138 THE STRUCTURE OF THE FOWL 



the oviduct ; the right atrophies ; and the 

 tubules of the Wolffian bodies degenerate. 



The Embryonic Circulatory System. — The 

 first indication of a blood -vascular system 

 is the appearance, at an early period, of 

 " blood -islands " in the opaque area of the 

 blastoderm. These become confluent and 

 bounded by a peripheral venous terminal sinus, 

 and thus give distinctive character to the 

 area vasculosa of the blastoderm. When the 

 horizontal splitting of the mesoblast takes 

 place, and the somatopleure and splanchno- 

 pleure separate from each other, the primitive 

 blood-vessels are confined to the splanchno- 

 pleure, i.e. they are present only in that 

 extra- embryonic membrane which surrounds 

 the yolk. The vascular network produced by 

 the confluence of the " blood-islands " graduafly 

 extends inwards from the opaque area until 

 the embryo itself is invaded. 



During the first half of the second day the 

 initial steps in the formation of the heart take 

 place. The heart arises in the form of two 

 thin-walled vessels running parallel to the 

 long axis of the embryo about the place where 

 vessels first reach the embryo from the vascular 

 area. The two tubes approach each other 



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