curves have been prepared for the pigeon l>y 

 Schoger (1939) who gave the range in length 

 as 7.5-8.5 microns and the width as 3.0-4.5 

 microns. 



In birds, thromljocytes develop as mononu- 

 cleated cells and remain so throughout their life 

 span. Unfortunately, for purposes of study, 

 they acquire early in their development a trigger- 

 like fragility that makes them as readily reac- 

 tive to damage in the immature as in the mature 

 stages. In contrast, during the process of mam- 

 malian evolution a shift in fragility apparently 

 arose, so that developmental stages represented 

 by the megakaryocyte lost this high degree of 

 fragility and retained it only in the functional 

 platelet. It is because the avian thrombocyte is 

 so easily and quickly damaged at all stages that 

 we know so little about its cytomorphosis. Fewer 

 than half a dozen papers have been written on 

 the subject. 



When the thrombocyte disintegrates, not only 

 does the cytoplasm go to pieces but the nucleus 

 rapidly reaches a pyknotic condition. Because 

 the disintegration mechanism is held in such deli- 

 cate balance the thrombocyte would seem to be 

 an ideal tool for the physiologist in his study of 

 cell equilibrium and disruption. Because die 

 avian thrombocyte has many points of association 

 and similarity with erythrocytes and with lym- 

 phocytes they have been placed between these 

 two cell types in the arrangement of subject mat- 

 ter in the Atlas. Bradley (1937) and Blount 

 (1939b) regard thrombocytes and erythrocytes 

 as closely related genealogically, and the latter 

 author mentions the existence of thromlioplas- 

 tids, but enucleated thrombocytes have not been 

 observed in these studies. Hartman ( 1925) and 

 Gordon (1926) extensively reviewed the various 

 theories of the origin of thrombocytes. From 

 Gordon's own experiments, he concluded that 

 thrombocytes were derived from erythrocytes. 

 This conclusion was based in part on the fact 

 that when he bled a heath-hen repeatedly the 

 number of thrombocytes increased when the num- 

 ber of erythrocytes decreased. The ratio of 

 thrombocytes to erythrocytes was 15:1000 at the 

 beginning of the experiment and after 5 bleed- 

 ings it was 45:1000. The absolute number of 

 thrombocytes had increased from 35,000 to 

 86,000 and the number of erytluocytes had de- 

 creased from 2,350,000 to 1,900,000. Hart- 

 man (1925) concluded from his studies that they 



were of extravascular origin and thus thrombo- 

 cytogenesis was not related to eryduocytogenesis. 



Normal mature thrombocytes (figs. 73, 74) 



The typical thrombocyte (fig. 73) has been 

 described many times as a cell slightly smaller 

 than an erythrocyte, elongated with rounded 

 ends, but not having the regular oval contour of 

 the erythrocyte. The thrombocyte nucleus also 

 has a slightly oval shape but is not as elongated as 

 that of the erythrocyte. The cytosome usually 

 consists of a framework with large spaces. Some 

 have called them vacuoles but they do not have 

 the discrete nature and regular contour of vacu- 

 oles. Sometimes the cytosome shows structural 

 uniformity. Considerable variability in color 

 is taken on by the cytoplasm — pale blue as in 

 figure 73 or pale purple as in figure 74. Often 

 the cell membrane appears as a distinctly pur- 

 plisli line (fig. 85 ) ; this is especially true in cells 

 in which disintegration is just beginning. 



Thrombocytes contain specific granules that 

 take a pink to reddish purple color. The vari- 

 ability in number, size, intensity of color, and 

 position in the cell is extremely great. In figure 

 73 there is a single compact granule at each 

 nuclear pole with a suggestion of diffuse orange 

 material beside the lower one. In figure 74 

 there is a definite single granule jjetween the nu- 

 cleus and the side wall, and at the lower pole 

 there are four granules surrounding a lighter 

 stained homogeneous mass of similar material. 

 In figure 81 there is a chain of four rings. The 

 intensity of staining is less than in the two cells 

 previously described. 



The outline drawings (figs. 88 as) have been 

 arranged so that the cells in the first horizontal 

 row are examples illustrating a single granule. 

 The second row shows 2 granules per cell, the 

 third row shows 3 granules, and the cells in 

 the fourth row contain 4 or more granules. The 

 granules may lie at the poles of the cell or at the 

 side. If multiple, they may be close together or 

 far apart, and they may be compact and dense or 

 diffusely organized. 



The significance of the specific granulation is 

 not known. Blount (1939b) is of the opinion 

 that these granulations do not represent hemo- 

 globin. Possibly it is part of the trigger mech- 

 anism that brings about the rapid disintegration 



42 



