INTRODUCTORY. BONE. 9 



II. MESOBLASTIC. 



The most primitive type of a mesoblastic endoskeleton 

 consists of a membranous sheath surrounding the notochord, 

 as in Myxine and its allies. The first stage of complication 

 is by the development of cartilage in the notochordal sheath, 

 as in Petromyzon. Often the cartilage becomes calcified in 

 places, as in the vertebral centra of Scyllium and other Elas- 

 mobranchs. Lastly, the formation of bone takes place ; it 

 generally constitutes the most important of the endoskeletal 

 structures. 



Bone may be formed in two ways : 



(1) by the direct ossification of pre-existing cartilage, 

 when it is known as cartilage bone or endochondral bone ; 



(2) by independent ossification in connective tissue ; it is 

 then known as membrane or dermal or periosteal bone. 



With the exception of the clavicle 1 all the bones of the trunk 

 and limbs, together with a large proportion of those of the 

 skull, are preformed in the embryo in cartilage, and are grouped 

 as cartilage bones : while the clavicle and most of the roofing 

 and jaw-bones of the skull are not preformed in cartilage, being 

 developed simply in connection with a membrane. Hence it 

 is customary to draw a very strong line of distinction between 

 these two kinds of bone; in reality however this distinction 

 is often exaggerated, and the two kinds pass into one another, 

 and as will be shown immediately, the permanent osseous 

 tissue of many of those which are generally regarded as 

 typical cartilage bones, is really to a great extent of periosteal 

 origin. The palatine bone, for instance, of the higher verte- 

 brates in general is preceded by a cartilaginous bar, but is 

 itself almost entirely a membrane bone. 



Before describing the development of bone it will be well 

 to briefly describe the structure of adult bone and cartilage. 



1 It is usual to regard the clavicle as a membrane bone, but Kolliker 

 has shown that in rabbit embryos of about the 17th day it is cartilaginous. 



