DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 99 



This torsion is more clearly seen on figure 2 of plate 5 and figure 1 of plate 4. 

 These two specimens are from the same litter. They measure 7.1 mm. in oil, and 

 have 27 somites. Figure 1 of plate 4 is given because of an extravasation in the 

 vessels of the head in the specimen of figure 2 of plate 5. In this latter figure 

 the sinusoids of the liver have markedly developed. The sinusoids of the left 

 side anastomose across the ventral line with those of the right side. The opening 

 of the left umbilical vein into the liver is directly mesial to the umbilical vein 

 itself and is hidden by it, while the opening into the duct of Cuvier is plainly visible. 

 There is also a plexus from the umbilical vein in the somatopleure connecting it 

 with the posterior cardinal vein and with the duct of Cuvier; but this is omitted 

 in the drawing. 



There is a well-marked constriction between the sinus venosus and the atrium. 

 The change in the heart is due to the twisting of the obliquely placed ventricular 

 arch, whereby the point which marks the beginning of the f return Halleri comes 

 to he exactly opposite the opening of the atrio-ventricular canal into the ventricle. 

 The bulbus cordis lies far to the right and its connection with the fretum Halleri 

 is hidden by the ventricle, while the opening of the auricular canal is far to the 

 left. These relations as seen from the other side are shown in plate 4, figure 1. 

 From these two figures it is obvious that a still further twisting of the heart must 

 take place before the arterial orifice comes to lie directly anterior. 



VENTRAL BRANCHES OF THE AORTA, INCLUDING THE ALLANTOIC ARTERIES 

 AND THE SUBINTESTINAL ARTERY. 



One of the most interesting subjects in connection with these injections has 

 been the study of the ventral branches of the aorta, or the branches to the yolk- 

 sac, the gut, and its derivatives. 



The study of the early vessels of the embryo emphasizes the fact that the 

 vessels should be considered in relation to the organs which they supply. The 

 fundamental relations of the ventral branches of the aorta to the yolk-sac and 

 to the allantois are shown in two total preparations of injected pig embryos 

 (plate 5, fig. 1, and plate 1, fig. 1) and in two sections (text-figs. 5 and 6). Plate 5, 

 figure 1, is from a specimen of approximately the same stage as in Evans's figure 

 394 in the "Manual of Human Embryology," which shows the state of develop- 

 ment of vessels of the brain at this stage. 



The position of the embryo should be carefully noted. The caudal half of 

 the specimen is seen from the direct ventral aspect, while the cephalic half is from 

 a direct lateral view. The place of rotation is around the ninth somite. 



Extending from the level of the eleventh somite to the caudal end of the 

 embryo there is a series of tiny ventral arteries from the two aortae. These are 

 of uniform size and are placed at regular intervals, approximately one opposite 

 an interspace and one opposite a somite. In this particular embryo only a few 

 of these ventral branches are injected; but other specimens show that the entire 

 length of both aortse gives rise to branches like those shown opposite the twelfth, 

 thirteenth, and fourteenth somites. From the region of the eleventh to the four- 



