Development of the Notochord 269 



portion of the syncytial tissue and, beginning at the center, is gradually 

 coming to surround the notochordal tissue instead of being surrounded 

 by it. The mucin vacuoles at first are imbedded in the syncytium; the 

 vacuoles, enlarging, touch and finally unite with one another, leaving a 

 coarse network of syncytium; the strands gradually become attenuated 

 and finally break, so that masses of notochordal tissue, which are very 

 small near the center and large at the periphery of the nucleus pulposus, 

 are isolated. 



The cartilaginous portion of the intervertebral disc is represented 

 by a very thin sheet of fibro-cartilage which lines the cavity in which 

 the notochord lies. The portion of this sheet which lies upon the calci- 

 fied cartilage of the epiphysis is more fibrillar than the portion which 

 stretches around the notochord from vertebra to vertebra. The fibrous 

 portion of the disc is very dense, and its inner portion, like the carti- 

 laginous portion of the disc at an earlier stage, is being pressed radi- 

 ally outward by the notochord and forms a capsule around the noto- 

 chord. 



In the adult (Fig. 8) the notochordal enlargement is relatively thicker 

 than in the half-gi'own pig. It is also relatively smaller, for it now 

 forms but .4 of the diameter of the disc, the fibrous tissue forming the 

 remaining .6. The expansion of the fibrous tissue is produced by the 

 thickening of its various layers, not by the addition of new layers. The 

 fibro-cartilage forms, as before, a thin lining of the notochordal cavity. 



The mucin of the notochord retains the same character, but the tissue 

 has undergone a most astonishing modification. The mucin now di- 

 vides the notochordal tissue into a great number of relatively large 

 masses which in turn are divided by smaller quantities of mucin into 

 subsidiary masses. Each of the latter consists of a number of very peculiar 

 cells or cell-like structures (Fig. 16) which are bound together by 

 small quantities of syncytial cytoplasm. These cells each contain two, 

 or more rarely one or three, large vacuoles which are surrounded by thin 

 cytoplasmic walls and are separated by a small amount of cytoplasm in 

 which lie, in the great majority of, if not in all cells, two small nuclei. 

 All efforts to determine the nature of these vacuoles have failed. 

 No stains affect them. They do not shrink in absolute alcohol but 

 they do swell in water. Each cell of material which has been fixed in 

 formalin and immersed in water for some time, owing to the swelling 

 of the vacuoles, becomes elongated and constricted in the middle. Cells 

 with but one vacuole resemble fat cells, but the vacuoles are not fat. 



