This very interesting phenomenon of ciliary motion in the neck of the tube and in the Malpighian capsule was discovered by Mr. Bowman in the frog ; and at the time when his paper was published he had not observed it in any other animal. Mr. Simon* wards observed it, as he says, " at the origin of each uriniferous tubule in the kidneys of various other reptiles, and also with perfect distinctness in the skate." Biddert has since observed the same phenomenon in the triton ; has described it in the embryo ard ; and I have seen it in the common snake.
I am not aware that ciliary motion has been detected in the kidneys of Mammalia or Birds. I shall presently show that in certain fishes and reptiles the cilia are not confined to the situation in which they were first discovered by Mr. Bowman ; but that they exist through out the greater part, if not the whole length, of the uriniferous tubes.
It appears desirable to allude here to some observations which have been made since the publication of Mr. Bowman's paper, and to inquire how far certain staternents which have been opposed to his account of the Malpighian bodies are worthy of consideration. It is not my intention to occupy time and space, by giving a history of all the contradictory opi nions which this subject has elicited. It may be fairly inferred that inability to detect ciliary motion within the Malpighian capsule, or to verify any observation in reference to which several competent authorities are agreed, is the result of some defect in the microscope employed, or in the eye or mind of the ob server.
Objections have been made to two parts of Mr. Bowman's description ; first, to his account of the relation which the Malpighian capsule bears to the basement membrane of the tube and to the blood-vessels; and, secondly, to his statement that the Malpighian capillaries lie uncovered within the capsule.
Bidder* made his observations on the kid ney of the male triton (Triton teeniatus). The anterior part of the kidney of this animal is exceedingly well adapted for the investigation in question, since it is very thin and transpa rent, and is thus fitted for microscopical ex amination without further artificial preparation by- tearing or other means. Bidder believes that the vessels do not perforate the capsule to enter its cavity, as described by Mr. Bow man ; but he considers the basement membrane to be introverted so as to form a covering for the vessels and a complete partition of a semi lunar forin between the cavity of the tube and that of the Malpighian capsule. I have exa mined the kidney of the triton with great care, and have satisfied myself that Mr. Bowman's account of the perforation of the capsule by the vessels is strictly ,correct, and that there is no partition, but, on the contrary, a free communication between the cavity of the capsule and the orifice of the tube. When the vessels are distended with blood, they almost fill the capsule ; on the contrary, when they are empty, they shrink into a small com pass. I have examined them under both conditions, and could never detect any ap pearance of a membrane reflected over them. The free communication between the cavity of the capsule and the orifice of the tube is sufficiently shown by, two phenomena which I have repeatedly witnessed : first, when the cilia are in action, the liquid filling that part of the capsule which is unoccupied by the vessels is freely propelled from the cavity of the capsule into the tube; and, secondly, when water is added to the specimen, loose particles of epithelium from the tube are often driven into the capsule, until they fill that part of its cavity which is not occupied by the collapsed blood-vessels.
Dr. Gerlach* describes and figures the Mal pighian capsule as being not a blind termi nation of the uriniferous duct, hut a lateral diverticulum of the same structureless base ment membrane which forms the duct ; and he believes that the capsule communicates with the duct by means of a short neck. It is not impossible that there may, in some rare instances, be a diverticulum from a tube as represented by Gerlach ; but as I have never yet seen such a mode of connexion between a tube and a capsule, and as I have seen numberless instances of tubes terminating directly in the dilatation which constitutes the Malpighian capsule, I do not hesitate to de clare my decided conviction that Mr. Bow man has correctly described the structures in question.1 With reference to the second point above alluded to, namely, to Mr. Bowman's state ment that the Malpighian capillaries lie un covered within the capsule, the observations of Gerlach deserve more consideration. Ile states that when the Malpighian capillary net work is examined after the capsule has been entirely detached from it, it may be seen in its whole extent covered by a thick layer of nucleated cells, which are continued from the inner wall of the capsule upon the Malpighian vessels ; so that the latter lie introverted with in a layer of cells like the intestine within the peritontim (fig. 160.); and he supposes that the secreting structure of the Malpighian bodies differs from the ordinary structure of glands only in the absence of the basement brane between the vessels and secreting cells. Dr. Gerlach's figure (fig. 160.) exhibits an appearance which every one must have seen in the tuft of vessels extended from the cap sule, but which fails to establish the existence of this epithelial investment of' the tuft ; for at the border of the figure the wall of the capillaries is seen actually bare, as described by Mr. Bowman. The fact, however, seems to be, that there do exist, here and there, upon the outside of the capillaries of the tuft, nu cleated particles, of an extremely delicate nature, the nuclei sometimes lying isolated in the fork of two vessels, and the substance of the cell not expanding into a continuous co vering of the whole tuft. It is possible that these nucleated particles may be rather the nuclei belonging to the capillary wall, than a modified representation of the epithelium of the tube. It is at least certain that they lie sparingly upon the individual vessels of the tuft, and do not form a membranous invest ment of it as a whole. Mr. Bowman showed me these particles, as I have now described them, some years ago. Their existence does not affect the substantial accuracy of his ac count of the anatomy of the tuft, nor his view of its special share in the secretion of urine.