reaches maturity; then they differentiate rapidly 

 to become dominant cells at 140 to 142 hours; 

 from that time on these cells may be found in the 

 circulating blood in considerable numbers up to 

 16 days of incubation. Occasionally they have 

 been seen after hatching. 



The tabulation shown in figure 232 gives us 

 some information aliout the processes of intra- 

 vascular differentiation, but the picture of decline 

 is always masked by the rise of the next higher 

 stage of development. Undoubtedly much of 

 the apparent decline in mature primary erythro- 

 cytes is due to the increase in the proportion of 

 subsequent generations since the data for these 

 curves have been collected as percentage values. 

 A study is definitely needed in which the abso- 

 lute number of each cell type is oljtained, as Kin- 

 dred (1940) has done with the rat. Then the 

 true picture of the rise and decline of cell types 

 can be visualized; in addition, rate of differentia- 

 tion and the true life span of the cells during 

 embryonic life will be known. 



Following the primary generation there is a 

 succession of generations — secondary, tertiary, 

 and others. Since it is practically impossible to 

 separate them except in the fully differentiated 

 cell, and then only in a general way, all genera- 

 tions after the first have been grouped together. 

 Graphically this produces long low curves in- 

 stead of a succession of sharp peaks. 



Erythroblasts with nucleoli and basophilic 

 cytoplasm are picked up occasionally at between 

 141 and 214 hours (fig. 227). They may be 

 seen even later but would not constitute a signifi- 

 cant percentage in a differential count. Early 

 polychromatic erythrocytes are never abimdant; 

 between 160 and 243 hours they vary from to 

 11 percent, and occasionally cells may be found 

 up through the second week of embryonic life. 

 Most of the cells of these two stages are retained 

 at the site of origin in the yolk sac. Differentia- 

 tion processes within the circulating blood are 

 limited largely to development from the mid- 

 polychromatic erythrocyte on to the mature form. 



The percentage of mid-polychromatic erythro- 

 cytes of the later generations never reaches a 

 high level — 38 percent at 160 hours is the max- 

 imum in our data. The talmlar data from which 

 the graphs of figure 232 were constructed sug- 

 gested that there was a sharp rise in percentage 

 level at about 160 hours, followed by a sharp 

 decline that does not continue downward because 



the later embryo generations of mid-polychro- 

 matic erythrocytes continue to be present at a 

 low but fairly constant level until about 285 hours 

 of incubation and then drop off gradually. 



The frequency distribution curve for the late 

 polychromatic erythrocytes of later embryo gen- 

 erations does not show a sharp peak. Late poly- 

 chromatic erythrocytes are present in the blood 

 at a high level throughout the period from 212 

 to 312 hours, after which they decline rapidly 

 at first but later continue to be present at a lower 

 percentage level. The course for these cells 

 trails out up to the time of hatching. They are 

 still present in variable numbers after hatching 

 and practically any slide, even from older birds, 

 will always show a few. 



Mature late embryo erythrocytes do not appear 

 until after 210 hours. Later they rise rapidly 

 and from about 375 hours until hatching main- 

 tain a high level in the blood. On aljout the 

 16th day of incubation, they constitute about 90 

 percent of the cells present. On the day before 

 hatching close to 100 percent of the erythrocytes 

 present belong to this stage of development and 

 this, of course, is the condition that continues 

 after hatching (fig. 230). 



No one, to our knowledge, has as yet attempted 

 to prepare a table of hematologic values for the 

 chick embryo at various ages. The data pre- 

 sented by Flemister and Cunningham (1940) 

 are at least a beginning. They found that in the 

 allantoic circulation at 8 days incubation there 

 were 1,210.000 erythrocytes/mm\ and at 10 

 days, 1,880,000. The hemoglobin at 8 days was 

 9.3 grams/100 cc. and at 10 days, it was 14.7. 

 Their percentage values for the types of leuko- 

 cytes listed are such as to indicate that they did 

 not identify the nonhemoglobin containing cells 

 the same way we have in this study. 



Data for each day of the last week of incuba- 

 tion were given by Roberts, Severens and Card 

 (1939). They presented erythrocyte, total 

 white cell, heterophil, and lymphocyte counts for 

 two lines of chicks. Up to the time of hatching, 

 75 to 96 percent of the white cells were het- 

 erophils. The total white cell counts were gen- 

 erally below 10,000 per cubic millimeter from 

 the 15th through the 19th day of incubation. On 

 the 20th day the count reached about 11,000 and 

 on the 21st day al)out 16,000 cells. It has been 

 our impression diat heterophils were not such a 

 constant constituent of embryo blood, and that 



114 



