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 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 

 the ccelomic " meso- 

 derm" and the blood islands 

 forms the 



| rudiment of the yolk-sac en- 

 3 | doderm, the development of 

 3 1 1 1- which we shall describe pres- 

 ently. 



At this point we should 

 consider briefly the relation 

 of the primitive streak and 

 associated structures to the 

 rudiments of the true em- 

 bryo. From what has been 

 said in the early part of this 

 ^J chapter, we should not ex- 

 * | pect to find that all of the 

 a structures mentioned in the 

 o> 1 * preceding paragraphs will be 

 43 ^ 8 ^ found to contribute to the 

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



0<a W -K I after 

 *j S'Sjs'S 



C8 T3 



> . i fl S 

 w ^ a are differentiated 



