In the interval between the protovertebrse and the cleavage of the mesoblast into its somatopleural and splanchnopleural layers, a mass of cells arranges itself into a longitudinal ridge. This ridge, which lies beneath the epiblast, becomes hollow, and thus a tube is produced, called the Wolffian duct.

From this tube diverticula arise, which extend into the surrounding mesoblast; they are tubular, and communicate with the central duct. The processes become twisted, and at their extremities the neighboring mesoblast undergoes differentiation, and forms vascular capsules corresponding in structure to the Mal-pighian corpuscles. This part of the Wolffian duct, which has acquired a glandular structure, is the Wolffian body or primitive kidney of the embryo, while the Wolffian duct corresponds to the primitive ureter.

Transverse section through the embryo of a chick on the second clay, where the medullary canal is closed.

Fig. 294. Transverse section through the embryo of a chick on the second clay, where the medullary canal is closed. (Kolliker).

mr. Medullary canal, h. Epiblast. wwh. Cavity of protovertebra uw. ung. Wolffian duct. nip. Mesoblast dividing, hpl. Somatopleure. df. Splanchnopleure. sp. Pleuroperitoneal cavity, dd. Hypoblast, ch. Notochord.

The epithelium lining the interval between the somatopleure and splanchnopleure (pleuroperitoneal cavity) becomes columnar in character close to their origin from the uncleft mesoblast. It receives the name of the germinal epithelium. An involution of this takes place into the mesoblast, just below the somatopleure, and becomes shut off, forming a hollow cylinder.

Section of the inner part of the pleuroperitoneal cavity through the origin of the genitourinary organs.

Fig. 295. Section of the inner part of the pleuroperitoneal cavity through the origin of the genitourinary organs. {Waldeyer).

L. Somatopleure. m. Splanchnopleure. a. Germinal epithelium. C, o. Primitive ova. E. Mesoblast forming the ovary. IVK. Wolffian body. y. Wolffian duct. a'. Epithelium giving rise to the duct of Muller z.

By this means a second duct is formed in close relation to the first; this is the Mullerian duct. This duct is developed from before backward.

According as the embryo is a male or a female, so one or other of these ducts develops. In the male the Wolffian duct remains as the vas deferens, and the Mullerian duct becomes atrophied.

In the female, on the other hand, the Mullerian duct forms the organs for the conveyance of the ova out of the body, and the Wolffian duct is represented by a rudimentary structure near the ovary.

Transverse section through the lumbar region of an embryo chick at the end of the fourth day.

Fig. 296. Transverse section through the lumbar region of an embryo chick at the end of the fourth day. {Foster and Balfour).

W. R. Wolffian ridge, g. e. Germinal epithelium. A. O. Dorsal aorta. M. Mesentery. SP. Splanchnopleiue. d. Alimentary canal. V. Vessels, m.p. Commencing Mullerian duct. So. Somatopleure. W. b. Wolffian body. W. d. Wolffian duct. V. c. a. Posterior cardinal vein. c. h. Notochord. A. IV. C. Anterior white column of spinal cord. a. r. Anterior root. A. G. C. Anterior gray column. p. r. Posterior root.;m.p. Muscle plate. nc. Canal of spinal cord.

Part, however, of the Wolffian duct in both sexes develops similarly: this, the metanephros, corresponds to that part of the duct nearest to the tail end of the embryo. It forms part of the urinary organs, and develops into the permanent ureter and the kidnev.

From the metanephros a projection arises, which grows quickly and opens into the cloaca; this remains as the ureter. From the upper part of the ureter arise small caecal evolutions, which become convoluted at certain points and surrounded by mesoblast; these canals are the urinary tubules, and at the extremity of each is developed a tuft of vessels, which thus forms a Malpighian corpuscle.

The straight tubes group themselves together at the inner part of the gland, while the convoluted tubules, with the Malpighian corpuscles, are aggregated at the periphery of the gland.

At the junction of the ureter to the glandular mass, changes take place by which this tube is split up into several subdivisions, which are the primitive calices of the kidney, the dilated part of the ureter forming the pelvis.

The testicle arises partly from the germinal epithelium lining the inner extremity of the pleuroperi-toneal cavity, lying close to the splanchnopleure, and partly from the mesoblast surrounding the Wolffian body.

The germinal epithelium, the cells of which are not so well developed as in the female, sends processes into the mesoblast, and these are said to form the spermatic cells, the mesoblast becoming differentiated around them to form the walls of the tubuli seminiferi.

Diagram of the genital organs of an embryo previous to sexual distinction.

