a smudged erythrocyte too closely to permit iden- 

 tification, and it has already been pointed out 

 that, after the cytoplasm is gone, all smudged 

 nuclei look very much alike. 



In conclusion, it should again be emphasized 

 that less is known about thrombocytes — their cy- 

 tology, physiology, development, and reactiv- 



ity — than about any other cell in avian blood. 

 For the solution of some of these problems the 

 phase microscope would be useful in that it re- 

 veals in the fresh living cell nearly all the details 

 that can be seen in the stained preparation, and 

 with it many of the rapid changes in the cell could 

 be followed directly. 



Nongranular Leukocytes 



The term "nongranular leukocytes" groups 

 lymphocytes and monocytes under one heading. 

 The term is applicable to these two cell types be- 

 cause stainable granules are usually absent from 

 the cytoplasm of these cells; however, this char- 

 acteristic should not be applied too broadly, be- 

 cause some lymphocytes contain magenta-colored 

 bodies and some monocytes contain azurophilic 

 granules. In mammalian hematology these two 

 cell types in some theories of hematology are said 

 to arise within the lymph nodes, and have a com- 

 mon cell of origin. A review of the theories of 

 hematopoiesis of lymphocytes and monocytes was 

 given by Bloom (1938) and Bessis (1956). 



The nongranular leukocytes oifer many com- 

 plex problems and some answers must be given 

 to them before any workable basis of identifica- 

 tion can be set up. It is true that practically any 

 answers given will be arbitrary or empirical; 

 nevertheless, they are indispensible and nuist be 

 presented and discussed before considering the 

 individual figures and the accompanying descrip- 

 tive text. 



Kasarinoff (1910) has reviewed the earlier 

 literature on the numjjer of types of leukocytes 

 in avian blood. He demonstrated by colored 

 drawings his division into six types: (1) Small 

 lymphocyte, (2) large lymphocyte, (.3) lympho- 

 leukocyte, (4) mast cell, (5) pseudoeosinophil, 

 and (6) true eosinophil. In this study, small 

 and large lymphocytes have been grouped to- 

 gether and his lympholeukocytes have been iden- 

 tified as monocytes. Mast cells are called baso- 

 phils, pseudoeosinophils are called eosinophils, 

 and true eosinophils, heterophils. 



Shall lymphocytes and monocytes be treated as 

 a continuous series extending from lymphocyte 

 to monocyte? Maximow and Bloom (1931) 



have stated (p. 63), "When preparations of 

 blood are examined and viewed ojjjectively, it is 

 seen that the nongranular leukocytes consist of a 

 series of transition forms which begins with the 

 smaller lymphocytes and ends with larger cells of 

 quite different appearance, the monocytes, . . . 

 But in the midportion of this series of transitions 

 is a group of cells which cannot be classified as 

 either typical lymphocytes or typical mono- 

 cytes." The same statement is applicable to 

 avian blood, and this might be considered as 

 lending weight to the ideas expressed by the 

 unitarian school, but it does not exclude assess- 

 ment of the problem in a different way, as fol- 

 lows: Lymphocytes and monocytes show but little 

 structural differentiation — neither cell type is 

 far removed from the conventional generalized or 

 typical resting cell so frequently depicted in 

 textbooks. 



In the blood there are no topographic tissue 

 relationships. It is as if a smear were to be pre- 

 pared for study from cells of the respiratory 

 mucosa, epithelium of the digestive tract, of the 

 liver, connective tissues, reticular tissues, and 

 thyroid, and these cells had all been isolated from 

 their usual haJiitat, allowed to round up and then 

 spread out and dried for identification. Identi- 

 fication in many instances would be difficult. 

 Another example could be cited that is probably 

 more pertinent — the similarity of appearance be- 

 tween certain heterophils and eosinophils. This 

 similarity will be discussed more fully later but 

 it may be said here that these cell types also 

 could be arranged in a structural series from 

 heterophil to eosinophil, and the literature on 

 avian blood reflects this confusion; yet at present 

 there are no suggestions that the mature form of 

 one develops into the other. From what has been 



47 



