72 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



In the region of the head, which can not be analyzed in the living blastoderm, 

 the angioblasts representing the heart are well known and easily identified. The 

 two cardiac primordia have met in the mid-ventral line and can be followed a short 

 distance into a ventral cephalic aorta, which gradually becomes too indefinite for 

 recognition. The dorsal cephalic aorta is very clear opposite the region of the heart , 

 gradually disappearing farther forward. Thus, within the embryo there are chains 

 of angioblasts representing the heart, most of the ventral cephalic aortse, and a part 

 of the dorsal cephalic aortse. Opposite the region of the heart the two dorsal aortee 

 are definite, tiny vessels which emerge from under the head-fold and are continued 

 partly as a plexus of solid angioblasts and partly as a vessel along the ventro-lateral 

 border of the myotomes. The entire plexus which is exposed on the ventral aspect 

 connects with the plexus of angioblasts of the area pellucida. In this account I wish 

 to emphasize the very early appearance of angioblasts along the hindbrain the 

 forerunner of the so-called vena capitis medialis, which I prefer to call the primitive 

 vessel of the hindbrain. I have not yet a sufficient number of observations to 

 prove, first, whether the transitory vessel of the hindbrain does differentiate in situ 

 while the aorta is differentiating, and secondly, whether it is established earlier 

 than the vessels of the f orebrain ; but both of these propositions seem to me to be 

 very probable. 



In this study I have found Williams's (1910) very careful description of the vas- 

 cular system of the early chick embryo of great value. His specimen of 6 somites 

 is clearly a little farther advanced than mine. He found that at 6 somites the aortse 

 were established, but were still small and irregular. He then observed a vessel 

 along the neural tube (hindbrain) connected with the aorta in the first and second 

 interspaces, the vessel in the first interspace being nearly as large as the aorta itself. 

 It is now important to consider how the plexus of angioblasts increases. This 

 occurs first by cell division and secondly by the differentiation of new angioblasts. 

 Cell division in the plexus of angioblasts is very extensive, for in watching the living 

 specimens it is seen that large areas of the plexus divide at the same time, and in 

 these cycles of cell division every cell of the plexus divides. Besides this very exten- 

 sive cell division new angioblasts differentiate and join the plexus. This process can 

 best be observed along the mesial border of the dorsal aorta itself, near the lowest 

 myotome. Here practically every blastoderm between 6 to 10 somites will show one 

 or two isolated angioblasts which are very readily marked from the dense mesoderm 

 beneath. Out in the zone of the developing ccelom the distinction is by no means 

 so easy. These angioblasts are either single, spindle-shaped cells or clumps of two 

 or three cells. When observed they are seen to put out tiny filaments toward the 

 wall of the aorta, which at once responds by putting out a filament toward the young 

 cells. These tiny filaments meet halfway, and the new angioblasts thus join the 

 wall of the aorta, They gradually approach the wall and become incorporated 

 into the vessel. As the new cells become a part of the wall their protoplasm becomes 

 less granular and they acquire a lumen. The exact process by which angioblasts 

 acquire a lumen is extremely difficult to determine, and concerning this point 

 nothing has yet been added to the original description of His. 



