THE MANDIBULAR AND HYOID MUSCLES OE MAMMALS. 575 
(which are proliferated from tlie intermandibiilaris) with tlie 
hyoid muscles. 
In stage A (just born) of Dasyurus (figs. 1 and 2 ), there is 
a cartilaginous ala orbitalis, orbito-parietal commissure, and 
- parietal-platte, continuous with one another. The ala temporalis 
forms a ventro-lateral process of the presphenoid region of 
the chondrocranium ; its free extremity turns forward. There 
is no line of demarcation between the processus ahiris and ala 
temporalis. The incus is a precartilaginous mass. Meckel’s, 
cartilage is a continuous cartilaginous structure, and has not 
divided into malleus and portion in front. The Anlage of the 
mandible is a group of cells dorsal to and extending down a little 
distance on the outside of Meckel’s cartilage. In this Anlage, 
dorso-lateral to Meckel’s cartilage, ossification is just visible 
in the region of attachment of the lateral muscle; anteriorly 
it is more marked and extends forward almost to the front 
end of the cartilage. Two masticatory muscles are present, 
medial and lateral. The medial muscle arises fi-om the ala 
temporalis ; its most anterior fibres pass downwards and 
outwards to the inner surface of Meckel’s cartilage, whilst 
the succeeding ones pass more and more obliquely backwards 
to the bar; the hindmost fibres are attached as far backwards 
as the malleus portion ; the fibres form one continuous sheet. 
The lateral muscle — the front end of which is anterior to that 
of the medial muscle — ai-ises from the orbito-parietal com- 
missure, passes downwards, and is attached to the upper 
border of the Anlage of the mandible. Both muscles consist 
of cross-striated muscle-fibres. The third division of the fifth 
cranial nerve passes downwards between the two muscles, 
giving off the ramus medialis to the medial muscle and the 
ramus lateralis to the lateral muscle. The intermandibularis 
forms a ventrally curved transverse sheet between Meckel’s 
cartilages ; it has a median raphe, and is attached laterally 
to the inner surface of the cartilage. The mylohyoid nerve 
passes down on its outer surface. 
In stage C (figs. 6 to 11) the medial muscle has separated 
into anterior and posterior portions. The former is the 
