Book - Text-Book of the Embryology of Man and Mammals 15
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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- 1 The Organs of the Middle Germ-Layer
- 1.1 The Development of the Voluntary Musculature
- 1.2 The Development of the Urinary and Sexual Organs
- 1.3 The Metamorphosis in the Male. Descensus testiculorum
- 1.4 The Metamorphosis in the Female. Descensus ovariorum
- 1.5 The Development of the External Sexual Organs
- 1.6 Summary
- 1.7 Literature
The Organs of the Middle Germ-Layer
THE organs which take their origin from the middle germ-layer stand in the closest genetic relation to the morphological products of the entoblast. For, as was stated in the first part of this work, the middle germ-layer is developed by a process of evagination from the inner germ-layer, and is therefore, like the latter, an epithelial membrane, which serves as the boundary of a cavity. In view of its origin, is it remarkable that the organs arising from it are of a glandular nature, and such as produce excretions by means of genuine epithelial glandular cells. In earlier times; this phenomenon was the cause of a good deal of difficulty, bee: i use since the time of HEMAK there had been an endeavor to bring the middle germ-layer as a non-epithelial structure into contrast with the other germ-layers. Attempts were also made to explain this supposed contradiction by assuming that the glandular organs in question were derived, sometimes in one way, sometimes in another, from the outer germ-layer. With the acceptance of the coclom-theory, however, the theoretical objections to the production of glands by the middle germ-layer have ceased to have any foundation.
Out of the middle germ-layer, or, otherwise expressed, out of the epithelial wall of the embryonic body-sacs, are developed aside from the mesenchyme, concerning the source of which an extended account was given in the ninth chapter three very different products : first the whole voluntary musculature, secondly the urinary and sexual organs, thirdly the epithelial or endothelial linings of the large serous cavities of the body.
The Development of the Voluntary Musculature
The total, transversely striped, voluntary musculature, aside from a part of the muscles of the head, arises from those parts of the middle germ-layer which have been differentiated as primitive segments, and with their appearance have effected the first primitive and most important segmentation of the vertebrate body. As has been previously stated, the segmentation affects the head as well as the trunk, so that trunk-segments and head-segments must be distinguished. Since the latter are in many points distinguished in their origin and metamorphosis from the former, a separate description of the two is fitting. I begin with the history of the metamorphosis of the primitive segments of the trunk, and treat of the same first in Amphioxus and the Cyclostomes, which furnish the simplest and most easily interpreted conditions, and then in the Amphibia, MIR! finally in the higher Vertebrates.
Primitive Segments of the Trunk
In Amphioxus the primitive segments (fig. 103 usli) are sacs, which are provided with a large cavity, and the walls of which are composed of a single layer of epithelial cells. The latter are further developed in two ways, for an accurate knowledge of which we are indebted to HATSCHEK. Only the cells (fig. 189) which abut upon the chorda (c/i) and the neural tube (n) are destined to form muscle-fibres; they increase considerably in size, project far into the cavity of the primitive segment, and assume the form of plates ; these lie parallel to one another and to the longitudinal axis of the body ; and one margin, which I shall designate as the base, is placed perpendicularly upon the surface of the chorda. Very early (in the stage with ten primitive segments) the cell-plates begin at their bases to be differentiated into transversely striped rnuscle-fibrill(B, with which the embryos are already able to execute feeble contractions. By the continual addition of new fibrillse to those which are formed at the surface of the chorda, and by an extension of the differentiation to both the surfaces of the cell-plates which are in contact with each other, there arise the transversely striped muscle-layers (Muskelblatter) which are characteristic of the musculature of Aniphioxus. These are attached to the chorda on the right and left like the leaves of a book. The more the fibrillse increase in number, the more the protoplasm of the formative cells between them diminishes in amount and the more is the nucleus with a remnant of protoplasm forced toward that edge of the cell which faces the cavity of the primitive segment.
The remaining cells of the primitive segment are converted into a low pavement-epithelium, which neither now nor later takes part in the formation of muscles. (Cutis-layer of HATSCHEK.) Having arisen in the vicinity of the chorda, the muscle-layer in older animals spreads out both clorsally and ventrally, and thus furnishes the total voluminous musculature of the trunk, which, like the cellular primitive segments from which it is derived, is separated into successive portions (the myorneres).
In general the Cyclostomes (rig. 190) agree in the development of their muscles with Amphioxus. Here, as there, one must distinguish between an inner muscle-forming epithelial layer (nif), which bounds the chorda (Ch) and the neural tube (A 7 ), and an outer indifferent epithelial layer (ae), which occupies the side toward the epidermis. The latter (ae) consists of low flat cells, the former of very broad and elongated plates (???,/<;), which as in Amphioxus are arranged perpendicularly to the surface of the chorda and neural tube. Since in Petromy/on the primitive segments arc destitute of cavities, the two epithelial layers lie immediately in contact, and are continuous with each other, both dorsally and ventrally, by means of transitional cells (ir/?), in the same way that in the fundament of the lens its epithelium is continuous with the lens-fibres. Muscle-fibrillse (mf) are now differentiated on both the broad surfaces of the cell -plates.
Fig. 189. Cross section through the middle of the body of an Amphioxus embryo with 11 primitive segments, after HATSCHEK.
nfc, ifc, Outer, inner germ-layer ; mk l , 5/tt 2 , parietal, visceral lamella of the middle germ-layer ; us, primitive segment ; n, neural tube ; ch, chorda; (h, body-cavity; dft, intestinal cavity.
Thus arise muscle-layers (Muskelblatter) which are perpendicular to the chorda. These layers are each composed of two sheets of the finest fibrillse, running parallel to one another. The two sheets are separated from each other by a delicate film of cementing substance ; one of them owes its existence to one formative cell, the other to an adjacent cell. In older larvse the primitive segments spread out both above and below ; accompanying this process there is a continual formation of new muscle-layers from the previously mentioned cells (ir/?). The upper and lower margins of the primitive segments therefore constitute a zone of proliferation, by means of which the musculature of the trunk is continually growing further dorsad and ventrad.
Fig. 190. Cross section through the trunk-musculature of a larva of Petromyzon Planeri 14 days old. Magnified 500 diameters.
2V and Ch, the part of the cross section which is adjacent to the neural tube and the chorda ; chs, skeletogenous sheath of the chorda; ep, epidermis ; ae, outer epithelial layer of the primitive segment ; ink, nuclei of muscle-cells ; mf, muscle-fibrillse in cross section ; WZ, zone of growth transition from the outer cell-layer to the muscle-forming layer of the primitive segment.
At a later stage of development, in larvae six weeks old (fig. 191), the muscle-layers are converted into Muskelkfistchen (k), as SCHNEIDER has named these peculiar definite structural elements of the Cyclostomes. The facing fibrillse-sheets of two adjacent layers (Blatter) unite with each other along their margins. Since these sheets have been produced on the two sides of one cell-plate, each formative cell is now surrounded on all sides, as though with a mantle, by the fibrillse which it has generated.
Finally, three alterations of the Muskelkastchen take place. The homogeneous cementing substance, which was indicated during the first stage by only a line line between the two fibrillae-sheets of a muscle-layer, increases and produces the partition by means of which the individual Muskelkastchen are separated from each other, and in which afterwards connective-tissue cells and blood-vessels are also to be found. Secondly, the protoplasmic matrix of the formative cells is almost completely consumed in the continued production of numerous fine fibrilla3, which finally fill the whole interior of the Kastchen. One can now distinguish two different kinds of fibrillse -those that are centrally located, and those that are firmly attached to the partitions. Thirdly, there are to be found scattered between the fibrilla? numerous small nuclei, which probably are descended from the original single nucleus of the formative cell by frequently repeated division.
The development of the muscle-segments takes place in the remaining Vertebrates in a somewhat different manner from that of Amphioxus and the Cyclostomes. For the study of this process
Fig; 191 M sect j" through the trunk-musculature of a the tailed Amphibia furnish the mOSt larva of Petromyzon Planeri , i T m L ic. i A/" 6 weeks old. Magnified r>00 instructive obiects. In Iriton (figs. 106, J diameters.
105 lisll) each of the primitive segments A 1 , Muskelkastchen; ml; nuclei ill ., i i of muscle-cells; iiif, musclecontains a considerable cavity, which is nbrmfe cut crosswise, bounded on all sides by large cylindrical epithelial cells. In somewhat older embryos active cell-multiplication takes place in the part of the epithelium which is adjacent to the chorda and neural tube, and which, therefore, corresponds to the previously described muscle -forming layer of Amphioxus and the Cyclostomes. By this growth the cavity of a primitive segment becomes entirely filled. At the same time the cells lose their original arrangement and form ; they are converted into longitudinally arranged cylinders, which correspond in length to a primitive segment and are located by the sida of and above one another on both sides of, and parallel to, the spinal cord and chorda dorsalis (fig. 192). Each cylinder, which in the beginning exhibits only a single nucleus (mk), becomes surrounded with a mantle of the finest transversely striped fibrillre (mf) ; it is now comparable with a Muskelkastchen of the Cyclostomes (fig. 191). A series of further alterations also takes place in this instance as in the former. In older larvae there are continually being formed more fibrillse (fig. 193), which gradually fill the interior portion of the cylinder. Only in the axis of the latter are there places left free, in which the small nuclei (ink) come to lie ; these, formed by division of the single mother-nucleus, increase considerably in number. Moreover, connective tissue with blocdvessels now penetrates between the muscle-fibres or the primitive bundles (pb), as the finished elements are subsequently called.
Fig. 192. Cross section through the musculature of the trunk of a larva of Triton taeniatus 5 days old. Magnified 500 diameters. mk, Nuclei of muscle-cells ; mf, muscle-fibrillae ciit crosswise ; dk, yolk-granules.
Fig. 193. Cross section through the musculature of the trunk of a larva of Triton taeniatus 10 days old. Magnified 500 diameters.
pb, Primitive bundle of muscle-fibrillfe (Muskelprimitivbundel) ; ;/;/, muscle-fibrillaj cut crosswise ; mk, nuclei of muscle-cells.
If we consider from a general point of view the facts here presented, which have been acquired in the study of the lower Vertebrates, we arrive at two propositions of importance concerning the origin of the musculature :
- In Vertebrates the elements of the musculature of the trunk are developed out of epithelial cells which are derived from a circumscribed territory of the epithelium of the body-cavity, a territory that is constricted off from the latter to form the primitive segments.
- The epithelial products become surrounded and enveloped on all sides by connective tissue, just as do the glands and gland-ducts that bud forth from an epithelium.
A comparison with the condition and development of the musculature of some classes of Invertebrates leads to a still better comprehension of the above propositions. In most of the Coelenterates the muscular elements are components of the epithelium, not only during their development, but also in the adult animal, so that the designation epithelio-in/u/scular cells is suitable for them.
The characteristic feature of these consists in their being simple sometimes cubical, sometimes cylindrical, sometimes thread-like epithelial cells, the outer ends of which, ordinarily reach the surface of the epithelium and are here provided with cilia, whereas their basal ends lie upon the sustentative lamella (Stittzlanielle) of the body and are there differentiated into one or several either smooth or transversely striped muscle -fibillla?. Inasmuch as the fibrillas of numerous cells lie parallel and close to one another, muscle-lamellce arise, by the activity of which the changes in the form of the body are produced. In Caelentcrates both the outer and the inner germ -layers can \ develop muscle-cells.
When one turns to the Vermes it is seen, in those groups in which a bodycavity (an enterocosl) is formed by an infolding of the inner germ-layer, that the parietal wall of the body-cavity, or the parietal lamella of the middle germ-layer, has assumed the production of the entire musculature of the trunk. Here also, for example in the Chastognatha, etc., the epithelial cells differentiate at their basal ends, which are directed toward the surface of the body, a lamella of ruuscle-fibrillffi, whereas their other ends bound the body-cavity. Thus from the lower to the higher animals the capability of producing muscles is, with the progressive differentiation of the body, more and more restricted to a limited special territory of the total epithelial investment of the body.
This process has proceeded furthest in the Vertebrates, for in them the musculature of the trunk is no longer furnished by the whole parietal lamella of the middle germ-layer, but by only a small detached part of it, the primitive segments. Consequently in Vertebrates the musculature spreads out from a small region where it originates, distributes itself first in the trunk, and then from the latter grows out into the extremities.
In the Vertebrates we recognised two different forms of voluntary musculature, the muscle-layer (and the Muskelkiistchen derivable from it) and the primitive bundle (Muskelpriniitivbundel). Parallels to this are found in the Invertebrates, both in Ccelenterates and in Worms. In Coslenterates both forms are derived from the primitive smoothly outspread muscle-lamella by the formation of folds, and are to be explained in the same way as the formation of those folds which in epithelial lamellre play such an important part in the origin of the most various organs. When certain tracts of a muscle-lamella are called upon to execute additional labor, this can be effected only by an increase in the number of the fibrillse lying parallel to one another. But a greater number of fibrillae can be brought into a circumscribed territory only in one or the other of two ways : either by their coming to lie in several layers one above another, or if the more simple arrangement of lying side by side is to be retained by the folding of the muscle-lamella. The folding exhibits two modifications. Sometimes there are produced parallel daughter-lamellae placed side by side and perpendicular to the mother-lamellae ; sometimes the folded lamellae become wholly detached from the parent-layer and converted into muscle -cylinders, which imbed themselves in the underlying sustentative lamella.
