60 



THK FIRST DAY. 

 Fig. 19. 



[chap. 



Teansvbrsb Section through the Embryonic Region of 

 THE Blastoderm of a Chick shortly prior to the 

 Formation op the Medullary Groove and Noto- 



CHOED. 



M. median line of the section ; ep. epiblast ; LI. 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 peUucida. 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 knowii as the n otocbord (ch). In most 

 instances the commencing notocbord remains attached 

 to the hypoblast, after the mesoblast has at the sides 

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

 the notocbord 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 



