252 OUTLINES OF CHORDATE DEVELOPMENT 



the anterior end remaining relatively fixed. The blastoderm, 

 too, shares in this elongation and becomes irregularly oval, and 

 then pear-shaped, the blunter end being anterior. 



FIG. 93. Total views of chick blastoderms. A. After Duval (modified) ; 

 B-D, after Lillie. A. Unincubated blastoderm, with primitive streak just form- 

 ing. B. Primitive streak formed, head process not yet indicated. C. Head 

 process formed; head-fold just commencing. D. Just before the establishment 

 of the first mesodermal somites, ac, Amnio-cardiac vesicle; cr, crescent-shaped 

 thickening at the posterior side of the blastoderm, in the region of endoderm 

 and mesoderm formation; g, primitive groove; hf, head-fold; hp, head process; 

 i, blood islands; m, axial thickening of mesoderm; mp, medullary plate; mr, 

 margin of mesoderm; n, Hensen's node; o, area opaca; p, area pellucida; pa, 

 proamnion; pi, primitive plate; pp, primitive pit; s, primitive streak; t, sinus 

 terminalis (marginal sinus); v, area vasculosa; vi, area vitellina interna. 



As the primitive streak elongates a narrow transparent line 

 appears along its middle; this is the primitive groove. Anteriorly 



