THE EARLY DEVELOPMENT OF THE CHICK 277 



encroaching upon the pellucid area. The broad area vitellina 



is further separable into two regions, 



an inner band, the vitellina in- 



terna, and a narrow area vitellina 



externa. 



The examination of sections 

 through the blastoderm shows that 

 the areas pellucida and vasculosa are 

 composed of the three germ layers, 

 the area vitellina of ectoderm and 

 endoderm only. Except in the re- 

 gion about the head of the embryo, 

 the area pellucida is extensively 

 vascularized, but the vessels are still 

 comparatively free from corpuscles. 

 The pellucid and opaque areas are 

 further characterized by the pres- 

 ence of extraembryonic ccelom be- 

 tween the two sheets of mesoderm 

 (Fig. 108). The vascular area is 

 definitely limited peripherally by the 

 sinus terminalis. The area yitellina 

 externa is the region where the ex- 

 tension of the blastoderm is actually 

 being carried forward. Here are 

 found conditions similar to those de- 

 scribed in the early part of this 

 chapter. The extreme margin 

 ("margin of overgrowth" Lillie) is 

 formed by a single layer of super- 

 ficial cells called "ectoderm/ 7 while 

 between this margin and the area 

 vitellina interna is a band (" zone of 

 junction" Lillie) which is the 

 equivalent of the marginal periblast, 

 where the processes of true blasto- 

 derm formation are chiefly localized, 



