STRUCTURE OF TWENTY-FOUR HOUR CHICKS 51 



The Area Vasculosa. In a 24-hour chick the boundary be- 

 tween area opaca and area pellucida has the same appearance 

 and significance as in chicks of 1 8 to 20 hours. There is, how- 

 ever, a very marked difference between the proximal portion 

 of the area opaca adjacent to the area pellucida and the more 

 distal portions of the area opaca. The proximal region is much 

 darker and has a somewhat mottled appearance (Fig. 15). The 

 greater density of this region is due to its invasion by mesoderm 

 which makes it thicker and therefore more opaque in transmitted 

 light (Fig. 17, D). The boundary between the inner and outer 

 zones of the area opaca is established by the extent to which 

 the mesoderm has grown peripherally. The distal zone is 

 called the area opaca vitellina because the yolk alone underlies 

 it. The proximal zone into which mesoderm has grown is 

 known aslhe area opaca vasculosa, because it is from the meso- 

 derm in this region that the yolk-sac blood vessels arise. The 

 mottled appearance of this region is due to the aggregation of 

 mesoderm into cell clusters, or blood islands, which mark the 

 initial step in the formation of blood vessels and blood corpus- 

 cles. Later in development the formation of blood islands and 

 vessels extends in toward the body of the embryo from its 

 place of earliest appearance in the area opaca and involves the 

 mesoderm of the area pellucida. The histological nature of 

 the blood islands will be taken up in connection with later 

 stages where their development is more advanced. 



