144 CHARLES R. STOCKARD 



lie, as described below, in small groups with originally no endo- 

 thelial cells around them. The groups are later either sur- 

 rounded by endothelium or taken into the early vessels as already 

 indicated. Nothing has been observed during a long study of 

 these cells on the living-yolk sac of normal embryos with a free 

 circulation, or on the yolk-sacs of embryos experimentally pre- 

 vented from establishing a circulation, or finally in sections of 

 embryos of various ages, that would indicate even a tendency 

 of endothelial cells to change into any type of blood corpuscle. 

 The endothelial cell, whether in the free and wandering mesen- 

 chymal state or constituting a part of the vessel wall, presents 

 a typical shape and clear appearance entirely distinct from the 

 wandering erythroblasts. 



In observing the early yolk vessels certain things may be seen 

 which are most important in interpreting sections supposedly 

 showing the transition of vascular endothelial cells into erythro- 

 blasts or primitive blood cells. Frequently, one or more cor- 

 puscles become entangled within the spaces and filaments existing 

 between the cells of the vessel wall. Other corpuscles flowing 

 in the current strike these entangled ones and beat against them 

 sometimes for hours before they become disentangled from the 

 vascular pits and holes, to flow again in the current. This is an 

 extremely common sight during the early hours of the circulation 

 of blood in any of the yolk vessels. 



It may now readily be imagined that if the embryo was killed 

 and f xed while the corpuscles were tangled in the spaces of the 

 vascular endothelium, a study of sections might produce the 

 impression that the cells of the vessel wall were " protruding into 

 the lumen and assuming the typical characters of primitive blood 

 cells," according to the description of many that imagine the 

 occurrence of such things. I have previously offered another 

 explanation of these appearances and many phenomena observed 

 on the living yolk-sac bear out the point of view. It may some- 

 times happen when the vascular endothelium encloses a group of 

 primitive corpuscles that one or more of these future blood 

 corpuscles come to lie in the plane of the vessel wall, or may 

 actually seem to form one of the cellular components of the wall. 



