Figs. 8 and 9 Successive 

 through the second somite, 

 section 1, X 300. Reduced i- 

 diate cell mass. 



1 Ectoderm 



2 Parietal mesoderm 



3 Visceral mesoderm 



4 Entoderm 



5 Coelom 



6 Mesostroma 



7 Dorsal aorta 



8 



transverse sections of Embryo No. 409 passing 



Left side. Slide 4 row 6, section 12 and row 5, 



To show derivation of endothelium from intermc- 



8 Intermediate cell mass 



9 Projection from intermediate cell 



mass 



10 Endothelial vesicle continuous with 



the projection from intermediate 

 cell mass of the preceding section 



Fig. 10 Graphic reconstruction showing the distribution of endothelium 

 and vasofactive cells in the somatopleure of an embryo of ten somites. The cavi- 

 ties of angiocysts and clefts in vasofactive cells are shown in red. Columbia col- 

 lection No. 532. Left side, dorsal view, X 300. Reduced i. 



1 Mesal margin of coelom 



2 Lateral margin of coelom 



3 Angiocysts of umbilical line 



4 Angiocysts of cardinal line 



5 -Somite 1 



6 Somite 5 



64 