With the conception here presented of the origin of the transversely striped muscle-fibres of Vertebrates, it must be assumed as very probable that subsequently an increase in their number will take place as a result of constriction and detachment into two parts, as was first maintained by WEISMANX,
In Anipliioxns, I ho Cyclostomes, and the Amphibia the most iiiipori.-uit function of 1 he primitive segments is tin* production of the fundament of the transversely striped and voluntary musculature. On the other hand it is not very evident that the primitive segments also share, in the manner previously (p. 172) described, in the development of the mesenchyme ; this is correlated with the fact that in general the connective and sustentative substances play a slight role in the construction of the bodies of the lower Vertebrates, and especially during larval life are developed to only a very insignificant amount.
This is altered in the Selachians and the three higher classes of Vertebrates. Not only does the mesenchyme in the adult bodies of these attain a more voluminous development and a degree of differentiation that is in all directions more advanced, but it is also established earlier and likewise in greater abundance. Therefore the primitive segments here exhibit in their metamorphosis somewhat modified phenomena. At the same time with the differentiation of the muscular tissue, and in part even before that event, the development of mesenchyme is observable. The primitive segment (fig. 194) in this case is differentiated from the start into two equally distinct fundaments, of which the one is designated as sclerotome or skeletogenous layer (sk), the other as muscle-plate (mp). While referring the reader to the ninth chapter, 1 add to the presentation given there a few further statements.
Fig. 194. Cross section through the region of the pronephros of a Selachian embryo, in which the muscle-segments [myotomes] (mp) are in process of being constricted off. Diagram after WI.TIIE.
nr, Neural, tube ; ch, chorda ; o, aorta ; xcJi, subnotochordal rod ; mp, muscle-plate of the primitive segment ; ic, zone of growth, where the muscle-plate bends around into the cutisplate (cp) ; vb, tract connecting the primitive segment with the body-cavity, out of which are developed, among other tilings, the mesonephric tubules (fig. 205 uk) ; *&, skeletogenous tissue, which arises by a proliferation from the median wall of the connecting tract vb ; vn, pronephros; mk\ ml-", parietal and visceral middle layer, from whose walls mesenchyme is developed ; lit, body-cavity ; Ik, entoblast.
Ill the Selachians the skeletogenous layer, the origin of which has already been described, grows upward at the side of the chorda (fig. 195 F?-). Outside of this layer one finds the part of the primitive segment which serves for the formation of muscle. This consists of an inner layer (mp') and an outer layer (nip), which are separated from each other by the remnant of the cavity of the primitive segment (fig. 194 A). The inner layer (fig. 195 mp') is in contact with the skeletogenous tissue (Vr), and is composed of numerous, superposed, spindle-shaped cells, which are arranged longitudinally and give rise to transversely striped muscle- fibrillre ; they correspond to the inner wall of the primitive segment in the larvae of Amphioxus (fig. 189) and Cyclostomes, which is in direct contact with the chorda. The outer layer lies in contact with the epidermis, and remains for a long time composed of cubical epithelial cells. Dorsally and ventrally it bends around into the muscle - forming layer, and here contributes to the enlargement of the latter, as in Amphioxus and the Cyclostomes, by its cells becoming longer and being metamorphosed into muscle-fibres (fig. 185). The muscle - plate then spreads out farther into the wall of the trunk both above and below (figs. 185 and 205). At the same time its cavity (rnyoccel) gradually disappears. The muscle-forming layer (fig. 185 mj)) continues to increase in thickness, since the number of muscle-fibres becomes greater ; the outer layer also loses, rather late it is true, its epithelial character, and is concerned on the one hand in the development of the coriuin (fig. 205 cp), while on the other it furnishes an additional outer, thin musclelamella. This observation, made by BALFOUR, has often been called in question, but has recently been confirmed by VAX WIJHE.
In Reptiles, Birds, and Mammals the proliferation of the primitive segments which furnishes the skeletogenous tissue is still more extensive than in Selachians. Thereby the muscle-plate, or the dorsal plate, as it is also called, is crowded farther away from the chorda. The differentiation of muscle-fibres follows at a much later stage of development, in comparison with Aniphioxus and the Cyclostomes. The inner layer of the muscle-plate is converted into longitudinal muscle-fibres, the outer contributes to the formation of the corium (fig. 202).
Fig. 195. Horizontal longitudinal section through the trunk of an embryo of Scyllium, after BALFOUK.
The section is m:ide at the height of the chorda, and shows the separation from the muscle-plates of the cells which form the bodies of the vertebra.
ch t Chorda ; .^, epidermis ; FV, fundament of the bodies of the vertebra; ; /<_/>, outer cell-layer of the primitive segment ; */<//, portion of the primitive segment which has already been differentiated into longitudinal muscles (muscle-plate).
Let us now consider somewhat more in detail the original condition of the musculature. It shows at the beginning complete uniformity in all classes of Vertebrates. Everywhere there appears as its foundation a very simple system of longitudinal contractile fibres, which first appear near the chorda and neural tube and spread themselves out thence dorsally toward the back and ventrally in the wall of the belly. The muscle-mass is divided in a very uniform manner into separate segments or myomeres by means of connectivetissue partitions (ligamenta intermuscularia), which run transversely or obliquely to the vertebral column. In the lower Vertebrates this condition persists, in the higher ones it gives place to a more complicated arrangement.
We cannot recount more precisely the details of the manner in which the groups of muscles of the higher Vertebrates, so various in form and position, are derived from the original system, especially since this field of embryology has been as yet little cultivated ; let attention be here called to only two points, which come in question in the differentiation of the groups of muscles.
First, a very important factor is furnished in the development of the skeleton, which with its processes affords points of attachment for muscle-fibres. Some of these find in this way opportunity to detach themselves from the remaining mass.
Secondly, the development of the limbs, which arise as protuberances at the side of the trunk (figs. 157 and 158), operates toward a greater differentiation of the musculature. The limbs likewise, acquire their musculature, which in the higher Vertebrates has a very complicated arrangement, from the primitive segments, as has been learned through the investigations of KLEINENBERG and BALFOUR, as well as recently through the very convincing accounts of DOHRN.
In the Selachians, in which the processes are most clearly recognisable, cell-buds sprout forth out of the, still hollow primitive segments and grow into the paired and median Jins, in which they become metamorphosed into muscle-fibres. The fact that always from a large number of primitive segments buds are given oft' to a fin is worthy oj attention, because it demonstrates that the extremity is a structure that belongs to several somites.
The Segments of the Head
Important works on the development of the head have appeared in late years by GOETTE, BALFOUR, MARSHALL, WIJHE, FRORIEP, RABL, and others. They have led to the important conclusion that the head is made up of a large number of segments, in the same manner as the trunk. These conditions are most evident in the Selachians.
When in these animals the middle germ-layers have grown into the fundament of the head, they here, as in the trunk, early separate from each other, and thus embrace on either side a narrow, fissurelike space, the head-cavity. This is continuous posteriorly with the general body-cavity. It follows from this that the two primitive body-sacs (coelom-sacs) possess a greater extent in the embryo than they do subsequently, since they reach into the most anterior part of the embryonic fundament, the head.
In the further course of development the walls of the head-cavity are differentiated, in the same manner as the walls of the body-cavity, into a ventral portion and a dorsal portion, the latter of an embryo of PrisprotluciiiL' primitive segments. Then there arises, ti - after BAL ip, Epidermis ; re, inner however, an important difference between head visceral pouch ; and trunk ; in the trunk only the dorsal portion is segmented, but in the head both ventral and visceral arch; a vessel of the visceral dorsal portions are segmented, each in a manner arch (aortic an . h) peculiar to itself.
Fig. 196. Cross section
?""** the n f i *
the last visceral arch
The ventral part of the head-cavity is divided, in consequence of the development of the visceral clefts, into separate segments (branchiorneres AHLBORN), the first of which is situated in front of the first cleft, each of the remaining ones between two clefts. Each. segment (fig. 196) consists of a wall composed of cylindrical cells and encloses a narrow cavity. With its enveloping connective tissue it constitutes the visceral arches, which are separated from one another by the visceral clefts ; for this reason the fissures arising from the head-cavity have been designated by WIJHE as visceral-arch cavities. The latter communicate for a time under the gill-pouches with the pericardial chamber surrounding the heart. But then they begin to be closed ; their walls come into contact ; and out of the cylindrical epithelial cells are developed the transversely striped muscle fibres which produce the muscles of the jaws and gills.
Consequently there results for the head-region of Vertebrates this important proposition : the head-musculature is developed not only out of the primitive segments, but also out of a part of the epithelium of the head-cavity which corresponds to the lateral plates of the trunk ; whereas the latter do not contribute to the formation of muscles.
So far as regards the dorsal part of the middle germ-layer in the head-region, it is divided, as in the trunk, into primitive segments, which in the Selachians are nine in number and embrace each a cavity, with the exception of the first, which is solid. They arise first in the posterior region of the head, and increase from there forward. The segmentation of the whole body is therefore accomplished in the Selachians and the same is likewise true for all the remaining Vertebrates in such a manner that it begins in the neck-region, and proceeds thence on the one hand backward to the tail, on the other forward.
The walls of the primitive segments of the head in part furnish muscles, in part degenerate. Out of the first three pairs arise the eye-muscles, as MARSHALL and WIJIIE have demonstrated in detail. The first segment envelops the primitive eye -vesicle like a cup, and is differentiated into musculus rectus superior, rectus inferior, and obliquus inferior. The second pair gives origin to the obliquus superior, and the third pair to the rectus externus. The segments from the fourth to the sixth inclusive disappear, while out of the last three are developed muscles which extend from the skull to the pectoral girdle.
In the remaining Vertebrates the metamorphosis of the middle germ-layer in the head has not been investigated in so exhaustive a manner as in the case of the Selachians. There do not appear to be any head-cavities developed, because the middle germ -layers remain at all times pressed together. However, we know that primitive segments are demonstrable even here. GOETTE describes four pairs of them in Bombinator ; FRORIEP finds in Mammals in the occipital region alone on either side four muscle-segments, of which the two most anterior are believed subsequently to degenerate. In individual cases there still remains much to be elucidated by more exhaustive investigations.
RABL has recently expressed dissent in some points from the exposition of the head-segments as given by WIJIIE. He divides the head-segments into two groups four anterior or proximal, and five posterior or distal. Only the latter are according to RABL to be compared with the trunk-segments ; whereas the first, owing to their method of origin, must take a separate position.
The Development of the Urinary and Sexual Organs
The development of the urinary and sexual organs cannot be discussed separately in two chapters, because these systems of organs are most intimately connected with each other, both anatomically and genetically.
First, both take their origin at one and the same place on the epithelial investment of the body- cavity ; secondly, parts of the urinary system subsequently enter into the service of the sexual apparatus, for they furnish the passages or canals which are entrusted with the evacuation of the eggs and semen. In anatomy also one therefore properly embraces the two genetically united systems of organs under the common name of urogenital system or apparatus.
Again in this subject we turn to one of the most interesting portions of embryology. The urogenital system claims an interest particularly from a morphological point of view, because a great number of important metamorphoses are effected in it during embryonic life, In the higher Vertebrates the pronephros and the mesonephros are formed first ; they are organs of an evanescent nature, which in some cases disappear and are replaced by the permanent kidney, in other cases their ducts alone are preserved. But these transitory structures correspond to organs which are permanently functional in the lower Vertebrates.
In late years, the attention of investigators having been directed to a series of entirely new and unexpected phenomena, by the excellent researches of WALDEYER and SEMPER, the topic " urogenital organs " has been carefully worked out by very many different observers through the investigation of each separate class of Vertebrates. There has arisen a voluminous literature, and many important facts have been brought to light. Nevertheless it is not to be denied that conceptions concerning many fundamental questions are still very divergent.
As in several previous chapters, I shall also here give to the discussion a broader foundation by treating somewhat more exhaustively of the lower Vertebrates in certain questions.
The Pronepliros and the Mesonephric Duct
The first thing that becomes noticeable in the origin of the urogenital apparatus is the fundament of the pronephros [head-kidney]. This is a structure which has now been demonstrated in the embryos of all Vertebrates, but which plays in some a greater part, in others a lesser one. In some Vertebrates (Myxine, Bdellostorna, Bony Fishes) it is retained permanently ;- in others, as the Amphibia, it grows during larval life to an important organ, which disappears after the animal's metamorphosis ; finally, in the Selachians and Amniota its fundament is from the beginning very rudimentary. In the latter case it was held to be the front end of the inesonephric duct, until through comparative embryology the right view had been attained.
I select as types of the development of the pronephros the Selachians, Amphibia, and Birds.
