264 Leonard W. Williams 



and outward into the neural and costal processes which belong to the 

 posterior part of the segment. These relations persist nntil the forma- 

 tion of precartilage begins, when the blastemal primitive vertebra breaks 

 up into the intervertebral disc, the neural arches, ribs, myosepta and 

 such diverse structures as cartilage, fibrocartilage, fibrous connective 

 tissue, and perichondrium. 



Finally, I am convinced that the delimitation of the "^primitive ver- 

 tebra" is not due to its becoming differentiated before the surrounding 

 structures, but to the more rapid differentiation of the definitive verte- 

 brae which leaves the more slowly developing blastemal tissue between 

 the successive vertebrae as the "primitive vertebra." These considera- 

 tions suggest that the scleromere is not a morphological unit or anlage; 

 it is rather a residual mass of undifferentiated sclerotomic tissue which 

 later forms such diverse morphological units as the annulus fibrosus and 

 the fibro-cartilage of the intervertebral discs, the rib, the neural arch 

 and the myoseptum. In short, the "primitive vertebra" is a lager 

 rather than an anlage, a store of rudiments, not a rudiment. If this is 

 true, the conception of the resegmentation of the primitive vertebrae is 

 without foundation, for the "primitive vertebra" is not a vertebra at 

 all. Moreover, the evidence presented by Bardeen in support of his 

 belief that the intervertebral disc is formed by the union of the tissue 

 from the anterior surface of the primitive disc and from the posterior 

 surface of the anterior half of the sclerotome, is not conclusive. He 

 says (p. 165) : "During the period of differentiation of the scleromeres 

 the myotomes undergo a rapid development. The median surface of 

 each myotome gi-adually protrudes opposite the fissure of von Ebner. 

 The dorsal and ventral processes of each scleromere are then slowly 

 forced into the interval between the belly of the myotome to which it 

 belongs and the one next posterior, and thus finally they come to occupy 

 an intersegmental position." Again (p. 166), "During the develop- 

 ment of the interdiscal membranes, the primitive discs become hollowed 

 out on the posterior surface. A comparison of Fig. 2 with Fig. 3 

 demonstrates this." On page 167, "Each primitive disc has become 

 further hollowed out at its posterior surface, owing in all probability 

 to the conversion of its tissue into that of the area between the discs. 

 The tissue of each segment immediately anterior to the primitive disc 

 has become greatly thickened and the line between it and the disc 

 indistinct." These facts are summarized on page 167, "The primitive 

 discs become hollowed out posteriorly by a loosening up of their tissue 



