THE NOTOCHORD. 



67 



minority represents the cells of the notochordal canal. The continuous layer 

 of cells is known as the permanent entoderm. At about this time, probably 

 sometimes earlier, sometimes later, according to the species, the blastopore 

 becomes permanently closed and the entodermal cavity no longer has an open- 

 ing to the exterior. 



The cells on the dorsal side of the notochordal canal have a different destina- 

 tion, for they become thickened to make the anlage of the future notochord. It 

 is to this fact that the canal owes its name. 



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 some of the lower vertebrates and as a temporary one in 

 the embryos of 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 medul- 

 lary groove (compare page 70) 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 meso- 

 derm of the primitive axis. The cells 

 of the anlage are larger than those 

 of the adjacent entoderm (Fig. 23, 

 nch) . The differentiation of the noto- 

 chordal cells begins usually at the 

 anterior end of the canal and pro- 

 gresses backward. It appears merely 



as a specialized part of the entoderm of the blastodermic vesicle, but has a very 

 sharp demarcation. 



The notochordal anlage separates off and the entoderm proper closes across 

 under it, so that the notochordal band 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. The separation of the anlage does not take place at the anterior 



Fig. 23. — Transverse Section of a Mole Embryo 

 (Heape's Stage H). 



atn. Amnion. Md, Medullary groove. Aly, Primitive 

 segment. Cce, Coelom. En, Entoderm, nch, Noto- 

 chord. ao. Aorta, vt.a. Vitelline artery. Som, 

 Somatic mesoderm. Spl, Splanchnic mesoderm. — 

 [After IV. Heape.) 



