266 Leonard W. "Williams 



The chondrostyle is now complete anteriorly, for the tissue of the 

 inner portion of the intervertebral disc is passing in this stage into 

 true cartilage. The outer part of the disc is more and more fibrillar 

 and is clearly destined to form the annulus fibrosus. The edge of the 

 disc is still attached to the head of the rib, but the formation of the 

 articulation is indicated by a small cavity which has now appeared be- 

 tween the two structures. 



The cartilage of the vertebrte is more advanced. A large quantity of 

 matrix intervenes between the cell-capsules, and many cells, owing to 

 division, have two nuclei. The notochord forms approximately .2, 

 the cartilage .3, and the 'fibrous tissue .5 of the diameter of the inter- 

 vertebral disc. 



Comparatively slight changes are seen in the notochord of an embryo 

 of 39 mm. The sjTicytial network has been enlarged both by growth 

 and by the formation of a relatively greater number of vacuoles of 

 mucin, and, consequently, the nuclei are farther apart. The cytoplasm 

 of the syncytium forms in places a regular continuous boundary, but 

 at other points the vacuoles seem al)out to escape to the exterior. The 

 small fragments of the notochord which have been enclosed within the 

 vertebrae are degenerating. The first step in this process is apparently 

 indicated by the separation of the cells from one another and by a 

 change in the C}i;oplasm and nucleus which causes the former to take 

 Orange G. more intensely and the latter to stain more deeply with hsema- 

 toxylin. Somewhat later the nucleus becomes deeply and irregularly 

 constricted and it is finally broken up into small pieces. The cyto- 

 plasm later is fragmented in the same manner. 



Calcification of the centers of the vertebrse and of the notochordal 

 sheaths within them begins in embryos of 32 mm. (Series 136) and has 

 affected the greater part of the vertebral bodies in an embryo of 39 mm. 

 Within the intervertebral disc, the outer notochordal sheath is no longer 

 recognizable in the larger embryos, having been, in all probability, 

 stretched to excessive thinness. The inner sheath has the same charac- 

 ter and the same relative size as before. 



Mitotic figures are very frequent in the cartilage just outside the re- 

 gion of calcification, and many characteristic cell-nests have been formed. 



The cartilaginous portion of the intervertebral disc has increased in 

 volume with advancing chondrification much more rapidly than its 

 fibrous portion, and now forms about .41 of the disc, the notochord form- 



