(1) Small, round, or polyhedral cells are about the size of pus-corpuscles, or a little larger, with a single round nucleus. Such cells may come from the deeper layers of any part of the urinary tract. They are uncommon in normal urine. When they are dark in color, very granular, and contain a comparatively large nucleus, they probably come from the renal tubules, but their origin in the kidney is not proved unless they are found embedded in casts. Renal cells are abundant in parenchymatous nephritis, especially the acute form. They are nearly always fatty--most markedly so in chronic parenchymatous nephritis, where their {122} substance is sometimes wholly replaced by fat-droplets ("compound granule cells") (see Figs. 44, 48, 60, and 61). (2) Irregular cells are considerably larger than the preceding. They are round, pear-shaped, or spindle-shaped, or may have tail-like processes, and are hence named large round, pyriform, spindle, or caudate cells respectively. Each contains a round or oval distinct nucleus. Their usual source is the deeper layers of the urinary tract, especially of the bladder. Caudate forms come most commonly from the pelvis of the kidney (see Figs. 49, _b_, 50, 62, and 63). [Illustration: FIG. 49.--Epithelial cells from urethra and bladder: _a_, Squamous cells from superficial layers; _b_, irregular cells from deeper layers (Jakob).] (3) Squamous or pavement cells are large flat cells, each with a small, distinct, round or oval nucleus (Fig. 49, _a_). They are derived from the superficial layers of the ureters, bladder, urethra, or vagina. Those from the bladder are generally rounded, while those from the vagina are larger, thinner, and more angular. Great numbers of these vaginal cells, together with pus-corpuscles, may be present when leukorrhea exists. {123} [Illustration: FIG. 50.--Caudate epithelial cells from pelvis of kidney (Jakob).] 3. Pus-corpuscles.--A very few leukocytes are present in normal urine. They are more abundant when mucus is present. An excess of leukocytes, mainly of the polymorphonuclear variety, with albumin, constitutes _pyuria_--pus in the urine. When at all abundant, pus forms a white sediment resembling amorphous phosphates macroscopically. Under the microscope the corpuscles appear as very granular cells, about twice the diameter of a red blood-corpuscle (Figs. 51 and 63). In freshly voided urine many exhibit ameboid motion, assuming irregular outlines. Each contains one irregular nucleus or several small, rounded nuclei. The nuclei are obscured or entirely hidden by the granules, but may be brought clearly into view by running a little acetic acid under the cover-glass. This enables one to easily distinguish pus-corpuscles from small round epithelial cells, which resemble them in size, but have a single, rather large, round nucleus.