spicuous nucleoli of figure 251. It is generally 

 believed that some change has taken place in the 

 cell metabolism that prevents the normal process 

 of differentiation. The great mass of data on 

 the cytology and propagation of leukemic and 

 other neoplastic cells amply substantiates the 

 idea that an intracellular change can and does 

 occur; but this does not exclude the presence in 

 the plasma of a "differentiating" factor, and 

 the sequence of events observed in embryonic 

 blood before and after circulation is established 

 suggests that such a factor does exist. Wliether 

 such a factor is sufficiently potent to force the 

 differentiation of a leukemic cell to the point 

 where it performs the equivalent function of the 

 normal cell is, of course, unknown. 



Dantschakoff (1909a) made an interesting ob- 

 servation on erythropoiesis. The phenomenon 

 she saw is undoubtedly an expression of the dif- 

 fei'ence between intravascular and extravascular 

 environment. In the early chick embryo, pri- 

 mary erythroblasts are being rapidly produced 

 both inside and outside the vessels of the vitelline 

 membrane and of the embryo. In both en- 

 vironments, differentiation to the extent of taking 

 up hemoglobin is started but only inside the ves- 



sels does the process go to completion. The 

 cells of the primary erythrocyte series in the 

 mesenchyme tissue outside the vessels are de- 

 stroyed, either by disintegration or by phagocy- 

 tosis. 



Let us return to a description of the changing 

 picture of the circulating blood. The rapid 

 shift between 48 and 65 hours has already been 

 indicated. The mid-polychromatic primary 

 erythrocytes are present at a higher percentage 

 level at 65 hours than are the early polychromatic 

 eiythrocytes and, with variations, remain high 

 after the latter have declined to a low level. The 

 maximum reached in the differential counts was 

 71 percent at 93 hours. There is an irregular 

 decline in mid-polychromatic primary erythro- 

 cytes at 97 to 120 hours, and there is a tapering 

 off at 120 to 142 hours. Late primary poly- 

 chromatic erythrocytes are present only in small 

 numbers at 65 hours, and the number increases 

 gradually until it reaches a peak at about 120 

 hours (fig. 226). Beyond that age there is a 

 rather rapid diminution in number for this stage 

 of development and by 160 hours (fig. 227) all 

 have disappeared. It is not until after 100 hours 

 that the primary generation of erythrocytes 



ER^ THR0C^TE5 .n CHICK EMBRYOS 



Luc pnmari| nnXwi pnniari| 



pnm:iry I polychromUic ^/'e,.i,throci|h5 \,.- 

 erL)throbln5ts ' ' ' " " '' 



and c.irl 

 primarL| 

 pokj chromatic 

 cri|tliroci)t"C5 



late polqchrom.itic 

 cruthrocijtcs of latcr 

 (^cncrations 



niid-pok|chrcinintic 

 criithrociitcs of Infer 

 i;cnerntions ; 



maTure cn.|Hirocutcs 

 of Inter ii,eneiMtlons 



200 



250 



500 



550 



400 



A50 



500 



INCUBATION AGE In HOURS 



Figure 232. — Curves showing the percentage rise and fall of three stages of 

 maturity in the primary and later embryo erythrocytes. Solid line — ■ 

 primary erytlu-ocyte. Dotted line — combined later generations of embryo 

 erythrocytes. 



113 



