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DEVELOPMENT OF THE VERTEBRAL COLUMN. 



the mesoblastic tissue beyond the commencement of the basis of the 

 cranium, the mass of blastema which there surrounds the prolongation of 



Fig. 5'28. — Sections op the Vertebral 

 Column of a Human Fcetus of eight 

 ■WEEKS (from Kolliker). 



A, transverse longitudinal section of 

 several vertebrre. 1, 1, cliortla dorsalis, its 

 remains thicker opposite the intervertebral 

 discs ; 2, is placed on one of the bodies of 

 the permanent vertebrce ; 3, on one of the 

 intervertebral discs. 



B, transverse horizontal section through 

 a part of one dorsal vertebra. 1, remains of 

 the chorda dorsalis in the middle of the 

 body ; 2, arch of the vertebra ; 3, head of a 

 rib. 



the notochord (the investing mass of 

 Rathke) remaininoj one and undi- 

 vided, or being devoid at least of 

 the marked cleavage which occurs 

 in the strictly vertebral part. 



It is fi'om this protovertebral plate 

 on each side, whether in its entire 

 primitive condition, or in its later 

 and divided state, that the material 

 is derived for the formation of the 

 bodies and laminae of the vertebra? and the muscles which cover them. 

 This is effected by the rapid increase of the mesoblast, and by the 

 extension of that structure beyond the immediate confines of the 

 vertebral laminae in an inward and downward direction, so as to throw 

 a quantity of new mesoblastic material round the notochord, and 

 inwards and upwards, so as to pass in between the primary medullary 

 canal and the enveloping layer of epiblast. 



The muscular plate. — Shortly after this extension of the mesoblast 

 in the two directions before mentioned, another separation, or rather 

 differentiation, is observed to take place in the direction of its length, 

 in the formation along the dorsal surface, and below the epiblast, of a 

 series of circumscribed plates which form the foundation of the erector 

 muscles of the spine, and the great dorsal muscles of the trunk. These 

 constitute together the muscular, or rather the musculo-cutancovs plate, 

 for it appears also to include the formative rudiment of the true skin. 



There is thus deposited the formative material for the vertebral 

 bodies, the vertebral arches, and the muscles which immediately sur- 

 round them, together with the general integument. 



Meanwhile the vertebral segmentation goes on progressing from 

 before backwards, extending through the dorsal, lumbar, sacral and 

 coccygeal vertebrfB, till the process is complete ; but this is accom- 

 panied by other changes having reference to the separation of the nerve- 

 roots and ganglia from their formative tissue, and the development of 

 the elements of the permanent vertebra?. 



In the outer portion of each protovertebral mass a transverse partition 

 arises which separates the anterior part, as ganglion and nerve root, from 



