THE DEVELOPMENT OF THE VASCULAR SYSTEM. 195 



to this accumulation, pressure would be exerted upon the cells bordering 

 the spaces. Seeking outlet from the confines of the spaces, the waste 

 products would move, or flow, and cause friction against the cells past 

 which they flow. Similarly, pressure and friction would result from the 

 movement of nutritive materials to and through the tissue. The plastic 

 mesenchymal cells, reacting to these mechanical influences, would tend to 

 become flat, and the continued operatic of the factors would result in a 

 smooth- walled tube in which the movement of fluid is greatly facilitated. 



The reaction of the irregular mesenchymal cells to the mechanical in- 

 fluences of pressure and friction is, of course, the crux of the question. It 

 has been shown experimentally that cells of this type do react to mechanical 

 stimuli. Smooth non-irritating foreign bodies have been imbedded in the 

 loose connective tissue of an animal and the cells in contact therewith be- 

 came flat and formed a mosaic apparently identical with simple squamous 

 epithelium or endothelium. In the growth of mesenchymal tissue outside 

 of the body (in vitro) it has been observed that the cells flatten against 

 foreign substances which may be present. 



In the embryo it has been observed that where blood vessels disappear, 

 which they do in certain regions, the endothelium does not degenerate but 

 that the cells assume irregular branching forms. This would indicate that 

 endothelium comprises merely modified mesenchymal cells and that upon 

 removal of the factors incident to the pressure and friction of blood flow 

 the cells reassume the indifferent character of mesenchyme, thus reverting 

 to the mesenchymal type. It militates, therefore, against the view that 

 endothelium is a specific tissue. 



It is generally recognized, whether or not the endothelium originates 

 in situ, that a capillary network precedes the formation of larger vessels. 

 For instance, the vitelline plexus of capillaries (p. 186) antedates any of the 

 larger vitelline vessels which later carry blood to and from the embryo. 

 The establishment of vascular trunks in this plexus of small vessels seems to 

 be dependent upon the same mechanical factors that were considered as 

 operative in the origin of vessels; viz.: pressure and friction. If the volume 

 of blood that flows through a given capillary network at a given rate is in- 

 creased the flow will naturally follow the channels that offer the least re- 

 sistance, and these channels will increase in size sufficiently to accommodate 

 the greater volume. A few channels, or perhaps even only one, will form the 

 most direct course, and the angles in the course will be still further reduced 

 as the blood stream impinges upon the walls of the vessels. In this manner 

 a large vessel, or main vascular trunk, is established and the remaining 

 smaller vessels constitute its branches or tributaries. A rather crude analogy 

 would be the draining of a swamp in which a small rivulet, once gaming 



