STRUCTURE OF THIRTY-THREE HOUR CHICKS 



73 



fore-gut the ventral aortic roots turning dorsad curve around 

 it, and then extend caudad, dorsal to the gut, as the paired 

 dorsal aortae (Figs. 23, 24 and Fig. 28, B). Few conspicuous 



PT- 



? ,'*n 



yolk 



ectoderm 

 of blastoderm - 



head fold 

 of amnion 



made mesoderm I lateral plate 



splanchnic mesoderm J of mesoderm 



FIG. 28. Diagrams of sections of 33-hour chick. The location of each section 

 is indicated on a small outline sketch of the entire embryo.* 



branches arise from the aortae at this stage but as development 

 progresses branches extend to the various parts of the embryo 

 and the aortae become the main efferent conducting vessels of 



