Development of the Notochord 263 



mal tissue will later differentiate into the upper portion of the inter- 

 dorsal ligaments and of the neural arch which now extends but to the 

 middle of the side of the spinal cord. The outer blastemal tissue of 

 the neural process, as noted above, seems to form the myoseptum. A 

 column of blastemal tissue from which will be formed the transverse 

 process of the vertebra, the tubercle of the rib, the costo-transverse 

 ligaments, etc., extends from the rib to the neural arch. 



The notochord has also undergone fundamental alteration. The cell 

 walls, which up to this time have remained intact, are now breaking 

 down (or are being absorbed) and the mucin from the cell vacuoles 

 escapes. The cells are united by strands of cytoplasm and the noto- 

 chordal tissue now resembles mesenchyma. A part of the mucin re- 

 mains in the cytoplasmic mesh, some of it, escaping, helps to thicken 

 the inner sheath of the notochord, and a large quantity collects within 

 the intervertebral portion of the notochord whose sheaths are compressed 

 slightly by the intervertebral disc. The vertebral portion of the noto- 

 chord, owing to the escape of the mucin from its vacuoles into the 

 inner sheath, or into the intervertebral portion of the notochord, is 

 much reduced in size and is much denser than before, but the corre- 

 sponding portion of its sheaths is dilated. The notochord is thus 

 dilated intervertebrally and contracted vertebrally, but the reverse is 

 true of its sheaths, the outer sheath being of greater diameter, and 

 the inner sheath both of greater diameter and also of greater thick- 

 ness in the vertebra. The notochordal undulations are obliterated by 

 these changes in the notochord and vertebrae. 



A brief discussion and summary of the relation of the notochordal 

 undulations and the ribs and vertebra to the segments is desirable at 

 this point. 



Dr. Minot has shown that the segmental waves of the notochord 

 of the pig are somewhat different from those of other mammals. The 

 crests of the undulations are a very short distance in front of the inter- 

 segmental arteries, and are in the posterior fourth of the segment; 

 the troughs are in the second fourth; the ascending or anterior slope 

 in the third; and the posterior slope in the first fourth of the segment. 

 The intervertebral disc, which is formed from the transverse portion 

 of the primitive vertebra, is mid-segmental, and lies just behind the 

 trough of each notochordal undulation. The edge of the blastemal 

 intervertebral disc abuts upon the posterior edge of the spinal nerve and 

 from this point the dense tissue of the primitive arch extends backward 



