ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 255 



ORIGIN OF THE HEART AND AORTA. 



It may be interesting to sum up what can be seen of the origin and develop- 

 ment of the heart and the aorta in these living blastoderms. As has been shown, 

 it is possible to find the first angioblasts that differentiate from mesoderm in the 

 wall of the myocardium before the two myocardia have fused across the mid-line. 

 These cells appear first at the stage of 5 somites. At the same time isolated clumps 

 of cells appear along the ventro-lateral margin of the somites, the forerunners of the 

 aorta. These clumps of isolated angioblasts liquefy to form vesicles, which grad- 

 ually unite to form the endocardium and the aorta. By the time the chick has 9 

 somites a complete aorta can be seen along the ventro-lateral margin of the somites, 

 but this vessel is constantly increased by the addition of new angioblasts. The 

 stage of 9 somites is especially favorable for watching this process. Indeed, I have 

 found it rare that a chick of that stage does not show a few clumps of angioblasts 

 joining the aorta along its mesial border opposite the last two somites. Moreover, 

 practically all chicks with from 14 to 17 somites will show new angioblasts joining 

 the aorta opposite the undifferentiated mesoderm at the lower end of the embryo. 

 Therefore, any illustrations of an injection of the lower end of the aorta (such as 

 are shown in Evans's [1909] fig. 1) should have added to them a few masses of solid 

 angioblasts about to join the wall of the plexus in order to completely represent 

 the aorta in that area. It is interesting to note that in this area, when a mass of 

 angioblasts joins the wall of a vessel, sprouts from the older vessel and from the 

 young masses of cells meet half way between the two structures, and then the new 

 cells seem to be gradually drawn into the vessel wall. On the other hand, in the 

 growth of a plexus the new cells may remain at some distance from the older vessel, 

 while the protoplasmic bridge develops into a vessel connecting the two lumina. 

 This is a different process from the drawing of new cells into the wall of a vessel, such 

 as can be seen so readily in the case of the endocardium and the wall of the aorta. 



The study of the heart in these specimens is interesting. The early stages have 

 already been mentioned. There are at first isolated clumps of angioblasts which 

 form tiny vesicles, as shown in text-figure 1. These vesicles then unite to make a 

 plexus. The development of the later stages must be watched through the myocar- 

 dium and hence it can not be seen as clearly as the vessels themselves. After the 

 endocardium has formed a complete vesicle the heart curves usually to the right, 

 by an occasional anomaly to the left, as seen in figure 8. The very first beats of 

 the heart can be made out in these hanging-drop specimens. They occur at the 

 stage of 10 somites and always in the same position. The first twitching is along 

 the right margin (left side of the photograph), beginning just at the lower border 

 above the junction and between the vitelline vein and the ventricle. It is interesting 

 to note that there is no movement whatever in the vein, the entire twitching being 

 confined to the ventricle proper. That is to say, the myocardium over the ventricle 

 is formed long before there is any muscle at all in the wall of the veins. The beat 

 is at first slow but rhythmical, and gradually involves the entire wall of the ventricle, 

 spreading from the posterior to the anterior end. The heart beats throughout the 

 stages of from 10 to about 16 somites without actually pumping any blood into the 

 aorta. At the stage of 16 to 17 somites circulation begins. 



