From the standpoint of comparative pathology attention can be drawn to the glomerulus, to the interstitial tissue, and to the character of the epithelium. The tuft of intricately wound capillaries called the glomerulus has always been viewed as the part of the secretory unit chiefly concerned in urine production whether one accept the older idea that it excretes only fluid or the modern belief of many observers that all parts of the urine go out through it. In the mammal the tuft is closely wound, is surrounded by a distinct space and a limiting membrane of appreciable width called Bowman’s capsule; all this so-called Malpighian body has a breadth varying from 120 to 300 micra. There is however great variation in the size of this body when seen in the peripheral and deeper zones of the same organ, amounting at times to seventy per cent. of the diameter. The capillary congeries forming the avian tuft is by no means so delicate and one can see individual capillaries with more ease. It may be impossible to discover a space between the tuft and its exceedingly delicate capsule, the latter being usually applied closely to the vascular corpuscle. The whole breadth varies from 70 to 140 micra with an average of 110. There is more uniformity in size than in the mammalian organ. Supporting tissues between the tubules seem less definitely nuclear in the bird than in the mammal, at least in so far as connective tissue is concerned, there being in the former only a few groups of mononuclears to be seen in the cortex. Perivascular tissues are reasonably rich. The epithelium of proximal and distal tubules is not easily fixed by our customary laboratory techniques, the best results being obtained by Zenker’s fluid. As seen in a routine specimen of a normal organ it is vacuolated or very palely stained. The individual cells stand out clearly and many present a pointed end to the tubular lumen. KIDNEY WEIGHTS. According to the work of Mangan[35] and of Alezais[36], the bird has an average kidney-to-body weight of 6.9 grams per kilo while man has a ratio of 4.3 grams, dog 5.9 grams and guinea-pig 8.5 grams. The first author would show that the fish-eating birds have the heaviest and vegetarian birds the lightest organ. Our own figures are limited to the weights of apparently normal organs in thirty-one mammals and five birds; they are as follows: