Figs. 2.S t(i 3o Serial transversa sections nl :iu cnibrvu oi twelve somites, ('ol- 

 umbia Collection Xo. 'i-iT. slide 7, row L', sections 7 to 12, X o(KI. licihiced ',. 



4 N'otocord 



5 Entoderm 

 (1 Coelom 



1 Aorta 



2 Cells derived from visceral meso- 



derm in process of accretion t<i 

 the aorta 

 .'i Cells of sclerotome in syncytial con- 

 tinnit V with endothelium of aorta 



7 Angiocyst added to aorta; a ren 

 nant of se|ituni is still visible 



St 



