162 DEVELOPMENT OF THE VERTEBRAL COLUMN. 



Development of the vertebral column. The vertebral column is developed 

 around the notochord, except at the anterior end of that structure, which is imbedded 

 in the basis cranii. It is formed from protovertebral mesoblast. The outer part of 

 each protovertebra is transformed into a muscle-plate, and thus the original meso- 

 blastic segmentation is maintained. The inner parts of the protovertebrae do not, 

 however, remain distinct, but blend with one another on each side of the neural 

 canal to form a longitudinal mass, which extends to the side of and subsequently 

 encloses the notochord, and finally sends dorsal prolongations over the neural canal, 

 so that this also receives a continuous mesoblastic investment, forming the membrana 

 reuniens superior of Kemak. The investment is only incomplete opposite the points 

 where the nerve roots are connected with the spinal cord, and in it there is no sign 

 of any differentiation into vertebrae. It is continuous with a similar investment 

 within the cranium, which extends in front of the notochord into the fronto-nasal 

 process. The mass of mesoblast, which thus encloses the notochord and neural 

 canal, is often spoken of as the membranous vertebral column and cranium, but it 

 represents much more than the cartilaginous and bony structures of those parts, all 

 the investing membranes of the cord and brain and the ligaments of the vertebra? 

 being also derived from it. From it septa pass between the muscle-plates and serve 

 to give attachment to the developing muscle-fibres. 



The first appearance of the permanent vertebrae is in the form of cartilage, which 

 becomes formed in this mesoblastic investment on either side of the neural canal, 

 nearly opposite the interval between each two muscle-plates, to form the neural 

 arch. This part of the vertebra therefore alternates with the original mesoblastic 

 somites as represented by the muscle-plates. 



According to Froriep, the lateral halves of each cartilaginous neural arch become 

 united below the notochord before the appearance of the rudiments of the cartilagi- 

 nous bodies, and the latter appear as median accumulations of cartilage, immediately 

 posterior to the hypochordal part of the cartilaginous arch. In most of the vertebra? 

 this hypochordal part of the arch soon disappears as a distinct structure, but in the 

 atlas vertebra the primitive condition is maintained. 



The serial arrangement of the musculature represents phylogenetically the original 

 segmentation of the vertebrate body. The segmentation of the vertebral column, 

 on the other hand, has been arrived at later, and has been carried out in depen- 

 dence upon the muscular segmentation. 1 



The cartilage makes its appearance on the fourth day in the chick, on the 

 eleventh or twelfth day in the rabbit, and in the fourth or fifth week in man 

 (Kolliker). It is completed by the sixth or seventh week, soon after which ossifica- 

 tion commences. To form the intervertebral discs, the mesoblast between the 

 bodies of the vertebrae acquires a fibre-cartilaginous character, while at the same 

 time the notochord, which gradually elsewhere becomes reduced in size and eventu- 

 ally disappears, here undergoes enlargement, and its cells form an irregular network 

 in the central intervertebral pulp. Its remains are found at all periods of life in 

 the middle of the discs (Luschka). 



In the osseous fishes there is an intervertebral dilatation of the notochord, the growth of 

 which pi-oceeds to such a considerable extent as to give rise to a mass of soft gelatinous 

 substance, which occupies the conical hollows of the biconcave vertebral bodies. But in 

 birds, reptiles, and amphibia, where synovial articulations are developed between the vertebral 

 bodies, the notochord soon disappears from the intervertebral spaces, although its remains are 

 seen for some time in the bodies themselves (Gegenbaur). 



In mammals, the notochord is constricted within the cartilaginous vertebral body, but 

 dilated in the intermediate parts of this rod, the whole cord being monilif orm. At a somewhat 

 later period small dilatations are also to be seen in the epiphysial cartilages (fig. 193). 



1 A. Froriep, "Zur Entwickl. der Wirbelsaule, &c.," Archiv f. Anatomie, 1883 and 1886. 



