THE MANDIBULAR AND HYOID MUSCLES OE MAMMALS. 601 
zygoinatico-manclibiilaris is formed subsequently^ in stage H, 
by a separation of the ventral fibres of the temporal muscle, 
dorso-posterior to the masseter muscle. The fibres of the 
zygomatico-mandibularis have the same direction as those of 
the temporal, and up to stage J — the latest investigated — the 
separation was not complete. 
In the rabbit and pig the process is a little different. The 
Anlage of the lateral masticatory muscle is inserted by a 
forked lower edg’e into the mandible. A groove appears in 
the posterior edge of the external fork, and spreading upwards 
and forwards separates the common Anlage of the masseter 
and zygomatico-mandibularis from the temporal. This Anlage 
divides into masseter and zygomatico-rnandibnlaris, the latter 
being cut off from the upper posterior part of the common 
mass. In view of the many modifications associated with the 
prolonged intra-uterine development, it is probable that 
the phenomena occurring in Dasyurus more closely represent 
the phylogenetic history of the muscles than do those of the 
rabbit and pig*. Both masseter and zygomatico-mandibularis 
have their first insertion into the Anlage of the mandible. 
Neither has any temporary insertion into Meckefis cartilage. 
The detr aliens mandibulm is present only in Mono- 
tremes. No trace of it was seen in the developmental stages 
of Dasyurus, pig and rabbit. It is innervated by the fifth 
nerve (VYestling, Fiirbringer, Gaupp, Schulrnan). Schulman 
regarded it as belonging to the dorsal (in this paper regarded 
as lateral group of masticatory muscles as — like the tem- 
poral;^ external pterygoid, masseter, and zygomatico-mandi- 
bularis — he found it to be supplied by the dorsal s. lateral 
division of the mandibular nerve. 
Owing to want of material its development could not be 
completely followed in either Ornithorhynchus or Echidna, but 
the stages described above permit of an approximate account. 
The muscle can be identified with the hinder fibres of the 
lateral masticatory muscle in 8*5 mm. embryos of Ornithor- 
hynchns — those fibres which pass downwards and forwards 
towards the outer surface of the hind end of the Anlage of 
