Development of the Notochord 277 



embryo of 15 mm., and the discs are precartilaginons. The interver- 

 tebral expansions (Fig. 19) are forming, but, unlike those of other mam- 

 mals, the vertebral expansions persist at least until the cartilaginous 

 vertebrae are calcified (in embryos of 39 mm.) (Fig. 11). The first 

 indication of the intervertebral expansion is a slight angular point which 

 appears upon the convex or upper side of the crest of the notochord 

 at the middle or near the anterior edge of the disc. As this increases 

 in size, a similar but smaller and usually more rounded ventral point 

 appears somewhat behind the first. The two angles usually become 

 nearly equal, but they retain for a long time their asymmetrical position, 

 and the enlargements are usually more flattened antero-posteriorly than 

 in other mammals. There is also greater variation of the shape of 

 the enlargement than in other mammals. 



It should be noted in passing that a large part of the upper edge 

 of the annulus fibrosus of the eighth to the seventeenth discs of the cat, 

 and of the eighth to the sixteenth in the rabbit is converted into a 

 transverse intercostal ligament which binds together the heads of each 

 pair of ribs. In older rabbit embryos these ligaments are less dis- 

 tinct than in younger embryos. 



The notochord of a sheep embryo of 14.6 mm. is of uniform diameter, 

 and the chondrification of the vertebras has just begun. In an embryo 

 of 16.1 mm. enlargements have appeared in the first few vertebrae, 

 but at 17 mm. the enlargements are being compressed. The chondrifi- 

 cation of the vertebrae is far advanced in the next older embryo in the 

 collection, 26.1 mm. (Fig. 14). The centers of the intervertebral discs 

 are precartilaginons and a somewhat top-shaped enlargement of the 

 notochord has formed in each at the summit of the notochordal wave. 

 The vertebral enlargement has been divided and the parts have been 

 driven forward and backward toward, but not into the adjacent discs. The 

 notochordal enlargements are thus intervertebral, but each is a deeply 

 constricted cord, consisting of the top-shaped central or intervertebral 

 lobe and a pair of somewhat irregular and larger lobes which lie at a 

 considerably lower level in the ends of the adjacent vertebrae. 



The human notochordal expansions are of yet another type. The 

 notochord is situated considerably below the center of the vertebral col- 

 umn and vertebral expansions do not occur. In an embryo of 22 mm. 

 chondrification of the vertebrse has advanced considerably and the noto- 

 chord is sharply compressed in the center of each vertebra. It is cor- 

 respondingly and symmetrically dilated intervertebrally. The centers 



