DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 73 



These observations on the origin of the aorta, as well as the observations 

 indicating that the transitory vessel of the hindbrain differentiates from angio- 

 blasts in xi'tn, at once lead to the general question of the origin of the vascular system 

 All are agreed on the foundation of the work of von Baer, Remak, and His that 

 certain cells of the embryo differentiate to form angioblasts or vasoformative cells 

 in the early stages of embryonic life, and that these angioblasts increase by cell 

 division. There has, however, been a wide divergence of opinion as to whether the 

 differentiation of new angioblasts continues throughout life or whether there is a 

 limit to the period of differentiation, after which all the new angioblasts must come 

 from the growth of preceding endothelium. 



It is in relation to these two theories that I am making these studies on the 

 living blastoderm. It is, I think, clear that the study of blood-vessels in the stages 

 of their differentiation does not prove that they continue to differentiate out of 

 mesoderm throughout life, any more than the finding of several primordia for the 

 thy mus proves that new thymus glands continue to arise throughout life. The 

 question of the origin of the blood-vessels is now an exact one namely, which 

 vessels arise in the embryo (as does the aorta, at least, in part) by differentiation 

 of angioblasts, and which grow from previous vessels. In other words, how long 

 does the period of differentiation of angioblasts continue? 



His formulated the theory that the embryo itself is invaded by angioblasts 

 from the yolk-sac. This theory was based on the following observations: First, 

 that along the myotomes in the early stages angioblasts can be seen streaming 

 toward the axis of the embryo from the outer margin of the area pellucida; second, 

 that he observed no such streaming of angioblasts toward the axis of the embryo 

 in the zone between the head-fold and the first myotome (here, as a matter of fact, 

 a few angioblasts can be found in early stages, but are much scantier in number than 

 lower down) ; and third, that the most cephalic part of the head does not receive 

 angioblasts from the membranes. From these observations he concluded that the 

 vessels of the axis of the embryo must arise from a growth of the angioblasts which 

 could be seen to enter the embryo at certain places. 



Although these observations of His are for the most part correct, that a differen- 

 tiation of new angioblasts does take place along the axis of the embryo was shown 

 by two series of experiments. First, those of Hahn, who cut out the membranes of 

 one side of a chick in the stage of the primitive streak and obtained a few specimens 

 in which the membranes were entirely lacking, but the aorta was formed on the 

 injured side. Second, the experiments of Reagan, in which he cut off a part of the 

 head of the chick in the stages just before and just after the head-fold is visible, 

 and allowed the isolated parts to remain in the egg and develop. In these isolated 

 fragments he obtained vessels. 



The fact that angioblasts do differentiate in the axis of the embryo is con- 

 clusively proved by my observations, having watched certain cells differentiate 

 and join the aorta in the living blastoderm. In what I have called the second 

 zone of the axis of the embryo that is, the zone between the head-fold and the 

 first myotome the process can not be followed with such minute detail as is 



