Figs. 28 to 33 Serial transverse sections of an embryo of twelve somites, Col 

 umbia Collection No. 547, slide 7, row 2, sections 7 to 12, X 300. Reduced J. 



1 Aorta 



2 Cells derived from visceral meso- 



derm in process of accretion to 

 the aorta 



3 Cells of sclerotome in syncytial con- 

 tinuity with endothelium of aorta 



4 Notocord 



5 Entoderm 



6 Coelom 



7 Angiocyst added to aorta; a rem- 



nant of septum is still visible 



87 



