10 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



that are still isolated, in the latter case resulting in small vesicles that subsequently 

 join the main plexus. These observations were made for the most part in the area 

 pellucida of the yolk-sac, but the same phenomena were seen also within the body 

 of the embryo. The dorsal aorta in the trunk region and a portion of it within 

 the head was seen to differentiate in situ in the above manner. 



The angioblasts are not all converted into endotheUum and blood-plasma; 

 some of them take part in the formation of red blood-corpuscles. During the 

 lumen formation in the angioblastic strands, small clumps of the original angio- 

 blasts become partially separated by the liquefaction of the cytoplasm around 

 them. Such masses soon show the presence of hemoglobin and constitute blood- 

 islands which eventually break apart as free red blood-cells and float away in the 

 blood-plasma. Blood-islands can also be seen to be derived secondarily from the 

 endothelium. 



Concerning the distribution of the earUest blood-vessels and concerning the 

 form and development of the primordial vascular system of the head, I have 

 followed the descriptions published by Evans (1912) and Sabin (1917a), both of 

 whom studied injected specimens of the chick and pig. On account of the 

 difficulty of making observations of this region in living preparations, the details 

 in the growth of this endothehal meshwork have not been actually seen. The 

 plexiform transformation of the aortic arches has been demonstrated only partly 

 and it is not known how much of the first endothelial system of the head is derived 

 from angioblasts that are differentiated locally and how much to the proliferation 

 of the cells of the aortic arches. It is possible, even after the laying-out of the main 

 parts of the primordial system, that angioblasts continue to differentiate in the 

 new territory around its margins and become incorporated with it. The essential 

 features, however, in the development of this s^ystem are admirably shown by 

 injected material, as can be seen in figures 398 (duck, 13 somites) and 393 (chick, 

 15 somites) of Evans (1912), and plate 1, figure 3 (chick, 9 somites), and plate 2, 

 figure 1 (chick, 14 somites) and figure 2 (chick, 16 somites) of Sabin (1917a). 



At about the time that the dorsal aortse become established in the head region, 

 it is seen that the first pair of aortic arches connecting them with the heart are 

 plexiform in character. Sprouting from this plexiform area an endothelial mesh- 

 work arises and extends dorsalward and caudalward toward the forebrain and 

 midbrain, to become the primary head-plexus. Very soon afterwards, a slender 

 longitudinal channel is formed bilaterally along the ventro-lateral margins of the 

 hindbrain which communicates with the primary head-plexus in front and cau- 

 dally with the anterior cardinal vein by way of the transverse veins of the first 

 two interspaces. A few slender communications are established very early between 

 it and the dorsal aorta of its respective side. This channel has been designated by 

 Sabin (1917a) as the "vasa primitiva rhombencephali." It seems probable from 

 her observations that this channel is not a derivative from sprouts from the dorsal 

 aorta or from the primary head-plexus, but is differentiated i7i situ, and its com- 

 munications with them are secondary. This slender channel, together with the 

 primary head-plexus, constitutes the primordial system from which all the blood- 