Fig. 297. Diagram of the genital organs of an embryo previous to sexual distinction. {Allen Thomson).

W. Wolffian body. 3. Ureter 4. Bladder. 5. Urachus. gc. Genital cord. m. Miillerian duct. w. Wolffian duct. ug. Urogenital sinus. cp. Clitoris, or penis, i. Intestine, cl. Cloaca. Is. Part from which the scrotum or the labia majora are developed, ot. Origin of the ovary or testicle respectively, x. Part of Wolffian body subsequently developed into the coni vasculosi.

The Wolffian duct, which persists as the vas deferens, aids in forming the testicle, the epididymis being merely a convoluted part of it, and the vas aberrans one of the caecal tubes in connection with the duct. The coni vasculosi are thought to be formed from some of the tubules of the Wolffian body; they are connected to the testicle by means of a tube which is split up into a number of divisions forming the vasa efferentia.

Diagram of the sexual organs of the male embryo. {Allen Thomson).

Fig. 298. Diagram of the sexual organs of the male embryo. {Allen Thomson).

3. Ureter. 4. Bladder. 5. Urachus. t. Testicle, in. Atrophied duct of Muiller (hydatid of Morgagni). e. Epididymis, g. Gubernaculum testis, as. Vesicula seminalis. i. Intestine, pr. Prostate. W. Organ of Giraldes. vh. Vas aberrans. vd. Vas deferens. C. Cowper's gland, cp. Penis, sp. Spongy part of the Urethra, t'. Position the testicle ultimately assumes, s. Scrotum.

The Wolffian duct forms, beside the vas deferens, the vesicula seminalis (which is merely a blind diverticulum from its extremity), and terminates in the ejaculatory duct.

The two Mullerian ducts, in the male, join and form a single tube; this is not further developed, but atrophies, leaving as its representative the sinus pocularis, which is situated in the floor of the prostate. The upper extremities of the Mullerian ducts form the hydatids of Morgagni.

The ovary, like the testicle, is formed from the germinal epithelium, which multiplies and forms a projection close to the Wolffian body. The cells of the epithelium become involuted and surrounded by the uncleft mesoblast, to form ova and Graafian follicles. The glandular part of the ovary thus arises from the germinal epithelium, and its stroma springs from the mesoblast in the neighborhood of the Wolffian body.

Diagram of the sexual organs of a female embryo. {Allen Thomson).

Fig. 299. Diagram of the sexual organs of a female embryo. {Allen Thomson).

f. Fimbriated extremity of the left Fallopian tube. W. Remains of the Wolffian tubes, z. Round ligaments, o. Ovary, po. Parovarium, u. Uterus. dG. Remains of the Wolffian duct, or duct of Gaertner. m. Right Fallopian tube cut short, w. Right obliterated Wolffian duct. va. Vagina. 3. Ureter. 4. Bladder. 5. Urachus. h. Inferior opening of vagina. C. Gland of Bartholin, v. Vulva, sc. Vascular bulb. cc. Clitoris, n. Nympha. /. Labium, i. Rectum.

The ducts of Muller are the precursors of the female genital passages. They approach one another and unite along a certain distance at their lower extremities. Of this united part, the upper end forms the uterus, and the lower the vagina, while the ununited parts of the Mullerian ducts form the Fallopian tubes, which become connected with the ovaries, while their cavities remain continuous with the pleuroperitoneal space.

In the female, the Wolffian duct and body atrophy, the parovarium being in the adult the representative of the Wolffian body.

The bladder is merely a dilated portion of that part of the allantois which is in immediate connection with the alimentary canal, and the urachus is the narrowed part of the allantois connecting the bladder to the remainder of the allantois which is without the body walls of the foetus.

While the alimentary canal is in connection with the allantois, the intestinal and genito-urinary passages open into a common cavity at their termination; this is the cloaca, and it is in the further development of the embryo that a septum arises, dividing this into an alimentary or anal portion, and an anterior or urinary portion. The septum, dividing the urogenitary from the alimentary portion of the cloaca, forms, externally, the perinseum.

At the aperture of the cloaca an eminence arises which develops into the penis in the male, the clitoris in the female. Around this eminence is a fold of integuments, which forms the labia in the female, the scrotum in the male.

In the female this integumentary covering enlarges much more than the clitoris and covers it in, the urethral orifice opening just below the clitoris.

In the male, the urethral orifice at first opens at the base of the penis, but eventually a groove is formed on the under surface of this organ, which becomes converted into a canal, and forms the urethra.