74 



DEVELOPMENT OP PRIMITIVE BLOOD-VESSELS. 



possible opposite the myotomes, because in the former case the heart lifts this zone 

 of the embryo from the cover-slip; but every specimen shows chains of angioblasts 

 which are apparently differentiating in situ in this area. None of the experiments 

 or observations here recorded take into account the ultimate point of origin of the 

 cells which differentiate into angioblasts. 



By the time the chick has 9 somites the dorsal aorta is readily seen behind 

 the head-fold as a complete vessel if the living embryo be viewed from the ventral 

 aspect. Opposite the upper myotomes the aorta is directly ventral to the myo- 

 tomes, but it gradually curves outward, so that opposite the ninth myotome and 

 the undifferentiated mesehchyme it lies along the lateral border of the myotomes. 

 Along its entire lateral border it is connected with the plexus of the area pellucida. 

 As has been shown by Evans, the entire 

 caudal portion of the aorta is a part of 

 the capillary plexus of the area pellucida. 

 In summing up the question of the origin 

 of the aorta it may be said that it differ- 

 entiates as a part of the plexus of angio- 

 blasts, extending over the entire area vas- 

 culosa, and is increased by the addition 

 of new angioblasts along the axial line of , 

 the embryo. 



By the time the chick has 9 somites 

 the aorta can be injected; it forms from 

 the plexus of angioblasts while the seventh, 

 eighth, and ninth somites are forming. 

 While the dorsal aorta of the region of 

 the myotomes is best seen in the living 

 chick, the cephalic aorta is best ob- 

 served in an injection. As can be seen 

 in plate 1, figure 3, the heart is a simple 

 tube. In some specimens, even with 

 10 somites, it is in the exact mid-ventral line; in others, as in plate 1, figure 3, 

 it is slightly to the right. In some of my injections the ventral aorta has numerous 

 mesial and lateral sprouts; in this particular specimen these sprouts are more 

 numerous along the dorsal cephalic aorta. 



In one of my injections the heart itself shows a little of the primitive plexus. 

 The dorsal cephalic aorta shown in plate 1, figure 3, is still in the form of a plexus; 

 from the arch of the aorta two very constant sprouts extend to the ventral surface 

 of the forebrain. The development of these sprouts is well shown in a figure by 

 Evans from a duck embryo with 13 somites (fig. 398) in the ' 'Manual of Human 

 Embryology" (Keibel and Mall). 



The mesial sprouts do not form permanent vessels ; but in one very interesting 

 abnormal embryo which I injected these mesial sprouts had formed anastomoses 

 across the mid-line. They are thus, in this specimen, analogous to the vessels 



FIG. 1. Transverse section of an injected chick of 12 

 somites, passing through the middle of the mesencephalon, 

 to show the vascular plexus on the mesencephalon. On 

 the left, the dotted area shows how far the ink passed 

 through a dorsal artery from the aorta into the plexus on 

 the midbrain. The section is from the same series as 

 those in figures 2 and 3, and it is to be compared with the 

 total preparations shown on plate 1, figure 2, and on 

 plate 2, figure 1. The section is 50/ thick and is un- 

 stained. X133. A. me., artery to the plexus on the 

 mesencephalon; Ao. d. c., aorta dorsalis cephalica; Ao. 

 v. c., aorta ventralis cephalica; Me., mesencephalon; P, 

 pharynx; PI. me., plexus on the mesencephalon. 



