us to say that, when a differential count is made, 

 the lymphocytes of medium size with reticular 

 nuclei should be classed as immature and only 

 small lymphocytes with dense chromatin should 

 be called mature. 



The medium-sized lymphocytes of figure 331 

 (cells 4-6 as examples) show a degree of 

 chromatin clumping intermediate between the 

 immature lymphocyte (cell 3) and the small 

 lymphocyte (cells 7-10) but the character of the 

 cytoplasm does not change significantly from 

 one stage to the other as seen in the spleen ; how- 

 ever, in the lymphocytes of the thymus some 

 differences in cytoplasmic structure did occur. 



It is generally agreed in mammalian hema- 

 tology that monocytes arise in the same organs 

 where lymphocytes have their origin, and this 

 appears to be true in birds also, but in avian 

 species there are no lymph nodes, which are the 

 organs chiefly responsible for lymphocytes and 

 monocytes in mammals. The spleen of birds 

 after hatching is a lymphogenic organ and it is 

 not surprising that developmental stages of mono- 

 cytes should also be present in this organ. In 

 the protocol above, developmental stages of the 

 monocytes were mentioned as occurring in the 

 circulating blood of the young chick. Jordan's 

 (1938) statement (p. 731) concerning the de- 

 velopment of blood cells in the lung fish (Pro- 

 topterus ethiopicus) is interesting: "While mono- 

 cytes are probably formed in the general cir- 

 culation, they may also arise in the spleen." 

 The idea of the maturation of monocytes in the 

 circulating blood was expressed also by Dawson 

 (1933a). His studies of hematopoiesis were 

 based on the amphibian, Necturus. In this 

 species, some monocytes arose in the spleen but 

 most of them differentiated in the lymphogranu- 

 lopoietic aggregations scattered through the 

 tissues. 



A superficial examination of the cells in figure 

 331 might lead to the suggestion that large cells 

 of the type shown at 16 are metagranuloblasts, 

 but a closer examination shows that this is a dif- 

 ferent cell. The nucleus is a typical monocyte 

 nucleus even at this early stage and there is no 

 tendency toward vacuolization of the cytoplasm 

 as in metagranuloblasts. In the early immature 

 monocyte (cells 17-18) there is some fuither 

 condensation of the nucleus to a condition seen in 

 many monocjiies but no bodies characteristic of 

 granulocytes appear in tlie cytoplasm. The late 



immature stage (cell 19) is readily recognizable 

 as a monocyte and cells like this are commonly 

 seen in the circulating blood. The Hof, azuro- 

 philic staining of the cytoplasm, and nuclear 

 indentation have not yet appeared. 



The implication was given under the discus- 

 sion of thrombocytes in bone marrow that it was 

 difficult to see how such labile cells as throm- 

 bocytes could retain their normal morphology 

 during the procedure leading up to the fixation 

 of tissue. Impression smears fix the cells more 

 quickly than the average fixative, which pene- 

 trates a block of tissue relatively slowly (Under- 

 bill, 1932, and Medawar, 1941) ; yet even in the 

 smear practically every thrombocyte appears as 

 a naked nucleus {12-14 are examples). Even 

 11, which has been labeled as a naked lympho- 

 cyte nucleus because of its larger size, may actu- 

 ally be the nucleus of an immature thrombocyte. 



A few mature heterophils may be seen in 

 spleen smears at this age but whether they repre- 

 sent cells brought in by way of the splenic arteries 

 or are cells that developed in this locus during 

 embryonic life, and have not yet been discharged 

 into the circulation, cannot be determined. 



LYMPHOCYTOGENESIS IN THE 

 THYMUS 



Cells of the thymus have been called thymo- 

 cytes but they are identical with lymphocytes, 

 and the latter term has been used here. Wise- 

 man (1931b and 1932) differentiated 5 stages 

 in the cytomorphosis of lymphocytes: lympho- 

 blast, young cell, mature cell, old cell, and de- 

 generate cell. The classification presented for 

 the lymphocyte series in table 2 (p. 10) includes 

 the first 3 of these 5 stages. 



Many smears were made before a satisfactory 

 one was obtained; most of them failed to show as 

 many intact cells as have been illustrated in 

 figure 332 A, which came from an embryo in- 

 cubated 9 days 4 hours. The fragility of the 

 cells continued well past the age of figure 332 B 

 (11 days 10 hours) ; yet in the older ages large 

 lymphoblasts were relatively rare. 



The high-power drawings (figs. 334-338) 

 were made after searching the slide to find suit- 

 able stages in an embryo that had been incubated 

 14 days 11 hours. At the earlier ages, because 



167 



