THE SPINAL COLUMN AND BACK. 145 



first of the sacral series in the early embryo; later the 25th 

 throws out great lateral masses, and thus forms a connection with 

 the ilia. In the lower primates (monkeys) the 27 th forms the 

 1st sacral; with the evolution of man the 26th, then the 25th 

 underwent sacral modifications, the trunk being correspondingly 

 shortened. It will be seen that the number is not yet definitely 

 fixed. The anterior point of attachment of the ilium fluctuates 

 from the 24th to the 26th vertebra in man. With the sacral 

 transformation of the 25 th and 26 th (lumbar) vertebrae, there 

 was a corresponding movement forwards of the sacral plexus. 



II. Sacro-coccygeal. — The 30 th vertebra forms the 1st coccy- 

 geal ; not uncommonly this vertebra is sacral in type and forms 

 part of the sacrum. 



III. Dorso-lumbar region. — This region is less liable to variation ; 

 the 20 th vertebra instead of forming the 1st lumbar, may 

 simulate the last dorsal in the type of its articular processes, 

 and may bear ribs, probably an atavistic form, or, on the other 

 hand, the 12th dorsal vertebra (19th) may not carry ribs. 

 About 2 °/ of bodies show variations of this kind. 



IV. Dorso-cervical. — The 7th vertebra may carry ribs ; rarely 

 the 8th vertebra has no ribs attached to it and is cervical in 



type- 

 Evolution and Development of the Spinal Column. — -The 

 human spinal column in the process of development passes 

 through three distinct phases : 



(1) It is membranous; (2) it becomes cartilaginous; (3) it 

 becomes bony. In the evolution of vertebrates the same three 

 stages are observed. In Amphioxus and Marsipobranchs (except- 

 ing the neural arches), the spinal column is membranous ; in 

 Elasmobranchs it is cartilaginous ; in other fishes and in all other 

 vertebrates it is ossified. In the human embryo, as in every other 

 vertebrate, the spinal column is developed from the mesoblast 

 which surrounds the notochord and neural canal. 



The Notochord. — It has already been seen that the notochord 

 is formed at a very early stage (before the 3rd week, Fig. 69, 

 p. ,90) by a tubular invagination of the hypoblast under the 

 neural canal. The notochord, with the mesoblastic tissue round 

 it, represents the most primitive type of spinal support. It is 

 hollow— the canal of the notochord runs from end to end, and 



