278 Leonard W. Williams 



of the discs are composed of precartilage. In an embryo of 33 mm. 

 (Figs. 12 and 20) the chondrostvle is practically complete. The other- 

 wise fusiform intervertebral expansion of the notochord is compressed 

 above as by the sharp edges of the vertebras, and consequently bears 

 dorsally a small angular process which projects into the broad but thin 

 cartilage of the disc. 



The shape of the notochordal enlargements, in the mammals which 

 have been studied, is perfectly characteristic at each stage of their 

 development until they are transformed into the nuclei pulposi of the 

 intervertebral discs. 



The Eelation of the Notochord to Chordoma. 



The course of the notochord in the skull of the human embryo, taken 

 in connection with the results that have been reached in the preceding 

 part of this paper, offers some suggestions as to the origin and nature 

 of chordomia. It will be seen in Fig. 20 that the notochord makes a 

 single large sigmoid curve in the base of the skull and that it lies near 

 the surface of the cartilage at four points. It is near the upper sur- 

 face, in the hypopliysial fossa, a short distance behind the fossa, and near 

 the foramen magnum and near the lower surface at a point midway 

 between the hypophysial fossa and the foramen magnum. This is the 

 normal course of the notochord in the skull of human embryos. Its 

 curve is due to the fact that after the formation of the notochord, the 

 mesenchyma, growing inward between the base of the brain and the 

 pharynx, surrounds the anterior and posterior ends of the cranial por- 

 tion of the notochord ; but, since the notochord is attached to the epithe- 

 lium of the vault of the pharynx longer than elsewhere (until embryos 

 are 9 or 10 mm. long), it collects above the central portion of the noto- 

 chord. As the parachordal cartilages unite, they surround the notochord 

 and hold it in this position. These facts were discovered by Froriep 

 (1882), who described the later history of the notochord in the 

 skull, and Gaupp cites them in his article upon the skull in Hert- 

 wig's "Handbuch der Entwickelungslehre."' The middle section bears 

 a large but variable number of kinks, short branches, thickenings, and 

 other irregularities. These I find often involve the pharyngeal epithe- 

 lium, which is here thickened and often invaginated. This section of 

 the notochord has been found by Froriep to degenerate first, but in 

 one of the embryos in the H. E. C, Xo. 851, 22 mm., it forms small 



