from all subsequent generations that the occur- 

 i-ence of slight differences in these later genera- 

 tions is often overlooked. The existence of a 

 succession of generations brings our terminology 

 into question. In both birds and mammals the 

 term "primary" for the first generation of red 

 cells is satisfactory, but in the chick the cells that 

 follow cannot be called definitive generations 

 since actually it is not until near the end of the 

 embryonic life that the definitive type of red 

 blood cells appears. Probably the term "em- 

 Ijryo erythrocytes" or "embryonic erythrocytes" 

 would more accurately designate these cells in 

 the chick. In the embryonic span there are nu- 

 merous generations beginning with the primary, 

 so that first, second, third (and so on) genera- 

 tions could be designated. Actually, the term 

 "generations" in reference to red blood cells is 

 somewhat misleading since the cells are being 

 produced continuously and not periodically. 

 The term is applied in the same sense as it is used 

 for human populations in that one speaks of dif- 

 ferent generations; whereas, in fact, there is a 

 complete and unbroken frequency distribution 

 in age from youngest to oldest. Even the mi- 

 totic periodicity known to exist in adult birds and 

 mammals is not present in the embryo. 



Evidence for the existence of successive gen- 

 erations of red cells following the primary gener- 

 ation is based almost entirely on the appearance 

 of the mature cell, the shade and hue of the cyto- 

 plasm, the shape of the nucleus and cell, the nu- 

 cleocytoplasmic ratio, and the tendency of the 

 nucleus to become dense and pyknotic. Reali- 

 zation that there are successive generations is one 

 thing, but saying that a particular cell belongs 

 to a particular generation — such as the third or 

 fifth — is another. It has not been possible to say 

 this. Moreover, it has not been possible to say 

 for sure how many generations there are during 

 embryonic life. We know that there are more 

 than 2, and the total is probably about 4. 



Embryonic erythrocyte generations can be fol- 

 lowed quite well in Ambly stoma. Cameron 

 (1941) demonstrated that cells of the first gen- 

 eration contained 128 yolk plates and each suc- 

 ceeding generation held half the number con- 

 tained in the parent cell from which it arose by 

 division. After eight generations the yolk plates 

 were reduced to one per cell and then the plates 

 degenerated. At the time of hatching, definitive 

 erythrocytes appeared that lacked yolk plates. 



The early generations of embryonic erythrocytes 

 retained round forms, whereas later genei'ations 

 took on elliptical forms. Yolk plates are absent 

 from avian emljryonic erythrocytes but the possi- 

 bility exists that tlie pattern of reproduction in 

 amphibians may be carried over to some extent 

 in birds. 



Figure 232 was prepared so as to sliow the 

 succession of forms. For the sake of simplicity, 

 the embryonic erythrocytes have been divided 

 into only two groups — the primary generation 

 and succeeding generations. Under the age 

 baseline the hours given are 3 less than the actual 

 duration of the egg in the incubator. This pe- 

 riod of time was estimated to be required for the 

 egg to become warm and for developmental proc- 

 esses to get under way. All embryonic ages 

 given in captions or text are estimated on this 

 basis. Incubation age is given either in hours 

 or in days and hours. Table 9 has been included 

 because it is useful in converting from one kind 

 of time scale to the otlier. 



Dawson (1936a) made a similar statistical 

 and cytological study on the shift of erythrocyte 

 stages during embryonic development. His 

 table 1 should be compared with our figure 232, 

 and his photographs of smears from embryos 

 that ranged between the age of 4 days of incuba- 

 tion and liatching age should be compared with 

 figures 226-230, which cover essentially the 

 same range. Under each group of erythrocytes, 

 whether primary or later generations, there are 

 five subdivisions — erythroblasts; early, mid-, 

 and late polychromatic erythrocytes; and mature 

 erythrocytes. The data included in the differ- 

 ential counts included all these stages but the 

 only stages selected for illustration in figure 232 

 were those that during embryonic life became 

 dominant cells. Each curve was plotted as a 

 hand-drawn average from a large number of dif- 

 ferential counts made from closely spaced ages. 

 Sometimes, among the differential counts for a 

 particular cell type at a particular age, the range 

 of variability would run almost 100 percent, but 

 in other cases all the values wou'd '>p closely 

 clustered. 



The first samples of blood were taken from the 

 dorsal aorta of 46- and 47-hour embryos. The 

 technic employed for obtaining intraembryonic 

 blood at this early age is given in chapter 7. 

 Low-power drawings from such embryos are 

 shown in figure 224, A and B. Although these 



105 



