6 DIRECT GROWTH OF VEINS BY SPROUTING. 



It thus becomes clear that in the study of the development of the vascular 

 system as a whole there are three great stages: First, a primary stage before the 

 circulation begins, when there is a differentiation of angioblasts and the formation 

 of a very primitive vascular system, including the heart, aorta, and primary veins; 

 second, a long stage of invasion of the entire body by the vascular system, a process 

 accomplished by both a progressive differentiation of new vessels and the continued 

 division and growth of the vessels already formed; and third, the final stage, in 

 which new growth or repair of the system is from preexisting endothelium. 



An exceedingly valuable analysis of these recent modifications on the subject 

 of the development of the vascular system was given by Streeter in 1918, in a study 

 on the developmental alterations in the vascular system of the brain of the human 

 embryo. He divided the development of the vessels of the brain into five suc- 

 cessive periods: First, a stage of differentiation of primordial endothelial blood- 

 containing channels, in which there are neither arteries nor veins and in which 

 it is practically impossible to make out a vascular pattern that is even a forerunner 

 of the pattern of the adult. This is the more strictly angioblastic phase. Second, 

 a stage characterized by the formation of certain primitive arteries and veins 

 and a capillary bed, through which blood circulates; the pattern is related to the 

 existing functional needs of the tissues and yet is not to be interpreted too closely 

 with reference to the adult pattern. Third and fourth, stages involved in the 

 adaptation of the vascular pattern to changes in the general region, and later 

 to changes in the specific developing organ, the vessels always conforming to alter- 

 ations in structure and to the immediate functional requirements of the organ. 

 Fifth, a period of the final histological differentiation of the ultimate, permanent 

 arteries and veins. It is clear that the entire vascular system must be restudied 

 with some such outline. 



These new concepts, in connection with the blood-vascular system as a whole, 

 apply with equal force to the subject of the lymphatic system. It has, I think, 

 become clear that the fundamental concept that the lymphatic system is a part 

 of the blood-vascular system, subject to the same laws of development, has been 

 strengthened rather than weakened by these new studies; that is to say, all the 

 observations that have gradually accumulated in connection with the develop- 

 ment of the lymphatic system fall into line with the idea that the lymphatics 

 also differentiate from angioblasts and develop as do the veins. In 1911 Hunting- 

 ton discussed the development of the lymphatic system from the standpoint of 

 the two processes of differentiation and growth and has throughout believed that 

 the Meyer-Lewis primordia that is, the isolated vesicles shown by Lewis (1906) 

 to characterize the pathway of developing lymphatic vessels arise locally. That 

 these isolated vesicles of Lewis do arise locally in the origin of the main lymphatic 

 trunks is undoubtedly true, since the time of their development corresponds with 

 periods during which blood-vessels themselves have been proved to be increasing 

 by a differentiation of angioblasts in loco. Their method of origin, however, has 

 proved to be the most important point. In connection with the origin of blood- 

 vessels it has been proved that these isolated vesicles of Lewis arise by a liquefaction 



