:

presently projects as a distinct vascular organ along the dorsal part of the peritoneal cavity on either side of the mesentery. Subsequently another duct becomes developed along the outer side of the Wolffian body, along which it runs backwards to open also into the cloaca in front it communicates with the pleuroperitoneal cavity by one or more funnel-shaped apertures (fig. 143, z). This is the Müllerian duct, so named after Johannes Müller; in some of the lower vertebrates it arises in common with the Wolffian duct. From the lower end of each Wolffian duct a hollow protrusion (fig. 125, C and D, N) grows upwards into a mass of mesoblast continuous with that of the Wolffian body; with the branches of this protrusion glomeruli and convoluted tubes also become connected, and thus the permanent kidney (hind-kidney, metanephros) is produced. Lastly, the cœlomic epithelium covering the inner side of the Wolffian body becomes thickened (fig. 143, a), and within it are found larger cells, from which the generative products in both sexes (ova and spermatozoa) arc eventually derived. This epithelium is accordingly known as the germinal epithelium. The duct of Müller becomes in the female the oviduct or Fallopian tube; in the male it becomes atrophied. The Wolffian duct in the male becomes the epididymis and vas deferens; while the vasa efferentia and tubes of the rete testis are formed as outgrowths from the Wolffian body; in the female these parts have no permanent function.

The head kidney, although permanent and functional in fishes, is only a rudimentary organ in the embryo of higher vertebrates, and soon disappears. The Wolffian body is well developed in all vertebrates; in fishes and amphibia it is an important part of the permanent urinary apparatus, and also serves to carry away the male sexual products (fig. 138). In higher vertebrates (amniota) it no longer continues to perform excretory functions, but still supplies the efferent apparatus of the testis.

The details of the development of these parts may next be considered.

The Wolffian duct and body.-The commencement of the Wolffian duct is seen at a very early period of development (second day in the chick, eighth day in the rabbit) as a thickening of the intermediate cell-mass in the anterior region of the trunk (fifth somite) (fig. 39, Wd). The outgrowth projects towards the epiblast, and

Fig. 139.-TRANSVERSE SECTION OF AN EMBRYO CHICK OF THIRTY-SIX HOURS. 150 (E. A. S.)

n.c, medullary tube; p, protovertebra; ep, epiblast; me, lateral mesoblast split into splanchnopleure and somatopleure; ca, pleuro-peritoneal cavity between them; ca', cavity of protovertebra, continuous on the right side with the lateral mesoblastic cleavage; W.d., Wolffian duct; W.b., mesoblast of Wolffian body; ch, notochord.

developes from before backwards; a solid cord of mesoblast thus becomes formed, which gradually becomes detached from the remainder of the intermediate cell-mass, lying close to the epiblast (fig. 139, ung). Soon after it is thus formed, a lumen appears in it and extends both forwards and backwards. The posterior end, which is still solid, is presently found to be attached to the epiblast, and apparently continues to grow backwards along and at the expense of the epiblast until it reaches the posterior end

of the body, where it becomes detached from the epiblast, and is connected with and opens into the hind-gut (cloaca).

I have here followed what has appeared to me the most probable account of the origin of the duct (Martin, Strahl), but it is right to state that in the opinion of some observers (Hensen, Spee, Flemming) the formation and growth of the duct in connection with the epiblast is primary, especially in mammals, and the duct is originally formed by a longitudinal thickening and involution of the epiblast, which only secondarily becomes connected with the intermediate cell-mass. Compare also Haddon, Origin of segmental duct, Proc. Roy. Dublin Society, Vol. V.

In teleosteans (Rosenburg) and amphibia (Götte) the Wolffian duct has been described as developing in the form of a longitudinal groove-like invagination of the somatopleural mesoblast (Balfour, Comp. Emb., vol. ii., pp. 580, 582), but more recent researches appear to indicate that in these animals also the epiblast may be concerned in its formation.

