but may be seen in spleen and bone marrow oc- 

 casionally. According to T. Makinodan (per- 

 sonal commmiication), plasma cells are abun- 

 dant in the spleen under experimental conditions. 

 These cells are only rarely observed in our stock 

 of untreated adult chickens. The pure light-blue 

 color of the cytoplasm and the few mitochondrial 

 spaces of the early immature plasmocyte sepa- 

 rate it from other cells. Other identifying fea- 

 tures are the clear area adjacent to the nucleus 

 and the presence of vacuoles of uniform, small 

 size in the cytoplasm. An early immature plas- 

 mocyte with one vacuole is shown in cell 1 of fig- 

 ure 331. Early in the differentiation process, 

 the nucleus shows a contraction of chromatin into 

 large, dense clumps and the cell shows an amount 

 of cytoplasm relative to nuclear size that exceeds 

 the proportion found in other cells, especially at 

 the corresponding stage of differentiation. 

 Other cells at the same stage of development 

 would show a reticulum of chromatin in the 

 nucleus. 



From this early stage to the late immature plas- 

 mocyte there is a diminution in size of both nu- 

 cleus and cytosome (cell 2) . Usually the Hof is 

 definite but often the dense blue of the cytosome 

 masks it. A plasmocyte at this stage resembles 

 a mature osteoblast and it has been suggested that 

 they may have a common cell of origin. Plas- 

 mocytes and osteoblasts look so much alike that 

 they could be confused readily in the bone mar- 

 row, but in the spleen it is assumed that osteo- 

 blasts are not present. In the bone marrow it is 

 the clear blue color of the plasmocyte that aids 

 in distinguishing it from the osteoblast, which 

 takes a violet hue. The observations on the cy- 

 tology of plasma cells, made by Dantschakoff 

 (1909b) from sectioned material, agrees closely 

 with the description given here, based on impres- 

 sion smears. From sections of bone marrow, 

 she observed that plasma cells are always located 

 inside vessels and that they became especially 

 numerous when the bird suffered from genei'al- 

 ized exhaustion. Irradiation by X-rays caused 

 the formation of plasma cells in the thymus 

 (Danschakoff, 1916b). 



Lymphocytes are the dominant cells of the 

 spleen beginning with the second day posthatch- 

 ing. Mjassojedoff (1926) found the spleen of 

 the adult fowl to be predominately a lymphocyte- 

 producing organ. This is true to a greater de- 



giee in birds than in mammals. At 35 days of 

 age (fig. 331) developmental stages can be seen 

 in abundance. Previous to this study of the 

 spleen, the thymus had been examined by the 

 smear method and drawings had been made of 

 the changes in cell morphology from lympho- 

 blast to small, mature lymphocyte (figs. 334- 

 338). These cells will be described in more 

 detail when the plates illustrating the thymus are 

 described, but several lymphocytes in figure 331 

 are worthy of mention here. 



Cell 3 is an immature lymphocyte; the narrow 

 rim of cytoplasm in a cell of this size with this 

 peculiar nuclear pattern (partly reticular and 

 partly clumped) is typical for the young lympho- 

 cyte. Somewhat similar examples of this were 

 found in the circulating blood (figs. 96-98) and 

 under these conditions they were called mature 

 lymphocytes of medium size, but as found in the 

 young spleen they are called immature lympho- 

 cytes. This certainly appears to be an inconsis- 

 tency in cell identification but it is more a re- 

 flection of the fact that in spite of a great deal 

 having been written about the lymphocyte, we 

 actually know less concerning its developmental 

 stages than we do of any other leukocyte. Per- 

 haps this is in part due to the fact that those who 

 have written most about the lymphocyte have 

 regarded all sizes from the largest to the smallest, 

 including all stages of cytoplasmic and nuclear 

 differentiation, as insignificant in comparison 

 with the fact that the cell is totipotent in its 

 capacity to produce other cells, and that every 

 lymphocyte is a hemocytoblast, or common stem 

 cell, to all other cells. 



In the circulating blood of one chicken the 

 lymphocytes may be small and in another they 

 may be predominantly of medium size. The 

 graph (fig. 152), based on numerous slides, 

 shows a typical distribution curve for size. More 

 on the size of lymphocytes will be given in chap- 

 ter 6. Careful statistical studies on the relation 

 of size to the health of the individual are needed. 

 From studies on the thymus at least three stages 

 of development can be recognized — lympho- 

 blast, immature, and mature. This represents 

 fewer subdivisions than for any other cell type. 

 It is suggested that lymphocytes of the type 

 shown in figures 96-98 are immature and to re- 

 gard them as such makes the developmental se- 

 ries consistent, but we do not yet have experi- 

 mental data for the circulating blood that enable 



166 



