CHAPTER 3 



Circulating Blood of the Embryo 



ERYTHROCYTE CHANGES DURING 

 INCUBATION 



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 develop- 

 ment is well under way before the first circula- 

 tory arc 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 hemo- 

 globin 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 termin- 

 ology and to error in identification of the stage 

 of development. As successive generations ap- 

 pear this source of error becomes less and less. 



Dantschakolf's (1907 and 1908a and b) ob- 

 servations of the early transformation of mesen- 

 chyme into blood islands and thence into intra- 

 vascular 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 Analogic zwischen der ersten 



Blutbildung ausserhalb und innerhalb des em- 

 bryonalen Korpers annehem. Die ersten Blut- 

 zellen treten hier wie dort zuerst als Lymphozy- 

 ten kleinerer mid grosserer Form auf, — diese 

 mussen also als die Stammzellen aller Blutele- 

 mente betrachtet iverden und erzeugen durch Pro- 

 liferation und Differenzierung in verschiedenen 

 Richungen die mannigfaltigen Formen der roten 

 und weissen Blutkorperchen, die wir im erwach- 

 senen Organismus finden." ^ 



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 undiffer- 

 entiated cells, he obtained cultures that went 

 through all the early stages of blood island for- 

 mation, with angioblasts enclosed within an endo- 

 thelial boundary. These early stages agreed 

 very closely with the observations made by Sabin 

 on the living chick. Murray obsei-ved in tissue 

 culture all the steps in primary erythrocyte dif- 

 ferentiation 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 cul- 

 tures 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 chick- 

 ens there was a succession of generations by 

 which the transition from the primary to the de- 

 finitive type was accomplished. Each genera- 

 tion of cells attained maturity within its life span. 



The primary generation of red blood cells in 

 the chick embryo is so conspicuously different 



'Translation: On the basis of the discussion we can assume 

 a complete analogy between the first blood formation outside 

 and inside the embryonal body. The first blood cells in both 

 places first appear as lymphocytes of large or small size, — 

 these mast be considered the parent cells of all the blood ele- 

 ments, and through proliferation and differentiation in differ- 

 ent directions they make the numerous forms of red and white 

 blood corpuscles such as we find in the mature organism. 



104 



