VERTEBRAL COLUMN 45 



mentioned (p. 41), bone may be developed in connection with 

 this primordial skeleton and may arise in two different ways. It 

 may be formed in parts which are not superficial ; or bony elements 

 may become associated with the cartilaginous skeleton which are 

 derived phylogenetically from the exoskeleton but which- in course 

 of time have taken up a deeper position : these become secondarily 

 connected with the bones which have arisen independently in this 

 position. In order to determine as to which of these two 

 categories any particular bone belongs, an appeal to comparative 

 Embryology is necessary. 



The relations of a bone to the cartilaginous skeleton may be of 

 such a nature that it merely becomes applied to the outside of the 

 cartilage, when it may be described as an investing bone. A bone 

 may, however, originate in the perichondrium or membrane which 

 covers the cartilage, and then, in the course of phylogenetic 

 development, may invade and replace the cartilage : in other 

 words, perichondral bone may become endochondral. 1 The cartilage 

 beneath investing bones may gradually disappear in the course of 

 time, and occasionally a bone which was originally perichondral 

 may attain apparent independence by the loss of the cartilage 

 around which it was formed in the first instance. 



I. VERTEBRAL COLUMN. 



An elastic rod, the notochord, or chorda dorsalis, lying in the 

 longitudinal axis of the embryo between the neural and visceral 

 tubes (cf. p. 6), is the first part of the endoskeleton to be formed, 

 and is the primitive forerunner of the vertebral column. It is 

 developed as a ridge of the primary endoderm, from which it 

 becomes constricted off, and is therefore of epithelial origin. In 

 the large parenchyma-like cells of which it is composed vacuoles 

 soon appear, and eventually only the walls of the cells persist in 

 the greater part of the notochord; these become flattened by 

 mutual pressure, so that they appear like a meshwork of pith-cells 

 (Fig. 34, A, B). At the periphery, however, the cells retain their 

 protoplasm, becoming flattened and arranged like an epithelium. 

 Around the notochord two homogeneous, cuticular sheaths are 

 successively developed from its cells. The primary sheatk is first 

 secreted by the peripheral notochordal cells : the thicker secondary 

 sheath, which has a similar origin from the so-called " notochordal 

 epithelium," appears later. 



From the surrounding mesoderm a skeletogenous layer is 

 developed : this not only surrounds the notochord, but extends 

 dorsally to it as well as ventrally. Thus a continuous tube of 

 embryonic connective tissue is formed enclosing the spinal cord, 



1 The unsatisfactory terms "membrane-bones" and "cartilage" bones are 

 usually used in describing the investing and replacing bones. 



