said it is apparent that inability to catalog with- 

 out question, or the mere existence of a structural 

 series, is not in itself proof of genetic continuity. 

 Having applied this reasoning to lymphocytes 

 and monocytes, it must be pointed out at once 

 that studies made on spleen, bone marrow, and 

 early embryonic blood make use of a structural 

 series of cells to trace developmental stages. 

 On the basis of such series one is led to an opin- 

 ion, but the diversity of opinions on hematologic 

 principles of cell relationships is evidence 

 enough that such an approach falls short of carry- 

 ing the weight often assigned to it. 



The most convincing evidence that lympho- 

 cytes and monocytes are as distinctly different 

 as are any other two cell types came from their 

 treatment with X-rays (Lucas and Denington, 

 1957). Single total body irradiation from 50r 

 to 300r applied once to chickens at different ages 

 has clearly demonstrated that lymphocytes are 

 highly susceptible to this treatment, and by 1 to 

 3 days after irradiation had reached a maximum 

 deo^ree of depression in which the normal value of 

 20t000 to 30,000 cells had dropped to about 

 2,000 to 3,000 cells. The recovery was rapid 

 and, depending upon age of the bird and severity 

 of treatment, was half to fully completed by the 

 15th day after irradiation. The monocytes re- 

 acted entirely differently; only the higher doses 

 gave any indication that these cells had been af- 

 fected at all, and the drop in number reached its 

 maximum between the third and eighth days. 



The typical lymphocyte and the typical mono- 

 cyte are easily distinguished but to describe and 

 illustrate merely the typical cells would defeat an 

 important function of an atlas on blood, that of 

 showing variability so that questionable and un- 

 identified cells in differential counts are reduced 

 to a minimum. As Lucas and Denington ( 1956) 

 noted in a study on the morphology of the liver, 

 the "typical" is very rare. On the other hand, 

 what is normal may include a wide variability as 

 was shown to be true for the avian erythrocyte. 

 In the case of lymphocytes and monocytes there is 

 overlapping in respect to the various character- 

 istics commonly associated with each of these 

 cells. 



It is simple and convenient to classify cells on 

 the basis of a chief and supporting characteristic 

 as was done for polychromatic erythrocytes. 

 Unfortunately, this method will not work on 

 lymphocytes and monocytes because their vari- 



ability ranges overlap, and one is forced to use 

 multiple characteristics with the confusion that 

 often results. Characteristics of monocytes and 

 lymphocytes have been presented in table 3. 

 Relatively few cells will show all the character- 

 istics listed under each cell type. The confusion 

 comes when cells show characteristics partly of 

 one cell type and partly of the other. Examples 

 will be pointed out when individual figures are 

 discussed (pp. 50-73). 



In table 3 the characteristics are set up on tlie 

 basis that all mature lymphocytes are either small 

 or medium in size. A group referred to as 

 "large lymphocytes" has not been considered in 

 the preparation of criteria for the table for 

 several reasons. It is only rarely that cells of 

 this type have been seen in the circulating blood 

 of normal adult birds. Four of them are illus- 

 trated (figs. 121-124) but it is misleading to call 

 them lymphocytes when obviously it is much 

 more probable that they are immature cells in 

 the process of development toward some other 

 particular cell type. To make the term "lym- 

 phocyte" so inclusive that it designates both a 

 fully mature functional cell of circulating blood 

 and also the other extreme, a stem or blast cell 

 from which other cell types arise, may be entirely 

 justifiable when propounding a unitarian theory 

 of blood-cell interrelationships.* However, this 

 is not workable when the term is used merely 

 to identify a definite structural cell type without 

 reference to any hematopoietic theory. It is for 

 this reason that the "large lymphocyte" is treated 

 separately from the small and medium-sized cell. 

 To do this will not change the values obtained for 

 differential counts since, as already pointed out, 

 the "large lymphocyte" occurs so rarely in the 



'Tlie unitarian point of view lias been clearly stated by 

 Dantschakoff (1909a, p. 157): ". . . Speziell ist der kleine 

 Lymphozyt im erwachsenen Organismus nicht eine reife Zell- 

 form, sondern, im Gegenteil, es ist eine ganz differente junge 

 Zelle mit sehr mannigfaltiger, prospektiver Entwicklungspotenz. 

 Gewiss entstehen die kleinen Lymphozyten in den Keimzentren 

 durch Wucherung der grossen, aber sie konnen sich dann 

 •.pater, vielleicht nacli einer liingeren oder kUrzeren Ruhe- 

 periode wieder durch Hypertrophie in typische grosse Lym- 

 phozyten zuruckverwandeln und zum Ausgangspunkte der 

 Haniatopoese werden." 



Translation: In particular, the small lymphocyte in the ma- 

 ture organism is not a mature cell form, but on the contrary 

 it is a quite distinct youthful cell with most manifold prospec- 

 tive developmental potential. Certainly the small lymphocytes 

 arise in the germinal center through proliferation of the large, 

 but they can later change back again through hypertrophy into 

 typical large lymphocytes, perhaps after a short or long period 

 of rest, and become the point of emergence of hematopoiesis. 



48 



