Development of the Xotochord 267 



ing about .25 and the fibrous tissue .33. The cartilage of the disc now 

 has the structure which was earlier (17 mm. embryos) characteristic 

 of the cartilage of the vertebrae; that is, each cell contains one or two 

 vacuoles as large as or larger than the nucleus. A few cell capsules 

 contain two nuclei. The fibrous tissue has begun to assume its charac- 

 teristic arrangement in alternating layers whose fibers almost form 

 right angles with one another and are inclined at an angle of 15° or 

 20° to the longitudinal axis of the spine. 



A great change in the shape of the notochordal enlargement has oc- 

 curred in an embryo of 75 mm. (Fig. 7). It is flattened antero-poste- 

 riorly and in vertical or horizontal section it is elliptical except for slight 

 projections forward and backward into the open ends of its sheaths. 

 The notochord now forms .41 of the diameter of the disc, the fibrous 

 tissue .36, and the cartilage .23. The notochordal sjiicytium is larger 

 but retains the same genera] character. It is boimded by a clearly de- 

 fined cytoplasmic layer which has fewer vacuoles and more nuclei than 

 the central . portion of the notochordal tissue. The cells of the tibro- 

 cartilage of the disc are flattened as though by the radial pressure pro- 

 duced by the gro^vtll of the notochord, and the cells of the numerous 

 cell-clusters are arranged in rows parallel to the adjacent portion of the 

 notochordal sheath. Periosteal buds have filled the centers of the 

 vertebrae and bone formation has begun. In a few places the calcified 

 iiotochordal sheaths have been destroyed by the periosteal buds. This 

 process finally destro3'S completely the vertebral portion of the noto- 

 chord and hereafter the notochord is confined to the intervertebral discs. 



The Formation of the ISTucLErs Pui.rosus of the xAdltlt Pig. 



The flattening of the notochordal enlargement continues until, in the 

 cervical region of an embryo of 150 nmi., it is thrice as broad as thick 

 and forms one-half the diameter of the disc, the cartilage having 

 shrunken to .15 and the fibrous tissue having remained of the same rela- 

 tive size (.36). The notochord in an embryo of 250 mm. forms .58, 

 the cartilage only .08 and the fibrous tissue .3-4 of the disc's diameter. 

 In short, the notochord is expanding at the expense of the fibro-carti- 

 lage, which, being attached to the cartilaginous faces of the vertebrae 

 near their common axis, is stretched over their faces by the expanding 

 notochordal tissue. Consequently the cartilage finally forms a thin cap- 

 sule which surrounds the notochordal disc. The diameter of the mass 