The Wolffian body developes in the intermediate cell-mass between the Wolffian duct and the body-cavity as a series of transverse tubes which lie at right angles to the course of the Wolffian duct, and open into it at regular intervals. The usual

mulation at regular intervals, corresponding with the somites, of rounded masses of mesoblast on the mesial or ventral side of the Wolffian duct (fig. 142, w.b.), which masses become afterwards hollowed out so as to form small vesicles, at first isolated, but afterwards growing towards and opening into the Wolffian duct (fig. 140). Corresponding with these vesicles there become formed invaginations of the epithelium of the body-cavity (fig. 141, st), which is thickened along the inner side of the Wolffian projection, and grows at regular intervals towards the vesicles. These ingrowths may at first communicate by funnel-shaped openings, which in some lower vertebrates are lined by ciliated epithelium, with the body-cavity, but the openings in higher vertebrates become closed again before communication with the Wolffian duct is established. Finally, the connection. between the Wolffian tubes and the peritoneal epithelium is completely severed, and the condition of simple or curved transverse tubes, blind at their inner ends and opening at their outer ends into the Wolffian duct, is produced (fig. 142, B). After a time the blind extremities are seen to be enlarged and spoon-shaped, and glomeruli

1 According to v. Wijhe the hollow condition is the primary one in elasmobranchs, and the cavity of each vesicle represents an intermediate part of the cœlem of the segment (meso-colom), the dorsal cœlom being represented by the cavity of the proto-vertebra and the ventral colom by the pleuroperitoneal space. I have myself observed this condition of a hollow intermediate cell-mass communicating on the one hand with the cavity of the protovertebra and on the other with the cleft of the lateral mesoblast, in a chick of 36 hours (see fig. 139).

In mammals the Wolffian vesicles are more numerous than the segments.

are observed developing in the bowl of the spoon from mesoblast cells, which presently become entirely enclosed by the end of the tube. Subsequently a second

Fig. 141.-TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES. (Balfour.)

am, amnion; so, somatopleure; sp, splanchnopleure; wd, Wolffian duct; st, segmental tube with peritoneal involution; ca.v, cardinal vein; m.s, muscle-plate; sp.g, spinal ganglion; sp.c, spinal cord; ch, notochord; ao, aorta; hy, hypoblast.

and a third set of tubes become developed in a similar manner, but without peritoneal invaginations, and also open directly into the Wolffian duct. Lastly, other tubes

Fig. 142.-TRANSVERSE SECTIONS OF SHEEP EMBRYOES, SHOWING TWO STAGES IN THE DEVELOPMENT OF THE WOLFFIAN BODY. (Bonnet.)

w.d, Wolffian duct; w.b, Wolffian body; p. v, proto vertebra; ch, notochord; n. c, neural canal; am, amnion; ao, aorta; i, intestine; y.s, yolk-sac.

with glomeruli become formed between, and open into those which are already connected with that duct. All these tubes are short and straight when first developed, but afterwards lengthen and become converted into convoluted uriniferous tubes, which, like those of the permanent kidneys, begin in a dilated extremity enclosing a tuft of capillary blood-vessels (glomerulus), which are supplied by branches of the primitive aorta.

When completely formed, the Wolffian bodies are seen on opening the abdomen. of the embryo as long prominent vascular organs projecting into the peritoneal cavity on either side of the intestine, and showing in section numerous Malpighian corpuscles and uriniferous tubules variously cut (fig. 143).

Soon after having attained its complete condition of development, the Wolffian body begins to undergo atrophic changes. These proceed much further in the

Fig. 143. TRANSVERSE SECTION OF THE

WOLFFIAN BODY OF THE CHICK ON THE

FOURTH DAY. (Waldeyer.)

m, mesentery; L, body wall; a', thickened epithelium from which the involution of the anterior part of the Müllerian duct z, is taking place; a, thickened germinal epithelium in which are seen primitive ova, o; E, modified mesoblast which will form the stroma of the ovary; WK, tubules of Wolffian body variously cut; y, Wolffian duct. Two glomeruli are shown in the Wolffian body.

female sex than in the male, but the tubules of the organ do not entirely disappear in either sex. In the female they form the rudimentary organ which is known as the parovarium (epoophoron of Waldeyer), while the main tube of that structure represents a remnant of the Wolffian duct. But in many animals, e.g., the sow, the Wolffian duct remains as the duct of Gartner, a

strong, slightly undulated tube, which is traceable, at first free in the broad ligament of the uterus, and lower down becoming incorporated with the wall of the uterus and vagina, upon which last it becomes lost. Traces of this tube can sometimes be seen in sections across the body or cervix of the adult human uterus, and even lying in the wall of the vagina.

