78 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



ORIGIN OF THE CARDINAL VEINS. 



It is now important to consider the cardinal veins how they arise, how 

 they become related to the primitive neural vessels, and how they become con- 

 nected with the heart through the duct of Cuvier. The general relations of the 

 cardinal veins are best shown in plate 2, figure 1, from a chick of 14 somites, but 

 their origin can be traced back to the stage of 9 somites. They form as a longi- 

 tudinal anastomosis which connects diverticula of the aorta that project dorsal- 

 ward between the somites. In 1906 these dorsal diverticula were described by 

 Grafe, who stated that the cardinal veins arose from sinus-like projections from 

 the aorta. That the cardinal veins arise from dorsal intersegmental branches 

 of the aorta was shown by Rabl in 1892 and by Hoffmann in selachians in 1893. 



The condition of the aorta just before the diverticula arise is of importance. 

 Up to the stage of 9 somites it is clear that the entire aorta which can be seen 

 from the ventral aspect in the living blastoderm is connected with the plexus of 

 the area vasculosa through so-called ventral branches which extend lateralward. 

 Even cephalic to the first myotome a few chains of angioblasts connect the aorta 

 with the plexus of the area vasculosa. However, these tiny branches all along 

 the lateral border of the aorta are seldom injected, except opposite the caudal end 

 of the aorta (plate 1, fig. 2). 



When the chick has 9 somites a new set of aortic branches begins to form, 

 which are very distinct from the lateral vessels. In the living blastoderm of from 

 9 to 12 somites it can be seen that diverticula of the aorta project dorsalward 

 into the interspaces. The more cephalic of these diverticula are dorso-lateral, 

 as shown on the right side in text-figure 2, from a section through the first inter- 

 space; the more caudal ones are distinctly dorsal, as seen for the twelfth interspace 

 in text-figure 3. This is due to the fact that at the stage of 12 somites the aorta 

 is obliquely placed with reference to the lateral margin of the myotomes. As 

 shown in text-figure 2, in the first interspace the lateral margin of the aorta is in 

 the lateral line, while in the twelfth interspace, as shown in text-figure 3, the aorta 

 is directly under the lateral line. The first two of these diverticula have been 

 seen at the stage of 9 somites; and they are present in all of the interspaces at the 

 stage of 12 somites. In a total preparation of a chick of 12 somites the ink lodges 

 in these dorsal diverticula and forms dark streaks across the aorta from the dorsal 

 aspect; these streaks are very characteristic, but are difficult to indicate in a draw- 

 ing. The specimen of plate 1, figure 2, shows such streaks across the aorta in the 

 interspaces. 



The diverticula begin at the time when the first two somites lie within the 

 arch formed by the two omphalo-mesenteric veins where they join the heart. In 

 this connection I have tried to determine whether there is a constant relation in 

 regard to the time when the cardiac or head fold reaches the level of the first 

 somite; and in this regard the figures in His's " Untersuchungen ueber die erste 

 Anlage des Wirbelthierleibes " (1868), plate xu, and those in Lilly's "Develop- 

 ment of the Chick" (1908) are the most helpful. In general, at the stage of 9 

 somites the position of the first somite is about as shown in Lilly's figure 61, page 



