94 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



DEVELOPMENT OF THE SPINAL ARTERIES. 



It will now be necessary to go back and follow the development of the spinal 

 arteries. This can be done in a series of injections of the stages from 15 or 16 

 somites upward, or the entire process can be followed in any one chick up to the 

 stage of about 30 somites. The process is easier to illustrate after the embryo 

 has rotated so that the lateral instead of the dorsal surface is presented. The 

 entire process can thus be readily followed in plate 3, figure 1, from a chick of 25 

 somites, with six sections from two different series of chicks of 25 and 30 somites. 



The general stage of development of the vessels of the head of the embryo at 

 the stage of 25 somites can be seen in Evans's figure 6 (Anat. Record, 1909, III, 

 p. 505) ; or can be estimated from my plate 6, the stage of 25 somites being just 

 before the superficial capillaries which make the middle segment of the primary 

 head-vein begin. The deep vessel of the hindbrain is still the vein for the brain, 

 and is shown in its relation to the capillary plexus on the lateral surface of the 

 spinal cord in plate 3, figure 1. 



The general development of the area vasculosa at this stage is also of interest 

 in following the vessels in sections. The roots of the omphalo-mesenteric arteries 

 at the stage of 25 somites are opposite the twentieth and twenty-first somites. 

 As was indicated above, in the earlier stages the entire lateral border of the aorta 

 opposite the somites was originally connected by direct lateral (that is, ventro- 

 lateral) branches with an arterial plexus of the area vasculosa. In this plexus, 

 on either side of the embryo, the omphalo-mesenteric veins gradually extend 

 caudalward from the region of the sinus venosus and thus are formed two veins, 

 or a plexus of veins, with direct short connections with the aorta. This process 

 explains the large veins of the splanchnopleure shown in figure 3, plate 2, and 

 figures 2 and 3, plate 3. 



In the chick the spinal arteries do not arise as direct dorsal arteries from the 

 aorta to the cord, but the direct dorsal arteries make a primary arch to the dorsal 

 border of the nephrotome, where they give rise to the cardinal veins. The spinal 

 arteries then arise from these arches instead of from the wall of the aorta itself. 



In following the development of the spinal arteries I shall begin with the 

 more caudal segments in plate 3, figure 1, because they show the earlier stages. As 

 has been described in connection with the origin of the cardinal veins, the first dorsal 

 branches of the aorta are direct dorsal diverticula of the wall of the aorta into 

 the interspaces, as is shown best in text-figure 3 for the stage of 12 somites. Figure 

 2 on plate 4 and figure 4 on plate 2 are both from the lower segments of a chick 

 of 30 somites. They are both taken below the origin of the omphalo-mesenteric 

 arteries in the zone where the arteries of the posterior limb-buds are forming. 

 The cardinal veins are also developing in this area. Figure 2 on plate 4 passes 

 through the twenty-fifth interspace, and plate 2, figure 4, is still lower down and passes 

 through the twenty-seventh interspace. In plate 4, figure 2, it can be seen that 

 even in later stages the dorsal branches start as direct diverticula of the aorta. 

 These diverticula soon arch lateralward and, as can be seen in plate 2, figure 4, 

 dilate slightly just dorsal to the nephrotome. These dilated portions of the arches 



