632 
Y. H. EDGEWORTH. 
muscles iu Stages A and B of Dasyurus. Indifferent meso- 
blast cells lie between the lateral surface of the hyoid ventral 
constrictor and the epidermis (fig. 5). In stage C (fig. 13) 
the Anlage of the sphincter profundus and platysma muscles 
is developed in this mesoblast, about midway between the 
lateral surface of the hyoid ventral constrictor and the 
epidermis, and extending dorsally lateral to the submaxillary 
gland. No evidence is afforded by the sections that these 
embryonic muscles are proliferated or delaminated from the 
hyoid ventral constrictor — they are developed in the sub- 
epithelial mesoblast. In stage D (fig. 18) the embryonic 
muscle-cells are better developed and have spread forwards 
into the mandibular segment. In stage E (figs. 20 and 21) 
they have spread still further forwards in the mandibular 
segment, and down the neck. In the mandibular and hyoid 
segments they show evidence of division into two layers — 
an inner, of cells elongated in the transverse plane, the 
sphincter profundus ; and an outer, of cells elongated in the 
longitudinal plane, the platysma. In stage F (figs. 22 and 
23) these two sheets are more widely separated, and the 
platysma has spread dorsally over the temporal muscle and 
forwards, forming the Anlage of the sphincter palpebrarum ; 
whilst the sphincter profundus does not extend dorsally 
above the level of the zygomatic arch. The sphincter 
profundus is not developed behind the external ear ; its hind- 
most fibres form the depressor auris s. pars auricularis of 
the constrictor profundus. The Anlage of the auriculo- 
mandibularis is formed as a forward and downward growth 
from the platysma sheet, opposite the incus (fig. 23). In 
stage H (fig. 28) and in the 10 mm. stage of Didelphys 
aurita (fig. 39) it has separated from the platysma and 
extended forwards and backwards; its anterior end is 
attached to the posterior edge of the mandible and its 
posterior end to the internal surface of the aural cartilage. 
In stage H the buccinator and maxillo-labialis are formed 
from the sphincter profundus. 
The muscles of the external ear begin to be differentiated 
