consist throughout life of two separate portions, one median and single, the other, derived from the sympathetic ganglia, paired; in birds, reptiles, and mammals these distinct portions are combined into the two paired organs (Balfour). But in these also, as has been shown by Mitsukuri for mammals, the medullary or nervous part is at first distinct and outside the cortical, into which it gradually insinuates itself, retaining, however, its connection with the neighbouring sympathetic ganglia.

The permanent kidneys arise (1) as protrusions from the posterior end of the Wolffian ducts (see fig. 125, C and D, N), which grow forwards towards the lower part of the Wolffian bodies, and form the ureters and the collecting tubules of the kidney; (2) from a portion of the intermediate cell-mass situated posterior to the Wolffian body, and within which convoluted tubes and Malpighian corpuscles, and eventually the remaining parts of the uriniferous tubules become developed. But before these changes occur in this intermediate cell-mass, it shifts its position relatively to the Wolffian body, eventually coming to lie above and behind that organ. The convoluted tubes, with their Malpighian corpuscles, appear to be developed independently of the ureter and collecting tubes, as in the case of the Wolffian tubules and the Wolffian duct, a communication between them being only subsequently established.

The glomeruli are apparent in the eighth week in the human foetus. In the third month the papillæ are formed, and in the fourth month the loops of Henle are seen. The tubes are wider in the foetus than in the adult; the expansion of the kidney as growth advances must therefore be due mainly to an increase in length of the tubules, since new tubules and glomeruli do not appear to be formed. The human kidney is at first lobulated, the lobules corresponding in number to the Malpighian pyramids, but by the end of the first year after birth, the kidneys have usually nearly lost their lobulated appearance.

The urinary bladder is formed by a spindle-shaped dilatation of the stalk of the allantois (second month). The upper pole of the spindle extends as the urachus into the umbilical cord; it not unfrequently remains hollow for some length within the cord (Luschka). The lower pole of the spindle which passes towards the cloaca becomes the first part of the urethra of the male, and the whole of the urethra of the female. The rest of the male urethra is formed and enclosed by the folds of integument which produce the penis (see p. 128). The ureters, which are originally prolonged from and open into the Wolffian ducts, subsequently become shifted in position, so as eventually to open into the enlargement of the allantoic stalk, from which the bladder is formed.

The Müllerian duct.-In lower vertebrates, as was shown by Balfour for elasmobranchs, this duct takes origin by the splitting off of the ventral part of a longitudinal segmental or Wolffian duct, the dorsal part remaining as the Wolffian duct proper, and receiving the segmental and uriniferous tubes, while the ventral part retains the funnel-shaped orifice, by which the segmental duct communicated anteriorly with the body cavity, and comes to open posteriorly into the cloaca by an orifice distinct from that of the Wolffian duct (fig. 138 and fig. 147). In amniotic vertebrates, the process of formation of a Müllerian duct is somewhat different. It arises on the outer side of the already fairly well developed Wolffian body, and some little distance from the anterior end of that organ, as a thickening of the peritoneal epithelium (fig. 143, a'), which thickening becomes invaginated towards the adjacent Wolffian duct, in the form of three successive funnel-shaped depressions (fig. 148), somewhat similar to those which are connected with the previously formed Wolffian segmental tubes. The invaginations are connected together by a continuous epithelial ridge, forming a cord which becomes disconnected from the peritoneal cavity except at the anterior invagination, and subsequently acquires a lumen. The short tube which is thus formed, soon begins to grow backward as a solid rod of cells, which comes in close contact as it proceeds with the Wolffian

duct (fig. 149). To this duct it presently adheres intimately, and then continues

Fig. 147.-FOUR SECTIONS THROUGH THE ANTERIOR PART OF THE
SEGMENTAL DUCT OF A SCYLLIUM EMBRYO. (Balfour.)

The figure shows how the segmental duct becomes split into the
Wolffian duct dorsally and the Müllerian duct or oviduct ventrally;
Wd, Wolffian duct; od, Müllerian duct or oviduct; sd (in D),
segmental duct.

to grow backwaras for a certain distance as a thickening of the epithelium of that tube, the thickening becoming gradually separated off from before backwards, and the lumen passing along it. Further back it ceases to grow thus in connection with the Wolffian duct, but is prolonged as an independent cellular cord, which lies in a groove along the side of the Wolffian duct (Balfour and Sedgwick).

Entering the genital cord,1 the two Müllerian ducts lie at first on the mesial side of the corresponding Wolffian ducts, but lower down pass behind them; they finally come again between these ducts, lying close together, and, according to Mihalkovics, approach close to the sinus urogenitalis, which by this time is formed out of the ventral part of the cloaca (see p. 128) without actually opening into it for some time. The Müllerian ducts fuse together below into a single tube (fourth month); the fusion begins not at the lower end,

Fig. 148.-SECTIONS FROM THE CHICK SHOWING TWO OF THE PERITONEAL INVAGINATIONS WHICH GIVE RISE TO THE ANTERIOR PART OF THE MÜLLERIAN DUCT. (Balfour and Sedgwick.)

gr2, second invagination; g3, third invagination; 2, epithelial ridge between them; Wd, Wolffian duct. These structures form the pronephros of Balfour and Sedgwick (see note, p. 115).

A name given to the thickened mass of tissue which surrounds the Wolffian ducts as they course together to the cloaca behind the stalk of the allantois (afterwards the base of the bladder).

certain length, the amount of this upward extension of the fused ducts varying in different animals.

The united part of the Müllerian ducts afterwards forms the foundation of the vagina and uterus in the female, and the prostatic vesicle, or uterus masculinus in

believed to represent in the male the remnant of part of the Müllerian duct.

