenoids are narrower and the
Laryngei take origin from the Dilatator.
In Siren (vide Druner) there are, in front of the Dilatator, Laryngei,
arising partly from the arytenoid and partly from the Dilatator, but no
Constrictor. There are, however, outside the Laryngei, sphincter fibres,
which have no homologues in other Amphibia. The condition needs embryological
investigation-possibly the sphincter fibres are proliferated or separate
from the Laryngei.
From the slightly different innervation of the Dilatator laryngis and of
the Constrictor laryngis and Laryngei Druner inferred videe supra, p. 134) that
these muscles have been derived from different segments. This is not borne
out by developmental phenomena. Further, in Anura and Gymnophiona all
the laryngeal muscles are innervated by the N. recurrens. The phenomena
in Urodela are simply due to a slightly earlier giving off of the branch for
the Dilatator from the N. intestino-accessorius, and have no morphological
importance.
Cartilago lateralis. Anura. Martens (1895) stated that in Rana temp.
there is no continuous cartilage lateralis which afterwards separates into
cartilage arytenoidea and C. laryngo-trachealis, but that these elements are
separate from the first, the arytenoid developing during metamorphosis, and
the four elements which fuse to form the annulus shortly afterwards.
I find that the arytenoid is present in a 12 mm. larva, in a precartilaginous
condition, quite distinct from the surrounding mesoblast. There is no trachea
at this stage, and during the rest of larval life the larynx immediately bifurcates
into the two bronchi (fig. 17). In a larva during metamorphosis, at the
stage when the tail has shrunk to half its original length, the trachea develops
and the elements of the annulus appear. They are continuous with the dorsal
end of the obliquely placed arytenoid by precartilaginous tissue (figs. 21 and
22).
Gymnophiona. In a 3-5 cm. larva of Ichthyophis the trachea is already
formed. There is a precartilaginous arytenoid, which anteriorly is circular in
shape, then broadens (with attachment of the Dilatator laryngis to its outer
edge, and the Laryngei above and below) and then contracts. It is continuous
posteriorly with a cellular sheath, thicker dorsally than ventrally, round the
trachea. In a 5-9 cm. larva the arytenoid has chondrified and is continuous
with a cellular sheath round the trachea. In this sheath are incomplete ringshaped
patches of cartilage.
In a 7 cm. larva of Siphonops the conditions are similar to those of the