THE .AIANDir.Ul.AR AND HY01J:> AlUSCLES OF AIAAIAIAES. 593 
pterygoid bone, bat grows round the hamulus into the soft 
palate ; its distal end does not gain any insertion to either 
Meckel’s cartilage or the tympanic bone, but becomes attached 
to the tensor tympani by tendon. In Dasyurus, Didelphy s, 
rabbit and pig, the development of the hamulus is synchronous 
with the ingrowth of the muscle into the soft palate. 
On comparison of the descriptions of the pterygo-tympanicus 
given by Kostanecki, Schulman and Lubosch, those of the 
tensor veli palatini given by Kostanecki, and the embryo- 
logical phenomena described above, it ma.y be concluded that 
the muscle is derived from one taking origin from the ala 
temporalis and inserted into Meckel’s cartilage. This stage 
is not present in any of the animals investigated, but its 
existence, in phylogeny, tnay be inferred from the phenomena in 
early stages of Ornithorhynchus and Dasyurus, i.e. an 
undivided medial muscle passing from the ala temporalis to 
Meckel’s cartilage, and after separation of this into parts, the 
insertion of the pterygo-tyrnpanicus,in Echidna and Dasyurus, 
into Meckel’s cartilage. 
On the occurrence of ossification the muscle extended from 
the (Mammalian) pterygoid bone to the tympanic bone. In 
Dasyurus this stage is present as regards the origin of the 
muscle but insertion into the tympanic bone is passed over ; 
in the rabbit it is present as regards the insertion but passed 
over as regards the origin ; in the pig neither attachment 
occurs. 
The Anlage of the tympanic bone in Dasyurus is formed 
as a straight rod ventro-lateral to the hinder part of Meckel’s 
cartilage, with its anterior part overlapped by the Anlage of 
the mandible; subsequently an outgrowth downwards and 
backwards occurs, originating just behind the hind edge of the 
mandible. This method of formation is in harmony with the 
theory of van Kampen that the tympanic bone was primarily a 
covering bone for the hinder part of Meckel’s cartihige, and 
only subsequently — on development of a squamoso-mandibular 
jaw joint — became a part of the wall of the tympanic cavity. 
He suggested that it was derived from the supra-angulare or 
