CHORDA DORSALIS 



343 



are carried up with the curling edges, and constitute the medullary 

 folds. Later the edges of the medullary plate meet each other, 

 and join to form a closed canal, the neural or medullary canal. 

 The edges of the medullary folds unite above, so that the neural 

 canal comes to lie underneath the surface ectoderm. (Fig. 100.) 

 The neural canal is the forerunner of the whole nervous system. 

 Chorda Dorsalis. The method of formation of the chorda 

 dorsalis, or notochord, is very similar to that of the neural canal. 



FIG. 100. Transverse section through dorsal region of embryo chick 



(45 hours) . 



One-half of the section is represented; if completed it would extend as far to the 

 left as to the right of the line of the medullary canal (Me). A, epiblast: t7, hypoblast, 

 consisting of a single layer of flattened cells; Me, medullary canal; Pv, protovertebra ; 

 Wd, Wolffian duct; So, somatopleure; Sp, splanchnopleure; pp, pleuroperitoneal 

 cavity; ch, notochord; ao, dorsal aorta, containing blood-cells; v, blood-vessels of 

 the yolk-sac. (Kirkes after Foster and Balfour.) 



It is a solid, instead of a cylindrical, longitudinal collection of 

 cells, extending along the dorsal aspect of the celom. It is 

 developed from the entoderm. A thickening of the cells of this 

 layer constitutes the chordal plate. Its edges curl up in a direc- 

 tion opposite to those of the medullary plate, and carry with them 

 chordal folds of the entoderm. When the curling edges have 

 joined to form a solid cylinder of cells, the chordal folds unite 

 over the ventral surface of the cylinder. Figures 99 and 100 



