290 



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 vertebree 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. 12G). 

 The original axis of the vertebral column, 

 the notochord, is more or less compressed 

 by the cartilaginous tissue which grows 

 vigorously round it. In lower Verte- 

 brates (i.e., in Primitive Fishes) a more 

 or less considerable portion of the noto- 

 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, ch) 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 



. .ili.v.»» 

 Tig. 261.— Three 

 breast-Tertebrte of a 

 human embryoof eight 

 weeks, in lateral lon- 

 gitudinal section : if, 

 cartilaginous vertebral 

 bodies ; li, interverte- 

 bral discs ; ch, noto- 

 chord. (After Koel- 

 liker.) 



