DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 79 



106; but I have chicks of 10 somites in which the first somite is farther from the 

 head-fold than in the usual specimen of 9 somites. As a rule the head-fold is 

 along the cephalic border of the first somite when the embryo has 12 somites; but 

 in some specimens, such as in His's plate xn, figure 20, there is at this stage 

 an interval between the first somite and the head-fold. After the cephalic curve 

 of the midbrain has formed, as shown in plate 2, figure 2, the embryos are not 

 as flat from the direct dorsal aspect and the point can not be tested with the 

 same definiteness. 



The longitudinal vessel which connects these diverticula in the lateral line 

 of the embryo is the common cardinal vein (plate 2, fig. 1). The cardinal vein 

 has two fundamental relations on the one hand to the primitive vascular channel 

 of the hindbrain and on the other hand to the venous end of the heart. As is 

 shown in plate 2, figure 1, and in the section in text-figure 2, the cardinal vein 

 becomes connected with the neural vessel by two cross-anastomoses in the first 

 and second interspaces. Of these vessels the one in the first interspace is the 

 larger and more important. The cardinal vein itself is not shown on the left side 

 in text-figure 2 (right side of the embryo), since the transverse vein of the first 

 interspace is slightly oblique, as is plainly seen in plate 2, figure 1. 



The transverse vein of the first interspace has been described and illustrated 

 by Evans; and has been traced back to the stage of 6 somites by Williams. In 

 the chick it is an important channel in the second and third days of incubation, 

 for it is the channel by which all of the blood for the brain drains into the cardinal 

 vein and thence to the heart. The transverse vein of the first interspace is char- 

 acteristic of the chick. It does not form in the pig where the transitory vessel of 

 the hindbrain connects with the cardinal vein in front of the first somite instead 

 of in the first interspace. 



At the stage of 12 somites the dorsal diverticula of the aorta are present in 

 all the interspaces, but there is not yet a continuous vein connecting them opposite 

 the lower interspaces. The cardinal veins begin to form at a very early stage, 

 when the zone along which they form is close to the aorta (text-fig. 3), so that the 

 primitive common cardinal vein is an accompanying vein to the aorta. It is 

 this accompanying vein of the aorta which connects with the venous end of the 

 heart, forming the duct of Cuvier. So close is its relation to the aorta that the 

 duct of Cuvier may be regarded as a direct connection between the dorsal aorta 

 and the omphalo-mesenteric veins. 



The position of the duct of Cuvier is well known, and is shown in plate 2, 

 figure 1. At the stage of 14 somites, as shown in this figure, the common cardinal 

 vein opposite the second, third, and fourth somites is in the form of a plexus; 

 and it will be noted that there is a vessel extending lateralwards from this plexus 

 opposite the cephalic border of the omphalo-mesenteric veins, and a similar vessel 

 opposite the caudal border of the vein. These two vessels are in the somato- 

 pleure dorsal to the omphalo-mesenteric veins. This is very clearly shown in 

 the section in text-figure 2. The more cephalic of these two vessels (V. so.) 

 develops, as I shall show for the pig, into veins which drain the body-wall over 



