pression with a rounded base as often occurs in 



monocytes ( ^•^ ^ ). In lymphocytes 



there is usually no Hof opposite the depression ; 

 the cytoplasm appears the same opposite the de- 

 pression as in the more lateral areas, but some- 

 times the nucleus shows an indentation and there 

 is a rarefied area opposite. In figure 131 there is 

 such a cell and it has been classed as a monocyte, 

 but some might be of the opinion that this cell 

 should be included with the lymphocytes because 

 of its small size and high nucleocytoplasmic 

 ratio. Fortunately, borderline examples of this 

 type are not numerous. 



Nuclear structure. — The nuclei of lympho- 

 cytes are typically described as filled with dense 

 chromatin clumps. Probably figure 102 fits the 

 usual description best and a similar elfect is given 

 in figure 90. Dense clumps of basichromatin 

 are common in small lymphocytes but rare in 

 larger cells. In lymphocytes of medium size the 

 reticular network with small chromatin masses 

 at the interstices is the usual arrangement (figs. 

 95-99 and 101 ) . As we study the change in cell 

 size from the larger to the smaller, it is easy to 

 find all corresponding degrees of change in chro- 

 matin clumping. Figure 98 shows two phases of 

 the process in the same nucleus — on one side 

 there is the reticular appearance of the larger 

 cell and on the opposite side are dense clumps 

 commonly associated with the smaller cell. 



When there is a delicate reticular pattern in 

 the nucleus the spaces between are clear and 

 transparent — the nucleoplasm is colorless (figs. 

 97-99 and 101). Wlien the nuclear pattern 

 takes the form of dense clumps there is some dis- 

 solution of basichromatin, which gives to the nu- 

 clear sap a color almost as dense as the chro- 

 matin itself (figs. 100 and 104) . The wide vari- 

 ation in size of chromatin clumps and density of 

 nuclear pattern is clearly demonstrated when a 

 smear containing lymphocytes of small and me- 

 dium sizes has been fixed in Petrunkevitch No. 2 

 and stained in May-Griinwald Giemsa (figs. 199 

 and 200) . When this technic is used the nucleus 

 of the medium-sized lymphocyte may resemble 

 closely the nucleus of the monocyte (compare 

 figs. 200 and 201). 



Cell division. — Cell division is rare in cir- 

 culating blood and in fact its occurrence in 

 lymphocytes has been questioned. One of the 



arguments against the lymphocyte as an undif- 

 ferentiated blast cell has been the absence of 

 mitosis, a process that has been observed only a 

 few times and then in birds that had been ir- 

 radiated. Cells undergoing mitosis are usually 

 difficult to name because identifying character- 

 istics are lost in the process; however, in the 

 case of figure 108, magenta bodies were present 

 that have a high specificity for lymphocytes. 

 Blood stains and air-dried fixation do not give a 

 sharp delineation of the individual chromosomes 

 but there is no question that the cell is dividing 

 mitotically. It is interesting that the magenta 

 bodies should arrange themselves approximately 

 midway between the poles. The division of 

 lymphocytes by mitosis is in contrast to that of 

 monocytes that divide by nuclear constriction. 



Developmental stages found in circulating 

 blood 



Probably less is known about the cytomor- 

 phosis of lymphocytes than about any otlier leu- 

 kocyte. Immature lymphocytes have been iden- 

 tified in the thymus where they are developed in 

 large numbers, and it has been suggested that 

 figure 122 represents a young lymphocyte, but 

 in general they are difficult to identify. There 

 are identifying structural features, it is true, but 

 fitting them into a maturation series is not easy. 

 The developmental series of lymphocytes de- 

 picted in atlases of mammalian hematology are 

 usually derived from cases of lymphoblastic 

 leukemia. Here the association with many other 

 cells, all developing in the same direction, makes 

 a kind of pure culture, as it were, from which a 

 developmental series can be constructed. After 

 it has been put together there still is not much in 

 the way of specific cell identification by which an 

 occasional immature lymphocyte, which one 

 might find in the circulating blood, can be satis- 

 factorily distinguished from various types of 

 blast cells. 



The cuiTe for the sizes of avian lymphocytes 

 shows approximately a normal distribution (fig. 

 152). It is generally recognized that specific 

 criteria useful for the identification of age and 

 maturation are generally lacking in the lympho- 

 cyte series. Wiseman (1931b and 1932) con- 

 sidered that the degree of basophilia of the cyto- 

 plasm could be used as such a measure and under 



53 



