142 FOUNDATIONS OF BIOLOGY 



ing framework of the body, they are entirely inadequate 

 to knit together the organism into a working unitrf We find 

 therefore various kinds of CONNECTIVE TISSUE interwoven 

 between the integral parts of the body. These tissues form 

 sheaths about most of the organs and also supply the con- 

 necting links between muscle and muscle, muscle and bone, 

 and bone and bone. Skeletal tissues, of which bone, cartilage, 

 and connective tissue form the chief groups, are distinguished 

 from the other body tissues by the development of large 

 amounts of non-living material in or between the component 

 cells themselves; the character of the skeletal tissue being 



determined chiefly by the 

 nature of this matrix. 



The primitive axis of 

 the skeleton consists of 

 a cylindrical eord or rod 

 Extent of one vertebra of cells (NOTOCHORD), 



FIG. 78. Diagram of a longitudinal section which HeS in the mid- 



through a developing vertebral column to show dorgal Une of the 



the invasion of the notochord by cartilage to 



form the centra of the vertebrae. (From wall JUSt below the dorsal 



nerve tube (SPINAL CORD) 



and above the coelom. In most Vertebrates, however, 

 the notochord in its original form is only a temporary struc- 

 ture, being partially or completely replaced during later 

 development by a linear series of cartilaginous or bony ele- 

 ments, known as VERTEBRAE, which form the VERTEBRAL 

 COLUMN, or backbone. This is the most characteristic struc- 

 ture of Vertebrates as compared with Invertebrates, or back- 

 boneless animals. (Figs. 74, 78.) 



A typical vertebra of the higher animals consists of a basal 

 portion, known as the CENTRUM, and a NEURAL ARCH which 

 it supports. These form a protecting ring of bone about 

 the spinal cord. From various parts of the vertebra as a 



