DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 77 



beat of the heart. The mounting of the blastoderm on a cover-slip, however, 

 interferes with the circulation much more than with the heart-beat, because the 

 flattening of the blastoderm tends to flatten the vessels and thereby impede the 

 circulation. This is often strikingly shown when through mechanical difficulties 

 the circulation is entirely cut off on one side of an isolated blastoderm and not 

 on the other. It is therefore probable that the circulation begins in the chick 

 about at the stage of 15 or 16 somites. It is interesting to note that it is at this 

 stage that the duct of Cuvier breaks through into the omphalo-mesenteric veins, 

 whereby the dorsal aorta and the veins of the embryo become connected with the 

 venous end of the heart. 



It is thus clear that at the stage of 12 somites, when the head of the embryo 

 contains a complete aorta and a neural system of vessels which consists of a plexus 

 of large vessels on the forebrain and midbrain, and a single channel on the hind- 

 brain, there is no circulation through these vessels due to the beat of the heart. 



The connections of the vessels of the brain with the aorta are of importance. 

 The arteries connecting the vessels of the forebrain with the aorta consist of a 

 group of vessels just at the primitive arch of the aorta. These are shown in plate 

 2, figure 1, and have been thoroughly demonstrated by Evans. These arteries 

 connect with the neural vessels at the base of the optic cup, in the groove repre- 

 senting the line between the telencephalon and the diencephalon. Subsequently 

 this group of vessels divides into two arteries, one of which encircles the optic 

 stalk and the other extends caudalward along the ventral border of the thalamus 

 and the midbrain (plate 6). Opposite the midbrain there is a group of tiny 

 arteries connecting the plexus with the aorta, one of which is shown injected in a 

 chick of 12 somites in text-figure 1. 



It is very clear (in the section of text-figure 1) that the vessels to the neural 

 plexus are direct, dorsal branches of the aorta. The vessel along the hindbrain 

 connects with the aorta by two groups of tiny branches, one cephalic and the 

 other caudal to the otic vesicle. These branches are also for the most part direct 

 dorsal branches. In one of my sections, however, two arteries to the vessel of the 

 hindbrain are placed with reference to the aorta, as are the vessels on the left 

 side in text-figure 2 that is, one is dorsal and the other dorso lateral. These 

 connections between the aorta and the primitive vessel of the hindbrain are 

 shown, injected, by Evans, in his figure 393 in the "Manual of Human Embry- 

 ology" (Keibel and Mall). 



As far as the vessels which connect the vascular channel of the hindbrain 

 with the aorta are concerned, it has been shown that they differentiate as angio- 

 blasts at the stage of 6 somites, while the aorta and the neural vessels are differ- 

 entiating. The origin of the primary plexus of deep vessels on the surface of the 

 forebrain and midbrain requires more careful study during the stages of from 

 6 to 12 somites. It is probable that these vessels differentiate, and that their 

 connections with the aorta differentiate, as does the preliminary vascular channel 

 of the hindbrain. The development of the deep neural vessels and the origin of 

 the superficial plexus of vessels opposite the brain, as well as the origin of the 

 primary head-vein, will be taken up subsequently. 



