344 



REPRODUCTION 



neath the primitive groove, the cells of the ectoderm become 

 thickened to form the medullary plate. The edges of this 

 longitudinal plate soon begin to curl up, and thus form the 

 medullary furrow, or groove. (Fig. 99.) The margins of 

 the adjacent ectoderm 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 



a.O 



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; 

 C, hypoblast, consisting of a single layer of flattened cells; Me, medullary canal ; 

 Pv, protovertebra; IV d, Wolffian duct; So, somatopleure; Sp, splanchnopleure; 

 pp, pleuroperitoneal cavity; eh, riotochord; ao, dorsal aorta, containing blood- 

 cells; v, blood-vessels of the yolk-sac. (Kirkes after Foster and Balfour.) 



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. It is a solid, instead of a cylindrical, longitudinal col- 

 lection 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 direction opposite to those of the medullary plate 



