THE NOTOCHORD. 375 



fibrous character, and is distinguishable as a separate layer through 

 both the vertebral and the intervertebral regions. As a result of 

 these changes a transverse section through the centre of the 

 vertebral regions now exhibits three successive rings (vide PI. 

 13, fig. 1 1), an external ring of hyaline cartilage invested by " the 

 membrana elastica externa " (m.el), followed by a ring of calcify- 

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

 which adjoins the now slightly constricted notochord. A trans- 

 verse section of an intervertebral region shews 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 notochord. 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 longitudinal section of the verte- 

 bral 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 however persists till the hyaline cartilage has 

 become a very thick layer (PI. 13, fig. u), 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 changes in the notochord itself during the stages 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 



