AVES. 



157 



from the front end of the primitive streak, but stopping short at 

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

 the area pellucida. In section (fig. 100) this opaque line is seen 

 to be due to a special concentration of cells in the form of a cord. 



FIG. 99. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE 

 BLASTODERM OF A CHICK SHORTLY PRIOR TO THE FORMATION OF THE MEDULLARY 

 GROOVE AND NOTOCHORD. 



m. median line of the section; ep. epiblast; //. lower layer cells (primitive hypo- 

 blast) not yet completely differentiated into mesoblast and hypoblast ; n. nuclei of 

 germinal wall. 



This cord is the commencement of the notochord (ch\ In some 

 instances the commencing notochord remains attached to the 

 hypoblast, while the mesoblast is laterally quite distinct (vide 

 fig. 100), and is therefore formed in the same manner as in most 

 Ichthyopsida ; while in other instances, and always apparently 

 in the Goose (Gasser, No. 127), 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 hypoblast, and that the notochord originates as a 

 median plate, simultaneously with the mesoblast, with which it 

 may sometimes be at first continuous. 



Kolliker holds that the mesoblast of the region of the embryo is derived 

 from a forward growth from the primitive streak. There is no theoretical 

 objection to this view, and I think it would be impossible to shew for certain 

 by sections whether or not there is a growth such as he describes ; but such 

 sections as that represented in fig. 99 (and I have series of similar sections 

 from several embryos) appear to me to be conclusive in favour of the view 

 that the mesoblast of the region of the embryo is to a large extent derived 