In the male the Wolffian duct forms the tube of the epididymis, the vas deferens, and the ejaculatory duct; the seminal vesicle being formed as a diverticulum from its lower part. The coni vasculosi and tubuli efferentes are in all probability formed by the persistence of some of the tubules of the Wolffian body. The Malpighian corpuscles of these tubules have long disappeared, but previous to their disappearance solid columns of epithelial cells, afterwards becoming tubules, grow from the walls of those corpuscles towards the germinal epithelium (fig. 153), where, in the male, they become continuous with and enclose cells derived from that epithelium (which subsequently form the epithelium of the seminiferous tubes), and thus produce the walls of the seminiferous tubules and the rete testis. In the female sex there is also a growth of solid cellular columns towards the germinal epithelium, but no connection becomes established between them, and the columns do not become tubular. The organ of Giraldés and the vasa aberrantia of Haller are probably the remains of one or more Wolffian tubules.

Suprarenal capsules.-These organs are intimately connected in their development with the Wolffian bodies. According to the observations of Weldon some of the cellular columns which grow from the Malpighian corpuscles of the upper part of the Wolffian body towards the germinal epithelium give offsets which pass upwards towards the inferior vena cava, and there become developed into the cortical substance of the suprarenal capsules. (Mihalkovics, on the other hand, states that the strands of cells which grow from the upper part of the

Wolffian body to take part in the formation of the suprarenal capsules have been derived by proliferation from the germinal epithelium.) It had long been believed that the two parts of these organs, cortical and medullary, are separate in origin; the former being derived, as was thought, from cells which are of mesoblastic origin, the latter being developed in connection with the sympathetic ganglia. In elasmobranchs and some other lower vertebrates, they

Fig. 144.-INTERNAL ORGANS OF A FEMALE HUMAN FETUS OF 3 INCHES LONG, OR ABOUT 14 WEEKS. MAGNIFIED (from Waldeyer).

o, the ovary full of primordial ova; e, tubes of the upper part of the Wolffian body forming the epoophoron (parovarium of Kobelt); W, the lower part of the Wolffian body forming the paroophoron of His and Waldeyer; w', the Wolffian duct; M, the Müllerian duct; m', its upper fimbriated opening.

Fig. 145.-INTERNAL GENITAL ORGANS OF A MALE HUMAN EMBRYO OF 3 INCHES LONG (from Waldeyer). t, body of the testicle with seminal canals formed; e, epididymis, or upper part of Wolffian body; W, Wolffian body, lower part, becoming paradidymis or organ of Giraldès: w Wolffian duct, becoming vas deferens; g, gubernaculum.

Fig. 146.--Two FIGURES EXHIBITING A COMPARISON BETWEEN PARTS OF THE GENERATIVE ORGANS IN THE TWO SEXES (from Farre, after Kobelt).

A.-ADULT OVARY, PAROVARIUM AND FALLOPIAN TUBE.

a, a, Epoophoron (parovarium) formed from the upper part of the Wolffian body; b, remains of the uppermost tubes, sometimes forming hydatids; c, middle set of tubes; d, some lower atrophied tubes; e, atrophied remains of the Wolffian duct; f, the terminal bulb or hydatid; h, the Fallopian tube, originally the duct of Müller; i, hydatid attached to the extremity; 1, the ovary.

B. THE ADULT TESTIS AND EPIDIDYMIS.

a, a, convoluted tubes in the head of the epididymis developed from the upper part of the Wolffian body b and f, hydatids in the head of the epididymis; c, coni vasculosi; d, vasa aberrantia; h, remains of the duct of Müller with i, the hydatid of Morgagni, at its upper end; I, body of the testis.

: