58 LEONARD W. AVILLIAMS 



uotochord laterally, dorsally and backward to the following inter- 

 segmental fissure. Its central portion (the {)riinitive vertebral 

 centrum of Remak) soon encircles the notochord and then dif- 

 ferentiates into a fibrous ring, surrounding the notochord, and a 

 transverse bar of cartilage. The former becomes the interverte- 

 bral disc, or takes part in the formation of the intervertebral joint; 

 the latter, the intercentrum or subnotochordal bar {hypochordale 

 Spnnge) usually degenerates without first losing its connection 

 with the lateral parts of the scleromere, but in connection with 

 the first two cervical vertebrae it persists, forming the body of the 

 atlas and a part of that of the axis. The vertebral centrum, aris- 

 ing in the loose tissue between the midsegmental condensations, 

 fuses, Froriep believed, with the preceding scleromere, forming in 

 this way the definitive vertebra. The centrum of the atlas, how- 

 ever, does not fuse with the preceding but with the succeeding 

 scleromere, that of the axis. The only criticism I have of Froriep 's 

 work is the one made in a former paper, namely, that the sclero- 

 mere which ultimately gives rise to the intervertebral disc (or 

 ligament), the intercentrum, the neural arches, the ribs, and the 

 myoseptum, cannot be regarded as a morphological unit. The 

 only actual units with which we are here concerned are the cen- 

 trum, the neural arches, the ribs, and the intercentrum. Fro- 

 riep's description of these and of their relation to the definitive 

 vertebrae is correct. 



Remak's theory received new support from Von Ebner ('88) 

 whose discovery of a midsegmental diverticulum of the cavity of 

 the somite, which divides the sclerotome into essentially equal 

 anterior and posterior parts, reopened the whole question of ver- 

 tebral formation. Von Ebner found this fissure, which he 

 named the intervertebral fissure, in the lizard, chick, mouse, and 

 bat. Schultze ('96) claimed that in birds the fissure arises inde- 

 pendently of the cavity of the segment and forms a connection with 

 it later, but in mammals is entirely without connection with it. 

 I have pointed out elsewhere that there is really no fissure in mam- 

 mals and the same is true, I find, in birds. Von Ebner, like Gegen- 

 l)aur, thought that the sclerotome is the structure which Remak 

 called the primary vertebral centrum; consequently the fissure 