In Selachians of about twenty - seven somites the pronephros begins with the third or fourth trunk - segment and is developed from there backwards. At the, place where the segmented portion of the middle germ - layer is continuous lateral portion, there grow out of its parietal lamella a number of cell -cords (fig. 197 vn) segmentally arranged one behind another, in Torpedo six, in Pristiurus four, which bend backwards and become united into a longitudinal cord. Soon afterwards the fundaments acquire small cavities with the unsegmented through disassociation of the cells. In this manner there has now arisen between epidermis and parietal middle layer a longitudinal canal, which stretches over several segments of the trunk and communicates with the body-cavity by means of several successive openings, the pronephric funnels (fig. 194 vn). At one place the pronephric duct comes close up to the epidermis and fuses with it (fig. 198 vg). Although an actual opening is never formed here, still, supported by this fact, one may express the conjecture that originally the pronephros in Vertebrates opened out at a point far forward on the body (VAN WIJHE, RUCKERT).
Figs. 197 and 198. Two cross sections through an embryo of Pristiurus, after BABL. Cross section fig. 198 lies a little farther back than section fig. 197.
ck, Chorda; spy, spinal ganglion; mp, muscle-plate of primitive segment ; W, skeletogenous tissue which has grown forth from the median wall of the primitive segment ; sck, subnotochordal rod ; ao, aorta ; ik, inner germ-layer ; pmb t vmb, parietal, visceral middle layer ; vn, pronephros ; <<!, pronephric duct ; x, fissure in the primitive segment, which is still in communication with the body-cavity.
A short time after its formation the fundament undergoes in its anterior half a complete degeneration ; the posterior half, on the contrary, is further developed and enlarges, but remains in connection with the body-cavity by means of a single funnel only (fig. 194 vn), either because, as VAN WIJHE asserts, the several funnels are fused into a single one, or because, in accordance with the account of RUCKERT, all the funnels except a single one become closed and degenerate.
In the Amphibia, with which the Bony Fishes exactly agree in this point, the pronephros is established in the most anterior part of the trunk as an organ that is from the beginning hollow (fig. 199). Below the primitive segments, which have already been differentiated into muscle-fibres (m), there appears a groove-like evagiiiation (u) of the parietal layer of the peritoneum, which stretches from in front backward over several somites. By detaching itself from its parent-tissue at several places, and remaining in connection with it at others, it is converted into a longitudinal canal, which in Rana and Bombinator communicates with the body-cavity by means of three pronephric funnels, in Triton and Salamander by means of two. The whole fundament soon after, during the larval life, acquires ample proportions, owing to the fact that the nephridial funnels grow out into long and very tortuous tubes (pronephric canals). (FURBRINGER, GOETTE.) In Birds, with which Reptiles and Mammals agree, the pronephros appears, much as in Selachians, in a more or less rudimentary form (SEDGWICK, GASSER, RENSON, SIEMERLING, WELDON, MIHALKOVICS). It is first observable in embryo Chicks having eight primitive segments and in the region of the seventh somite ; in older embryos it is developed from this place backward into the region of the twelfth somite. At the place where the primitive segments (fig. 200 P.v) are constricted oft' from the lateral plate ($.0), but still remain for some time in continuity with it by means of a connecting region (the middle plate), there grows out from the parietal lamella of the middle germ - layer (somatopleure) a ridge of cells (W.cV), which is directed toward the overlying epidermis. Later, like the corresponding furrow in the Amphibia, it becomes detached in places from its parent - tissue, and when, meanwhile, the primitive segments have likewise wholly detached themselves from the lateral plates, it is converted into a longitudinal cord, which is united with the epithelium of the body-cavity by means of short transverse branches. Similar conditions exist in Reptiles and Mammals.
+++++++++++++++++++++++++++++++++++++++++ Fig. 199. Cross section through a very young Tadpole of Bombinator in the region of the anterior end of the yolk-sac, after GOETTE.
a, Fold of the outer gerui-layer that is continued into the dorsal fin ; ('*, spinal cord ; HI, lateral muscle ; as*, outer cell-layer of the muscleplate ; s, niesenchyniatic cells ; b, transition of the parietal into the visceral middle layer; u, pronephros; /, intestinal cavity; c, entoblast, which is coiitimious with the mass of yolk-cells (</.) ; f, ventral ccecal pouch of the intestine, which becomes the liver.
Finally, the pronephros subsequently acquires a peculiar condition from the fact that there are developed out of the wall of the bodycavity, in the vicinity of the openings of its tubules, one or several vascular glomeruli. In the Chick for example (fig. 201), in the region from the eleventh to the fifteenth somites, there is a proliferation of connective tissue on either side of the mesentery (me), by means of which the right and left pronephridia are separated from each other, -which grows into the body-cavity as a spheroidal body (gl).
A blood-vessel from the aorta penetrates into each proliferation and is here resolved into a tuft of capillaries, which are then united again into an efferent .vessel. Only in those Vertebrates in which the pronephros is functional, as in the larvae of the Amphibia, in the Cyclostomes and the Teleosts, does the glonierulus attain to a considerable development, whereas in the Selachians and Amniota it remains rudimentary. In the first case fluid or urine is probably secreted by this apparatus, and then taken up by the openings of the pronephric tubules and conducted outside the body by means of the pronephric duct, which is to be discussed directly. There is one point in this connection that is noteworthy and characteristic of the structure of the pronephros : the glomerulus is developed, not in the wall of the pronephric tubule itself, as is the case in the tubules of the mesoiiephros, but in the wall of the body-cavity, so that the urine can be evacuated only through the agency of the latter.
But in what manner does the pronephros communicate with the outside ? This communication takes place by means of a longitudinal canal, which is developed in immediate continuation with the pronephros, and, beginning in front, gradually grows backwards until it reaches the proctoda3um and opens into the cloaca. It is found in all Vertebrates in the region where the primitive segments abut upon the lateral plates. At the time of its origin it is always close under the epidermis, later it is farther and farther removed from the latter by the ingrowth of embryonic connective tissue, and comes to lie very deep (fig. 202 wd and fig. 205 ?(//). This canal has acquired a number of different names, and is cited in the literature as pronephric, mesonephric, Wolffian, or segmented duct. The different designations are explainable from the fact that the canal alters its function in the course of the development of the iiephridial system, serving at first as an outlet for the pronephros only, afterwards for the mesonephros.
+++++++++++++++++++++++++++++++++++++++++ Fig. 201. Cross section through the external glomerulus of a pronephric tubule of an embryo Chick of about 100 hours, after 15 U.KOUK.
gl, Glomerulus ; ge, peritoneal epithelium ; Wd, naesonephric (Wolffian) duct ; ao, aorta ; me, mesentery. The pronephric tubule and its connection with the glomerulus are not shown in this figure. +++++++++++++++++++++++++++++++++++++++++
Views concerning the origin of the canal were for a time conflicting. According to one supposition, which a few years ago almost all investigators entertained, the longitudinal canal of the pronephros, when it had been constricted off from the parietal wall of the bodycavity, protruded with its posterior end as a free knob into the space between outer and middle germ-layers, and gradually grew out independently, by multiplication of its own cells, as far as the hind gut (prdctodseum). It was said, therefore, to be constricted off from neither the outer nor the middle germ-layers, nor yet to derive from them cell -material for its increase.
This interpretation has recently become untenable. As is reported in an entirely trustworthy manner concerning several different classes of Vertebrates, for Selachians (WiJHE, RABL, BEARD), for Amphibia (PERENYI), for Reptiles (MITSUKURI), and for Mammals (HENSEN, FLEMMING, GRAF SPEE), the posterior end of the pronephric duct in process of growth is in these cases by no means an entirely isolated structure, but is in close union with the outer germ-layer. Attention has already been called to this fact apropos of the development of the pronephros. In a Selachian embryo the condition which is represented in fig. 197 is soon followed by a condition (fig. 198) in which, in a series of cross sections, the pronephric duct now appears as a ridge-like thickening of the outer germ-layer. By a study of various older embryos it can be further established, that the ridge-like thickening of the outer germ-layer is prolonged backwards by means of cell-proliferation in that layer, while in front it is being constricted off from the parent-tissue. The pronephric duct therefore grows at the expense of the outer germ-layer, and moves as it were along the latter, with its terminal opening behind, as far as to the hind gut.
When HENSEN, FLEMMING, and GRAF SPEE made their observations on Mammals, they were thereby led to adopt the view that the mesonephric duct, as well as the whole urinary system, was derivable from the outer germ-layer. The union with the middle germ-layer they regarded as one that had arisen secondarily. But their conception cannot be brought into unison with the conditions of the pronephros which have been found in the remaining and especially in the lower Vertebrates (Selachians, Teleosts, Amphibia, Birds) ; on the other hand allowance is made for all observations, if we summarise them as follows : that the pronephros is developed from the " middle plate," and that then its posterior end comes into union with the outer germ-layer and in conjunction with the latter grows farther backward as the pronephric duct.
If this explanation, which has also been expressed by WIJHE and RticKERT, is correct, then one can designate the pronephric duct at its first appearance as a short canal-like perforation of the wall of the body, which begins in the body-cavity with one or several inner ostia and opens out upon the skin by a single external orifice. Originally the outer and inner openings lay near together, later they moved so far apart that the outer opening of the canal united with the hind gut. It may be said, in favor of the view here presented, that in the Cyclostomes the more primitive condition, that is to say, the union with the skin, has been preserved. For in them the mesonephric duct opens to the outside at the abdominal pore.
That openings should arise between the cavities of the body and its outer surface is in no way remarkable. I call to mind the intestinal tube, at various place s in the territory of which there are formed openings, as mouth, anus, and branchial clefts. Still more frequent are passages through th;> body-wall of Invertebrates. As such, arise the openings at the tips of the hollow tentacles of the Actinia, on the ring-canal of the Mt'dusre, and the canals (segmental organs) which in Worms lead out from the body-cavity and serve for the elimination of the sexual products and the excretions.
The Mesonephros. (Wolffian Body.)
Following upon the origin of the pronephric system there is developed in all Vertebrates, after the lapse of a longer or shorter interval of time, a still more voluminous gland, serving for the secretion of urine, the primitive kidney (mesonephros) or Wolffian body. It is developed earlier in those cases in which the fundament of the pronephros is from the beginning only rudimentary, as in the Selachians and Amniota ; it appears relatively late, on the contrary, in those Vertebrates in which the pronephros attains to a temporary functional activity, as in the Amphibia and Teleosts.
The mesonephros is established on the portion of the pronephric duct immediately behind the pronophric tubules. The duct consequently serves from this time fonvard as an outlet for the newly formed glandular organ also, and can therefore be designated as mesonephric or Wolffian duct.
When it is stated that a gland is developed on the mesonephric duct, one at first thinks that lateral buds grow out from its wall and give forth branches, as occurs in the fundaments of glands formed from the outer or the inner germ-layers. Nothing of the kind takes place here. All observers with the exception of a few earlier investigators agree rather that the glandular tubules of the mesonephros arise independently of the mesonephric duct. The source of its material is either directly or indirectly the epithelium of the body-cavity, as it has been possible to prove in many cases in Cyclostomes, Selachians, Amphibia, and Amniota.
There are formed, following one another in immediate succe.-sion, short transverse tubules (fig. 202 st), which are at one end continuous with the epithelium of the body-cavity, and at the other end, which remains for a long time closed, are joined to the rnesonephric duct (wd),
Fig. 203. Embryo of a Dog of 25 days, straightened out and seen from in front, after BISCHOFF.
Magnified 5 diameters. J. Intestinal tube ; ds, yolk-sac ; al, allantois ; un, mesonephros ; /, the two lobes of the liver, with the lumen of the vena omphalomesenterica between them ; re, he, anterior and posterior extremities ; h, heart ; m, mouth ; au, eye ; g, olfactory pit. +++++++++++++++++++++++++++++++++++++++++
which runs close to them, but somewhat more laterad. The mesonephros elongates from before backward and attains a great length on both sides of the mesentery, for it reaches back from the region of the liver nearly to the posterior end of the body-cavity ; it acquires a very delicate, regular condition, as the figure of an embryo Dog twenty-five days old shows (fig. 203 un), and can be designated as a comb-shaped gland, composed of a lateral collecting tube, running lengthwise of the body at a little distance from the mesentery, and, attached to the median side of it, short transverse branches, which we shall d< signate a,s mesonephric tubules.
Whereas there can no longer exist any doubt about the origin of the mesonephric tubules from the middle germ-layer, the statements concerning the method of their formation are still at variance with one another. In accordance with the fundamental investigations of SEMPER, it was generally believed that the mesonephric tubules either were evaginated in metameric sequence along the dorsal wall of the body-cavity out of its epithelial lining, or grew forth as originally solid buds, as glandular sacs do from the outer or inner germ-layer.
