cells are known to carry hemoglobin, they fill the 

 requirements of an erythroblast in that they have 

 a strongly basophilic cytoplasm and contain nu- 

 cleoli. At this age practically all cells show an 

 almost identical stage of development, and yet 

 there is a wide range of size as a result of cell 

 division. 



A digression for discussion of the implications 

 of these facts may be desirable. Sabin (1920) 

 has shown that the first erythroblasts appeared 

 even before 24 hours of incubation. She also 

 indicated that new erythroblasts continuously de- 

 veloped from their stem cells and that while this 

 was taking place there was a rapid multiplication 

 of cells that had already begun differentiation. 

 This information might lead to the conjecture 

 that, at a later time, when these cells were thrown 

 into the circulation, there would be a wide range 



of developmental stages present. Instead, one 

 actually finds a remarkable degree of uniformity 

 in the level of differentiation. Therefore, it is 

 suggested that antecedent to the completion of the 

 embryonic and yolk sac circulatory arcs the ery- 

 throblasts lie dormant within the vitelline chan- 

 nels and that cytoplasmic and nuclear differen- 

 tiation is inliil:)ited, although mitosis continues at 

 a rapid rate. Only one phase of differentiation 

 is taking place — that in which the synthesis of 

 hemoglobin is involved — but the nuclear and 

 cytoplasmic changes that ordinarily accompany 

 this process are lacking in these cells. Once 

 circulation is established and the cells begin to 

 move with the flowing blood that has now created 

 a new environment, differentiation proceeds, and 

 about a day and a half later (at 65 to 69 hours, 

 fig. 225) the erythroblasts are transformed into 

 the early, mid-, and late polychromatic erythro- 

 cytes of the primary generation. At 48 hours 

 practically all the cells are erythroblasts, but at 

 65 hours only a few erythroblasts are present, and 

 after that they disappear from the circulating 

 blood. From this time on there is an ever- 

 increasing tendency for immature stages of 

 erythrocyte development to be retained at the 

 loci of their origin. 



Apparently blood cells in themselves are not 

 fully equipped to bring about their complete 

 differentiation but something is added while they 

 are being circulated that enhances differentia- 

 tion. Moreover, as development of the embryo 

 proceeds it is no longer necessary for the im- 

 mature cells to enter the circulation in order to 

 receive the chemical stimulus that is to be found 

 there. It is no longer necessary because the cir- 

 culating blood is brought into intimate contact 

 with the areas where the cells are being produced 

 and thus more and more of the immature stages 

 are almost completely differentiated before lieing 

 released into the circulation. ( See Gordon, 

 19.59.) 



It is obvious that the suggestions made need 

 tlie sujiport of additional evidence obtained from 

 experimentation. Tlie effort required to sort 

 out the factors involved in stimulating differen- 

 tiation would be justified since it is apparent that 

 the same problems of differentiation that have 

 been solved by the embryo are unsolved in cases 

 of leukemia. The cells seen in leukemia of birds 

 and mammals often closely reseml)le those of 

 figure 224 and may have the large size and con- 



112 



