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

138 F. H. Edgeworth
extended forwards to Meckel's cartilage and separated into Genio-hyoid and
Sterno-hyoid. The hind end of the Genio-hyoid has grown backwards a little,
ventral to the Sterno-hyoid. In a larva of 18 mm. (fig. 38) (in which the
sixth gill-cleft is still continuous with the ectoderm) the Subarcualis rectus iv
has begun to grow forward. Its hind end is continuous with the lateral end
of the Transversus ventralis iv which has begun to grow transversely inwards
(fig. 39). In a larva of 19 mm. Subarcualis rectus i has grown forwards to the
second gill-cleft, Subarcuales obliqui ii and iii have grown forwards and downwards
and meet, laterally to the Sterno-hyoid (fig. 42). The fourth bronchial
bar now passes from the fourth bronchial segment backwards, outside the
stump of the sixth gill-cleft on the left side, into the fifth bronchial segment,
and then upwards; i.e. on the total or partial atrophy of the sixth gill-cleft
it bulges backwards into the fifth bronchial segment (figs. 43, 44). Correspondingly,
the hind end of the Subarcualis rectus iv and the lateral end of
the Transversus ventralis iv have shifted back into the fifth bronchial
segment (figs. 43, 44). The front end of Subarcualis rectus iv has grown forwards
into the second bronchial segment. The Transversus ventralis iv has
now further developed, and passes transversely inwards to the middle line,
on the left side under, and behind, the stump of the sixth gill-cleft. In a larva
of 22 mrp. the Urobranchiale has developed (see later, p. 141), and the hind
end of the Genio-hyoid has grown further back. In a larva of 24 mm., where
the seventh gill-cleft has disappeared, the hind end of Subarcualis rectus iv
and the lateral end of Transversus ventralis iv are relatively further back,
with the result that the anterior edge of Transversus ventralis iv is oblique
and lies, on the left side, posterior to the stump of the sixth gill-cleft (figs. 48,
49). The anterior end of Subarcualis rectus iv (a) reaches the first bronchial
bar, whilst off-shoots (b and c) are given off to the second and third bronchial
bars (fig. 51). The Subarcuales obliqui ii and iii unite and pass to the sheath
of the Sterno-hyoid.
There is little further change in the Subarcuales; in a 32 mm. larva the
ventral end of the second gill-cleft is shallower and the anterior end of Subarcualis
rectus i is attached to the Ceratohyale. Transversus ventralis iv
gradually spreads backwards, forming a broad sheet; in a 34 mm. larva its
posterior edge underlies the Laryngeus ventralis. In the adult (Druner) the
muscle underlies the trachea.
Miss Platt (1897) stated that in Necturus Subarcualis rectus i is developed
from the ventral end of the mesothelial tissue of the glosso-pharyngeal arch.
Subarcualis obliquus ii grows forwards from the mesothelium of the first
vagus arch near the point where this tissue joins the wall of the pericardium.
Subarcualis iii (a and b) arises as a single muscle from the wall of the pericardium
in the region where the mesothelium of the second vagus arch unites
with the pericardial wall.
She did not mention the Transversus ventralis iii, nor state how many
gill-clefts are developed, but the figures given show five.
My observations in regard to the Subarcuales of the first two bronchial