No. 2304. REPTILIAN CHARACTERS IN MAMMALS— WORT AI AN. 29 



be overlooked that there are many cases in which it is difficult if not 

 impossible to decide to which particular category a given bone is to 

 be assigned because of the intergradation of the various processes 

 involved in its production or development. 



Parker in his consideration of the osseous skull of the vertebrates 

 makes the following classification:^ "Calcareous deposit occurs in 

 vertebrates in the following tracts: (1) Epidermis or epithelium 

 (enamel of the teeth, and outer layer of Ganoid scales) ; (2) dermis 

 (dentine of teeth, Ganoid and Teleostean scales) ; (3) subcutaneous 

 fibrous mesh, immediately outside the perichondrium, and eating into 

 cartilage (ectostosis) ; and (4) deep in its substance (true endostosis, 

 central or subcentral) . In most of these tracts the calcification may 

 be such as not to gain the title of bone; but in all except the first, true 

 bone may result from the process." 



While this classification is in the main correct and m general, 

 accord with the more modern views of the subject, it is at the same 

 time hardly explicit enough to serve our present purposes. Accord- 

 ing to the researches of histologists the formation of bony tissue, 

 outside of Parker's first group, is divided into two categories, namely, 

 an intramembranous and an intracartilaginous ossification. The chief 

 and most important distinction between these two catagories is that 

 in the former there is no cartilaginous mold or matrix which precedes 

 the appearance of the bone tissue; while in the latter a cartilaginous 

 mold or matrix is always present. In the intramembranous division, 

 the membrane which occupies the place of the future bone consists of 

 white fibrous connective tissue and ultimately forms the periosteum 

 from which the osteoblasts are derived. At first a series of fine bony 

 spicules are seen radiating from the point or center of ossification, 

 known as the osteogenetic fibers, which are deposited under the 

 influence of the osteoblasts. As these osteogenetic fibers grow out to 

 the periphery they continue to ossify and give rise to fresh bony 

 spicules. Subsequently successive layers of bony tissue are deposited 

 beneath the periosteum and around the larger vascular channels, so 

 that the bone increases much in thickness. It is further stated that 

 the process of bone formation spreads laterally to the future suture, 

 and here between the various bones a layer of fibrous tissue, the 

 cambium layer, is maintained until the full size of the bone is reached. 

 The cambium layer then ossifies and the bone ceases to grow at its 

 edges. The persistence of this cambium layer and its failure to 

 undergo final ossification is the cause of the maintenance of the 

 sutures between bones which so frequently results in the anomalies 

 which have been discussed in the preceding pages. 



In the intracartilaginous method of ossification, on the other hand, 

 as already stated, the future bone is preceded by a cartilaginous mold 



I Morphology of the Skull, IS77, p. 343. 



