256 OUTLINES OF CHORDATE DEVELOPMENT 



^^ 



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 lacunae. 

 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 coelomic "meso- 

 I -! derm" and the blood islands 

 I ^ * J a are differentiated, forms the 

 ^ 2S "I jj rudiment of the yolk-sac en- 

 '! | 1 1 | doderm, the development of 

 "8 1 1 which we shall describe pres- 

 1 1 1. -gently. 



"S ! 2 =5 1 At this point we should 

 Jj *- consider briefly the relation 

 !! J 1 1 of the primitive streak and 



"^ M bC 



I M S ..2 associated structures to the 



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formation of the 



* '! 23 - a rudiments of the true em- 



\^ A fe | bryo. From what has been 



l'fi"S'l * said in the early part of this 



J> g J chapter, we should not ex- 



( * I ! 1 pect to find that all of the 



l^iSi ^ structures mentioned in the 



H 1 preceding paragraphs will be 



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



definitive embryo. As a matter of fact, of 



