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JANE I. JIOBERTSON. 
region can already be distinguished ; the coeloniic chinks are 
just appearing, and, lying between the columnar layer and 
the endoderm are the comparatively large, flattened, heavily 
yolked cells, similar to the young mesenchyme cells that 
later form the rudiments of the vitelline veins. 
These vessel-cells, therefore, appear synchronously with the 
definition from the yolk or endoderm of that part of the 
splanchnic mesoderm that will, later, constitute the myocar- 
Text-fig. 27. 
Section showing vacnolation and fusion of adjacent vessel-cells. 
my. Myocardiac layer, v. c. Vessel-cells, y. Yolk. 
dium: they are closely wedged between the mesoderm and 
endoderm, and, posterior to the cardiac area, they merge into 
the ordinary cells over the yolk. A little later the endothe- 
lial vitelline vessels are formed from the vessel-cells between 
the yolk and the myocardiac mesoderm, apparently by a 
process of intra-cellular vacuolation combined with a 
syncytial fusion of adjacent cells and the budding off into 
the lumen of the vessel of free blood-cells from the inter- 
lacing branches of the syncytium (Text-fig. 27, v. c.). 
The endothelial cells of the lateral ventral aortas are 
similarly formed, while, once the ventral aort^ extend 
