THE MANDIBULAR AND HYOID MUSCLES OF MAMMALS. 591 
cartilage. This is not present in tlie ral)bit and pig, being- 
passed over in the Anlage stage. 
The medial muscle or muscle-Anlage in Dasyurus, rabbit 
and pig, separates into three — from before backward, the 
internal pteiygoid, tensor veli palatini, and tensor tym]mni. 
The internal pterygoid is the first to separate, the division 
into tensor veli palatini and tensor tympani occurring later. 
The internal pterygoid of Dasynrns first forms a muscle 
passing from the ala temporalis to Meckel’s cartilage. On 
the occurrence of ossification it passes from the alisphenoid 
bone to the mandible and also gains an additional origin, ft-om 
the palate and pteryg'oid bones. This is also the case in 
Didelphys aurita. In the rabbit and pig, tlie muscle at 
first arises from the ala tempoi-alis and is inserted into the 
mandible — the stage of insertion iiito Meckel’s cartilage being- 
passed over. On ossification the muscle arises from the 
alisphenoid bone, with — in the case of the rabbit — an addi- 
tional head from the palate bone. 
In adult forms of Ornithorhynchus and Echidna there 
exists a muscle which was stated by Meckel and by Toldt to be 
homologous with the internal pterygoid of other mammals. 
Schulman stated that it is a true member — the caput auterius 
— of the temporalis group, as shown by its connection with 
the rest of the muscle and by its innervation by the N. 
temporalis profundus. This latter statement is confirmed by 
the evidence of embryos of Echidna in Stages 47 and 50 (of 
Semon) ; in these there is no muscle arising from the ala 
temporalis or (Mammalian) pterygoid bone and inserted into 
Meckel’s cartilage or mandible. An internal pterygoid is thus 
absent. 
An 8*5 mm. embiyo of Ornithorhynchus (fig. 80) shows 
that the anterior fibres of the median masticatory muscle 
descend vertically to Meckel’s cartilag-e, i . e . fibres are present 
homologous with those which form the internal pterygoid in 
Dasyurus. It may be concluded that this muscle atrophies 
in Monotremes, though whether its development in 0 r n ithor- 
hynchus advances a stage further than that just mentioned 
