tween the shape of the indentation in lymphocytes 

 and in monocytes has been mentioned, and the 

 examples shown in the various figures of mono- 

 cytes bear out the observation that the nuclear 

 depression is usually broad with a rounded bot- 

 tom, as diagramed on page 53. 



The presence of this type of indentation is 

 helpful in identifying monocytes but certainly 

 it cannot be relied upon entirely; many mono- 

 cytes have round or elongated nuclei without any 

 depression (figs. 151, B, d, C, c, D, c, and E, c). 

 Some have irregularly shaped nuclei (figs. 151, 

 A, d, B, b, D, b and e, and F, d) or double in- 

 dentations (figs. 136, 137, and 151, C, e), and 

 sometimes the indentation cuts the nucleus into 

 two equal or unequal parts (figs. 138, 151, B, e 

 and F,b). This variability must be kept in mind 

 when a differential count is being made so that 

 some monocytes will not be omitted from the 

 count because they have atypical nuclei. 



Nuclear structure. — Nuclear structure is a 

 pattern that is often viewed impressionistically 

 without giving delil^erate attention to tlie parts. 

 Wlien the pattern is carefully studied, it breaks 

 down into a complex of interrelated details sucli 

 as size, shape, and distribution of chromatin 

 clumps, the character of the reticulum and its re- 

 lation to the basichromatin and the tinctorial re- 

 actions of the nucleoplasm. When viewed 

 superficially the monocyte nucleus gives the im- 

 pression that it has a delicate lacelike reticular 

 pattern of chromatin and a transparency that 

 is not generally obsei-ved in the lymphocyte 

 nucleus. 



Upon close examination, it may lie observed 

 that the clumps of clu'omatin at the interstices of 

 the network are often small, as in figure 127. 

 Sometimes they may be relatively large and 

 dense, as in figure 125, which has a highly 

 colored nucleoplasm and thus would not give a 

 transparent effect. In lymphocytes a reticular 

 appearance was often found associated with 

 larger nuclei. Whether the more open reticulum 

 and smaller chromatin clumps commonly found 

 in monocytes represent a characteristic differ- 

 ence between tlie two cell types, or are nothing 

 more than a reflection of the larger nuclear size, is 

 undetermined. Nuclear pattern carries relatively 

 little weight in the separation of the two cell 

 types, chiefly because a definite type of chromatin 

 organization cannot be considered specific for 



each of these cells. Yet any sui-vey involving 

 hundreds of monocytes and lymphocytes will 

 clearly reveal thai each type has its own general 

 pattern. It will also reveal that monocyte nu- 

 clei fall into two groups — those with a delicate 

 open reticulum and those with coarse blocks. 

 The same differences may be found in the im- 

 mature stages (figs. 139-144). 



Cell division. — The indentation of the nucleus 

 so common in monocytes is often carried further, 

 leading to various degrees of constriction that 

 may approach the center from two sides (figs. 

 136-138) or cut the nucleus from one side only 

 (fig. 151, B, e). This may lead to complete di- 

 vision of the nucleus into two parts. When they 

 are equal in size, they suggest cell division by 

 amitosis; but the occurrence of unequal nuclei 

 (fig. 151, F, b) raises the question of whether 

 this type of nuclear duplication has any more re- 

 lationship to amitosis than has the lobulation of 

 the granulocyte nucleus. No actual pulling 

 apart of the cystosome to form two cell l^odies has 

 ever been observed. Constriction of the nucleus 

 is much more frequently found in monocytes than 

 in lymphocytes, and this is a useful morphologic 

 feature that aids in separating these two 

 leukocytes. 



Conclusions derived from use of table 3. — Fre- 

 quent mention has been made of the fact that one 

 cannot decide whether a particular cell is a mono- 

 cyte or a lymphocyte without considering numer- 

 ous characteristics, which must be balanced 

 against one another. A few cells, questionable 

 ones as well as those that are obviously of one 

 type or the other, have been presented in talndar 

 form in table 5 to show how the various charac- 

 teristics given in table 3 have been applied. Ta- 

 ble 5 should make clear why some questionable 

 cells have been classed as monocytes instead of 

 lymphocytes and vice versa and it brings out that 

 oftentimes a cell will sliow a characteristic that is 

 just as frequently found in monocytes as in lym- 

 phocytes, and tliere may be a "-!-" in both rows 

 of the table. 



Developmental stages found in circulating 

 blood (figs. 139-144) 



Lymphocytogenesis and monocytogenesis are 

 vague and controversial subjects, chiefly because 



ro 



