HISTOGENESIS OF CIRCULATORY SYSTEM 733 



ferentiated, mesenchymal cells which give origin to a mother cell, the 

 hemocytoblast. From the hemocytoblast, four main stem cells arise, lympho- 

 blasts, monoblasts, granuloblasts, and erythroblasts, each of which differen- 

 tiates into the adult type of blood cell as shown in fig. 333A. Such an inter- 

 pretation is the basis for the monophyletic or unitarian th:;ory of blood cell 

 origin. Some observers, however, believe that the erythrocyte, granulocyte, 

 and the monocyte each have a separate stem cell. The latter view is the basis 

 of the trialistic theory. (Consult Maximow and Bloom, '42, pp. 107-116 for 

 discussion relative to blood-cell origin.) 



b. Places of Blood-cell Origin 



1) Early Embryonic Origin of Blood Cells. It long has been recognized 

 that the yolk-sac area is a region of early blood-cell development. This is 

 one aspect of the angioblast theory of His, referred to on page 731. In the 

 teleost fish, Fundulus, Stockard ('15) reports the origin of red blood cells 

 from two main sources: 



( 1 ) an intermediate cell mass or blood string in the vicinity of the noto- 

 chord and 



(2) the blood islands in the yolk sac. 



However, the yolk-sac area appears to be the primary source for the early 

 phases of hematopoiesis in most vertebrate embryos. In the human embryo, 

 both red and white cells have been described as arising from primitive hemo- 

 cytoblasts in the yolk sac by Bloom and Bartelmez ('40). These authors 

 report the origin of primitive erythrocytes as arising primarily intra-vascularly, 

 although some develop extra-vascularly. Definitive erythrocytes develop, ac- 

 cording to these authors, in the entoderm and within blood vessels of the 

 yolk sac (fig. 333B). In the 24-hr. chick embryo, the blood islands in the 

 area vasculosa of the blastoderm show a direct conversion of mesodermal 

 cells into primitive blood cells and the endothelium of the forming blood 

 capillaries (fig. 333C). In the frog, blood islands appear in the mesoderm 

 and entoderm of the ventro-lateral areas of the body of 3- to 4.5-mm. embryos. 

 These islands are extensive, extending from the liver area caudally toward 

 the tail-bud region. 



2) Later Sites of Blood-cell Formation. As indicated previously in teleost 

 fishes, early blood formation occurs in the region of the notochord near the 

 developing kidney tissue, as well as in the yolk-sac area. During later develop- 

 ment, hematopoiesis in teleost fishes is centered in the kidney area. The origin 

 of blood cells from kidney tissue also is true in the amphibian tadpole (Jordan 

 and Speidel, '23, a and b). The liver also functions in these forms to produce 

 blood cells. In the developing shark embryo, blood cells appear to be formed 

 around the heart and later in the esophageal area of the adult. In the adult 

 frog, the spleen functions as a center of blood-cell formation, although in the 



