THE DEVELOPMENT OF THE VASCULAR SYSTEM 225 



proliferation and migration of the endotheiial elements. This method, of 

 course, would demonstrate vessels only so far as the lumina are continuous. 

 Solid cords of cells which extend beyond the field of injection are interpreted 

 as cords of endotheiial cells which subsequently acquire lumina and become 

 capillary tubes. If this theory is correct then the vascularization of the 

 area pellucida and of the embryonic body would be effected through true 

 outgrowths of the original endothelium of the opaque area. Possible 

 exceptions to this, as noted above, are the rudiments of the heart, the aorta 

 and the cardinal veins which arise in situ as do the first vascular rudiments. 

 Observations upon growing vessels in living embryos, in which strands 

 of cells were seen to extend from the endothelium already present, have 

 also been accepted as evidence in favor of this view. 



The evidence afforded by injected specimens has been attacked by those 

 who believe in the in situ origin of vessels, on the ground that the injection 

 shows only vessels with continuous lumina and does not prove the non- 

 existence of isolated vascular rudiments beyond the field of injection. It is 

 claimed that the vascular field becomes more extensive through the gradual 

 addition of such isolated spaces to the channels already continuous, in the 

 same manner that the primitive blood spaces unite to form a network, and 

 the claim is supported by demonstration of these spaces in the mesenchymal 

 tissue with every gradation between the bordering flattened cells (endo- 

 thelium) and the branching irregular mesenchymal cells. The actual 

 formation of intercellular spaces with flat bordering cells and their union 

 with vascular channels have been observed in the living chick blastoderm. 

 Experimental evidence has also been brought to bear in favor of the view 

 that vessels arise in situ. The area opaca was entirely removed from the 

 chick blastoderm before any vascular rudiments had appeared in the area 

 pellucida and the blastoderm was then allowed to develop further; it was 

 found that vascular rudiments appeared both in the area pellucida and 

 embryonic body with practically the same disposition as in the normal 

 embryo. 



The concept that the vascular channels are structural expressions of the 

 functional necessity of carrying nutritive materials to the tissues and waste 

 products away from them leads to consideration of such factors as may be 

 involved in the formation of vessels; that is, factors that would cause plastic 

 cells, like those of the mesenchyme in which the earliest and simplest vessels 

 appear, to change in character and rearrange themselves to form capillary 

 tubes. In a mass of mesenchymal tissue, in which there is a resemblance 

 to a sponge with the cellular elements representing the parenchyma of the 

 sponge and the intercellular tissue spaces the interstices, the products of 

 cell activity naturally accumulate in the intercellular spaces. Incident 



