256 OUTLINES OF CHORDATE DEVELOPMENT 



11*3 



area derived from the primitive streak, it is commonly supposed 



that this relation is second- 

 ary. The blood islands later 

 become hollowed out, their 

 constituent cells forming 

 both the walls and the cor- 

 puscular contents of the ir- 

 regular vascular la ounce. 

 These soon anastomose with 

 one another forming a com- 

 plex network throughout the 

 area vasculosa; still later 

 this net connects with the 

 vascular structures of the 

 pellucid area and of the em- 

 bryo. The cellular portion 

 of the germ wall remaining 

 after the ccelomic "meso- 



CD 



T?1 derm" and the blood islands 

 4 * are differentiated, forms the 

 j> J ^ "g '3 rudiment of the yolk-sac en- 

 a J 1 2 a doderm, the development of 

 ' "" 1 3 f which we shall describe pres- 

 | g I. f. gently. 



~- a 

 bi 



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 rf c3 



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 o a " 



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should 



-g consider briefly the relation 

 b 1 of the iDrimitive streak and 



- - C 



structures to the 

 * a rudiments of the true em- 







fe " bryo. From what has been 



. -~ o ^ 



- said in the early part of this 

 j2 fi J chapter, we should not ex- 

 '. S * S pect to find that all of the 



iO O ^ * *" ^ 



03 7 3 1 structures mentioned in the 



>*} ^ ^ ra 



1 ^ preceding paragraphs will be 

 ^5 0*0 f oun( j to contribute to the 

 formation of the definitive embryo. As a matter of fact, of 