This view, according to the more recent investigations of SEDGWICK, WIJHE, and RtfCKERT for the Selachians and the three higher classes of Vertebrates, is no longer adequate. In these cases the development of the mesonephric tubules is intimately connected with that of the primitive segments. When the latter begin to be more sharply separated from the lateral plates, there arises at the place of constriction a narrow stalk, which maintains for a time a connection between the two parts (fig. 204 vb). In the Selachians it possesses a small cavity, which unites the cavity of the primitive segment with the body-cavity. In the Amniota it is solid (fig. 200). Inasmuch as the successive cords (stalks) are here closely pressed together, they appear like a continuous cell-mass interpolated between primitive segment and lateral plate, and have been previously mentioned under the name of the middle plate. On account of its relation to the mesonephric tubules, the middle plate is also designated as mesonephric blastema. The mesonephric duct, split off from the outer germlayer, is to be seen taking its way on the lateral side of and close to the connecting stalks of the primitive segments. Each of the connecting stalks, which RUCKERT names at once nephrotome, in contradistinction to the remaining parts of the primitive segment, which produce the muscle-plate (myotome) and the cell-material for the skeletogenous tissue (sclerotome), is afterwards metamorphosed into a mesonephric tubule. Whereas one of its ends remains connected with the body-cavity, the other becomes separated from the primitive segment (fig. 205 uk l ), then applies itself closely to the mesonephric duct, fuses with the wall of the latter, and opens into it. In the diagram (fig. 205) the detachment of the connecting stalk from the primitive segment is shown on the right, the fusion of the detached end with the mesonephric duct on the left. According to this whole process of development the mesonephros is from the very beginning a segmentally formed organ, as can be best followed in the Selachians ; for each mesoiiephric canal is developed in a single segment.
In Reptiles, Birds, and Mammals the connecting stalks are solid cords of cells (mesonephric cords). It is only when they have detached themselves from the primitive segment, arid their blind ends have united with the mesonephric duct, that they acquire a small cavity (fig. 202 si). Now they also become more readily distinguishable as separate canals, since they become farther removed from one another and are marked off from the surrounding tissue by sharper contours.
Figs. 204 and 205. Diagrams of cross sections through a younger and an older embryo Selachian to show the development of the principal products of the middle germ-layer. After WIJHE, with some alterations.
Fig. 204. Cross section through the region of the pronephros of an embryo in which the musclesegments 0<//>) are in process of being constricted off.
Fig. 205. Cross section through a somewhat older embryo, in which the muscle-segments have just been constricted off.
nr, Neural tube ; ch, chorda ; oo, aorta ; sch, snbnotochordal rod ; mp, muscle-plate of the primitive segment ; w, zone of growth where the muscle-plate bends around into the cutisplate (cp) ; rb, the connecting piece which unites the primitive segment to the walls of the body-cavity, and from which are developed, among other things, the mesonephric tubules (fig. 205 ulc) ; sk, skeletogeuous tissue, which arises by a proliferation of the median wall of the connecting piece vb ; vn, pronephros ; iuk\ //<F, parietal and visceral middle layer, out of which mesenchyma is developed ; Ih, body-cavity ; ik, entoblast ; h, cavity of the primitive segment ; vie, mesonephric tubules, which have arisen from the connecting piece <!> of the diagram fig. 204 ; <'&', the place where the mesonephric tubule has been detached from the primitive segment ; <</, mesonephric duct, with which, on the left side of the figure, the mesonephric tubule has runted ; t /, union, of the mesonephric tubule with the body-cavity (nephridial funnel) ; mes 1 , mes", mesenchyma that has arisen from the parietal and visceral middle layers.
Although it is often stated that in the Araniota the mesonephric tubules " are. differentiated out of" the middle plate or the mesonephric blastema, it is nevertheless to be observed that this is not a case of new formation out of undifferentiated cell-material. The so-called middle plate at the time of its origin, in the manner previously described, is at once separated into segmentally arranged cords, which are afterwards metamorphosed into the mesonephric tubules. The differentiation out of a blastema is therefore here, as in most cases, to be conceived of as an increase in the distinctness of already established structures, which constitute a cell-mass that appears undifferentiated, but only on account of our limited means of discrimination.
In the Amphibia, Teleosts, and Ganoids' the origin of the mesonephros deserves to be subjected to renewed investigation from the recently acquired points of view.
Soon after their union with the mesonephric duct the individual mesonephric tubules begin to grow somewhat in length, to take on S-shaped curves, and to be differentiated into three regions. The middle region undergoes a vesicular enlargement and is converted into a BOWMAN'S capsule. Individual transverse branches from the primitive aortre, which pass along close to the mesonephros, make their way to the capsules, and are there resolved into a tuft of capillaries. The knot of blood-vessels, or glomerulus, now grows into the epithelial vesicle, the median wall of which is pushed before it and invaginated into the interior. During this process the epithelial cells of the invaginated part of the wall become greatly flattened, whereas upon the opposite uninvaginated side they remain tall and cuboidal. Such a structure, consisting of a vascular glomerulus and the enveloping BOWMAN'S capsule, is called a Malpighian corpuscle, an organ that is exceedingly characteristic of the primitive kidney (mesonephros) and the permanent kidney (metanephros) of Vertebrates.
In addition to the enlarged middle part, there is to be distinguished on each mesonephric tubule a narrow connecting portion, which continues to increase in length, running to the mesonephric duct, and, secondly, a short portion connecting with the body-cavity. The latter is metamorphosed in different ways in the separate classes of Vertebrates. In some, as in many of the Selachians, it retains its original connection with the body-cavity even in the adult animals ; it begins at the peritoneum with an opening, surrounded with ciliate cells, which was discovered by SEMPER and has been designated nephridial funnel or nephrostome, and which in many respects recalls the similar structures of the excretory organs of segmented Worms. In the most of the Vertebrates, however, special nephridial funnels are no longer developed, inasmuch as the mesonephric tubules soon after their origin completely detach themselves from the epithelium of the body-cavity as well as from the primitive segments, and thereby lose all relation to the body cavity.
A mesonephros in the simple form in which it is at first produced develop mentally is retained permanently only in Bdellostoma, a representative of the Cyclostomes. It here consists, as JOHANNES MULLER has shown, of an elongated canal (fig. 206 A and B a) and short transverse tubules (b), which open into it at short intervals. The latter are no longer connected with the body-cavity by means of a nephridial funnel, but they enclose a vascular glomerulus at their blind end (fig. 206 B c), which is somewhat set off by a constriction.
In all remaining Vertebrates the mesonephros is metamorphosed into a more voluminous and more complicated organ. For the originally short tubules, which run transversely into the mesonephric duct, begin to grow in length, and at the same time to be thrown into numerous folds (fig. 207 s.t). Moreover there are formed mesonephric tubules of a second and third order. These again are also formed independently of the mesonephric duct dorsal to the first-formed transverse tubules ; their blind ends approach the primary urinary tubule and join its terminal part, which is thereby converted into a collecting tube. At the same time a Malpighian body is formed on each of them also.
Still more exhaustive investigations concerning the formation of the secondary and tertiary mesonephric tubules, especially for the higher Vertebrates, appear to me to be desirable. In t e Selachians, according to the statements of BALPOUR, which are also confirmed by others, the epithelium of the already existing Malpighian glomcruli is the starting-point of a proliferation. Cell-buds -TOW out from the latter and toward the urinary tubules lying in front of them, with which their blind ends fuse. After this union has been effected they detach their other ends from the parent-tissue.
Fig. 206. Parts of the mesonephros of Myxine, after J. MULLER.
a, Mesonephric duct ; 6, mesonephric tubules ; c, glomerulus ; d, afferent artery ; e, efferent artery.
B n part of A more highly magnified.
Through the development of compound urinary tubules, each of the branches of which is provided with a Malpighian corpuscle, the primitive kidney (mesonephros) acquires a complicated structure. But this is not uniform in all its parts; ordinarily the condition realised in the most of the Vertebrates is this : the anterior part, which afterwards enters into relation with the sexual glands, retains simple tubules, and only the posterior part passes into a more complicated form by the production of secondary and tertiary fundaments.
The more the mesonephros, with its tortuous tubules and its further differentiation, increases in volume, the more it becomes delimited from its surroundings and emerges from the wall of the body into the body-cavity as a distinctly differentiated organ, where it forms a protruding band on either side of the mesentery (fig. 210 ).
Fig. 207. Diagram of the original condition of the kidney in an embryo Selachian, aftar BALKOUR.
pd, Mesonephric duct, which opens into the body-cavity at o, and into the cloaca at the other end ; x t line along which the Miilleilan duct (lying below in the diagram) is divided off from the ruesonephric (Wolftian) duct; a.t, mesonephric (segmental) tubules, which on the one hand open into the body-cavity, on the other into the mesouephric duct. +++++++++++++++++++++++++++++++++++++++++
On a cross section one can recognise in the human embryo also (NAGEL) two distinctly separated regions on each urinary tubule (1) a larger one, which begins with the BOWMAN'S capsule and is lined with large epithelial cells containing abundant protoplasm, and (2) a narrower region with small cubical elements. The latter is the collecting tube, which unites with other collecting tubes before it opens into the mesonephric duct ; on the other hand, probably the former region alone has the secretory function, as also it is best developed at the time of the greatest prominence of the Wolffian body. The Malpighian glomeruli, likewise, attain at this time in human embryos a remarkable size (NAGEL).
The further fate of the primitive kidney is very different in the separate classes of Vertebrates. In the Anamnia, i.e., in Fishes and Amphibia, it becomes the permanent urinary organ, through which the excretions of the body are eliminated ; but besides that, it also acquires relations to the sexual apparatus, upon which, however, I shall not enter until later. In Birds and Mammals, on the contrary, the primitive kidney is functional only a short time during embryonic life ; soon after its establishment it undergoes profound regressive changes, and at last is preserved only in part, in so far as it enters into the service of the sexual apparatus, and, as we shall likewise see later, participates in conducting away the sexual products.
The Kidney. (Metanephros.)
The secretion of urine is assumed in the higher Vertebrates by a third gland, which is established at the posterior end of the niesonephiic duct the 'permanent kidney. The method of its formation, which appears to differ at first from that of the mesonephros, presents great obstacles to its investigation. It is most accurately known from studies on the development of the Chick through the works of SEDGWICK. At the beginning of the third day of incubation in the Chick there grows out of the [posterior] end of the uiesonephric duct, from its dorsal wall, an evagination the excretory duct of the kidney or ureter.
There are two conflicting views relative to its connection with the development of the kidney. According to the older view, which is still shared by many, the kidney is formed from the ureter in the manner of an ordinary glandular growth. It is maintained that evaginations take place which give rise to other evaginations, and thus produce the whole parenchyma of the kidney. According to the second view, which has been formulated especially by the more recent embryologists, by SEMPER, BRAUN, FURBRINGER, SEDGWICK, and BALFOUR, the permanent kidney is, on the contrary, developed out of two different fundaments, which come into relation with each other only secondarily : the medullary substance with its collecting tubules j j j ^^*^<^->^^&^^^^^ out of the ureter, the cortical substance with the tortuous tubules
- w-^^ -"*- a ~ *^^^^^*****"*H
and the loops of HENLE, 011 the other hand, out of a special fundament. According to this view there would be an agreement between the development of the kidney and primitive kidney, in as far as in the latter the mesonephric duct and the mesonephric tubules also arise separately, and only secondarily enter into relation with each other by means of fusion. The agreement here indicated is a not unimportant ground for my giving preference to the second rather than the first view.
As far as regards the details of the conditions, they are in the Chick according to the investigations of SEDGWICK, which BALFOUR has confirmed as follows : the ureter, which has arisen by an evagination from the end of the mesonophric duct, grows into that part of the middle plate which is located at the end of the Wolffian body in the region of the thirty-first to the thirty-fourth primitive segment. The fundament, however, is not at once and at this place converted into a kidney, but first undergoes, after the ureter has penetrated into it, a very considerable change in position ; together with the ureter it grows forward on the dorsal side of the mesonephric duct farther ; it meanwhile gradually enlarges, and begins to show internal differentiation only when it has come into this new position. One then sees that tortuous tubules become more and more distinct in the small-celled mass and that in their walls Malpighian corpuscles are established. One finds, in addition, that there are evaginated from the end of the ureter separate sacs, which grow out into collecting tubes, and probably later certainty in regard to this has not yet been established join the tortuous tublues which have arisen in the cortical portion of the kidney.
This voluminous organ, which has soon outstripped the mesonephros in size, is originally composed of individual lobes separated by deep furrows (fig. 208). The lobation is retained permanently in Reptiles, Birds, and some of the Mammals (Cetacea). In most Mammals, however, it disappears, in Man soon after birth. The surface of the kidney acquires an entirely smooth condition ; the internal structure (Malpighian pyramids) alone points to its composition out of individual portions, originally also separated externally.
JFor the sake of clearness the development of the three regions, pro-, meso-, and metanephros, has been treated as a whole up to this point. Consequently there have been left out of consideration for the time being other processes which are taking place in the vicinity of the fundament of the mesonephros at the same time. These have to do with the evolution of the Miillerian duct and the .sexual organs.
Fig. 208. Kidney and suprarenal body of a human embryo at the end of pregnancy.
nn, Suprarenal body ; n, kidney ; I, lobes of the kidney ; hi, \ireter.
