230 



TEXT-BOOK OF EMBRYOLOGY. 



Fig. 200. — Surface views of chick blastoderms. Riickert, Hertwig 



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



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. 



-» •**' ^ 



V: 



■4?-,- '"**> ■^■'. <!•!>' 

 • * - , k-" * 



ml 





•?»„ 



3*. 





Ectoderm 



Mesoderm 



Entoderm 

 (yolk cells) 



Blood island 



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



