4oo 



AN INTRODUCTION TO ZOOLOGY 



the appearance of the streak the blastoderm begins to increase fairly 

 rapidly, mainly by the extension of the area opaca equally in all 

 directions. The streak itself also grows quickly hi a posterior 



o 

 -P 



FIG. 138. Chick blastoderms. From Kellicott. 



A after Duyal (modified) ; B-D, after Lillie. A, unincubated blastoderm, with primitive 

 streak just forming. 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 meso- 

 dermal somites, ac, amnio-cardiac vesicle ; cr, crescent-shaped thickening at the posterior 

 side of the blastoderm, in the region of entoderm and mesoderm formation ; g., primitive groove; 

 hf, head fold ; hp, head process ; ., blood islands ; m., axial thickening of mesoderm ; mp, 

 medullary plate ; mr, margin of mesoderm ; n., Henson's node ; o., area opaca ; p., area pellu- 

 cida ; pa, proamnion ; pi, primitive plate ; pp, primitive pit ; s., primitive streak ; L, sinus 

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



direction its anterior end remaining relatively stationary. With this 

 growth goes a lengthening of the area pellucida, which first becomes 

 oval and later pear-shaped. As the primitive streak lengthens a 

 superficial gutter-like depression, the primitive groove, appears. 



