Figs. 21 to 23 Ventral view of a model of the mesoderm (white), splanchno- 

 pleuric mesenehyme (green), vasofactive cells (yellow) and endothelium (red) 

 of an embryo of two somites. Columbia Collection No. 539, X 300. Reduced 3. 



1 Anlages of ventral aorta and heart 5 From this point craniad the meso- 



2 Anlages of dorsal aorta derm is continuous across the 



3 First intersomitic cleft median line 



4 Second intersomitic cleft 6 From this point caudad the meso- 



derm is continuous across the me- 

 dian line 



76 



