11 



P'ig. 11 Transverse section of same embryo at level indicated in preceding 

 figure, X 300. Reduced j. Slide 6, row 2, section 8. 



1 Aorta 4 Angiocyst of cardinal line 



2 Omphalomesenteric vein 5 Angiocyst of intermediate position 



3 Angiocyst of umbilical line 6 Coelom 



Figs. 12 and 13 Ventral view of model of mesoderm, mesenchyme and vas- 

 cular anlages of an embryo of twelve somites. Columbia Collection No. 547. The 

 coelomic mesoderm from the nephrotomcs laterad has been omitted, to allow the 

 hypectodermal mesenchyme (green) to be seen from below, with its endothelium 

 (red), vasof active cells (yellow) and blood islands (blue), X 300. Reduced $. 

 The umbilical vein extends into the incipient limb bud. 



1 Dorsal aorta 



2 Truncus arteriosus 



3 Hypectodermal mesenchyme 



4 Angiocysts of umbilical vein 



5 Coelomic angle 



6 Cells derived from visceral meso- 



derm in process of accretion to 

 the aorta cf. figure 28 



7 Mesenchyme about foregut contain- 



ing anlages of ventral aortic root 

 and caudal aortic arches. 



66 



