ORIGIN OF BLOOD AND ENDOTHELIUM 247 



ment. The anterior end of the intermediate cell mass showing 

 erythroblasts is just seen where the body of the embryo turns 

 over the yolk sphere. 



Figure 9 illustrates a lateral view of a small embryo. In all 

 of these embryos in which the blood fails to circulate the fins 

 are much smaller and less well developed than in the control, 

 the entire body of the embryo is smaller and the whole appear- 

 ance is that of a general developmental arrest, the rate of develop- 

 ment being behind the normal. Yet such individuals have rather 

 perfectly formed bodies, are capable of movement and seem in 

 general to be very well developed, their only defect, so far as 

 can be determined in many cases, is the absence of the circulation 

 of the blood. 



In figure 9 the heart is again sheathed with pigment cells, the 

 blood cells in the intermediate cell mass are very ■ distinct^ 

 present in the posterior body region, and in this individual a 

 lateral vein in the position of the posterior cardinal also con- 

 tains blood corpuscles. This appearance is seen in a number 

 of individuals and may merely result from the fact that in 

 these the intermediate cell mass is bilateral or split rather 

 than entirely median in position. Such an explanation will 

 seem probable, I think, after a consideration of the embryos in 

 section. 



Figures 12, 13 and 14 show three individuals of ten days old. 

 These happen to be more or less abnormal. Figure 12 has very 

 small eyes but the general body structure and shape are fairly 

 normal. The pericardium is dilated, and the heart is small and 

 pulsating feebly with a little pigment towards its aortic end. 

 There is a great accumulation of pigment cells around the pos- 

 terior region of the yolk sphere and near the distended Kupffer's 

 vesicle in this embryo. Here again the cardinal veins are seen 

 to be loaded with blood and only in the posterior body region 

 do the two lateral masses come to unite into a median cell mass. 

 Figure 13 shows much the same conditions, the heart is a mere 

 filament indicated by the chromatophore along it. 



Figure 14 gives a dorsal view of an embryo of ten days. The 

 pigment spots are very few in number and the embryo has a pale 



