The formation of these cysts has been referred to an intra-uterine chronic nephritis, but another theory accounts for them by a vice of development. The fact that when the lesion is unilateral, as sometimes happens, there is apt to be a deficiency of some other part of the genito-urinary apparatus on the same side, and that several infants with cystic degeneration have been born of the same mother, speaks strongly in favor of the latter theory. Serous cysts of later origin do not usually attain so large a size, or rather the kidney does not, on account of their smaller number. They are lined with a thinner membrane, and their contents are nearly clear, but coagulable, comprising uric acid, carbonate of lime, and cholesterin. Occasionally a single cyst attains considerable dimensions and produces by its pressure atrophy of part of the kidney. These cysts are supposed to arise in consequence of the blocking of a tube. The third class of cysts closely resemble the first in appearance and in form, and contain more or less serous or gelatinous fluid, with albumen, blood-corpuscles, and pus, as well as the peculiar colloid bodies previously mentioned. They undoubtedly arise from the distension of tubes and of Malpighian bodies. These cysts are usually associated with chronic interstitial nephritis, and in fact they are rarely absent in cases of this kind, although the extreme degree--that is, where the cysts assume the most prominent position while the contracting nephritis falls into the background--are less common. In these latter cases the organ may be almost transformed into a mass of rounded bodies somewhat resembling a bunch of grapes. The SYMPTOMS of the first two of these conditions--that is, of the cysts which are not connected with an active nephritis and attract attention simply as tumors--depend on the pressure they exert; and a diagnosis is to be made by a knowledge of their history and by the rules already given. The symptoms and diagnosis of the third variety are involved in those of chronic interstitial nephritis. {64} There is no reason to suppose that any drug has any therapeutic action on such kidneys, so far as the cysts are concerned. It should always be remembered that a kidney may contain a large number of cysts, and yet scattered portions of secreting substance enough be left to carry on the function indefinitely. It might under some circumstances be justifiable to remove a cystic kidney on account of the pressure exercised on other organs, but as the cysts do not increase rapidly in size, punctures several times repeated, so as to empty a number of them, would in most cases prove as effectual an operation, and, what is of greater importance, would not involve the loss of any portion, even if small, of secreting structure which may be left. Tuberculosis.