74 THE WHENCE AND THE WHITHER OF 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 

 the notochord or skeleton lies 

 between these and the digestive 

 or alimentary canal. The skel- 

 eton of the clam or snail is 

 purely protective and a hin- 

 drance to locomotion. That of 

 the 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 

 system, although this system 

 especially required, and was 

 worthy of, protection. It does 

 not protect even the brain ; the 

 skull of vertebrates is an af- 

 ter-thought. It is almost the 

 deepest seated of 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 



10. CROSS - SECTION OP AXIAL 

 SKELETON OP PETROMYZON. 

 HERTWIG, FROM HIEDERSHEIM. 



SS, skeletogenous layer ; Ob, Ub, 

 dorsal and ventral processes of 

 S'.S' ; C, notochord ; Cs, sheath 

 of notochord ; Z?e, elastic ex- 

 ternal layer of sheath ; F, fatty 

 tissue ; M, spinal marrow ; P, 

 sheath of M. 



