THE NOTOCHORD. 



55 



The Notochord. 



The notochord (chorda dorsalis) is a rod of peculiar tissue constituting the 

 primitive axial skeleton of vertebrates. It begins in the embryo immediately 

 behind the pituitary body and extends to the caudal extremity. It occurs as a 

 permanent structure in all vertebrates, but undergoes much modification in the 

 amniota. It appears very early in the course of development, being differentiated 

 from the median dorsal wall of the notochordal canal, beginning at a time when 

 the medullary groove (compare page 68) is not fully marked out posteriorly, and is 

 nowhere closed. The notochordal anlage can be first detected as an axial band 

 of cells, which at first is not well marked off from the mesoderm of the primitive 

 axis. The anlage is thicker than the 

 adjacent entoderm (Fig. 22, nek). The 

 differentiation of the notochordal cells 

 begins usually at the anterior end of 

 the canal and progresses backward. It 

 appears merely as a specialized part of 

 the entoderm of the archenteron, but 

 has a very sharp demarcation. via 



FIG. 22. TRANSVERSE SECTION OF A MOLE EMBRYO 

 (HEAPE'S STAGE H). 



segment. CCR, Coelom. En, Entoderm. nch, 



Notochdrd. ao, Aorta, -vt.a, Vitelline artery. 



Som, Somatic mesoderm. Spl, Splanchnic meso- 

 derm. (After W. Heape.) 



The notochordal anlage separates off 

 and the entoderm proper closes across 



under it, SO that the notochordal band am, Amnion. Md, Medullary groove. My, Primitive 



lies between the entoderm and the over- 

 lying ectoderm (floor of the medullary 

 groove or canal). The two primitive germ- 

 layers come into actual contact in the 



median line, along which, therefore, when the notochord first separates from 

 the entoderm, there is no middle germ-layer present. This condition exists 

 in the chick with eight segments described in Chapter V. The separation 

 of the anlage does not take place at the anterior extremity of the notochord 

 until somewhat later, so for a considerable period the cephalic end of the noto- 

 chord remains fused with the entoderm. The separation from the entoderm is 

 effected in mammals by the entoderm proper shoving itself under the notochord 

 toward the median line. When the cells from one side meet those of the other 

 they unite with them and form a continuous sheet of entoderm below the noto- 

 chordal cells. The process of separation may be followed easily in the develop- 

 ment of the frog and toad. 



After its separation the notochord is a narrow band of cells, which starts 

 anteriorly from the entoderm (the future lining of the alimentary tract), running 

 backward to the blastopore. So long as the blastopore or neurenteric canal is 

 open the notochord terminates in the epithelium lining it. For a certain period 

 the notochord continues to grow tailward by accretion of cells from the walls of 

 the blastoporic passage; and after the canal is permanently obliterated, the noto- 