The Mullerian Duct
The Miillerian duct is a canal which is found lying at first parallel and close to the mesonephric duct in the embryos of most Vertebrates (Selachians, Amphibia, Reptiles, Birds, Mammals). It is a canal that is established in both sexes in the same manner, but subsequently acquires in each a different function. It takes its origin in the lower Vertebrates from the mesonephric duct, as can be most easily followed in the Selachians (SEMPER, BALFOUR, HOFFMANN). In this case the mesonephric duct becomes enlarged, acquires in cross section (fig. 209 4 ) an oval form, and presents a different condition in its dorsal (sd) and ventral (od) halves, the latter being at the same time in immediate con o tact with the peritoneal epithelium. The mesonephric tubules open into the dorsal half, while ventrally the wall is considerably thickened. Then a separation of the two parts takes place, which begins at a little distance from the anterior end (cross sections 3-1) and proceeds backward to the point of opening into the hind gut. Of the parts which result from the fission, that which lies dorsally is the permanent mesonephric duct (wd) ; it exhibits at first a broad lumen and receives the urinary tubules (fig. 207 sty. Ventrally, between it and the epithelium of the body-cavity, lies the Miillerian duct (fig. 209 od and fig. 207), which is at first only a narrow passage, but later a much enlarged one. In the process of fission the anterior initial part of the primary canal (fig. 207 pd), which was described at p. 353 as pronephros and which opens into the body-cavity by means of a ciliate funnel (o), becomes a part of the latter duct, and the ciliate funnel becomes the ostium abdominale tubte.
Also in the case of the Amphibia the Miillerian duct is developed by being split off (FiiRBRiNGER, HOFFMANN) from the mesonephric duct, with the exception of the anterior end. which bears the orifices leading into the body-cavity. A small territory of the epithelium of the body-cavity immediately adjacent to the pronephros serves for the construction of this portion. The epithelium becomes thickened, owing to the fact that its cells take on a cylindrical shape ; it sinks in to constitute a groove, and then becomes constricted oil' from the surrounding tissue in the form of a short funnel, which in front remains in connection with the body-cavity by means of a broad opening, but posteriorly becomes continuous witli the part of the Miillerian duct that is produced by fission. The pronephric tubules and the glomerulus degenerate.
Fig. 209. Four cross sections through the anterior region of the mesonephric duct of a female embryo of Scyllium canicula, after BALFOUR.
The figure shows how the Miillerian duct (od) is split off from the meaoiiephric duct (sd and wd).
The fission of the single mesonephric duct into two canals lying close together is a peculiar process, which is intelligible only upon the assumption that the mesonephric duct has possessed a double function. Probably it originally served as an outlet for the secretions of the mesonephric tubules, and also by means of its pronephric funnel took up out of the body-cavity the sexual products (eggs or seminal filaments) eliminated into it at their maturity, and conducted them to the outside. Similar conditions are often observed in Invertebrates, e.g., in various divisions of the Worms, in which also the segmental canals, which break through the body-wall, transmit to the outside both secretions from the body and sexual products. In Vertebrates each of the two functions is assigned to a special canal, one of which loses its communication with the bodycavity, but remains in connection with the transverse mesonephric tubules, while the other retains as its part the ciliate funnel of the pronephros, and thus is adapted to conducting away the sexual products (eggs).
In Reptiles, Birds, and Mammals the manner of the development of the Mullerian duct is still a subject of scientific controversy. Most observers (WALDEYER, BRAUN, GASSER, JANOSIK, and others) state that at no time was a process of fission observed. According to their representation the Mullerian duct arises in Birds and Mammals quite independently as a new structure, at a time when the mesonephros is already well developed and has the form of a bandlike body (the mesonephric fold) projecting into the body-cavity (fig. 210). One then sees on the lateral face of the anterior region of this body that the epithelium of the body-cavity over a limited area (') is thickened in a remarkable manner and composed of cylindrical cells, whereas elsewhere the cells are flattened. The thickened portion of the epithelium sinks down in the form of a funnel and applies itself closely to the mesonephric duct (?/), which is near at hand. The blind end of the funnel grows from this point backwards independently, as is usually asserted, by means of the proliferation of its own cells, and gives rise to a solid cord, which lies directly between the mesonephric duct and the peritoneal epithelium, which is here somewhat thickened. The funnel produced by the invagination now becomes the ostium abdominale tubse, but the solid cord of cells, which is soon hollowed out and finally opens behind into the cloaca, becomes the Miillerian duct.
If the representation just given is correct in all particulars, the Miillerian ducts in the Anamnia and the Amniota, although possessing the same location, form, and function, would still be nonhomologous organs, because their development is different.
Fig. 210. Cross section through the mesonephros, the fundament of the Mullerian duct, and the sexual gland of a Chick of the fourth day, after WALDEYER. Magnified 160 diameters.
m, Mesentery ; L, somatopleure ; ', the region of the germinal epithelium from which the Mullerian duct (2) has been invaginated ; , thickened part of the germinal epithelium, in which the primary sexual cells, C'and o, lie; E, modified mesenchyme out of which the stroma of the sexual gland is formed ; WK, mesonephros ; y, mesonephric duct.
For the one is split off from the mesonephric duct, the other is formed independently by a new invagination ot the epithelium. Such a surprising result appears to us, however, upongrounds of comparative anatomy, to be very improbable, and therefore the attempt made by some investigators to refer- back the conditions found in the Amniota to such as exist in the Anamnia deserves every attention. This would be possible if the statements of BALFOUR AND SEDGWICK, which have however been called in question by others (JANOSIK), should be confirmed. As we have previously seen, there are two different regions to be distinguished on the Mullerian duct an anterior, which is the degenerated pronephros and bears the orifice of the tuba, and a posterior, which is formed by being split off from the mesonephric duct. Such a double origin BALFOUR AND SEDGWICK endeavor to establish for the Miillerian duct in the Chick also. The part produced by invagination of the peritoneum (fig. 210 c) they interpret as pronephros. A similarity with the latter they find in the fact that this part does not, according to their investigations, consist of a single invagination of the peritoneal epithelium, but of three open imaginations lying one behind the other, which are joined together by ridge-like epithelial thickenings which afterwards become hollow (fig. 211 yr 2, yr 3, r 2). From this ridge is formed a slightly curved, short duct, which communicates with the body-cavity through three openings.
Fig. 211. Cross sections through two peritoneal invaginations out of which is formed the anterior region of the Mullerian duct (the pronephros) of the Chick, after BALFOUR AND SEDCW.CK.
A is the llth, B the 15th, C the ISth section of the whole series. gr 2, 3, Second and third furrows ; r 2, second ridge ; v;d, Wolffian duct.
If this explanation is right, the most anterior fundament of the excretory system of the Chick, which was described on page 356 as pronephros, must have undergone a change in position, and, with the appearance of the Wolffian body, have slipped backward somewhat along this organ. As long as this alteration of position is not demonstrated by the study of intermediate stages, the interpretation, however probable it may seem to us, still lacks actual proof, As far as regards the posterior, longer region of the Miillerian duct, SEDGWICK maintains that it arises by being split off from the mesonephric duct. One always finds, according to his researches, the pronephric part of the Miillerian duct in union at its posterior end with the ventral wall of the mesonephric duct. He maintains that it is enlarged at the expense of the latter in somewhat the same manner as the mesonephric duct grows from in front backwards by a proliferation of the outer germ-layer, The cross sections A and J3 of figure 212 exhibit this condition. Figure B shows the place where the ventral wall of the mesonephric duct is thickened into a ridge (md) by an increase of the epithelial cells ; upon a cross section (A} made farther forward the thickened part has become detached as a cord (met), which subsequently becomes still more isolated and acquires a cavity of its own. The condition recalls very clearly the appearances which the cross sections through embryo Selachians (fig. 209) gave.
According to the observations of SEDGWICK, therefore, the anterior end of the Mullerian duct would be derived from the pronephros, but the posterior end by a splitting off of cells from the mesonephric duct. Thus an agreement with the conditions in the non-anmiotic Vertebrates would be established.
Fig. 212. Two sections to show the union of the solid terminal part of the Miillerian duct with the mesonephric duct in the Chick, after BALFOUR AXD SEDGWH K. In A the terminal part of the duct is still finite distinctly separate ; in B it has united with thu wall of the mesonephric duct. md, Miillerian duct ; Wd, Wolman duct.
Fig. 213. Cross sections through the Wolman and Mullerian ducts of two human embryos, after NAGEL.
^4, A female embryo -21 mm. long. B, A male embryo 22 mm. long. W.g., Wolffian duct ; M.g., end of the Miilleriau duct in process of development.
+++++++++++++++++++++++++++++++++++++++++ It still deserves to be especially mentioned that in human embryos also the Miillerian ducts (fig. 213 A and B M.g.) during their development have their posterior ends fused for a short distance with the mesonephric duct (W.g.). NAGEL, to whom, we are indebted for this fine observation, expresses himself, it is true, against a splitting off ; liowever, the similarity with the conditions found in the Chick and the iion-amniotic Vertebratt-s is not to be denied, and has indeed been emphasised by NAGEL.
The Germinal Epithelium
In Vertebrates, at the time when the Mullerian duct is established, the first traces of the sexual glands are also to be recognised. The parent-tissue of these is likewise the epithelium of the body-cavity. This acquires for example in the Chick, which is to serve as the foundation for our description a different appearance in the various regions of the body-cavity (fig. 210). In most places the epithelia become extraordinarily flattened and assume the condition of the permanent " endothelium." Also on the mesonephros, which projects into the body-cavity as a thick, vascular fold, the epithelium is for the most part greatly flattened, but retains its original condition (1) on its lateral surface along a tract (a) from which, as we have previously seen, the Mullerian duct is formed, and (2) along a tract (a) which stretches from in front backward along the median side of the mesonephros ; the signification of the latter has been correctly estimated by BOENHAUPT and by WALDEYER, who have characterised it as germinal epithelium. From it are derived the yerm-cells : in the female the primitive ova, in the male the primitive seminal cells. It is only in the very earliest stages that it is impossible to distinguish whether the germinal epithelium will be developed into testis or ovary. Differences soon appear, which allow a positive determination. We shall take up first the development of the ovary, then that of the testis.
(/) The Ovary.
The development of the ovary is tolerably well known both in the lower and the higher Vertebrates, except for a few controversial points. I can therefore limit myself simply to the presentation of the results which have been acquired in the case of the Chick and Mammals.
At about the fifth day of incubation the germinal epithelium in the Chick increases a good deal in thickness, becoming two to three layers of cells deep. Certain elements in this thickening are prominent ; they are distinguishable (fig. 210 C and o) by their richness in protoplasm and by their large round nuclei. Because they stand in the closest relation to the development of eggs, they have been designated as primitive eyys by WALDEYER, who was the first to study them in detail.
Beneath the germinal epithelium there is to be found, even at that time, embryonic connective tissue with stellate cells (E), which are in an active state of proliferation. In. this way there arises on the median, side of the mesoiiephros the ovarian ridge, which -is separated from the urinary tubules by a small quantity of embryonic connective substance.
Changes similar to those of the Chick occur in Mammals, with this difference, that the germinal epithelium appears to J^^f attain a much greater thickness.
In older stages of development the boundaries between the germinal epithelium, which is in process of rapid proliferation and therefore exhibits numerous figures of nuclear division, and the underlying connective tissue become less and less distinct. This results from the simple fact that a process of mutual ingroivth now occurs between the e2)ithelium and the embryonic connective tissue (fig. 214). I purposely say a process of mutual ingrowth, for I leave it undetermined whether the germinal epithelium in consequence of its development grows into the embryonic connective tissue in the form of cords and distinct groups of cells, or whether the connective tissue penetrates with its projections into the epithelium. Probably both tissues are actively engaged in the process.
In the phenomenon of intergrowth, which continues for a long time during development, two chief stages can be distinguished.
At first there arise from the germinal epithelium both slender and stout cords and balls of cells (figs. 214 and 215), which have received from the name of their discoverer the designation PFLUGER'S egg-tubes. Occasionally these are joined to one another by means of lateral branches. Together with the connective tissue separating them, they form the foundation for the cortex of the ovary. Afterwards they are covered over on the side toward the body-cavity with a thick continuous layer of connective tissue, which becomes the albuginea of the ovary ; they are thereby more sharply separated from the germinal epithelium, (fig. 216 k.e), which is still preserved, even after this, as a layer of cubical cells upon the albuginea.
Fig. 214. Cross section through the ovary of a Rabbit 5 days old, after BALFOUR. Highly magnified.
k.e, Germinal epithelium ; u.ei, primitive (or primordial) ova ; ei.b, egg-nests ; bi, connective tissue.
Fig. 215. Section through an egg-nest of a Rabbit 7 days old, after BALFOUR. ei, Ovum, the germinative vesicle (kb) of which exhibits a filar network ; bi, connective-tissue stroma ; f.z, follicular cells.
There are two kinds of cells to be found in the Pfliigerian egg-tubes : follicular cells and primitive ova (fig. 215y. ande*). Concerning the source of the former opinions are still contradictory (compare p. 382) ; according to my view both arise from the germinal epithelium.
Whereas the follicular cells become by means of an uninterrupted process of division more numerous and smaller, the primitive ova increase in size continually, and their nuclei become very large and vesicular and acquire a distinctly developed filar network (&&). They rarely lie singly in the cords and balls of follicular cells, but ordinarily in groups, which are designated as egg-nests. One frequently observes in the nests, as has been announced by BALFOUR and VAN BENEDEN, that several primitive ova become fused into a common, multinuclear mass of protoplasm a syncytium. From this there is afterwards developed usually only a single egg. One of the numerous nuclei soon outstrips the others in size and becomes the germinative vesicle, whereas the remaining ones undergo degeneration and are dissolved. It is not to be concluded from these processes that the egg, as is occasionally asserted, corresponds to a multiple of cells ; the condition is more properly to be interpreted as follows : of the eggs contained in a nest, one outstrips the others in its growth and thereby represses them and employs them, in a certain sense as nutritive material, for its own growth.
