Development of the Notochord 275 



to admit ready flexion, and with them the chondrostyle would not offer 

 a sufficient resistance to axial stresses; hence the need of the nucleus 

 pulposus which, being incompressible and also being closely invested 

 by the fibro-cartilage and fibrous tissue of the disc, serves as a pad upon 

 which the vertebrse turn. When the spine is unbent, the nucleus pul- 

 posus forms a rounded mass which is bound in on all sides by the fibrous 

 tissue and fibro-cartilage of the disc, which, being attached to the 

 heads of the vertebrse, forms a capsule whose layers are concentric with 

 the nucleus pulposus. When the spine is bent forward, for example, 

 the posterior portion of the annulus fibrosus is stretched straight, forcing 

 the nucleus pulposus forward as a wedge-shaped mass between the in- 

 clined faces of the vertebrae, while the anterior part of the disc is pushed 

 forward, its surfaces being drawn together, in a sharp curve or in one 

 or more folds. Corresponding changes occur as the spine is flexed in 

 other directions or is circumducted. The nucleus pulposus contributes 

 largely to the strength of the spine and to its flexibility. The chon- 

 drostyle is partially replaced by bone, but its intervertebral portions 

 persist. 



The tissue of the notochord is at first cellular and epithelial. Later 

 it becomes syncytial and resembles closely mucoid connective tissue. It 

 finally becomes cellular a second time and then is very similar to 

 cartilage. Kotochordal tissue is perfectly distinct from all other tis- 

 sues of mammals, and passes through a very cliaracteristic cytomor- 

 phosis. 



The Shape of the jSTotochordal Enlargements in Mammals. 



A description of the shape of the notochordal enlargements is very 

 unsatisfactory alike because of the difficulty of accurate description; 

 because the shape of any particular dilation changes with growth; 

 and because, although the shape of the enlargement in each species 

 is remarkably characteristic, the great amount of variation which occurs 

 renders it difficult to determine the normal type of expansion. In the 

 opossum, as has been noted above, the crest of the notochordal undulation 

 makes the upper contour of the expansion. From this boundary the 

 enlargement gTows downward to the level of the troughs (Figs. 10 and 

 17). Later a small ventral process appears somewhat behind the crest 

 of the enlargement and, as this process enlarges, the expansion becomes 

 somewhat diamond-shaped, but the upper angle always remains in ad- 



