242 THE ("HICK. 



As shown in Figs. 107 and 109, the mesoblast very early 

 extends outwards beyond the area pellucida so as to underlie 

 the inner zone of the area opaca ; this three-layered zone of the 

 area opaca, represented by the dark shading in Figs. 107 and 

 109, is known as the area vasculosa, because the blood-vessels 

 which absorb the yolk and carry it to the embryo are very early 

 developed in it (cf. Figs. 98, 99, av). 



G. The Notochord. 



Before the sheet of mesoblast ceils, spoken of above as 

 group (ii), separates completely from the hypoblast, a distinc- 

 tion may be noticed in it between a median longitudinal rod of 

 cells, and two lateral tracts. This median rod is the notochord 

 (Fig. 117, CH) ; and the cells of which it consists are, from the 

 first, more closely compacted than those of the lateral tracts. 



The notochord sometimes remains attached to the hypoblast 

 after the lateral mesoblastic sheets have completely separated 

 from this ; in other specimens the entire sheet of cells separates 

 as one continuous layer, which then divides into the median 

 rod, or notochord, and the two lateral mesoblastic tracts. 



The notochord of the chick has, accordingly, been described 

 by some authorities as of hypoblastic, by others as of mesoblastic 

 origin ; the component cells are, however, in all cases derived 

 directly from the. hypoblast, and the difference is merely in the 

 relative times of separation of the notochord from the lateral 

 sheets of mesoblast, and from the underlying hypoblast respec- 

 tively. 



The notochord lies entirely in the part of the blastoderm 

 in front of the primitive streak ; its posterior end is, however, 

 directly continuous with the anterior end of the primitive streak. 

 Inasmuch as the primitive streak cells are continuous with the 

 epiblast, and the notochord is, at any rate at first, continuous 

 with the hypoblast, it follows that the three germinal layers, 

 epiblast, mesoblast, and hypoblast, are directly continuous and 

 fused with one another at this point, which marks the hinder 

 end of the chick embryo, and corresponds to the anterior or 

 dorsal lip of the blastopore in the frog (cf. Fig. 60, b). 



7. The Mesoblastic Somites and the Coelom. 



The mesoblast of either side forms at first a continuous sheet 

 of loosely arranged cells, which in transverse section is somewhat 



