have reached the late polychromatic stage hut a 

 few show the color typical of the mid-stage ( fig. 

 226, 1 and 2). A different type of cytosome 

 rarefaction appears at this age — the fracture 

 breaks (cells 10 to 12 and others) that in the pri- 

 mary erythrocytes are not as severe as in later 

 generations. Some cells, 3, for example, show 

 change toward the oval form; the final nucleo- 

 cytoplasmic ratio is not yet established but the 

 nucleus has undergone considerable contraction, 

 and the chromatin now forms coarse blocks. The 

 luicleoli have practically disappeared but mi- 

 totic figures are still large and conspicuous. The 

 capability of a cell for mitosis is considered by 

 Osgood and Ashworth (1937) to have consider- 

 able value for the proper placement of a cell in 

 its developmental series. 



Maturity of primary erythroblasts is attained 

 during the fifth day of incubation and reaches the 

 peak percentage by about the sixth day (fig. 232) . 

 Dantschakoft" (1908b) states that primitive eryth- 

 roblasts attain their complete maturity on the 

 fifth day. Figure 227 shows a typical low-power 

 field in which there is a mixture of mature first- 

 generation red cells and early stages of succeed- 

 ing generations. 



The primary erythrocytes still show a wide 

 range in size (fig. 227, 5 and 9) although they 

 are now all equally mature. Such variation, 

 were it to occur after hatching, would indicate a 

 pathological condition and would probably be 

 classed as anisocytosis. Among these mature 

 cells there are shades of color difference. Al- 

 though all have a deep orange base color, some of 

 them (cell 5 and the one just below 12) have 

 some gray mixed with them. The cytosome has 

 lost the coarse textural quality of the eryth- 

 rocytes at earlier embryonic ages (figs. 235, 236, 

 238, and 239) and taken on a uniform, finely 

 granular quality (fig. 243). This cytoplasmic 

 texture, characteristic of a cell that is mature or 

 nearly mature, may be observed at 5 days of in- 

 cubation (figs. 241 and 242). 



Since the study of reticulocytes in the blood of 

 the hatched chick came rather late in the program 

 of work, there was no opportunity to reexamine 

 the primary erythrocytes to determine whether 

 they passed through a reticulocyte stage; but 

 Dawson (1936a) showed a series of figures of pri- 

 mary erythroblasts and erythrocytes stained with 

 brilliant cresyl blue, and these closely resemble 



what we have pictured for the reticulocytes in the 

 definitive cells after hatching. 



The particles near the margin of cell 5 of figure 

 227 are merely foreign bodies that have fallen on 

 the surface. The nucleus now holds a central po- 

 sition in the cell. There is considerable varia- 

 tion in the ratio of nucleus to cytoplasm and in 

 some of the cells — for example, 2 and 11 oi 

 figure 227 — the micleus is smaller relative to the 

 size of the cell than in the definitive erythrocytes. 

 This is a characteristic of the mature primary red 

 cell (figs. 244 and 245). In general the cells 

 and their nuclei are still round, only a few show- 

 ing a slight elongation. 



Chromatin of the nucleus is not so intensel)' 

 stained in the primary erythiocytes as in the later 

 generations of red cells in the same blood. This 

 type of faint coloration of the nucleus is found 

 as a general characteristic of the primary cells 

 (figs. 244 and 250) ; where extreme, it has been 

 classified as indicative of deterioration and de- 

 generation, but as seen in figure 227, it hardly 

 seems likely that this is the case at this age. 

 Later generations of erythrocytes first appeared 

 at about 120 hours (5 days) and it is hardly to 

 be expected that the primary generation would 

 be disappearing 21 hours later at a time when 

 the succeeding generations of erythrocytes are 

 just making their appearance. It is true that 

 definite examples of degeneration can be seen at 

 all ages, such as karyorrhexis (fig. 252) and 

 erythroplastid production (fig. 249) in early 

 generations of erythrocytes, and multiple nuclei 

 (fig. 2.53) in later generations. 



Figure 228 shows that new generations have 

 been poured out into the blood and that they 

 have become Uie dominant cells; only a few 

 mature primary erythrocytes remain — 6 to 17 

 percent of the total number of cells. The nu- 

 cleus and cytoplasm have the same appearance 

 that they had earlier. It is a question whether 

 these cells ever become oval in large numbers. 

 Cells typical of this age are shown in figures 247 

 and 248; the latter is slightly ovoid. Degenera- 

 tion appears in erythroplastid formation, as weak 

 affinity of chromatin for stain, and as pyknotic 

 nuclei ( lower left-hand cell of fig. 228) . Mature 

 primary erythrocytes become increasingly scarce 

 as the embryo grows older; one is shown in figure 

 229 (cell 1) from an embryo 13 days 16 hours 

 old. They may be seen the first day after hatch- 



127 



