80 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



the region of the heart cephalic to the duct of Cuvier. They receive their blood 

 from lateral branches of the aorta (of which the lateral artery opposite the heart, 

 shown in plate 2, fig. 1, may be one) and are analogous to the branches of the 

 umbilical veins below the duct of Cuvier. 



Of the veins of the somatopleure, those which are opposite the caudal border 

 of the omphalo-mesenteric veins join the omphalo-mesenteric vein in the septum 

 transversum of His, as shown on the right side of text-figure 2. The connection, 

 which has not taken place in the specimen in plate 2, figure 1, at the stage of 14 

 somites, occurs at the stage of 15 somites, as was shown by Evans. In making 

 injections at the stage of 15 or 16 somites it sometimes happens that the ink first 

 injected does not fill the neural vessels, but runs from the aorta into the duct of 

 Cuvier through direct aortic branches, such as that shown in the third interspace 

 in plate 2, figure 2. 



One of the most interesting points in connection with the duct of Cuvier is 

 that it forms just about the time or just before the time when the circulation 

 begins, which is probably of great importance from the standpoint of the physi- 

 ology of the embryo. Thus plate 1, figures 2 and 3, and plate 2, figure 1, represent 

 the blood-vessels of the chick before the circulation has begun, while figure 2 of 

 plate 2 and figure 1 of plate 3 represent a series of chicks in which the circulation 

 has commenced. Inasmuch as the duct of Cuvier has not connected with the 

 omphalo-mesenteric veins (sinus venosus) at the stage shown in plate 2, figure 1, 

 the longitudinal plexus and vessel of the lateral line at this stage is a common 

 cardinal vein which will be divided into an anterior and a posterior division by 

 the position of the duct of Cuvier. From a comparison of figures 1 and 2 of 

 plate 2, figure 1 of plate 3, and plate 6 it is clear that the anterior cardinal vein 

 must increase in length at the expense of the posterior cardinal vein as the heart 

 shifts caudalward. In these figures it is plainly shown that the cardinal system 

 opposite the duct of Cuvier continues in the form of an extensive plexus (see also 

 plate 1, fig. 1. of the pig) and that the plexus ultimately gives rise to the umbilical 

 veins. 



This completes the general account of the blood-vessels of the chick before 

 the circulation has begun; that is, up to the stage of plate 2, figure 1. I shall 

 take up, under two headings, the study of the further development of the primitive 

 blood-vessels in the stages in which the blood is circulating; first, the vessels of 

 the brain and their relation to the primary head- vein; second, the vessels of the 

 spinal cord. The primitive vessels of the nephrotomes will be taken up in con- 

 nection with the pig embryos. It is of course evident that the two divisions 

 overlap, for the vessels of the brain begin in the period before the circulation 

 commences. 



VASCULAR SYSTEM OF THE BRAIN AND THE PRIMARY HEAD-VEIN. 



As has been shown, the neural vessels begin to form very early, before there 



is any circulation, and indeed before the heart has begun to beat. The primitive 



vessel of the hindbrain differentiates at the stage of 6 somites as a chain of angio- 



blasts along the border of the hindbrain, and at the time it is differentiating 



