142 



CHARLES R. STOCKARD 



Figs. 28, 29 and 30. Portions of vessels from three yolk-sacs of 6 day embryos. 

 The vessels show blind endothelial sacs projecting from their walls. The con- 

 stituent cells of these sacs are distinctly seen, and still retain their wandering 

 mesenchymal characters. Filamentous processes from these cells may extend 

 entirely across the lumen and fuse with processes from the cells on the opposite 

 side of the wall. Corpuscles are often entangled in the filaments as well as the 

 spaces between the endothelial cells. X, a resting corpuscle harbored behind 

 an endothelial projection. 



These porous or incomplete endothelial walls permit the blood 

 cells to occasionally escape from the vessel cavity and become 

 free within the space of the yolk-sac; or, on the other hand, a 

 growing vascular tip may be observed at certain stages to come 

 in contact with a group of ery.throblasts, or actually a blood 

 island unsurrounded by vascular endothelium. The tip of the 



