VI.] SECONDARY SEGMENTATION OF VERTEBRAL COLUMN. 155 



W. Schwarck (Bciirage zur EntwicMung der Wirbelscmle hex den Ydgcln. 

 Anatomiscltc Studien, Dr flutse, in. Heft, 1S72) states that both in the 

 intervertebral and the vertebral segments the cartilage is divided into two 

 layers, an inner, central, and an outer peripheral. This division is less marked 

 in the intervertebral than in the vertebral region. 



The inner layer in the vertebral region he speaks of as "the body of the 

 vertebra belonging to the notochord," and the external layer as "the skeleton- 

 forming layer." 



On the fifth day a division takes place of each of the in- 

 tervertebral segments into two unequal parts ; a larger one 

 appertaining to the vertebra in front and a smaller one to the 

 vertebra behind. To the larger segment the spinal ganglia 

 naturally remain attached, and thus comes about the altera- 

 tion of their place in relation to the vertebrae which we before 

 spoke of. 



This fresh segmentation is not well marked, if indeed it 

 takes place at all in the sacral region. 



Each arch at its first appearance corresponds to about the 

 middle of a vertebral portion of a proto vertebra, but after the 

 secondary segmentation the portion of each vertebra behind 

 its arch grows more quickly than that in front, and thus after 

 a while the arches seem to spring from the front rather than 

 from the middle of the vertebral segments. 



To recapitulate: — the original pro to vertebrae lying side by 



side along the notochord, after giving off the muscle-plates, and 



dividing lengthways into ganglionic and vertebral portions, 



grow around, and by fusing together completely invest, 



with mesoblast of protovertebral origin, both neural canal 



and notochord. 



This investment, of which bv reason of its greater growth 

 1 . . . 00 



the original bodies of the protovertebrse seem to be only an out- 

 lying part, becomes cartilaginous in such a way that while the 

 notochord becomes surrounded with a thick tube of cartilage 

 bearing no signs of segmentation, but having the ganglia 

 lodged on its exterior at intervals, the neural canal is covered 

 in with a series of cartilaginous arches, connected to each 

 other by ordinary mesoblastic tissue only, but passing at their 

 bases directly into the cartilaginous tube around the noto- 

 chord. 



By a histological process of differentiation the cartila- 

 ginous tube is divided into vertebral and intervertebral 

 portions, the vertebral portions corresponding to the arches 



