articular cartilage 



Meckel's cartilage 



prearricuiar 

 angular with tympanic process 



Figure 3-18. A stage in the development of the tympanic bone of the 

 opossum. (After Toeplitz, 1920) 



there is a dermal e.xtension from the petrosal occupying the 

 area of the alisphenoid, which is lacking. There are two 

 "pterygoid" bones in the palate, which arise separately and 

 remain separated by the palatine — except perhaps at their 

 extreme posterior ends. The bone on the lateral wall of the 

 nasal passage appears to be the pterygoid, but it lacks the 

 hamular process with its characteristic muscle associations. 

 The outer bone, which we have called the ectopterygoid, has 

 been compared with the hamular portion. From this com- 

 parison stems the concept that the mammalian pterygoid 

 is two parted. The solution to this problem will come only 

 through studies of the critical developmental stages of those 

 placental species described as having a two-parted pterygoid. 

 As a last point, the ear capsule of the monotreme is said to 

 arise from two centers of ossification comparable to those of 

 lower vertebrates. 



a postparietal and parasphenoid in marsupials suggests a 

 modified skull rather than a primitive one. 



In its visceral skeleton the opossum is of interest only in 

 the origin of the tympanic (Figure 3-18). Here there is an 

 anterior process extending along Meckel's cartilage, indicat- 

 ing the origin of this bone from one of the bones of the lower 

 jaw, generally identified as the angular of the reptile. 



Monofreme Details of the development of the montreme 

 skull reveal that there is no postparietal or lacrimal, but 



General observations From this examination of the carti- 

 laginous embryonic head skeleton and the development of 

 the bony head skeleton, we arrive at conclusions parallel to 

 those reached from study of the adult. Although there is a 

 great deal of variation among mammals, a basic general 

 plan is discernible. From this the marsupials show some 

 divergence and the monotremes show marked differences. 

 The origin of many parts of the mammalian skull will take 

 on more meaning after a review of the reptile skeleton and 

 its development. 



60 • OSTEOLOGY AND THE MAMMALIAN HEAD SKELETON 



