22 THE VERTEBRAL COLUMN. [CHAP. 



a very early period, traces only remaining in the axis of the 

 intervertebral substance, though in many of the inferior 

 Vertebrata it is persistent as a continuous rod for a longer 

 period, and sometimes permanently. 



The formation of the arches of the vertebrae in the 

 lamince dorsales is preceded by the appearance of dark- 

 looking cellular masses called proto-vertebrcR or somatomes, 

 corresponding in number, though not exactly in situation, to 

 the future vertebrae, and which undergo a series of changes 

 (for a description of which the student is referred to special 

 treatises on embryology) out of which ultimately results a 

 vertebra, similar in shape to that which it presents in adult 

 life, but formed of a continuous piece of hyaline cartilage. 



The mode of ossification of this cartilaginous vertebra in 

 the different groups of Mammals still offers an interesting 

 field for investigation, but the following is a summary of the 

 most important facts ascertained regarding it. 



Leaving out for the present the greatly modified two 

 anterior vertebras, the atlas and the axis, which must be 

 specially considered afterwards, and also the comparatively 

 rudimentary vertebrae of the caudal region, each vertebra 

 consists at one period of three pieces of bone, as distinct 

 from each other, and remaining so for as long a period, as 

 many of the separate elements of the skull. 



One constitutes the greater part, but usually not the whole, 

 of the body or centrum. Each of the others forms one side 

 of the arch, and usually more or less of the upper lateral 

 part of the body. These last ultimately unite to each other 

 in the middle line above, and to the central piece on each 

 side below. The line of union between them and the central 

 piece is readily distinguishable in all vertebrae up to the time 

 the animal is about half-grown, and is named by Professor 

 Huxley the neuro-central suture. (See Fig. 8, p. 33, ncs.} 