This is a process that occurs very frequently in Invertebrates, and in the phylum of the Arthropods has been studied with the greatest detail by WEISMANN. In these cases the lower Crustacea and Insects one can see how, step by step, out of numerous primitive ova which are originally contained in a germinal chamber of an ovariole, only one becomes the egg, whereas the. others from an early period lag behind in development, then undergo degeneration, and in the form of products of degeneration are taken up as yolk-material into the persisting egg-cell.
During the enlargement of the egg-cell the second stage of the process of intergrowth of epithelium and connective tissue is introduced : the stage of the formation of the follicle (fig. 216). At the boundary between the medullary and cortical zones of the ovary the surrounding connective tissue, carrying with it the blood-vessels, grows into the egg-tubes of PFLUGER (e.sch) and the nests (ei.b), and divides them all into spheroidal bodies, the individual follicles (f). Each such structure contains a single ovum, that is enveloped on all sides by a layer of follicular cells. The vascular connective tissue that grows around it becomes the follicular membrane or tkeca folliculi.
The resolution into follicles continually advances from the medullary substance toward the germinal epithelium ; however, there are preserved under it for a long time Pfliigerian tubes, which remain in connection with it by means of narrow epithelial cords (e.sch) and contain eggs in process of development.
Fig. 216. Part of a sagittal section of an ovary of a Child just born, after WALDEYER. Highly magnified. k.e, Germinal epithelium ; e.sch, PFLUGER'S egg-tubes ; v.e, primitive ova lying in the germinal epithelium ; c.xch', long PFLUGER'S tubes, in process of being converted into follicles ; ei.b, egg-balls [nests], likewise in process of being resolved into follicles ; /, youngest follicle already isolated ; gg, blood-vessels. In the tubes and egg-nests the primordial eggs are distinguishable from the smaller epithelial cells, the future follicular epithelium.
The formation of new Pniigerian. tubes and young ova is a process which continues in the lower Vertebrates throughout life, but in the higher appears to be limited to the period of embryonic development, or to the first years of life. In the first case, there being an unlimited capacity for the formation of new structures, egg-germs are found, even in the adult animal, sometimes in the most widely separated parts of the ovary, sometimes limited to definite regions of the gland. In the second case the period of forming primitive ova in the germinal epithelium bears a direct ratio to the total number of ova eliminated during the life of the individual. Thus WALDEYER states concerning Man that in the second year after birth the formation of new ova can no longer be shown.
Nevertheless in Man the number of ova contahu d in a single ovary is very great. They have been estimated to number in a sexually mature girl 30,000. In other Mammals the production of new ova appears to last longer. PFLUUER'S tubes which were still connected with the germinal epithelium and contained small primordial ova have been observed even in young animals (Dog, Eabbit, etc.). However, it has been questioned whether we here have really new structures or only primitive ova that in their development have remained stationary. It is maintained by VAN BEN EDEN with certainty for a few Mammals, e.g., the Bat, that in the sexually mature animal PFLUGER'S tubes and primitive ova still continue to be produced from the germinal epithelium.
In connection with the first formation of the follicle I will here add some statements about its further metamorphosis. This is very similar in the different Vertebrates, excepting Mammals.
In most Vertebrates the follicle (fig. 216 _/) consists at first of a small, centrally located egg-cell and a single layer .of small follicular cells enveloping it. Soon both are more sharply separated from each other by means of a vitelline membrane. In older follicles both parts have increased in size. The follicular cells ordinarily grow out into long cylinders, and appear to play an important part in the ' nutrition of the egg. In many animals, e.g., in Sharks and Dipnoi, yolk-granules have been found in them, as in the egg itself, and it has been concluded from, this, as well as from other phenomena, that the follicular cells take up nutritive substance from the vascular follicular capsule, and pass it along to the egg. Such a method of nutrition is made easier by the fact that the vitelline membrane (fig. 5 z.p) is traversed by tubules, through which the follicular cells (f.z) send protoplasmic filaments to the egg. When the egg has attained its full size, the follicular cells lose their significance as nutritive organs and become more and more flattened.
In the lower Vertebrates the mature ova are generally eliminated in great numbers all at once, frequently in the course of a few days or even hours. The discharge takes place by the rupture of the connective-tissue envelope, which causes the eggs to escape into the body-cavity, as in the Fishes and most of the Amphibia. After the elimination, the ovary, which up to this time Avas extraordinarily large and took up most of the space in the body -cavity, shrivels into a very small cord and now encloses only the young germs of ova, part of which are destined to mature during the next year.
The formation of the follicle takes place in a somewhat different way in Mammals. The follicle originally contains, as in the i emaining Vertebrates, only a single egg and a single layer of follicular cells, which are at first flat, then cubical, then cylindrical (fig. '21Q f). For a lone; time these cells envelop the egg as a single layer, but they then grow, undergo division, and are converted into a thick envelope of many layers. But the difference from the course of development described above becomes still greater, owing to the fact that a fluid, the liquor folliculi, is secreted by the proliferated follicular cells, and collects in a small cavity at the side of the egg (fig. 21 1 A JQ.
Fig. 217 A and B. Two stages in the development of the Graafian follicle. A with the follicular fluid beginning to be formed ; B with a greater accumulation of it. ei, Egg; fz, follicular cells; fz\ follicular cells which envelop the ovum and constitute the discus proligerus ; .#', follicular fluid (liquor folliculi) ; fk, follicular capsule (theca folliculi) ; zp, zona pellucida.
In consequence of a considerable increase of the fluid, the originally solid follicle becomes converted finally into a large or small vesicle (Fig. 217 JB), which was discovered more than two hundred years ago ' by the Hollander EEGNIER DE GRAAF and was held to be the human ovum. The structure has also been named after him the Graafian follicle. Such a follicle (fig. 217 B) now consists of (1) an outer connective-tissue, vascular envelope (fk], the theca folliculi ;
(2) lying on its inner surface, an epithelium composed of many layers of small follicular cells (fz), the membrana granulosa ; (3) the liquor folliculi (ff) ; and (4) the ovum (ei), which originally lay in the centre of the follicle, but which has now been crowded to the periphery. Here, enveloped in a great mass of follicular cells (fz l ), it causes an elevation of the wall, tho discus proliyerus, which protrudes into the cavity.
When the egg has reached complete maturity its elimination occurs by a collapse of the Graafian follicle, which has then attained in Man a diameter of about 5 mm. and causes an elevation at the surface of the ovary. The liquid of the follicle flows out through the rupture and at the same time carries away with it from the discus proligerus the egg, which comes first into the bodycavity, being surrounded by a small number of follicular cells, which still cling to the zona pellucicla (fig. 5). The egg is then taken up by the oviduct.
Into the cavity of the follicle produced by the flowing out of the liquid an effusion of blood takes place from the ruptured blood-vessels in the vicinity. The blood coagulates, and, accompanied by a proliferation of the adjacent tissue, is converted into the yellow body, or corpus luteum, which is a characteristic structure of the ovary of Vertebrates. Both the follicular cells (membrana granulosa) which are left behind and the connective -tissue follicular capsule participate in this proliferation. The follicular cells continue to multiply, penetrate into the interior of the coagulum, and after a time begin to undergo degeneration and to be dissolved into a granular mass. Vascular outgrowths from the capsule penetrate into the yellow body, and at the same time there is an extensive emigration of white bloodcorpuscles or leucocytes, which likewise undergo fatty and granulardegeneration at a later period.
It is of great importance for the further development of the yellow body whether the egg set free is fertilised or remains unfertilised. For according as the one or the other event supervenes, the corpus luteum is distinguished as true or false. In the first case it acquires a much greater size, the maximum of which is reached in the fourth month of pregnancy. It then appears as a fleshy reddish mass. After the fourth month a process of degeneration begins. The products of degeneration, which have resulted from the granular metamorphosis of the follicular cells and leucocytes, as well as from the coagulum of blood, are absorbed by the blood-vessels. Out of the decomposed coloring matter of the blood there have arisen hsematoidin crystals, which now give to the body an orange-red color. The connective tissue, originally with an abundance of cells, begins to shrivel, as in the formation of a scar ; as a result of these various processes of degeneration the yellow body, which projects beyond the surface of the ovary, begins to become considerably smaller, and is finally converted into a firm connective-tissue callus, which causes a drawing in at the surface of the organ.
When fertilisation has not occurred, the same metamorphosis and processes of growth it is true take place, but the false corpus luteum remains very much smaller. This is probably due to the fact that the afflux of blood to the sexual organs is very much less when there is no fertilisation than in case pregnancy takes place.
In addition to the tubes of PFLUGER, which arise from the germinal epithelium and produce the primitive ova, in most classes of Vertebrates ejnthelial cords of another kind and another origin enter into the composition of the ovary. As has been observed by various persons in Amphibia, Reptiles, Birds, and Mammals, there grow out from the Wolffian body, which lies in the immediate viciuity, epithelial shoots, the " sexual cords of the primitive kidney" and these penetrate toward the developing ovary even as early as the beginning of the intergro\vth between germinal epithelium and connective tissue. They arise from the epithelium of the Malpighian corpuscles, as BRAUN has shown for Reptiles, HOFFMANN for Amphibia, and SEMON for Birds. In Mammals, in which at present their subsequent fate has been most accurately traced out, they then unite with one another into a network at the base of the fundament of the ovary, which protrudes as a ridge into the body-cavity, and, pursuing tortuous courses, grow into contact with the tubes of PFLUGER. Whereas in Mammals the cortex of the ovary is developed out of the latter, the former share in the composition of the future medullary substance, and are on that account designated as niedidlary cords. In the vicinity of the follicle they remain solid, whereas the part near the primitive kidney acquires a cavity which is surrounded by cylindrical cells.
The medullary cords exhibit in different species of Mammals different degrees of development, as the comparative investigations of HARZ have established. In some animals, e.g., in the Pig and Sheep, they reach only to the base of the ovary, and therefore remain separated from the tubes of PFLUGER by a wide space ; in others they grow out into the vicinity of the latter, and in part apply themselves closely to them (Cat, Guinea-pig, Mouse, etc.), and take a very prominent part in the composition of the medullary substance.
There are two antagonistic views relative to the significance of the sexual cords of the primitive kidney, or the medullary cords, in the formation of ova. According to KOLLIKER and ROUGET the medullary cords early fuse with the tubes of PFLUGER and furnish to them the cells which become the follicular epithelium. The cells contained in a follicle would, according to this, come from two sources the follicular cells would arise from the primitive kidney, the eggs from the germinal epithelium. Most embryologists dispute this. According to their observations the medullary cords only exceptionally extend close up to a follicle, in many Mammals they do not reach it at all ; consequently not only the primitive ova but also the accompanying follicular cells must be furnished by the germinal epithelium. I also favor the latter view, which appears to me to be best supported by the facts. But what significance the medullary cords have will be better understood when we have become acquainted with the development of the testis, to which we shall now proceed.
I will at once state that our knowledge of the development of the testis is less complete than that of the development of the ovary.
The conditions appear to me to be the clearest in the non-amniotic Vertebrata. We possess here the pioneer researches of SEMPER and BALFOUR on the Selachians, and of HOFFMANN on Amphibia. All these investigators have, with one accord, come to the conclusion that the male sexual products, as well as the female, arise from the germinal epithelium of the body-cavity. In males also there is to be recognised in the region of the primitive kidney a special thickened band of tall epithelial cells, in which are imbedded larger cells with vesicular nuclei, the primitive spermatic cells. In the Sharks, the conditions of which I shall make the basis of the further description, they form irregular cords of cells, the " Vorkeimketten " of SEMPER (fig. 218 A]. Out of these are developed small, spherical, follicularlike bodies (fig. 218 B\ by the ingrowth of surrounding connective tissue into the cords, which are thereby divided up.
Thus far, therefore, complete agreement exists in the development of both kinds of sexual products. But whereas in the case of the ovary one cell in each follicle increases in size and is converted into the ovum, a like process does not take place in the male ; here the follicle-like structures become hollow and thus converted into seminal a inputted, whose epithelial cells gradually grow out into long cylinders. The greater part of these become seminal mother-cells, which by many repeated divisions are converted into sixty seminal cells, each of which is metamorphosed into a seminal filament. Since the filaments derived from each seminal mother-cell always arrange themselves parallel to one another, it is easily understood why before the ^ attainment of complete maturity the seminal filaments are found united in
gi-eat numbers into bundles. Whereas the testis, like the ovary, draws its specific histological components directly from the germinal epithelium, it acquires its efferent ducts from the primitive kidney. As in the female, so also in the male, epithelial shoots, the sexual cords (genital canals of HOFFMANN), grow from the primitive kidney toward the testis ; in the Amphibia they arise as proliferations from the cells of the wall of certain Malpighian corpuscles ; in the Selachians, on the contrary, they sprout out in a somewhat different manner from the ciliate funnels.
