COMPARATIVE HISTOLOGY AND PHYSIOLOGY OF THE SPLEEN. 287 



numbering several hundreds in each section. They may have one rather large nucleus, 

 or two or three smaller, and their protoplasm is usually coarsely granular and vacuolated, 

 sometimes showing differentiation into a hyaline rim and a coarsely granular core, and 

 always staining deeply with eosine. 



Yellow granules are imbedded in some cells at intervals in the granular pink-stained 

 protoplasm, often accompanied by a single nucleus and several vacuoles. Occasionally a 

 faintly blue-stained nucleus may be seen within a vacuole, and sometimes a round piece 

 of granular protoplasm. The pigment increases in amount until in some cells the 

 granules stud the small remnant of protoplasm surrounding the nucleus and separating 

 the vacuoles. 



In the spleen of a half-groivn Rat there are no giant cells, but numerous smaller 

 vacuolated cells containing pigment, which are like those of the adult rat, but are usually 

 slightly larger and more pigmented. They vary in size from about 12 ju, to 20 ju, in 

 diameter, and the larger are not of greater size than similar non- vacuolated cells. Some 

 of the vacuoles are twice or thrice the size of a red blood-corpuscle. The remnant of the 

 original protoplasm varies in amount, and the pigment granules appear to be in the proto- 

 plasm and not in the vacuoles. (Plate III., fig. 13.) There is usually one nucleus, but some- 

 times two or three, generally situated near the periphery of the cell. Similar pigmented 

 cells, some of which are vacuolated, are found in the follicles. 



In the spleen of a young Rat there are a few characteristic giant cells, about ten in a 

 section, but there are apparently no smaller vacuolated cells. 



In the spleen of the Mouse the reticulum of the pulp resembles very closely that in 

 the rat's spleen ; the cell processes are on the whole more plate-like. 



A characteristic feature of the spleen of the adult brown Mouse is the presence in its 

 pulp of giant cells ; this holds good too for the white mouse, as pointed out by J. Arnold. 

 In addition to the giant cells there are protoplasmic corpuscles, lymphoid cells, and 

 pigment-holding cells. Lymphoid cells of the follicles show numerous karyokinetic 

 figures ; and a similar association of giant cells in the pulp with active karyokinesis in 

 the follicles w r as noticed in the spleens of the young rat and water-vole. 



The giant cells number about fifty in a section, and while some are small, most are 

 large, measuring about 20 /a by 30 /*. Their protoplasm has a markedly yellow tint, 

 and is for the most part coarsely granular, although in some cells there is a very thin 

 hyaline rim. The outline of the cells is sometimes prolonged into spinous projections. 

 In the isolated unhardened giant cell, as seen on examining the fresh pulp teased in 

 methyl salt solution, numerous buds may be seen to project from the cell, perhaps four 

 or five in number ; they look like basins turned downwards upon the surface of the cell. 

 The periphery of the giant cell is seen in the fresh condition to have a distinct yellow 

 colour, apparently from the presence of haemoglobin. 



The nuclei of the giant cells after fixation show many different forms, many of which 

 are figured by Arnold ; the simplest form seems to be that of a horse-shoe shape — some- 

 times the nucleus has the shape of an hour-glass — but the most frequent arrangement is 



