11 



Fig. 11 Transverse section of same enihryn at level indieateil in prei'edinj; 

 figure, X 30(). Hedueed 1. Slide (i, row 2, section S. 



1 Aorta 4 Angioeyst of cardinal line 



2 OMiplialouiesentcric vein .') Angiocyst of intermediate position 



3 Angioeysl of UTid>dical line 6 Coelom 



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

 cular anlages of an embryo of twelve somites. Cohunbia Collection \o..)47. The 

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

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

 (red), vasofactive cells (yellow) and blood islands (blue). X 300. Re<l\iced \. 

 Tlir umbilical vein extends inlo the incipient limb bud. 



1 IJursal aorta 



2 Truncus arteriosus 



3 Hy])ecto(lernial mesenchyme 



4 AngiocysLs of umbilical vein 

 .^ C'lM'lomic angle 



fells derived from visceral meso- 

 derm in jiroce.ss of accretion to 

 the aorta cf. figure 2.S 



Mesenchyme about foregut contain- 

 ing anlages of ventral aortic root 

 and caudal aortic arches. 



66 



