276 Leonard W. Williams 



vance of the lower. Thus^ although the shape of the notochordal en- 

 largements is constantly changing, it is characteristic of the opossum 

 at each stage. 



In the pig, the trough of the notochordal wave lies slightly in front 

 of the center of the intervertebral discs, and, as in the opossum, the con- 

 vex wall of the notochord gives the cue to subsequent changes of form. 

 The upper or concave wall of the notochord moves upward and, at the 

 same time, the chondrification of the vertebra forces, or at least seems 

 to force, the notochordal crest or summit forward until it comes to 

 lie at the posterior edge of the disc. The notochord now makes a sharp 

 descent from the posterior to the anterior edge of the intervertebral disc, 

 and as it gradually expands the upper point moves forward to the 

 center of the disc and the lower moves backward until a symmetrical 

 diamond-shaped expansion is formed. 



The notochord of the rabbit is first enlarged vertebrally, as in the 

 pig and opossum, but to a greater extent. These dilations appear in the 

 anterior vertebrae of an embryo of 10.5 mm. which are just passing 

 over into precartilage. The convex side (the lower) of the notochord 

 is usually more expanded than the upper, and there is usually formed 

 a sharp ventral expansion of its sheath. The vertebral enlargements 

 are larger and more symmetrical in an embryo of 12 mm., in whicli 

 the vertebrse are precaTtilaginous. They occur throughout the trunk 

 and they have apparently obliterated the notochordal undulations. 

 Chondrification forces the notochord from the vertebrse into the inter- 

 vertebral expansions, and in an embr3'o of 14.5 mm. (Fig. 18) the ver- 

 tebral expansions have disappeared. The centers of the intervertebral 

 discs are now loosening up in preparation for the formation of precar- 

 tilage. The notochord first expands upward in the disc and later 

 downward also. In an embryo of 18.8 mm., the enlargements are 

 quite rounded and somewhat later they become of greater diameter 

 than length. The upper moiety is usually somewhat larger than the 

 lower, and somewhat in front of it (Fig. 9). The notochord is vastly 

 more vacuolated in the embr3'o of the rabbit, and the vacuoles are more 

 evenly distributed than in the other mammals studied. 



In the cat, as in the pig, opossum and rabbit, the first notochordal 

 expansions are vertebral. These appear in an embryo of 10.7 mm. 

 in the anterior vertebrae which consist of very loose mesenchymal tissue; 

 and in an embryo of 12 mm. they are larger and more numerous, the 

 vertebrae being precartilaginous. The vertebrae are cartilaginous in an 



