DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 143 



vessel seems to disorganize to some extent and its cellular ele- 

 ments slowly surround the group of corpuscles which are later 

 taken into the circulation as the current becomes established 

 in the including vessel. 



Unfortunately, I have never been able to observe the con- 

 secutive steps in any one case of this kind, so that an absolutely 

 positive statement cannot be made at present. Yet numerous 

 observations of the contact of vascular sprouts with groups of 

 uncovered corpuscles and the ends of such sprouts containing 

 corpuscles, as well as other arrangements, would indicate that the 

 behavior of the endothelium in surrounding the naked groups of 

 erythroblasts on the yolk-sac probably takes place about in the 

 manner just outlined. 



Figure 32, page 568, illustrates a highly magnified field on the 

 yolk-sac of a 90 hour embryo. This field shows a very interest- 

 ing composite of the vascular condition at such an age. The 

 rapidly flowing blood current is freely . passing through the 

 vessel on the right. A short circuit is forming across below the 

 curve of an arch in the vessel. This small vessel permits only 

 a single line of corpuscles to pass. At this time only one cor- 

 puscle has entered and it is caught in the narrow tube. This 

 corpuscle remained fixed for a long time and so enabled a com- 

 parison of its structure and appearance with the erythroblasts 

 forming the group just below the huge black chromatophore. 

 The cells of this group are uncovered by endothelium. On the 

 left of the figure a portion of a vascular net not yet connected 

 with the circulating current presents the typical appearance of 

 an early blood vessel formation. The individual cells are loosely 

 associated and the tip projecting towards the right slowly changes 

 position. This tip later approached the group of erythroblasts 

 and finally these cells were all included within the vessel by a 

 process which, as stated above, I was not able to follow definitely. 



After closely studying these early vessels in a large number 

 of living yolk-sacs, the observer is able to establish very clearly 

 the actual relationship between the vascular endothelial cells and 

 the erythroblast or early blood corpuscles. The corpuscles on 

 the yolk are always of a distinctly different shape and size, and 