In the human embryo of the third month the uterus is bifid, and it is by the upward extension of the median fusion that the triangular body of the uterus is produced. The bifid condition corresponds with the bicorned uterus of many animals, and the process of fusion above described explains the occasional malformation of a partial or complete division of the uterus and vagina into two passages. Up to the fifth month there is no distinction between vagina and uterus. Then the os uteri begins to be seen, and the cervix uteri subsequently becomes manifest as a part, which is at first thicker and larger than the rest of the organ. In some animals the prostatic vesicle of the male is prolonged into cornua and tubes like the uterus of the female.

The germinal epithelium.-This name was given by Waldeyer to the thickened epithelium lying along the inner side of the Wolffian projection (fig. 143, a). The cells become at first columnar, and then two, three, or even several layers thick, while at the same time the mesoblast below them becomes increased in amount, and thus a marked projection is produced, which in some vertebrates forms a distinct ridge the genital ridge. Amongst the cells of the germinal epithelium, some are seen which are larger and more spherical than the others, these are the primordial ova (fig. 143, o), and occur in both sexes; in fact, up to a certain point, the difference of sex of the embryo is not apparent.

Development of the ovary. In the female sex the germinal epithelium soon becomes much thickened, and begins to grow down into the mesoblastic stroma in the form of columns of epithelium cells, which enclose amongst them some of the primordial ova.1 These columns constitute the egg-tubes of Pflüger (fig. 152). They are separated from one another by mesoblast, which grows towards and into the germinal epithelium simultaneously with the down-growth of the egg-tubes, and there is thus produced a complete interlocking of strands of connective and epithelial tissue, which together constitute the ovary. The egg-tubes next become broken up into rounded groups or "nests" of germinal epithelial cells, each of which may enclose one or more primordial ova. The primordial ova eventually develope into ordinary ova, two or more frequently fusing together to form a single ovum (Balfour), while from the remaining cells in the "nest the epithelium of the Graafian follicle is eventually produced. In many of the cell nests, primordial ova cannot at first be

1 Mihalkovics states that the cells which are to form the follicular epithelium first sink into the stroma, and that afterwards the primordial ova follow them, and become enclosed by them.

distinguished, but become formed subsequently by an increase in size of one or more of the cells. The further changes which take place in the Graafian follicle arc described with the structure of the ovary (v. Splanchnology). The remainder of

Fig. 151.-TRANSVERSE SECTION THROUGH THE OVARY OF AN EMBRYO SHARK (SCYLLIUM), SHOWING THE GERM-EPITHELIUM FORMING PRIMITIVE OVA. (Balfour.)

At po, the germ-epithelium and primitive ova; the lightly-shaded part is the ovarian stroma, covered elsewhere by flattened epithelium.

the germinal epithelium which is left covering the surface after the formation of the egg-tubes, constitutes the permanent epithelium of the ovary.

Most, if not all, of the permanent ova are produced, at least in the human subject, long before birth. In the human ovary the nests of cells which are to form the Graafian follicles

are more equally diffused through the substance of the ovary than in most animals, in many of which the young follicles remain forming a stratum near the surface. In the human embryo of from four months up to the period of birth, the ovary seems to be formed of little else than a mass of young ova, closely surrounded by flattened cells of the germinal epithelium and constituting thus minute Graafian follicles; the amount of

stroma being at this time relatively small. It has been calculated that the ovaries may at this stage contain as many as 70,000 primordial ova.

Development of the Testicle.-The germinal epithelium does not undergo so marked an hypertrophy in the male as in the female. But it becomes thickened, and enlarged cells, corresponding to the primordial ova in the female, are found in it. Further, small strands of the epithelium dip down into the subjacent mesoblast,

which grows simultaneously into the epithelium, and eventually cell-nests are separated and included in the mesoblastic tissue. Whether these nests are derived from the division of the primordial ova only, or whether they also include other cells of the germinal epithelium is not clear. It would appear that from these cell-nests the epithelium of the seminiferous tubules is developed, although all stages of the process have not been observed. The cell-nests eventually become connected with the outgrowths from the Wolffian bodies (fig. 153, st), which as already mentioned,

Fig. 153. -SECTION OF THE GERMINAL EPITHELIUM AND ADJACENT STROMA IN A MALE CHICK EMBRYO.

(Semon.)

g.p, germinal epithelium forming a thickened ridge-like projection; pr.ov, primitive ova of various sizes, some in the germinal epithelium and others somewhat beyond the limit of this epithelium; st, strands of cells which have grown from the Wolffian body towards the germinal epithelium, and one of which appears connected with an enlarged primitive ovum.

form the rete testis and the efferent tubes of the testicle. The reproductive gland is in both sexes at first attached directly to the Wolffian body (fig. 156, A, of), which itself is attached by a fold of peritoneum to the back of the abdominal cavity. This fold becomes the mesovarium or mesorchium as the case may be. A band also passes from the Wolffian body upwards to the diaphragm, and another fold containing involuntary muscular fibres-the plica gubernatrix-runs down towards the groin from the lower part of the Wolffian body and the duct. This band, as the Wolffian body becomes atrophied, is found to be attached to the reproductive organ, constituting the gubernaculum testis in the male, and the round ligament of the ovary in the female (fig. 156, g).

Descent of the Testicles.-The testicles originally lie in the lumbar region of the abdomen. From this part they become shifted, at first to the intestinal abdominal ring, opposite which they are found in the sixth month, and which they enter in the seventh month, then down the inguinal canal into the scrotum, which