Arrived at the base of the testicular ridge, they are joined together into a longitudinal canal, from which fine tubules are sent still farther into the substance of the testis, where they unite with the structures that take their origin in the germinal epithelium. As figure 218 B shows, the efferent tubules (sc) in Selachians at first apply their blind ends to the ampullae, and enter into open communication with them, but only after tho maturation of the seminal filaments begins.
M:mv differences of opinion still prevail concerning the development of the testis in the higher Vertebrates. It is true that the presence of a, germinal epithelium upon the surface of the mesonephros has also been established in this case by WALDEYER for the male, but its participation in the fundament of the testis has been called in question. According to the original account of WALDEYER, which is still defended by many investigators, especially by KOLLIKER, the seminal tubules are morphological products of the primitive kidney. However, more recent researches, which it must be admitted do not yet harmonise with one another in all points, indicate that the development of the testis of Eeptiles, Birds, and Mammals agrees with that of non-amniotic Vertebrates in the main outlines. In continuation of the work of BORNHAUPT and EGLT, who it is true worked with incomplete methods of investigation, BRAUN has recently maintained for Reptiles, SEMON for the Chick, MIHALKOVICS and JANOSIK for the latter and for Mammals, that in the male also the germinal epithelium begins to proliferate, penetrates into the depths of the testis, and furnishes the primitive seminal cells. The tubules, which according to KOLLIKER and WALDEYER grow into the fundament of the testis from the primitive kidney, the sexual cords, serve only for carrying away the semen. As stated by BRAUN for Reptiles, and by SEMON for the Chick, they sprout out from the epithelium of Malpighian corpuscles, as in the case of the Amphibia.
Although according to these accounts the double origin of the substance of the testis, on the one hand from the germinal epithelium, on the other from the primitive kidney, can no longer be well called in question, nevertheless in the details many conditions, which are still differently described in the higher Vertebrates, demand renewed investigation. Before all else this point should be still further explained : In what proportion do the epithelial cells furnished by the germinal epithelium and those by the primitive kidney share in the formation of the testicular substance ? Are the tubules which produce the semen formed exclusively from germinal epithelium, or is it only the seminal mother-cells which have this origin, while there are associated with the latter indifferent cells from the " sexual cords of the primitive kidney " ? I hold it to be the more probable that the tubules producing the semen, the tubuli seminiferi, are derived from the (jerminal epithelium; the tubuli recti and the rete testis, on the contrary, from the primitive kidney.
N'AGEL has studied the development of the testis in human embryos. According to his description also, there arise from the actively proliferating germinal epithelium numerous cords, in which large primitive seminal cells are imbedded. The cords afterwards become the seminal tubules. In Man there prevails from the beginning, as XAGEL remarks, such a great difference between the two sexes, both in the form, of the original germinal ridge and in the whole process of its differentiation, that one can recognise in the anatomical structure of the sexual glands from a very early stage whether one has before him a male or a female.
Metamorphosis of the Different Fundaments of the Urogenital System into the Adult Condition
We have become acquainted in the preceding pages with the first development of the various parts which constitute the foundations of the urogenital system. These are (fig. 219) three pairs of canals -the mesonephric ducts (?/), the Miillerian ducts (ing), and the ureters (hi) and in addition a great number of glandular structures -pronephros, mesonephros (un), rnetanephros (n), and the sexual glands (kd), ovary and testis.
It will be my task in what follows to indicate how the ultimate t condition is derived from these embryonic fundaments. In this I shall limit myself, in the main, to Man, because we now have to do with more easily investigated, and in general well-known conditions.
In a human embryo eight weeks old (fig. 220) the fundaments, if we neglect differences which are recognisable only by the aid of the microscope, are so similar in male and female as to be i ndistinguishable.
All the glands lie at the sides of the lumbar vertebra? : farthest forward the kidney (n), which is a small bean-shaped body ; upon this lies the suprarenal body (nn), that at this time is disproportionately large and is to be seen only on the left half of the figure.
Somewhat lateral to the kidney one sees the primitive kidney (un) as an elongated, narrow tract of tissue. It is attached to the wall of the trunk by a connective-tissue lamella, a fold of the peritoneum, the so-called mesentery of the primitive kidney. In the middle of the gland it is rather broad, but above, toward the diaphragm, it is elongated into a narrow band, which KOLLIKER has described as the diaphragmatic ligament of the primitive kidney. Upon careful examination one also observes at the lower end of the primitive kidney a second fold of the peritoneum, which runs from it to the inguinal region (figs. 219 and 220 gh). It encloses a firm strand of connective tissue, a kind of ligament, that is destined to play a part in the development of the female and male sexual organs tie inguinal ligament of the primitive kidney. It subsequently becomes in man the gubernaculum Hunteri, in woman the round liyament of the iiterus (ligamentum teres uteri).
On the median side of the primitive kidney is found either the testis or the ovary (M], according 1 to the sex of the embryo, both sexual organs still being at this time small oval bodies. They also possess mesenteries of their own, a mesorchium or mesovarium, by means of which they are connected with the root of the primitive kidney. As long as the sexual organs retain their positions on each side of the lumbar vertebrae, the blood-vessels that supply them run in an exactly transverse direction : the arteria spermatica from the aorta to the ovary or the testis, the vena spermatica from the gland to the vena cava inferior.
Fig. 219. Diagram of the indifferent fundament of the urogenital system of a Mammal at an early stage.
/', Kidney ; kd, sexual gland ; iin, primitive kidney ; iig, mesonephric duct ; mg} Miillerian duct ; mg', its anterior end ; gli, gubernaculum Hunteri (mesonephric inguinal ligament); Id, ureter; Id', its opening into the urinary bladder ; iff/", mg", openings of the mesonephric and Miillerian ducts into the sinus urogenitalis (sue/) ; md, rectum ; cl, cloaca ; ghO, sexual eminence ; gw, sexual ridges ; cl', external orifice of the cloaca ; hbl, urinary bladder ; 1M', its elongation into the urachus(the future lig. vesico-umbilicale).
The various efferent ducts lie at this time close together at the margin of the mesonephric fold (fig. 219), the most anterior [ventral] being the Miillerian duct (mg). Farther backwards toward the pelvis the ducts of both sides approach the median plane (fig. 219), whereby the Miillerian duct (mg) comes to lie for a certain distance on the median side of and then behind [dorsal of] the mesonephric duct (uy) t so that altogether it describes around the latter a kind of spiral course. When they reach the lesser pelvis, the four ducts are united behind the bladder (hbl) into a fascicle, the tjenital cord ; this union is due to their becoming surrounded by the umbilical arteries which have at this time attained a large size, and which run from the aorta on both sides of the bladder up to the
umbilicus and to their being, as it were, tied up into a bundle by them. In a cross section through the genital cord (fig. 228) we find the mesonephric ducts (ug) somewhat more anterior [ventral] and at the same time farther apart than the Miillerian ducts (nig], which are a little behind them and pressed quite close together in the median plane.
In older embryos there arise in the evolution of the urogenital system differences between the two sexes which are visible even externally and which become more distinct from month to month. These result from fundamental metamorphoses, which the whole apparatus continually undergoes in its separate parts. In connection with this some originally quite large fundaments undergo almost complete degeneration ; of those which remain some are serviceable only in the female, others only in the male ; when not employed, they disappear. Moreover the conditions which were referred to at the beginning of the description are extensively altered by the fact that the sexual organs surrender their original position, on either side of the lumbar vertebrae, and move farther downward into the pelvic cavity.
I describe first the changes in the male, then those in the female.
Fig. 220. Urinary and sexual organs of a human embryo 8 weeks old, after KOLLIKER. Magnified about 3 diameters, and seen from the ventral side.
nn, Right suprarenal body; un, primitive kidney ; n, kidney ; ung, mesonephric duct ; gk, HUNTER'S directive or inguinal ligament (gubemaculum Hunteri or ligamentum uteri rotundum) ; m, rectum ; I, bladder ; kd, sexual gland.
The Metamorphosis in the Male. Descensus testiculorum
Whereas the testis (figs. 221 and 222) by conglomeration of the seminal tubules becomes a bulky organ (A), the mesonephros (nh+pa) is retarded in its development more and more, and is at the same time differently metamorphosed in its anterior and its posterior portions. The anterior or sexual part of tfie primitive kidney (nh), which lias come into communication Avith the seminal tubules by means of individual canals, in the manner previously described, and has thereby furnished the rete testis and the tubuH recti, is converted into the head of the epididymis. It exhibits in the tenth to the twelfth week from ten to twenty short transverse; canals, which are now to be designated as vasa efferentia testis. They unite in the mesonephric duct (fig. 222), which continues to have a straight course, and has now become the seminal duct (si, vas deferens). During the fourth and fifth months the individual canals begin to grow in length and thereby to become tortuous. The vasa efferentia in this way produce the coni vasculosi, which are at once the initial part of the vas deferens and the tail of the epididymis.
Incidentally let it be stated that near the external opening of the vas deferens, as it passes along the posterior surface of the bladder, there arises in the third month a small evagination, which becomes the seminal vesicle (sM).
Fig. 221. The internal sexual organs of a male human embryo 9 cm. long, after WALDEYER. Magnified S diameters.
It, Testis ; nh, epididymis (sexiial part of the primitive kidney) ; pa, jiaradidymis (remnant of the primitive kidney); si, vas deferens (duct of the primitive kidney) ; .'/, vasciilar bundle of connective tissue. +++++++++++++++++++++++++++++++++++++++++
The posterior region of the primitive kidney (pa} degenerates into very insignificant remnants. In older embryos one still finds for a time, between vas deferens and testis, small, tortuous canals, usually blind at both ends, between which degenerated Malpighian corpuscles also occur. The whole forms a small yellow body. In the adult these remnants are still further reduced ; they produce on the one hand the vasa aberrantia of the epididymis, and on the other the organ discovered by GIRALDES, the paradidymis. The latter consists, according to HENLE'S description, of a small number of flat, white bodies, lying in contact with the blood-vessels of the seminal cord, each of which is a knotted tubule blind at both ends ; each tubule is lined with an epithelium containing fat, and is enlarged at its blind ends into irregularly lobed vesicles.
The Miillerian ducts (fig. 222 mg] do not acquire in the male any function, and therefore, as useless structures, undergo degeneration ; the middle region in fact usually disappears without leaving a trace although it has been for a time during embryonic life demonstrable as an epithelial cord. GASSER indeed observed a rudimentary canal of considerable extent at the side of the vas clefereris in a recently born male child. Certain rudiments of the terminal portions, on T
the contrary, are preserved even in the adult individual, and in descriptive anatomies are called uterus masculinus (uni) and non-stalked hydatids of the epididymistyy). The posterior terminal parts of the two Miillerian ducts, which lie close together enclosed in the genital cord, are modified into the uterus masculinus (?;*). Owing to the disappearance of the partition separating them, they are united into a single small s;i,c, which is situated between the openings of the two vasa deferentia at the prostata and therefore still bears the name of sinus prostaticus. Extraordinarily inconspicuous in Man, it acquires in many Mammals, in Carnivores and Ruminants
Fig. 222. Diagram to illustrate the development of the male sexual organs of a Mammal from the indifferent fundament of the urogenital system, which is diagrammatically represented in fig. 219.
The persistent parts of the original fundament are indicated by continuous lines, the parts which undergo degeneration by dotted lines. Dotted lines are also employed to show the position which the male sexual organs take after the completion of the desoensus testiculorum.
n, Kidney ; h, testis ; nh, epididymis ; pa, paradidymis ; hy, hydatid of the epididymis ; si, vas def erens ; ing, degenerated Miillerian duct ; urn, uterus masculinus, remnant of the Miillerian ducts ; gh, gubernaculum Hunteri ; hi, ureter ; hi', its opening into the bladder ; sbl, vesiculse seminales ; hbl, urinary bladder ; hbl' ', its upper tip, which is continuous with the ligamentum vesico-umbilkale medium (urachus) ; In-, urethra ; pi' t prostata ; dej, external orifice of the ductus ejaculatorii.
The letters nh' , h', si' indicate the position of the several organs after the descent has taken place.
(\VEBER), a considerable size, and is differentiated, as in the female, into a vaginal and a uterine part. In Man it corresponds chiefly to the vagina (ToUBNEUx).
The non-stalked hi/datid (hjj) is developed out of the other end of the Miillerian duct. It is a small vesicle that rests upon the epididyruis, is lined with ciliate cylindrical epithelium, and is continued into a small, likewise ciliate canal. At one place it possesses a funnelshaped opening, which has been compared by WALDEYER to the pavilion of a Fallopian tube in miniature.
In order to complete the account of the development of the sexual organs, there still remain to be mentioned the important chanyes f position which the testis together with the attached rudiments undergoes. Since early times, these have been embraced under the name of descensus testiculorutn.
