I'iss. iS aiul 9 Successive transverse sections of ICinl) 



No. 4()!l i)assinK 



tlirough the second somite, 

 section 1, X 300. Reduced \ 

 diate cell mass. 



Left side. Slide 4 row (1, section 12 and row ."i 

 To show derivation of endothelivmi fioni iTitcrmc 



mas.s 

 intermediate cell 



S Intermediate ccl] 



9 Projection ficjin 



mass 



10 Endothelial vesicle continuous with 



the projection from interme<liate 

 cell mass of the preceding section 



Fig. 10 Graphic reconstruction showing tlie distribution of cndotiielium 

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

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

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



4 Angiocysts of cardinal line 



1 Mesal margin of coelom 



2 Lateral margin of coelom 



3 Angiocysts of umbilical lino 



Sonnte 1 

 Somite 5 



64 



