74 THE WHENCE AND THE WHITHER OP MAN 



later production. The primitive skeleton was the 

 notochord, still appearing in the embryos of all verte- 

 brates and persisting throughout life in fish. This 

 is an elastic rod of cartilage, lying just beneath the 

 spinal marrow or nerve-cord, 

 which runs backward from the 

 brain. The nerve-centres are 

 therefore here all dorsal, and 

 V, j^m tlie notochord or skeleton lies 



- "^^ between these and the digestive 

 y>--~<^ p or alimentary canal. The skel- 



jr.. J^^^^t"' -^ eton of the clam or snail is 



" "^ purely protective and a hin- 



- c.t drance to locomotion. That of 

 -^— JS^^^SH- '^ *'^® insect is almost purely lo- 

 comotive, but external, that of 



-^ the vertebrate purely locomo- 

 tive and internal. It does not 

 lie outside even of the nervous 

 10. cKoss- SECTION OP Axui system, although this system 



SKELETON OP PETEOMTZON. . ,, . . . 



HEBTwiQ, PKOM HiEDEESHBiM. espcciaJly requu'ecl, and was 



SS, skeletogenous layer; 06, m. ^O^thy of, protection. It doeS 



dorsal and ventral processes of not protect even the brain ; the 



&S'- C, notochord ; Cs, sheath iiij! i. ^ l • e 



of notochord; Ee, elastic ex- skuU of vertebrates IS an af- 



ternal layer of sheath ; J?-, fatty ter-thought. It is aim ost the 

 tissue ; M, spinal marrow ; P, , , . n » n 



Bheathofjtf. deepest seated oi all organs. 



But lying in the central axis of 

 the body it furnishes the very best possible attachment 

 for muscles. Around this primitive notochord was a 

 layer of connectile tissue which later gave rise to the 

 vertebrae forming our backbone. 



The nervous system on the dorsal surface of the no- 

 tochord consists of the brain in the head and the spinal 



