248 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



determined with the focusing screw in the living specimen. The endocardium, 

 then, is constantly increased in two ways: first by the division of its cells, and 

 second by the addition of new angioblasts to the outside. 



The aorta in the stage of 6 somites looks much like the endocardium shown in 

 text-figure 1, except that the clumps of angioblasts are more isolated. During the 

 stages from 6 to 9 somites the angioblasts along the axial line unite to become a 

 complete vessel. The liquefaction of angioblasts of the dorsal aorta opposite the 

 lower somites can be followed in chicks of 8 to 9 somites, and it is very interesting 

 to note that if a chick of about 9 somites is watched one will usually find a few small 

 blood-islands giving rise to erythroblasts in the lower end of the aorta. At this stage 

 only small islands are formed there, never larger than two or three cells and these from 

 the original angioblasts rather than from a new proliferation of the endothelium. 



That the endothelium does proliferate later in the lower aorta to form blood- 

 islands has now been abundantly proved. This was first discovered by Madame 

 Danchakoff (1907) in a study of the development of blood in the chick. She 

 described that in chicks of from 4 to 5 days of incubation there was an intensive 

 growth of the endothelium of the-vessels, especially great in the lower aorta, giving 

 clumps of young indifferent elements like the primitive blood-islands. These 

 masses of cells she described as becoming free both within and without the lumina 

 of the vessels, and as giving rise to both red and white blood-cells. In 1909 Maxi- 

 mow described these masses of cells in the aorta of the cat and the rabbit (p. 517), 

 while in 1911 and 1912 they were described by Minot in the human embryo. The 

 question brought up by the latter author as to whether these masses of cells or 

 blood-islands could be proved to be derived from endothelium has, I think, been 

 entirely settled by these observations in the living form. The blood-islands in the 

 aorta have been described more recently by Emmel (1915-1916) in other mammals, 

 especially in pig embryos, and by Jordan (1916-1917) in pig and mongoose embryos. 

 All of these observers describe the islands as giving rise to a primitive mesamceboid 

 cell capable of producing both red and white corpuscles. This point seems to be 

 now the most significant question in regard to these structures. It is generally 

 admitted that in the chick the red cells develop within the lumina of vessels, and I 

 think these observations on the living form make it possible to sharpen this con- 

 ception by stating that red cells in the chick during the first two days of incubation 

 come either from the original angioblasts or from the subsequent division of endothe- 

 lium, and that the development of hemoglobin in some cells within the vessels is 

 preceded by the liquefaction of some protoplasm to make plasma. Moreover, it 

 may be said that all of the cells that become free within the lumina of the vessels in 

 the first two days of incubation become red cells. This, of course, does not include 

 certain wandering cells from the endoderm, or germ cells which may get into the ves- 

 sels. That all of the primitive blood-cells are erythroblasts was pointed out by O. 

 Van der Stricht in 1892 for the chick and in 1895 for the rabbit. Thus the red cells 

 develop intravascularly in the chick because it is angioblasts that give rise to them. 



The proliferation of endothelium to make blood-islands, now abundantly 

 proved for the chick and shown to occur in mammals, becomes especially interesting 



