VII.J THE PERMANENT VERTEBRAE. 207 



with a number of cartilaginous arches which appear in 

 the mesoblastic investment of the neural canal. These 

 arches, which thus roof in the neural canal, are the 

 cartilaginous precursors of the osseous vertebral arches. 

 We further find that the portions of the cartilaginous 

 tube from which the arches spring come to differ histo- 

 logically from the portions between them not connected 

 with arches : they are clearer and their cells are less 

 closely packed. There is however at this period no 

 distinct segmentation of the cartilaginous tube, but 

 merely a want of uniformity in its composition. 



The clearer portions, from which the arches spring, 

 form the bodies of the vertebrae, the segments between 

 them the intervertebral regions of the column. 



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

 intervertebral segments into two parts, which respec- 

 tively attach themselves to the contiguous vertebral 

 regions. A part of each intervertebral region, immedi- 

 ately adjoining the notochord, does not however undergo 

 this division, and afterwards gives rise to the ligamen- 

 tum suspensorium. 



This fresh segmentation is not well marked, if in- 

 deed it takes place at all in the sacral region. 



To recapitulate: — the original somites lying side by 

 side along the notochord, after giving off the muscle- 

 plates, grow around, and by fusing together completely 

 invest, with mesoblast, both neural canal and notochord. 



This investment, of which by reason of its greater 

 growth the original bodies of the somites now seem to be 

 only an outljdng part, becomes cartilaginous in such a 

 way that while the notochord becomes surrounded with 

 a thick tube of cartilage bearing no signs of segmenta- 



