Circulating Blood of the Embryo
Early in the process of blood development, mesenchyme transforms into angioblasts or into primitive erythroblasts. The red blood cells-- in the first generation, at least--develop from the walls and within the lumen of the newly formed vessels ( Sabin, 1920). This process of development is well under way before the first circulatory are through the embryo has been completed. As soon as afferent and efferent channels have been joined, the heart pumps through them the cells that previously had accumulated in the yolk sac. This event begins at 33 to 36 hours and is well established by 48 hours incubation, which is the age when the first smears were made in this study. Before this, the development of hemoglobin has caused the blood to take on a red color. In the primary generation of red cells, the hemoglobin is acquired much more rapidly than in the later generations, with the result that, although the cell has a red color as seen with the unaided eye, the stained cytoplasm retains a strong affinity for basophilic dyes. It is this fact that leads to misunderstanding in cell terminology and to error in identification of the stage of development. As successive generations appear this source of error becomes less and less.
Dantschakoff ( 1907 and 1908a and b) observations of the early transformation of mesenchyme into blood islands and thence into intravascular and extravascular blood elements were based on celloidin-sectioned material. The statements agree closely with those of Sabin ( 1920) but the names and interpretations are different. Both authors agree that blood cells can be produced from the endothelium of the blood islands of yolk sac and the dorsal aorta of the early embryo.
Dantschakoff ( 1907) summarized her basic hematologic theory as follows (p. 166):
"Auf Grund des Erorterten konnen wir also eine vollstandige Analogie zwischen der ersten Blutbildung ausserhalb und innerhalb des embryonalen Korpers annehem. Die ersten Blutzellen treten hier wie dort zuerst als Lymphozyten kleinerer und grosserer Form auf,--diese mussen also als die Stammzellen aller Blutelemente betrachtet werden und erzeugen durch Proliferation und Differenzierung in versehiedenen Richungen die mannigfaltigen Formen der roten und weissen Blutkorperchen, die wir im erwachsenen Organismus finden."1
Murray ( 1932) analyzed the observations of Sabin and of Dantschakoff by studying early blood formation in tissue culture. He used parts of the primitive streak of the chick embryo before the head fold developed. From these undifferentiated cells, he obtained cultures that went through all the early stages of blood island formation, with angioblasts enclosed within an endothelial boundary. These early stages agreed very closely with the observations made by Sabin on the living chick. Murray observed in tissue culture all the steps in primary erythrocyte differentiation up to cells that had an oval shape and contained hemoglobin, even up to a fully mature cell. This entire process took place in his cultures within 24 hours.
It has been pointed out by Dantschakoff ( 1908b and 1909b) that whereas Maximow ( 1909) found but two generation types in the development of red blood cells in mammals-- primary and definitive--she found that in chickens there was a succession of generations by which the transition from the primary to the definitive type was accomplished. Each generation of cells attained maturity within its life span.
The primary generation of red blood cells in the chick embryo is so conspicuously different____________________