614 
F. H. EDGIEWORTH. 
no lateral attacliment. In stage E (fig. 21) the muscle has 
separated from the intermandibularis ; its ventral (inner) end 
has become attached to the ventral aponeurosis of the 
posterior digastric. 
In stage F (fig. 22) the muscle is more obliquely situated, 
the muscle-fibres passing from its posterior attachment to the 
posterior digastric forwards and outwards to the ventral edge 
of the mandible. A digastric muscle is thus formed. 
In an 8'5 mm. embryo of Ornithorhynchus (figs. 30, 31, 33), 
the hyoid bar, in a pre-cartilaginous condition, is a continuous 
structure passing from the stapedial portion above to near the 
middle line below. The upper, stapedial end abuts against the 
auditory capsule, which consists of aggregated mesoblast cells. 
There is no crista parotica and no latero-hyale. The hyoid 
muscles consist of a levator liyoideT and a hyoid ventral con- 
strictor s. styloideus, which are not quite separated from one 
another. The levator hyoidei lias no dorsal attachment ; it is 
inserted into the hyoid bar. The N. facialis passes back- 
ward lateral to the muscle. The hyoid ventral constrictor 
arises from the hyoid bar and passes down to a ventral apo- 
neurosis connecting together the muscles of the two sides. 
The hyoid muscles of Echidna, in stage 50 of Semon, con- 
sist of a levator hyoidei and hyoid ventral constrictor s. 
styloideus (fig. 37). The levator hyoidei arises from the 
outer end of the inturued crista parotica and is inserted into 
the upper end of the stylo-hyal cartilage. The hyoid ventral 
constrictor arises from the stylo-hyal cartilage just below the 
insertion of the levator hyoidei and passes downwards to a 
median ventral aponeurosis. 
The following is a summary of Futamura’s observations 
(1907) on the development of the hyoid muscles in the pig. In 
a 8‘4 mm. embryo their Anlage consists of an aggregation of 
myogenic cells which is continuous ventrally with its fellow 
across the middle line. It is penetrated by the seventh nerve. 
'■ The introduction of this term is explained later; the muscle is 
homologous with the stapedius of non-Monotreme Mammals, but never 
gains any insertion into the stapes. 
