Development of the Notoeliord 259 



mately the same density of tissue in them despite the rapid increase in 

 volume of this part of the embryo, but the cells of the vertebrae do not 

 keep pace with the general growth and are consequently drawn apart. 



The notochord is larger and about 15 cells occur at the periphery 

 and 3 or 4 at the center of each transverse plane. 



The fissure of von Ebner is present in the trunk. It scarcely reaches 

 downward to the level of the notochord and it does not reach inward as 

 far as the intersegmental arteries. Its position is such that, if it were ex- 

 tended downward and inward, it would divide the intervertebral disc. 

 Bardeen finds that in man this fissure is mid-segmental and that the 

 "primitive disc" lies in the posterior half of the sclerotome in early stages, 

 but that the intervertebral disc is later formed upon the site of the fissure 

 of von Ebner. In the pig, however, the fissure of von Ebner does not 

 divide the sclerotome into anterior and posterior portions ; on the con- 

 trary, the sclerotomes fuse with one another in the median line and longi- 

 tudinally, as we saw in the 5.5 mm. embryo, and, in the axial rod thus 

 formed, appear the loose transverse zones which will form later the 

 bodies of the vertebrte. 



The dense mesenchyma of the intervertebral disc extends outward to 

 the spinal nerves and then divides into an anterior and a posterior 

 process. The former, the interdiscal membrane of Bardeen, extends 

 forward on the inner side of the spinal nerve to the preceding disc. 

 The latter, representing the "costal and neural processes" of Bardeen, 

 extends outward and backward, and downward and upward. The upper, 

 or "neural" process, extends upward behind the spinal ganglion and upon 

 the inner side of - the posterior half of the myotome. The lower, or 

 "costal" process, extends downward and outward between the divergent 

 lower ends of the myotomes. 



As Bardeen found in man, all the axial mesenchyma is as yet blaste- 

 mal, and I believe that, although this tissue has greatly increased in 

 volume, almost all visible differentiation has been effected by the sep- 

 aration of its cells from one another. 



The apparent condensations give rise "to cartilage, perichondrium 

 and ligaments" (Bardeen) and consequently the blastemal "sclero- 

 mere" of Bardeen, which is composed of the intervertebral discs with 

 the costal and neural processes, cannot justly be regarded as a morpho- 

 logical skeletal unit. In short, definite skeletal differentiation, the for- 

 mation of cartilage or precartilage, has not as yet begun in the "primi- 

 tive vertebrae," but is foreshadowed in the definitive vertebrge by the 



