1166 



EMBRYOLOGY. 



are seen in the membranous vertebral column, one on each side of the notochord. 

 These small masses lie opposite to -the intervals between the muscle-plates and so 

 alternate with these structures. They are soon joined across the middle line on 



the ventral aspect of the notochord by a hypo- 

 chordal cartilaginous bar, which ultimately dis- 

 appears, except in the case of the atlas ver- 

 tebra, where it forms the anterior arch of that 

 bone. The vertebral bodies are formed imme- 

 diately to the dorsal aspect of these hypo- 

 chordal bars, alternating with the muscle-plates 

 which represent the original mesoblastic 

 somites. The notochord contained in the centre 

 of this chondrifying mass does not continue 

 to grow, but becomes in the human subject 

 relatively smaller, so as, at last, to form a 

 mere slender thread, except opposite the in- 

 tervals between the bodies of the permanent 

 vertebrae. Here it presents thickenings and 



d 



FIG. 799. Cervical part of the primitive 

 vertebral column and adjacent parts of an 

 embryo of the sixth day, showing the divi- 

 sion of the primitive vertebral segments. 

 (From Kolliker, after Remuk.) 1, 1, Chorda 

 dorsalis in its sheath, pointed at its upper 

 end. 2 points by three lines to the original 

 intervals of the primitive vertebrae. 3, in 

 a similar manner, indicates the places of 

 new division into permanent bodies of ver- 

 tebra, c indicates the body of the first cer- 

 vical vertebra ; in this and the next the 

 primitive division has disappeared, as also 

 in the two lowest represented, viz., d and 

 the one above ; in those intermediate the 

 line of division is shown. 4 points in three 



E laces to the vertebral arches ; and 5, simi- 

 irly, to three commencing ganglia of the 

 spinal nerves ; the dotted segments outside 

 these parts are the muscular plates. 



Vertebral body.-- f 

 Intervertebral disc.-- 



Notochord.-\ 





FIG. 710. Longitudinal section of vertebral column of an 

 eight weeks' human foetus. (Kolliker.) 



forms an irregular network, the remains of which are to be found at all periods 

 of life in the central pulp of the intervertebral disks (Figs. 709, 710, and 711). 



Development of the Ribs and Sternum. The ribs are formed from the muscle- 

 plates of the protovertebral somites, from which also the muscles of the back and 

 the true skin of the body-wall are formed. The ribs consist of extensions of this 

 mesoblastic material, which speedily undergo chondrification, and appear as carti- 

 laginous bars, which become separated from the vertebrae at their posterior extremi- 

 ties. At their anterior ends the nine upper costal bars turn upward and fuse 

 together so as to form a cartilaginous strip bounding a central median fissure. The 

 strips on either side then join in the middle line from before backward, and so give 

 rise to a longitudinal piece of cartilage, Avhich represents the manubrium and gladi- 

 olus of the sternum. In the process of development the sternal attachment of the 

 eighth rib disappears, while that of the ninth subdivides, one portion remaining 

 attached to the inferior extremity of the cartilaginous sternum and becoming 

 developed into the ensiform cartilage, the other portion receding from the sternum 

 and becoming attached to the cartilage of the eighth rib. 



The further development of the vertebrae, ribs, and sternum, and the ossifica- 

 tion of their cartilaginous framework, are described in the body of the work. 



Development of the Cranium and Face. It has been seen that the first trace of 

 the embryo consists in the formation of a longitudinal fold of the epiblast on either 

 side of the neural groove, and that these folds or ridges grow backward and meet 

 in the median line, thus forming the neural canal. This canal, at the cephalic 

 extremity of the embryo, is dilated and forms a bulbous enlargement. The bul- 

 bous enlargement soon expands into three vesicular dilatations, the three primary 



