more of the definitive bodies are visible. Some 

 of the bodies appear in the round form and some 

 in the rod form. The total number is less than 

 half of the number found in the mature heter- 

 ophil. A metamyelocyte is shown in the heter- 

 ophil above cell 28 of figure 321. This cell 

 does not yet have a full complement of rods but 

 the nucleus has condensed until its boundary 

 is distinct. 



Other cells {9-11) are mature, or nearly so, 

 but since they contain but a single nuclear lobe 

 they would be classed as juveniles or as band 

 cells in mammalian blood terminology. Mature 

 heterophils are abundant throughout the smear 

 and some of those having more than one lobe are 

 designated by the numbers 12—14. 



Eosinophils also are held in depots until after 

 hatching. Two cells each with a single nuclear 

 lobe are indicated by numbers 15 and 16 (fig. 

 321) and, in the same field, part of one other 

 cell is shown at the border. With the ex- 

 ception of cell 15, all these cells are of the small 

 type that, in the bone marrow, is considered to be 

 the source of the small eosinophils sometimes 

 found in the circulating blood (figs. 181-183). 



The basophil (fig. 321, 17) is an adult cell in 

 which the distortion due to action of water on the 

 granules is as great as in the circulating blood, 

 and the poor stainability of the nucleus is as evi- 

 dent here in the bone marrow of the embryo as 

 it was found to be in the circulating blood of the 

 adult (fig. 190). 



In this field of bone marrow, no erythroblasts 

 are present Imt there are three early polychro- 

 matic erythrocytes (fig. 321, 18-20). These 

 examples show considerable range in size, yet 

 all are already older than eiythroblasts. Cell 

 21 is a good example of a mid-polychromatic 

 erythrocyte. There are no late polychromatic 

 erythrocytes in this field, and tlie remaining 

 erythrocytes are mature. It is assumed, not that 

 these mature erythrocytes represent cells held in 

 storage, ready to be discharged later, but that 

 they came into the bone marrow by way of nu- 

 trient vessels from outside. 



Cell 26 (fig. 321) has been identified as a 

 thromboblast. It has the characteristics of the 

 early stage of this cell line in its densely stained 

 nucleus and cytojjlasm. Had the cell not been 

 pressed out of shape during the making of the 

 smear, its identification would have been more 

 certain, but it does not have the characteristics 



typical of the erythrocyte or the granulocyte line. 

 Previously it was stated that the erythrocjiies and 

 the thrombocytes of an early embiyo were dif- 

 ficult to separate; but with increasing age of tlie 

 embryo, separation becomes easier, and in the 

 adult marrow there was relatively little con- 

 fusion. 



Naked nuclei and smudged cells (fig. 321, 

 29 and 30) need no additional explanation, and 

 the primordial osteogenic cell {31) has already 

 been discussed. The most important cell re- 

 maining for consideration is 28, which has been 

 called a lymphocyte. It appears to be a small 

 cell undergoing l3leb formation. The nucleus 

 is too large for the definitive thrombocyte ; more- 

 over, if it were a thrombocyte the structure of the 

 nucleus would not have been so clear cut and 

 definite at this stage of cytoplasmic disintegra- 

 tion. 



EMBRYO SPLEEN 



The spleen is an organ that develops quite early 

 in embryonic life; according to Hamilton 

 (1952), it appears on the last half of the fourth 

 day. Efforts to procure satisfactory impression 

 smears before the beginning of the eighth day 

 were unsuccessful because the cells were so frag- 

 ile that all broke, and the naked nuclei were cov- 

 ered by a layer of blue-stained tissue fluid. The 

 smears resembled that portion of figure 329 

 where there are naked and ruptured nuclei and 

 strands of dissolved chromatin; therefore no in- 

 tact, recognizable cells could be seen. By the 

 eighth day some cells remained unbroken in care- 

 fully made smears. Why cells should be deli- 

 cate in early embryonic life and much tougher at 

 older ages is not known; cytologically they ap- 

 pear identical at both ages, and the size does not 

 change much with age. All the serum granules 

 and disturbing elements in the smear were in- 

 cluded when the drawing (fig. 329) from the 

 spleen of the 8-day-old embryo was prepared. 



Danschakoff (1916a) reviewed the early de- 

 velopment of the avian spleen and reaffirmed her 

 observations on the function of lymphocytes as 

 the progenitors of other cell types. Antibodies 

 against adult spleen tissue were produced by 

 transplanting pieces of the spleen to the allantois 

 of the embryo. 



155 



