THE MANDIBULAR AND HYOID MUSCLES OF MAMMALS. 633 
ill stage F, ami are well marked in stage H ; the depressor 
auris, as stated above, is developed from the sphincter pro- 
fundus; all the other muscles are developed from theplatysma 
sheet, which is solely present behind the ear, and in front 
solely present above the zygomatic arch. They consist ot‘ 
sculutaris, post-auricularis, auriculo- occipitalis, three little 
muscles on the outside, and two — the transversus and obliquus 
— on the medial side of the cartilage. 
These developmental phenomena in Dasyurus confirm the 
opinion of Boas and Paulli that the orbicularis palpebrarum 
is derived from the platysma sheet, the opinion of Ruge that 
the buccinator is derived from the sphincter profundus, and 
show that the maxillo-labialis is derived from the sphincter 
profundus. 
In an 8'5 mm. embryo of Ornithorliynchus the Anlage of 
the sphincter superficialis and platysma muscles is present 
in the hyoid segment outside the hyoid ventral constrictor 
(figs. 29-33). It has spread forwards into the mandibular 
ses:nient, and backwards into the neck. It shows sliofht 
indications of separating into the sphincter superficialis and 
platysma sheets. 
In 14 mm. embryos of the pig there is no trace of the 
Anlage of the sphincter profundus and platysma muscles ; 
the space between the outer surface of the Anlage of the 
stapedius and hyoid ventral constrictor and the epiblast is 
packed with mesoblast, in which no trace of differentiation 
can be seen. In 15 mm. embryos (figs. 56, 58) the Anlage 
of the sphincter profundus and platysma can be seen in the 
ventral part of the hyoid segment, close to the epiblast, and 
separated by an interval filled with indifferent mesoblast cells 
from the Anlage of the hyoid ventral constrictor. 
The auriculo - mandibularis muscle — either muscular or 
tendinous — is present in many Mammals. It takes origin 
from the cartilaginous portion of the external auditory meatus 
or cartilage of the outer ear, and is inserted into the ramus 
ascendens of the mandible (Bijvoet). It is innervated by the 
N. facialis, either by a branch from the rami auriculares 
