60 



THE FIRST DAY. 



I 



Fig. ly. 



[chap. 



Transverse Section through the Embryonic Eegion of 

 THE Blastoderm op a Chick shortly trior to the 

 Formation of the Medullary Groove and Noto- 



CHORD. 



m. median line of the section ; ep. epiblast ; l.l. lower layer cells 

 (primitive hypoblast) not yet completely differentiated into 

 mesoblast and hypoblast ; n. nuclei. 



fold the future head-fold near the front end of the 

 area pellucida. In section (Fig. 20) this opaque line is 

 seen to be due to a special concentration of cells in the 

 form of a cord. This cord is the commencement of an 

 extremely important structure found in all vertebrate 

 embryos, which is known as the notochord {ch). In most 

 instances the commencing notochord remains attached 

 to the hypoblast, after the mesoblast has at the sides 

 become quite detached {vide Fig. 20), but in other cases 

 the notochord appears to become differentiated in the 

 already separated layer of mesoblast. In all cases the 

 notochord and the hypoblast below it unite with the front 

 end of the primitive streak; with which also the two 

 lateral plates of mesoblast become continuous. 



From what has just been said it is clear that in the 

 region of the embryo the mesoblast originates as two 

 lateral plates split off from the primitive hypoblast, and 



