Fig. 25 Transverse section of an embryo of four somites at the level indicated 

 by leader 3 in figure 24. Columbia Collection No. 409, slide 5, row 6, section 3, 

 X 300. Reduced \. The several angyocysts arc in contact but their lumina arc 

 not contimious. 



1 Aorta 



2 Juxta-neural line 



3 Quintal anlage 



Vasofactive cells attached to juxta- 

 neural vessel; this cleft-like lu- 

 men is not continuous with that 

 of the vessel 



Fig. 26 Ventral view of a model of the mesoderm (white), mesenchyme (green), 

 vasofactive cells (yellow) and endothelium (red) of an embryo of eight somites. 

 Columbia Collection No. 530, X 300. Reduced \. 



1 Dorsal aorta 



2 Omphalomesenteric vein 



3 Omphalomesenteric plexus 



4 Projections of visceral mesoderm 



which later become connected 

 with the aorta 



5 Position of heart 



84 



