in the cells shown in the preceding plate. The 

 greater distinctness is due to tlie contraction of 

 the chromatin network into larger hut more 

 widely spaced clumps. Mitotic figures are still 

 abundant, and in cells 20 and 22 of figure 22.5 

 it is evident that division reduces the size of the 

 cell to half of the original volume. Undoul^t- 

 edly cell growth occurs during the interkinetic 

 period but average size decreases as the cells get 

 older (compare figs. 225 and 227). 



A statistical study on the percentage of mitosis 

 present in the early embryo has been prepared 

 by Dawson (1936a), who says (p. 262). "In the 

 blood stream there is little restriction of mitotic 

 capacity in the differentiating primitive line until 

 the mature stage is reached." 



Two types of cells are distinctly visible at 6.5 

 hours. One is the primary erythrocyte line, and 

 the other is the emlnyo thrombocyte line. The 

 cytology of the latter will be discussed after the 

 description of the red cell has been completed. 

 After 4 days incubation little change is visilile in 

 the primary erythrocyte except additional hemo- 

 globin accumulation, a greater range in size, and 

 a sharper distinction between red-cell and throm- 

 bocyte lines. The characteristic intensity of 

 cytosomal hue is shown in figures 238-240 and 

 ranges from early to late polychromatic. A 

 comparison of figures 238 and 239 brings out an 

 interesting point — the latter is a small cell with a 



small, densely stained nucleus and indications of 

 cytoplasmic differentiation, yet the hemoglobin 

 content is low in comparison with that of figure 

 238, which is not so well differentiated cyto- 

 logically. 



The primary erythroblasts at 65 to 93 hours 

 of incubation shown in figures 235, 236, and 238 

 may reveal a faint tinge of eosin coloration but 

 in some preparations made at this age the cyto- 

 plasm may take a completely basophilic colora- 

 tion. The benzidine test (p. 231) reveals that, 

 nevertheless, hemoglobin is present (fig. 237). 

 Thus this test demonstrates tliat basophilic color- 

 ation of the cytoplasm does not preclude the ex- 

 istence of hemoglobin and it is for this reason that 

 the stage of erythrocyte development called baso- 

 phil erythroblast has been omitted from tJie series 

 of erythrocyte stages presented in table 2. 



Hemoglobin is present in erythrol)lasts of em- 

 bryos incubated 48 hours, although the cytoplasm 

 is fully basophilic. No benzidine tests were 

 made at this age, but the blood flowing through 

 the vessels of the embryo and area vasculosa is 

 clearly red. 



Additional hours of development produce 

 definite changes, which are seen in figure 226 and 

 in figures 241 and 242, where the cytoplasm has 

 lost nearly all of its mitochondrial spaces and the 

 perinuclear, vacuolar space characteristic of 

 earlier stages, is nearly gone. Most of the cells 



Figures 233-243. — Stages in the differentiation of 



size and struc 



Figures 233, 234: Primary erythroblast from dorsal aorta 

 of an embryo incubated 1 day 22 hours. Same slide as 

 figure 224. 



233 Early primary er\'throblast. Large nucleolus fills 

 upper half of nucleus. 



234 Primary erythroblast. Lobulations are few and 

 small. 



Figures 235-237: Primary erythrocytes from embryo in- 

 cubated 2 days 17 hours. 



235 Late primary erythroblast. Same slide as figure 

 225. Rounded cytosome. The effects of an eeto- 

 plasmie mantle is characteristic of cells of this size 

 and degree of differentiation. 



236 Small early polychromatic erythrocyte. From same 

 slide as preceding figure. 



237 Late primary erythroblast. Ralph's benzidine 

 method and M. G. G. I^resence of hemoglobin in 

 cyto.some indicated by yellow color. Cell equivalent 

 to figure 235 — taken from same embryo. 



primary erythrocytes selected to show variations in 

 ture. 2,470 X. 



Figures 238, 239: Cells from an embryo incubated 3 days 

 21 hours. 



238 Mid-polychromatic ])riniary erythrocyte. Still shows 

 a refractile granule and mitochondrial spaces char- 

 acteristic of erythroblasts. 



239 Early polychromatic primary erythrocyte. Less 

 hemoglobin than preceding cell but further differen- 

 tiation. 



240 Late polychromatic primary erythrocyte in mitcsis. 

 Polar view of what is apparently metaphase. Em- 

 bryo incubated 4 days. 



Figures 241, 242: Late polychromatic primary erythrocytes 

 from an embryo incubated 5 days 3 hours. 



241 Average size. 



242 Small size. 



243 I^ate polychromatic primary erythrocyte. Larger 

 than average, almost a mature cell. From same 

 slide as figure 227 



]18 



