600 Comparative Morphology of Chordates 



disk of cartilage persists between the diaphysis and each epiphysis, 

 permitting growth of the shaft at its ends. When growth is completed, 

 the epiphyses usually fuse to the shaft. The advantage in this method 

 of development lies in the fact that the terminal region of the bone is 

 relieved of the necessity for growing and at the same time maintaining 

 a suitable surface for a working joint. Early ossification of the epiphy- 

 sis makes possible the establishing of a finished joint long before 

 longitudinal growth of the bone is finished. Mammals are usually most 

 active during youth, and they may well be so, for the joints are then 

 at their best even though the development of the skeleton is not 

 completed. Epiphyses are not known to occur in reptiles. It is to be 

 noted that epiphyses are absent or very poorly developed in the mono- 

 tremes — mammals which reproduce in the reptilian manner. 



Skull 



The mammalian skull has the same general structure as that of a 

 reptile, but the adult skull, in form and in details of structure, is 

 simpler than a reptilian skull and very much simpler than that of a 

 bony fish. Its simpler condition results in part from reduction in 

 number of dermal bones, especially in the region of the orbit, on the 

 roof of the mouth, and along the jaws. In part, however, the simplicity 

 is of secondary nature in that two or more bones which are quite 

 separate in the embryo may fuse to form a single bone in the adult. 

 In the human skull the sphenoid bone is a fusion of eight embryonic 

 parts (see Table 1, p. 122) of which all but one correspond to reptilian 

 bones. The mammalian temporal bone includes all the cartilage bones 

 formed in the auditory capsule, the squamosal bone, and, in some cases, 

 still others (see Table 1). 



The brain does not extend forward between the orbits — that is, the 

 skull is of the tropibasic type (p. 517), but the interorbital region is 

 usually not reduced to the extreme narrowness of the interorbital 

 septum of birds and many reptiles. Comparison with skulls of pre- 

 sumably ancestral theromorph reptiles justifies regarding the mam- 

 malian skull as of the synapsid type, but much disguised. The four 

 types of skull are defined on the basis of the number and positions of 

 "windows" (fenestrae) in the temporal arcade (p. 501). In extreme 

 contrast to the anapsid skull in which the whole temporal space is 

 covered laterally by a bony temporal arcade having no " windows " 

 at all (Fig. 394), the temporal arcade in the modern mammal (Figs. 

 436.B, 465) is nearly all "window" with little of its frame left except 

 the "sill," which appears as a relatively slender bony arch projecting 

 outward at the level of the upper jaw, the anterior end of the arch 

 springing from the maxillary bone and its posterior end consisting of 



