544 MEMBRANE BONES AND CARTILAGE BONES. 



the cartilage, and the ossification takes place around these 

 vessels within the cartilage; but in the lower types (Pisces, Am- 

 phibia) ossification is often entirely confined to the perichon- 

 drium ; and the cartilage is simply absorbed. 



The regions where ossification first sets in are known as 

 centres of ossification; and from these centres the ossification 

 spreads outwards. There may be one or more centres for a 

 bone. 



The actual causes which in the first instance gave rise to 

 particular centres of ossification, or to the ossification of par- 

 ticular parts of the cartilage, are but little understood ; nor have 

 we as yet any satisfactory criterion for determining the value to 

 be attached to the number and position of centres of ossification. 

 In some instances such centres appear to have an important 

 morphological significance, and in other instances they would 

 seem to be determined by the size of the cartilage about to be 

 ossified. 



There is no doubt that the membrane bones and cartilage bones can 

 as a rule be easily distinguished by their mode of development ; but it is 

 by no means certain that this is always the case. It is necessarily very 

 difficult to establish the homology between bones, which develop in one 

 type from membrane and in another type from cartilage ; but there are 

 without doubt certain instances in. which the homology between two bones 

 would be unhesitatingly admitted were it not for the difference in their 

 development. The most difficult cases of this kind are connected with the 

 shoulder-girdle. 



The possible sources of confusion in the development of bones are 

 obviously two. (i) A cartilage bone by origin may directly ossify in mem- 

 brane, without the previous development of cartilage, and (2) a membrane 

 bone may in the first instance be formed in cartilage. 



The occurrence of the first of these is much more easy to admit than 

 that of the second ; and there can be little doubt that it sometimes takes 

 place. In a large number of cases it would moreover cause no serious 

 difficulty to the morphologist. 



BIBLIOGRAPHY of the origin of the Skeleton. 



(405) C. Gegenbaur. " Ueb. primare u. secundare Knochenbildung mit be- 

 sonderer Beziehung auf d. Lehre von dem Primordialcranium." Jcnaischc Zcit- 

 schrifl, Vol. III. 1867. 



(406) O. Hertwig. " Ueber Ban u. Entwicklung d. Placoidschuppen u. d. 

 Ziihne d. Selachicr." Jenaische Zeitsckrift, Vol. viu. 1874. 



