Development of the Notochord 257 



In the head the notochord is nearly midway between the medulla 

 and the pharynx, but posteriorly it gradually approaches the cord until, 

 in about the 25th segment, the two are in contact. In front of this 

 point, the mesenchyma of the sclerotomes extends from the myotomes 

 to the notochord as a dense mass which is interrupted at the level of the 

 notochord by scarcely perceptible light transverse zones which connect 

 each pair of intersegmental vessels, and by a narrow longitudinal 

 median zone of similarly light tissue. 



In order to determine accurately the density of the nuclei I have 

 counted the nuclei in an area 24 microns square in sections 10 microns 

 thick, or in 5,760 cubic microns. In one or two cases the series are cut 

 at a different thickness, and it was necessary to calculate the number of 

 nuclei in this volume from the data for another volume. The percen- 

 tage of error in this calculation is less than that of the counting. In 

 every case the number given is the mean between at least two counts in 

 different places. At the level of the notochord there are 36 nuclei in 

 5,760 cubic microns in the transverse light zones, 48 in the longitudinal 

 zone and 63 in the denser regions. Below the notochord the mesen- 

 chyma is of nearly uniform density and has 54 nuclei in 5,760 culnc 

 microns. 



Higher up, beside the spinal cord, the structures of the anterior 

 half of the segment, the spinal ganglion and the roots of the spinal 

 nerves, are surrounded by loose mesenchyma; and the dense tissue, 

 like that below the spinal cord, is confined to the posterior half of the 

 segment. The cavities of the myotomes have closed. The fissure of 

 von Ebner, which in the Sauropsida divides the sclerotome into anterior 

 and posterior halves, has not appeared and consequently the sclerotome 

 is not divided into anterior and posterior portions. 



Behind the point where the spinal cord and notochord first come in' 

 contact, there is a region in which the notochord is not in contact with 

 the sclerotomes, but lies in a small cavity. Farther back the notochord 

 is in contact with the spinal cord, the post-anal gut, and the somites. 



Slight condensations of mesenchyma become visible in embryos of 

 6 mm. They occur in the middle of the segments and lie just under 

 and perhaps a trifle behind the troughs of the notochordal waves. They 

 are produced by the extension of the intersegmental zones of loose 

 tissue, forward and backward toward the centers of the sclerotomes, 

 and are the intervertebral discs of Froriep or the primitive discs of 

 Bardeen. Their position is mid-segmental in the pig as in the cow 



