106 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



exactly the same position as the umbilical veins; that is, in the somatopleure. 

 In the -specimen of plate 1, figure 1, this venous capillary plexus connects with a 

 tiny lateral aortic branch shown just opposite the zone of the second aortic arch. 

 This lateral artery is not the second aortic arch, which arises from the ventral 

 rather than from the lateral surface of the aorta. From this tiny lateral artery a 

 straggling chain of capillaries is injected within the somatopleure, out over the heart, 

 and down to the duct of Cuvier; they are omitted in the drawing. It is clear that 

 they are vessels for the body-wall analogous to the vessels which drain into the 

 umbilical veins; but they are cephalic to the duct of Cuvier. The venous end of 

 the plexus is injected in plate 5, figure 2. It was shown in the chick that the 

 corresponding vessel of the somatopleure over the heart develops very early. In 

 later stages these vessels in the somatopleure over the heart anastomose freely 

 with a plexus. of capillaries lateral to the occipital myotomes, as shown in text- 

 figure 5 in my article on the Origin and Development of the Lymphatic System, 

 1913. 



NEURAL BRANCHES OF THE AORTA AND THE PRIMARY HEAD-VEIN. 



In connection with the neural vessels, I have no specimens of embryo pigs 

 corresponding to the chicks of 6 somites in which to trace their beginning. I have 

 one litter of very young pigs, measuring 3 mm., in which the heart and aorta are 

 present; the neural folds are open at the cephalic end, and I can find no angio- 

 blasts along the closed hindbrain. 



At the stage of figure 3, plate 4, the vessels to the forebrain can be injected; 

 and the vessel of the hindbrain must be present, for it is seen in a human embryo 

 of the same stage of development. Opposite the third and fourth somites the 

 lateral plexus of the neural tube has been injected from the aorta. I found only 

 one specimen of the litter of figure 3, plate 4; but the fact that the posterior cardinal 

 vein is almost completely injected indicates that the anterior cardinal vein is 

 present and that it connects with the deep vein of the hindbrain. At the stage 

 of plate 5, figure 1, the vessels of the head are in about the stage of development 

 of those of plate 1, figure 1, as is proved by the injections of the same Utter. In 

 one specimen of the same litter as plate 5, figure 1, the anastomosis of the capillaries 

 around the optic stalk is complete, just as was shown by Evans for the later stage 

 of three aortic arches in his figure 395 (Keibel and Mall, Manual of Human 

 Embryology, II, p. 579). 



The best view of the early neural vessels in my series is given in plate 1, 

 figure 1 . In order to analyze the relations of the vessels of the head, I have used 

 gray to indicate all of the capillaries which are true neural vessels, in the sense of 

 lying close to the wall of the neural tube and giving rise to the vessels of the sub- 

 sequent pia mater. 



As can be seen in plate 1, figure 1, the deep capillary plexus of the forebrain 

 and midbram is covering the wall of the brain, and the form of this plexus indi- 

 cates the form of the brain. The vascular arch which surrounds the large 

 peduncle of the optic vesicle (see Evans's figure 395) is incompletely injected in 



