DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



75 



Vca 



which cause the fusing of the two aortse lower down. Opposite the region of the 

 heart some of the lateral sprouts extend out in the somatopleure, as shown in 

 plate 2, figure 1, and in sections in Duval's plate xvn, figure 276. Opposite the 

 midbrain some of these lateral sprouts may connect with the superficial plexus. 



The dorsal cephalic aorta itself, as seen in plate 1, figure 3, is very large. 

 From the dorsal aspect it is broad and flat and is placed in a nearly exact trans- 

 verse axis instead of in the oblique position which it subsequently assumes. 



The next stages in development, including the relations of the primitive cere- 

 bral vessels and the cardinal system of veins up to the stage of 14 somites, I shall 

 describe with the aid of two total preparations from chicks of 12 and 14 somites 

 and three sections from the stage of 12 somites (figure 2 of plate 1, figure 1 of plate 2, 

 and text-figs. 1, 2, and 3). 



At the stage of 12 somites the vt 



aorta is very readily injected. The 

 vessels to the brain, however, though 

 they connect with the aorta, are diffi- 

 cult to inject. In plate 1, figure 2, 

 is shown the usual result of injecting 

 a small quantity of ink into the 

 omphalo-mesenteric veins at the 

 stage of 12 somites; the ink passes 

 through the heart and the aorta into 

 the capillaries, which are the fore- 

 runners of the Omphalo-mesenteric FIG- 2. Transverse section of an injected chick of 12 somites, passing 

 m, . j i i through the first interspace to show the relations of the primitive 



arteries. I his is true, even though 

 vessels to the entire brain that is, 

 to the forebrain, midbrain (text-fig. 

 1), and hindbrain (text-fig. 2) are 

 present and connect with the aorta, 

 although the common cardinal vein 

 is present down to the twelfth interspace (text-fig. 3) and the entire lateral border 

 of the aorta opposite the myotomes is connected with the plexus of the area vascu- 

 losa. In plate 1, figure 2, the only branch of the aorta injected is an unusual dorsal 

 branch opposite the tenth somite, passing out into the somatopleure. In order 

 to fill these different branches of the aorta in the stages shown in figure 2 of 

 plate 1 and figure 1 of plate 2 before the circulation has begun, it is necessary to 

 introduce the needle into the aorta and inject, as it were, backwards. In this way 

 the pressure in the aorta is probably raised, the heart being sufficiently stimulated 

 by the ink to force the injection mass into the tiny channels that would otherwise 

 remain empty. Indeed, after the circulation has begun, if only a very small 

 quantity of ink enters the heart it will return to the area vasculosa without inject- 

 ing the branches of the aorta within the embryo. These fill up only as the injection 

 is continued and the heart becomes well filled with ink. 



vessel of the hindbrain, the transverse vein of the first interspace, 

 and the anterior cardinal vein. The section is from the same series 

 as those of figures 1 and 3, and is to be compared with the total prep- 

 arations shown on plate 1, figure 2, and on plate 2, figure 1. The 

 section is 50 n thick and is unstained. X133. Ao. d. c., aorta dor- 

 salis cephalica; C, ccelom; P, pharnyx; V. c. a., v.'cardinalis ante- 

 rior; V. om., v. omphalo-mesenterica; V. so., vein of the somato- 

 pleure; V. t., v. transversa of the first interspace; To. p. r., vasa 

 primitiva rhombencephali; Yen. c., ventriculus cordis. 



