2QO THE EVOLUTION OF MAN. 



In all Skulled Animals (Craniota), most of the soft, 

 undifferentiated cells which originally constitute the 

 skeleton-plate, afterwards change into cartilage cells, which 

 secrete a firm, elastic "intercellular sub- 

 stance," and thus produce cartilaginous 

 tissue. Like most other parts of the 

 skeleton, the rudimentary vertebrae soon 

 pass into a cartilaginous condition, and, 

 in the higher Vertebrates, the cartila- 

 ginous tissue is afterwards replaced by 

 ^ the rigid bony tissue with its peculiar 



radiate bone-cells (Fig. 5, vol. i. p. 126). 



FIG. 261. Three . . . \ to 



breast-vertebrae of a The original axis of the vertebral column, 



human embryoof eight the notochord, is more or less compressed 



Sj Ittl! ; *y the cartilaginous tissue which grows 



cartilaginous vertebral vigorously round it. In lower Verte- 



bodies ; K, intervene- brates - ^ Primitive Fi s h e s) a more 



bral discs ; ch, noto- ' 



chord. (After Koel- or less considerable portion of the noto- 

 liker -) chord remains within the vertebral 



bodies. In Mammals, on the contrary, it disappears almost 

 entirely. In the human embryo, even at the end of the 

 second month, the notochord is seen only as a thin thread 

 which passes through the axis of the thick cartilaginous ver- 

 tebral column (Fig. 261, ch). In the cartilaginous vertebral 

 bodies themselves, which afterwards ossify, the thin remnant 

 of the notochord (Fig. 262, di) soon disappears entirely. A 

 remnant remains, however, throughout life in the elastic 

 " intervertebral discs" which develop, from the skeleton 

 plate, between each pair of vertebral bodies (Fig. 261, li). 

 In a new-born child, a large, pear-shaped cavity, filled with 

 a gelatinous cell-mass, is visible in each intervertebral disc 



