236 



TEXT-BOOK OF EMBRYOLOGY. 



FIG. 209. Surface views ot chick blastoderms, Rtickert, Hertwig 



a, Blastoderm with primitive streak and head process; showing blood islands (dark spots in crescent- 



shaped area in lower part of figure). 



b, Blastoderm with 6 pairs of primitive segments. Reticulated appearance is due to blood islands 



(dark spots) and to developing vessels, the entire reticulated area being the area vasculosa. 



Blood island 



Fro. 210 Section of blastoderm (area opaca) of chick of 27 hours' incubation. Photograph. 



