154 THE NOTOCHORD. 



ring of calcifying cartilage, and internal to this a ring of fibrous 

 cartilage, which adjoins the now slightly constricted notochord. 

 A transverse section of an intervertebral region shows only a 

 thick outer and thin inner ring of fibrous cartilage, the latter in 

 contact with the sheath of the unconstricted notochord. 



The constriction of the notochord proceeds till in the centre 

 of the vertebrae it merely forms a fibrous band. The tissue 

 internal to the calcifying cartilage then becomes hyaline, so that 

 there is formed in the centre of each vertebral body a ring 

 of hyaline cartilage immediately surrounding the fibrous band 

 which connects the two unconstricted segments of the noto- 

 chord. The intervertebral tissue becomes more and more 

 fibrous. In Cartier's paper before quoted there is a figure 

 (fig. 3) which represents the appearance presented by a longi- 

 tudinal section of the vertebral column at this stage. 



The relation of the vertebral bodies to the arches requires a 

 short notice. The vertebral hyaline cartilage becomes almost 

 precisely similar to the tissue of the arches, and the result is, 

 that were it not for the 'membrana elastica externa' it would 

 be hardly possible to distinguish the limits of the two tissues. 

 This membrane how^ever persists till the hyaline cartilage has 

 become a very thick layer (PI. Xll. fig. 11), but I have failed 

 to detect it in the adult, so that I cannot there clearly dis- 

 tinguish the arches from the body of the vertebrae. From a 

 comparison however of the adult with the embryo, it is clear 

 that the arches at most form but a small part of what is usually 

 spoken of as the body of the vertebrae. 



The chano^es in the notochord itself durinof the stao^es sub- 

 sequent to K are not of great importance. The central part 

 retains for some time its previous structure, being formed of 

 large vacuolated cells with an occasional triangular patch of 

 protoplasm containing the starved nucleus and invested by 

 indurated layers of protoplasm. These indurated layers are 

 all fused, and are probably rightly regarded by Gegenbaur 

 and Gotte as representing a sparse intercellular matter. The 

 external protoplasmic layer of the notochord ceases shortly 

 after stage K to exhibit any traces of a division into separate 

 cells, but forms a continuous layer with irregular prominences 

 and numerous nuclei (PI. xii. fig. 9). In the stages subsequent 



