III.] 



THE NOTOCHORD. 

 FIG. 20. 



61 



TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE 

 BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION 

 OF THE NOTOCHORD, BUT BEFORE THE APPEARANCE OF 

 THE MEDULLARY GROOVE. 



ep. epiblast ; hy. hypoblast ; ch. notochord ; me. mesoblast ; 

 ylc. yolk of germinal wall. 



FIG. 21. 



TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 

 18 HOURS. 



The section passes through the medullary groove we., at some 

 distance behind its front end. 



A. Epiblast. B. Mesoblast. C. Hypoblast. 



m.c. medullary groove ; m.f. medullary fold ; ch. notochord. 



