264 DK A. J. WHITING ON THE 



substance. The intrafollicular reticulum is fibrous but not very strongly developed. 

 Many of the follicles possess a large-celled central area — the germinal centre of Flemming 

 — which tends to fall away from the section. The cells of the follicle are of two kinds : — 

 (l) Small, deeply stained lymphoid cells, which are often collected in clumps, and the 

 clumps sometimes appear to be surrounded by a capsule; and (2) large, faintly stained 

 cells, leucoblasts, grouped near the centre, but also occurring sparsely among the smaller 

 ones, which consist of a large vesicular nucleus surrounded by a varying amount of finely 

 granular protoplasm, that occasionally contains yellow pigment grains. There is a 

 distinct zone of fine spindle-shaped muscle fibre cells, in two or three layers, bounding 

 the follicle ; and outside that is a broad belt of tissue showing characters intermediate 

 between those of the pulp on the one side and those of the follicle on the other, and is, 

 in fact, the pulp containing the lymphoid cells, small and uninucleated, part of which 

 have probably been pushed out by the cellular proliferation within the follicle, and part 

 squeezed out by the contraction of the peripheral muscular layer. Although these cells 

 are small, they are apparently somewhat larger than those in the outer part of the 

 follicles. 



In the spleen of the Sheep the continuous adenoid sheath of the artery, along the 

 course of the artery, between the follicles and the hilus, is very well developed. It is 

 bounded externally by the thick muscular layer of the hilar sheath. The structure of the 

 follicle resembles very closely that in the ox. But the artery is apparently of larger 

 calibre, the capillaries are slightly larger, and the intrafollicular reticulum is somewhat 

 more strongly developed. The lymphoid cells show the same division into zones, a 

 large-celled germinal centre, a small-celled peripheral zone, and an extrafollicular 

 aureola. 



In the spleen of the Pig the follicles are not quite so numerous as in the ox and sheep, 

 and they are still less sharply defined from the surrounding pulp. Sometimes sections 

 of three or four arteries may be seen in one follicle ; moreover, the artery may often 

 be seen to branch within the continuous adenoid sheath without any division of the 

 hilar sheath. In structure the follicles are almost identical with those in the ox and 

 sheep ; the intrafollicular reticulum appears, however, to be more strongly developed. 

 In the spleen of the young Pig the peripheral zone of muscle is more distinct than in 

 the adult, as also is its origin from the hilar sheath. 



In the Cat the follicles are more numerous and larger, relative to the size of the 

 spleen, than they are in the Ungulata. The intrafollicular capillaries form a very 

 distinct network, and they may frequently be traced into the spaces of the pulp. 

 The cells of the follicle in the adult cat are clearly divisible into two zones. In a 

 central area, where the capillaries are most numerous, they are comparatively large, 

 although they vary considerably in size ; some of them are uninucleated and others are 

 multinucleated. The nuclei always possess a nucleolus, and the cells a definite outline. 

 Some of the smaller central cells show evidence of division by transverse fission. The 

 largest cells in the follicles are scattered and isolated, each contained in a cell space. 



