578 
V. H. EDGEWORTH. 
arise from this bone, and the upper fibres of the temporal 
also from the parietal bone. The exteimal pterygoid has two 
heads, separated from one another Ijy the buccal nerve. dTe 
lower portion of the temporal is beginning to be separated off 
as the zygomatico-mandibnlaris. The internal pterygoid has 
become partially separated into two portions, one head— the 
original one — arising from the alisphenoid bone, the other 
from the palate bone ; the latter head, in stage J, additionally 
arises from the pterygoid bone. The proximal portion of the 
tensor tympani, i. e. that attached to the pterygoid bone, has 
begun to disappear, and the proximal end of the persisting 
distal portion lies alongside the distal end of the tensor veli 
palatini which has lost its attachment to Meckelhs cartilage. 
This process continues, so that by staged (fig. 28) the adjaceiit 
ends of the two muscles join end to end, separated only by a 
tendinous intersection. In stages C to F the proximal end of 
the tensor veli palatini is, as stated abovm, attached to the 
pterygoid bone; in stage H (fig. 26) itlms additionally grown 
inwards below the pterygoid bone (iu which a nodule of car- 
tilage — the hamulus — has appeared) into the soft palate ; and 
in stage J a well-marked transverse aponeurosis connects 
together the inner ends of the muscles of the two sides. A 
similar process occurs in Didelphys aurita. 
In stage H there is a cap of dense tissue just above the 
dorsal end of the condylar process of the lower jaw, and into 
this the squamous bone is beginning to spread. In stage J the 
extension of the squamous inwards is more marked, and a 
split — the joint cavity — is developed iu the dense tissue. 
Cartilage is developed in the condylar process in stage J 
(fig. 28). The external pterygoid is inserted into the con- 
dylar process and its neck in both stages. 
In a 8’5 mm. embryo of Ornithorhy nchus (figs. 29-33) 
the Anlagen of the incus, MeckeTs cartilage, ala temporalis, 
and processus alaris are present in a precartilaginous condi- 
tion. Aggregated mesoblast cells connect together the ala 
temporalis and processus alaris. The Anlage of the mandible 
is present, dorso-lateral to that of MeckeFs cartilage. The 
