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STRUCTURE OF THIRTY-THREE HOUR CHICKS 



67 



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from each other and rounded. They soon come to contain 

 haemoglobin and constitute the primitive blood corpuscles. 

 The fluid accumulated in the blood islands serves as a vehicle 

 in which the corpuscles are suspended and conveyed along 

 the vessels. 



ectoderm 



yolk granule 





mesoderm 



blood island 



yolk 



B 



ectoderm 



central cells of 

 blood island 



peripheral cell 

 of blood island 



ectoderm 



blood cells 

 /entoderm cell 



somatic 

 mesoderm 

 coelom 



endothelial cell 

 lumen 



yolk 



''^.^i^^ 



Pig. 25. — Drawings to show the cellular organization of blood islands at 

 three stages in their differentiation. The location of the areas drawn with 

 reference to the body of the embryo and other structures of the blastoderm 

 can be ascertained by reference to Fig. 17, D. 



A, from blastoderm of 18-hour chick; B, from blastoderm of 24-hour chick; 

 C, from blastoderm of 33-hour chick. 



The differentiation of the blood islands in the manner de- 

 scribed begins first in the peripheral part of the area vasculosa 

 and from there extends toward the body of the embryo. By 

 33 hours of incubation the extra-embryonic vascular plexus has 

 extended inward and made connection with the omphalomesen- 

 teric veins which, originating within the body of the embryo 



