A R Y 



DIRECT GROWTH OF VEINS BY SPROUTING. 



In the chapter on the development of the vascular system in Keibel and Mall's 

 Manual of Human Embryology, published in 1911 and 1912, Evans gave an analysis 

 of the progress of embryology in connection with this system up to that time. 

 By his own work he then demonstrated in a series of beautiful studies that the 

 method of injection, as applied to the embryo, had made possible a great advance 

 in the phase of the subject concerned with the spread of vessels over the body. 

 In the introduction he said : 



"The two fundamental questions involved in the development of the vascular 

 system are (1) What is the origin of the blood-vessels in the body of the embryo? 

 (2) What is the primitive form of the vessels in any area, and the manner of change from 

 this to that of the adult? These two aspects of the subject thus concern themselves 

 with the problem of the cellular antecedents of the endothelium, on the one hand, and 

 with the principles governing the architecture of the vascular system, on the other. 

 To the former problem it is still impossible to give any decisive answer, but to the latter 

 I trust the reader will see that a flood of new light has come." 



It is now possible, I think, to give a definite answer to the first question; 

 we know just how blood-vessels begin, and it is therefore possible to show that 

 this knowledge of the fundamental genesis of the vascular system calls for certain 

 extensions and modifications of the prevailing views on the second question. 



A more careful examination of the old problem of angiogenesis, opened up by 

 the early embryologists, Wolff, Pander, von Baer, and others, in their studies on 

 blood-islands, has shown that blood-vessels begin by the differentiation of a new 

 type of cell, the angioblast of His or vasoformative cell of Ranvier. The final 

 proof that vessels are formed intracellularly was not obtained until the methods 

 of tissue-culture permitted the process to be actually watched in a living specimen. 

 The angioblast has certain characteristics. When it divides it forms syncytial 

 masses, which have two essential properties: (1) the power of liquefying in the 

 center, with the formation of plasma and vesicles; (2) the power of sprouting, 

 by which these groups of cells join similar groups, forming vessels or plexuses. 

 Both of these processes are necessary for the formation of the vascular system. 

 It has thus become clear that the most fundamental concept, in connection with 

 the vascular system, is that its essential tissue, endothelium, differentiates from 

 mesenchyme. This means that the processes by which vessels form are essentially 

 different from the processes by which the great tissue spaces (such as the arachnoidal 

 spaces and periotic spaces) form. Weed (1917) has followed the development of 

 the spaces of the arachnoid, Streeter (1917) the periotic spaces, and Shields, in 

 a paper now in preparation, the tendon-sheaths; in all of these structures the for- 

 mation or differentiation of a mesothelial lining is the last stage in the process, 

 while in the formation of the blood-vessels the differentiation of the lining-cell, 

 endothelium, is the first stage in the process. 



