abnormalities are completely missed or are not noted in the records. Abnormal arteries and persistent foetal lobulation are the two most common of these minor abnormalities. The figures for the major abnormalities are less erroneous, but they are vitiated by the difficulty of deciding as to whether atrophic kidneys and abnormally placed kidneys are due to congenital (germinal) or acquired (somatic) causes.

Henry Morris states that one instance of solitary kidney will occur in every 2400 autopsies, of "horse-shoe" kidney one in every 1000 autopsies, and of atrophy of one kidney even more frequently, these appearing to be the commonest abnormalities. Brewer, in a recent research, in 151 autopsies found aberrant arteries in 56 per cent., and four cases of split ureters. Our own experience in the post-mortem room, when looking out for such abnormalities, agrees with that of Brewer, and we found aberrant arteries to be extremely common.

Fortunately for the surgeon variations in one part tend to be correlated with variations in other parts-that is to say, when congenital defects of the kidney are present, similar defects will often be present, not only in the rest of the urogenital tract, but elsewhere in the body. Defects of the testicle and penis and of the female genitalia are especially common, and we would draw attention to the importance of noting not only minor degrees of hypospadias, but the presence of minute para-urethral follicles and canals round the penile meatus, which are very frequent and are congenital defects. Such conditions should be looked upon as stigmata of a tendency to congenital defect. Renal operations should never be undertaken lightly upon such subjects until the presence of two functioning kidneys has been adequately determined. The surgeon should accept such minor stigmata as danger signals.

Before proceeding to classify defects of the kidney we append a short account of the development of the kidney and ureter. This account is largely taken from the revised edition of Quain's Anatomy' by Bryce, from articles by Pohlmann and his pupils, and from Professor Arthur Keith's books and lectures.

Embryology; Development.1

"The excretory system first appears as a longitudinal duct and a longitudinal series of epithelial tubules in the part of the

1 Portions in inverted commas (and Figs. 2, 4, 5, 6) are taken from 'Quain's Anatomy,' vol. i, new edition, T. H. Bryce. We are indebted to Prof. Schäfer and to Messrs. Longmans for leave to make these extracts.

mesoderm named the intermediate cell mass [Fig. 2, w.d., w.b.]. This forms a continuous blastema from which every part of the system takes origin. The tubules are developed from before backwards, and are grouped as they appear in time and place into three systems, the pronephros, the mesonephros and the metanephros [Fig. 1].

"The pronephros is represented only by vestiges in the mammalia, and in the human embryo is probably to be recognised in a longitudinal duct, some blind tubules, a peri

FIG. 1.-Scheme of the Wolffian body on the right side. After Keith.

toneal funnel, and a vestigial glomerulus which have been described in the fifth, sixth and seventh segments.

"The segmental or Wolffian duct appears as a solid cord of cells which extends backwards close under the ectoderm external to the nephrogenetic cord as far as the cloaca. It soon acquires a lumen and comes to open into the cloaca [Figs. 1 and 2].”

It is disputed by embryologists as to whether this duct is formed by an ingrowth of ectodermal cells inwards into the mesoderm, or by the differentiation of mesodermal cells. ectodermal origin is suggested not only by the phylogenetic

An

origin of the duct but also by the tumours which arise from it (carcinomata of a squamous cell type [see Ch. X] which closely resemble the tumours of the anterior urethra, undoubtedly an ectodermal structure).

Mesonephros or Wolffian Body.

"The mesonephros appears as a ridge (Wolffian ridge) on each side of the attachment of the primitive mesentery, which extends from the fifth cervical to the fourth lumbar segment.

FIG. 2.-Rabbit embryo of eleventh day. After Bryce. w.d. Wolffian duct. w.b. Wolffian body. e. Entoderm. a. Aorta. c. Cœlom. m. Mesenchyme.

It is formed by the enlargement of the intermediate cell-mass, as the tubules and their Malpighian corpuscles develop in its tissues" [Fig. 2, w.d.]. (The tubules [Fig. 2, w.b.] are formed by the ingrowth of the colomic endothelium [Fig. 2, e.] into the mesenchyme of the intermediate cell-mass, to join outgrowths from the Wolffian duct.) "As the tubules increase in number the ridges become prominent bodies, which increase in size until the eighth week, after which they gradually diminish owing to the fact that the tubules undergo degenerative changes. These degenerative changes

begin at an early stage in the head end of the Wolffian body and by the fifth mouth the tubules have

almost entirely disappeared, and in the end only persist as accessory parts of the reproductive apparatus (organ of Giraldes, vasa aberrantia of Haller).”

FIG. 4.-Longitudinal section of embryo.

Metanephros; Permanent Kidney.

"The metanephros arises from the posterior part of the same nephrogenetic cord as forms the blastema of the mesophros [Figs. 2 and 4].

"The tissue, however, remains passive during the time when the tubules are forming in the Wolffian body, and becomes related to a diverticulum which grows out from the dorsomesial aspect of the Wolffian duct immediately in front of its opening into the cloaca [Figs. 3 and 4]. As the diverticulum increases in length in a dorsal direction, the nephrogenetic tissue becomes displaced and separated from the Wolffian duct until it comes to lie dorsal and mesial to it.

"The diverticulum becomes expanded at its distal end to form the primitive pelvis of the kidney [Figs. 4 and 5], while the stalk represents the future ureter.

FIG. 5.-g. Gut. b. Bladder. i.z. Inner nephrogenetic zone of mesenchyme cell. p. Pelvis. u. Ureter. w.d. Wolffian duct. m. Mesenchyme.

"The primitive pelvis is surrounded by the nephrogenetic tissue, which now shows a differentiation into an inner epithelioid and an outer mesenchymatous zone [Figs. 4 and 5].

"The primitive pelvis is at first simple, but soon becomes branched by the outgrowth from it of bulbous ampullæ, each of which is covered by a cap of the epithelioid inner zone, which has been broken up into separate masses during the process of branching [Fig. 6A].

"The collecting tubules are formed by a continuous process