Development of the Xotochord 265 



and strengthened anteriorly by a condensation of the tissue immediately 

 bounding the fissure of von Ebner." The evidence for the most impor- 

 tant point, whether or not the primitive disc is partitioned between the 

 vertebra and the intervertebral disc, is very slight; and if the process 

 described in the first and second quotations were to continue for some 

 time, it would perfectly account for the fact stated in the third and 

 fourth quotations. However, Weiss found in the white rat, and I find 

 in the pig, that the primitive disc becomes the intervertebral disc. The 

 fundamental mistake, I believe, in all work upon the primitive vertebrae, 

 is the assumption that the primary sclerotomic condensations either 

 are precartilage or are skeletal anlages. The fact is that they are 

 neither the one nor the other. Precartilage, of the mammalian vertebrae 

 at least, arises from such primary condensations only after a preliminary 

 loosening up and subsequent condensation. 



The Segmentation of the Notochord. 



The notochord shows a most marked change in pig embryos of 24 

 mm. (Fig. 6). The advancing chondrification of the vertebrae is the 

 apparent cause of a considerable expansion which, on the one hand, 

 presses the notochord, together with the greater part of its semifluid 

 inner sheath, from the vertebra toward the intervertebral discs; and 

 on the other hand draws the disc away from the notochord so that a 

 cavity is formed within the disc for the reception of the notochord. 

 The outer sheath seems not to be broken and the inner sheath adheres 

 to it so that, at no point, does the notochordal tissue come in contact 

 with the cartilage of the intervertebral disc or even with the outer 

 sheath. The intervertebral cavity is fusiform and the notochordal en- 

 largement is irregularly diamond-shaped. The dense tissue from the 

 vertebral portion of the notochord usually forms two slender cones whose 

 broadened bases, opposing the bases of similar cones from the adjacent 

 vertebrae, compress the intervertebral part of the notochordal tissue 

 and flatten antero-posteriorly the masses of mucin within it. The 

 two notochordal sheaths are very much compressed at the center of the 

 vertebra, but less and less so the farther from its center. The greater 

 part of the mucin which forms the inner sheath is forced into the inter- 

 vertebral cavity; but a small part of it, and occasionally a few noto- 

 chordal cells, are retained within the vertebrae. The notochordal tissue 

 retains its syncytial character, and there begins at this time a more 

 rapid increase of its mass, affecting alike nuclei, cytoplasm and mucin. 



