262 Leonard W. Williams 



in shape and arrangement, and an outer and larger region with nuclei 

 which are elongated longitudinally and are united by strands of pro- 

 toplasm into layers concentric with the center of the disc. The inner 

 portion later (in 24 mm. embryos) forms the cartilage which serves, 

 as has been shown by Schultze, Minot, and Weiss, to bind together the 

 successive vertebrae in a continuous rod of cartilage, the chondrostyle. 

 It should be noted in passing that the development of the cartilage of 

 the intervertebral disc at this late period, as compared with the verte- 

 bral cartilage, is another indication that the apparent condensation, 

 the scleromere, is not the first but the last portion of the vertebral 

 column to be differentiated. It is undifferentiated rather than pre- 

 cociously differentiated tissue. 



The vertebral precartilage is still further differentiated in embryos 

 of 17 mm. Each nucleus is now surrounded by a small cell body in 

 which are enclosed one, or more commonly two, vacuoles, each of which 

 is nearly or quite as large as the nucleus. The cytoplasmic network 

 has disappeared and the cells lie in a homogeneous matrix. 



The perichordal septum has ceased to be recognizable as such, but 

 its upper portion now remains as the fundament of the dorsal common 

 vertebral ligament. It lies in a deep groove in the precartilage of the 

 body of the vertebra. The ribs and vertebras are sharply marked off 

 by the perichondrium from surrounding tissues. 



Chondrification of the vertebrae begins before embryos are 20 mm. 

 long. (Fig. 5.) At this time the vertebral cells have much the same 

 character as before, but the vacuoles are less conspicuous and the cyto- 

 plasm is more granular and stains more heavily. Each cell, however, 

 ir now separated from the matrix by a heavily stained capsule. A con- 

 siderable space often separates the cell from its capsule; this, however, 

 may be due to shrinkage. 



The cartilage is now surrounded on all sides by a layer of small, 

 closely-packed, rounded nuclei. This layer, together with the fibrous 

 tissue surrounding the vertebra, forms the embryonic perichondrium. 

 The central portion of the intervertebral disc, wdiich includes from 

 one-third to one-quarter of its diameter, is now precartilaginous. The 

 outer portion of the disc is gradually becoming more fibrous. The 

 interdorsal ligament is now well differentiated from the blastemal 

 portion of the ligament and from the neural arch. Its upper edge 

 and outer portion still remain blastemal. The same is true of the 

 upper end and outer portion of the neural process. The upper blaste- 



