DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 95 



become connected with similar dilatations in the other interspaces and make the 

 cardinal veins. These observations indicate that the cardinal veins begin with 

 dilatations of the dorsal branches of the aorta that is, that they start as an out- 

 growth from the wall of the aorta in the different interspaces and that these inter- 

 segmental vessels become connected along the lateral line. 



Very soon the vascular arches which give rise to the cardinal veins give off 

 sprouts which extend toward the spinal cord, as is shown from the tenth to the 

 seventeenth interspaces in plate 3, figure 1. The position of these sprouts is 

 shown in section on plate 2, figure 3. This section passes through the twenty- 

 first interspace of a chick of 30 somites. These neural sprouts soon reach the spinal 

 cord, as is shown in section on plate 3, figure 4, which is to be compared with the 

 seventh interspace of plate 3, figure 1. The section shown in plate 3, figure 4, is 

 from a chick of 25 somites from another series than that of all the other sections 

 on the plates. It is from a series of nearly the same stage as that of plate 3, figure 

 1; it passes through the seventeenth interspace. It was selected because it shows 

 so well the double dorsal arch to the posterior cardinal vein, the primary direct one 

 and the secondary neural one. One has only to imagine the primary direct arch 

 disappearing to obtain the well-known pattern of the spinal arteries shown in 

 section on plate 3, figure 2, and in the upper interspaces of plate 3, figure 1. 



From this study it is, I think, clear that the spinal arteries of the chick arise 

 from dorsal intersegmental vessels which give rise to the cardinal veins, and not 

 directly from the aorta. In plate 3, figure 1, it is very evident that the capillary 

 plexus which forms along the lateral surface of the spinal cord is a direct continua- 

 tion of the primitive vessel of the hindbrain. The original simple chain of 

 capillaries on the lateral surface of the cord, such as is shown in plate 3, figure 1, 

 from a chick of 25 somites, very soon becomes a plexus on the neural tube, as 

 indicated opposite the second somite in plate 6. By the fourth day of incubation 

 this plexus covers the entire lateral surface of the spinal cord. The relation of 

 this plexus to the spinal arteries on the one hand and to the spinal veins on the 

 other is very regular and characteristic. Every spinal artery, on approaching 

 the cord, bifurcates into a short ventral branch and a longer lateral branch (plate 

 3, fig. 2). The ventral branch leads to a longitudinal neural artery which at the 

 stage of the fourth day lies on the ventral surface of the cord just lateral to the 

 notochord. In other words, there are symmetrical ventral longitudinal arteries. 

 These arteries form a ventral border for the plexus along the lateral surface of 

 the cord. The lateral arteries run in the plexus on the surface of the cord; they 

 lie just cephalic to each spinal ganglion and extend nearly to the dorsal border of 

 the cord. The veins which accompany these transverse arteries, in contrast to the 

 arteries, are lifted off from the surface of the cord, as it were. They also correspond 

 to the cephalic border of each ganglion, and they are more superficial in every case 

 than the corresponding artery. I am emphasizing the fact that the arteries lie 

 in the plexus on the surface of the cord and that the veins are more superficial, 

 because the same is true for the primary arteries and veins of the brain, as can 

 be seen very clearly in plate 7. 