Originally the testes (fig. 222 1i) lie, as previously stated, in the peritoneal cavity at the side of the lumbar vertebraB. In the third month we find them already in the greater (false) pelvis, in the fifth and sixth on the inner side of the anterior wall of the abdomen close to the inner abdominal ring (fig. 223). In consequence of these changes the nourishing blood - vessels, which at first ran transversely, have altered their direction and now pass obliquely from below upward, because their original place of attachment to the abdominal aorta and the inferior vena cava remains the same. How is the migration to be explained? I have already mentioned the inguinal ligament, or the gubernaculum Hunteri (fig. 222 and 223 yk), which puts the primitive kidney, or, when this has disappeared, the testis, into connection with the inguinal region. This ligament has in the meantime become a strong connective-tissue cord, in which non-striate muscles also lie. Its upper end is attached to the head of the epididyniis (nil) ; its lower end traverses the abdominal wall to be inserted into the corium of the inguinal region. Apparently this gubernaculum plays a part in the migration of the sexual organs. Formerly it was believed that it exercised a traction upon the testis, in which connection
Fig. 223. Human embryo of the fifth month, after BRAMANN.
Natural size. md, Rectum ; h, testis ; nh, epiilidymis ; si, vas deferens ; yk, gubernaculum Hunteri with proeessus vaginalis peritonei ; bl, bladder with lig. vesico-umbilicale medium.
attention was directed to the non-striate muscle-fibres contained in it, or a shortening of the connective-tissue cord by gradual shrinkage was assumed. But it is impossible for this very important change in position to have taken place in that manner. One therefore rightly seeks to explain the agency of the ligament in another way, without assuming an active shortening or a traction exercised by muscular action. We have to do here simply with processes of unequal growth. When, out of several organs originally lying beside one another in the same region of the body, certain ones in later months of embryonic life increase in size less, while others, on the contrary, grow extraordinarily in length, the natural consequence is that the more rapidly growing parts are shoved past those that grow more slowly. If, now, in the present case the skeletal parts and their accompanying muscles in the lumbar and pelvic regions become elongated, while the Huiiterian ligament does not grow and therefore remains short, the latter necessarily because one of its ends is attached to the skin of the inguinal region and the other to the testis draws down the testis as the movable part ; it draws the testis at first gradually into the cavity of the false pelvis, and finally, when the other parts have become still larger, when at the same time the abdominal wall has become much thicker, into the vicinity of the inner abdominal ring (fig. 223).
Fig. 224. Two diagrams to illustrate the descensus and the formation of the envelopes of the testis. A, The testis lies in the vicinity of the inner abdominal ring. B, The testis has entered the scrotum. 1, Skin of the abdomen; 1', scrotum with tunica dartos; 2, superficial abdominal fascia; 2', COOPER'S fascia ; 3, muscle-layer and fascia transversa abdominis ; 3', tunica vaginalis conumvnis with cremaster ; 4, peritoneum ; 1', parietal layer of the tunica vaginalis propria ; 4", peritoneal investment of the testis or visceral layer of the tunica vaginalis propria. Ir, Inguinal or abdominal ring ; /t, testis ; si, vas deferens. +++++++++++++++++++++++++++++++++++++++++
The testis migrates still farther in consequence of a second process, which begins even in the second month. For there is formed at the place where HUNTER'S ligament traverses the wall of the abdomen an evagination of the peritoneum, the processes vaginalis peritonei (fig. 224 ./I). This gradually penetrates the abdominal wall and enters into a fold of the skin, which is developed in the pubic region, as will be shown in a subsequent section (see fig. 231 yw). The opening of the hernia-like evagination, which leads into the bodycavity, is called the inner inguinal [abdominal] ring (lr) ; the portion which traverses the musculature of the abdominal wall, the inguinal caned; and the blind end which is expanded within the dermal fold, the scrotum.
In its migration the testis (fig. 224 ./>) also sinks down into this peritoneal fold, whereby it remains undetermined whether HUNTER'S ligament exercises an influence on it or not. The entrance into the inguinal canal usually takes place in the eighth month, into the scrotum in the ninth month, so that at the end of embryonic life the descent is, as a rule, completed. The canal then closes by fusion of its walls, and thereby the testis comes to lie in a sac constricted off from the abdominal cavity and enclosed on all sides.
The various enveloping structures of the testis also become intelligible from the sketch of the development just given. Since the cavity which shelters it is simply a detached portion of the bodycavity, it is, as a matter of course, lined by peritoneum (fig. 224 4 '). This is the so-called tunica vaginalis propria, on which, as on other regions of the peritoneum, we have to distinguish a parietal layer ( 4 ') lining the wall of the sac and a visceral layer ( 4 ") investing the testis. Outside of this follows the tunica vaginalis communis ( 3 ') ; it is the evaginated, and at the same time extraordinarily attenuated, layer of muscles and fascise ( 3 ) of the abdominal wall. Conset meiitly it also contains some muscle-fibres enclosed in it, which are derived from the musculus obliquus abdominis interims, and constitute the suspensory muscle of the testis or cremaster.
In the descensus testiculorurn, which should normally be com] Dieted in Man at the end of embryonic life, interruptions may, under certain circumstances, occur and produce an abnormal location of the testis, which is known under the name of cryptorchism. The descent remains incomplete. Then the testes of the recently born child are either found to be located in the body-cavity, or they still stick fast in the wall of the abdomen, in the inguinal canal. In consequence the scrotum feels small, flabby, and flaccid.
Such anomalies are designated as inhibition- nxilj'o filiations, because they are explained by the fact that the processes of development have not reached their normal termination.
The Metamorphosis in the Female. Descensus ovariorum
The metamorphosis of the primitive embryonic fundaments in the female is in many particulars the opposite of that in the male, inasmuch as parts which are made use of in the latter become rudimentary in the former, and vice versa (compare with one another the diagrams shown in figs. 219, 222, and 225). Whereas in m a 11 the mesonephr ic duct becomes the vas deferens, n woman the Miillerian duct (fig. 225 , ut, sch) assumes the function of conducting away the ova, while the mesoiiephric duct (ug] and the primitive kidney (ep, pa} become rudimentary.
Fig. 225. Diagram to illustrate the development of the female sexual organs of a Mammal from the indifferent fundament of the urogenital system, which is diagrammatically represented in fig. 219.
The persistent parts of the original fundament are indicated by continuous lines, the parts which undergo degeneration by dotted lines. Dotted lines are also employed to show the position which the female sexual organs take after the completion of the descensns.
n, Kidney ; ei, ovary ; ct>, epoophoron ; pa, paroophoron ; hy, hydatid ; t, Fallopian tube (oviduct) ; ug, mesoiiephric duct ; ut, uterus ; xch, \.igina ; hi, ureter ; hbl, urinary bladder; hbl', its upper tip, which is continuous with the ligamentum vesico-umbilicale medium ; hr, urethra; iv. vestibulum vaginae; rm, round ligament (inguinal ligament of the primitive kidney) ; lo', ligamentum ovarii.
The letters t', ej/, ei', lo' indicate the positions of the organs after the descent.
pkric duct in advanced human embryos of the female sex is still demonstrable as an inconspicuous structure in the broad ligament and at the side of the uterus ; in the adult it has, as a rule, entirely disappeared, except the terminal portion, which is enclosed in the substance of the neck of the uterus, where it is distinguishable, but only by 1111 ans of cross sections, as an extraordinarily narrow tubule (BEIGEL, H. DOHRN), In many Mammals, as in Ruminants and Swine, the mesonephric ducts persist even later in a rudimentary condition, and are here known under the name of GARTNER'S canals.
There are to be distinguished on the degenerating primitive kidney, as in Man, an anterior and a posterior region (WALDEYER).
The anterior region (figs. 225 ep, 226 ep), or the sexual part of the primitive kidney, which in the male becomes the epididymis, is also retained by the female as an organ without function and here becomes the parovarium (ep), which was first accurately described by KOBELT (the parovarium or epoophoron of WALDEYER). It lies in the broad ligament (fig. 226) between ovary (ei) and Miillerian duct (t), and consists of a longitudinal canal (ug), the remnant of the upper end of the mesonephric duct, and of ten to fifteen transverse tubules (ep). The latter have at first a straight course, but afterwards become tortuous (fig. 227 ep), in much the same way as the canals which in the male are converted into the coni vasculosi. The comparison between parovarium and epididymis may be carried still further.
Fig. 226. The internal sexual parts of a female human embryo 9 cm. long, after WALDEYER. Magnified 10 diameters.
ei, Ovary ; t, Miillerian duct or oviduct (Fallopian tube) ; t', ostium abdominale tubas ; ep, epoophoron (= epididymis of the male sexual part of the primitive kidney) ; ug, mesonephric duct (vas deferens of the male) ; jpa, paroophoron (paradidymis of the male rudiment of the primitive kidney) ; mk, Malpighian corpuscles.
As in the male tubules grow out from the latter into the cortex of the testis and are there differentiated into the rete testis and the tubuli recti, so there are also canals found in the female which proceed from the parovarium, enter the medullary substance of the ovary itself, and form here the previously (p. 381) described medullary cords, which are highly developed in many Mammals.
The posterior portion of the primitive kidney, which in the male (figs. 221 and 222 pa) furnishes the paradidymis and the vasa aberrantia, degenerates in the female (fig. 225 pa] in a similar manner into the paroophoron, and is still to be recognised for a long time in the human embryo as a yellowish body (fig. 226 pa), which lies media/awards of the epoophoron (ep) in the broad ligament, and is composed of small, tortuous, ciliate tubules (pa) and a few degenerating vascular glomeruli (mk). Certain canals and cyst -like structures, which are often found in the broad ligament of the adult close to the uterus, are to be referred to it.
The two Miillerian ducts (fig. 219 ing), which from the beginning lie in the margin of the peritoneal fold that serves for the reception of the ovary and subsequently becomes the broad ligament, undergo a very profound metamorphosis. It has already been mentioned that as they enter the lesser or true pelvis they approach the median plane, and are joined to the genital cord. We can therefore distinguish in them two different regions, one enclosed in the genital cord, the other lying in the margin of the broad ligament. The latter becomes the oviduct (the tuba Fallopise) with its funnel-shaped beginning (figs. 225 t, 226, 227 t, t'). The anterior end of the Miillerian duct, which in the embryo reaches far forward and is here enclosed in the diaphragmatic ligament of the primitive kidney, appears in the meantime to degenerate, whereas the permanent opening (figs. 225 t and 226 t') is probably an entirely new formation. MORGAGNI'S hydatid (fig.' 225 hy) is perhaps to be referred to the anterior rudimentary part the conditions here have not yet been made entirely clear. This structure is a small vesicle, which is joined, by means of a longer or shorter stalk, with one of the fimbrise of the funnel-shaped end of the oviduct.
Fig. 227. Broad ligament with ovary and oviduct in the adult condition, seen from behind. ei, Ovary ; t . oviduct ; t', ostium abdominale tubas with fimbriae ; f.o, n'mbriae ovarii ; l.o, liga menttim ovarii ; x, a portion of the peritoneal investment is dissected away, in order to see the epoophoron (parovarium), c^.
Out of the part of the Miillerian ducts enclosed in the genital cord (fig. 219 mg) are formed the uterus and the vagina (fig. 225 ut and sc/i), as THIEUSCH ami KOLLIKER have shown for Mammals, and as DOIIHN and TOUIINEUX ET LEGAY afterwards showed for Man. Their formation is accomplished by a process of fusion, which in Man is effected in the second month. When the Miillerian ducts (fig. 228 my) are closely pressed together, the partition between them becomes thin and breaks through at first in the middle of the genital cord. Thus there is developed out of them by an extension of this process a single sac (the sinus genitalis), which is also established in the male as a rudimentary organ, the previously mentioned sinus prostaticus or uterus inasculinus (fig. 222 um). In woman it begins to be differentiated in the sixth month into uterus and vagina. The upper portion, which receives the oviducts, acquires very thick, muscular walls and a narrow lumen, and is limited below by a reeiitering ring-like ridge that becomes the vaginal portion [of the uterus] from the lower portion, the vagina, which remains spacious and possesses a thinner wall.
Similarly to the testis, the ovaries also have to pass through a considerable change in position : the descensus ovariorum (fig. 225 ei', '), which corresponds to the descent of the testes. In the third month of embryonic life, at the time when the primitive kidney begins to disappear, the ovaries move from the region of the lumbar vertebrae down into the false pelvis, where they are found medianwards from the musculus psoas. Probably the above-described inguinal ligament of the primitive kidney (fig. 225 rm), which is not wanting in the female, participates in the change of position in this case also. As WIEGER has recently shown, the ligament is differentiated into three distinct regions by the fact that it acquires a firm union with the Miillerian ducts at the place where they meet to form the sexual cord. The uppermost region becomes a strand of non-striate musclefibres, which, arising from the parovariuin, is imbedded in the hilus of the ovary. This is continuous with the second region, or the ligamentum ovarii (lo r ), and the latter with the round ligament (rm) (ligamentum teres uteri). The round ligament, produced from the third and most developed region of the inguinal ligament, extends from the upper end of the genital cord to the inguinal region. Here there is usually, as in the male, a small e vagina tion of the peritoneum, the processus vaginalis peritonei, which occasionally persists even in the adult as the diverticulimi Nuckii, and then may likewise be the cause of the formation of an inguinal hernia in the female. At this place the round ligament passes through the wall of the abdomen and ends in the external skin of the labia majora.
In its last stages the descent in the female is accomplished in a manner different from that in the male. For instead of advancing like the testes toward the inguinal region, the ovaries, when the development is normal, sink down instead into the true pelvis. Here they are enclosed between bladder and rectum in the broad ligament, which is developed out of the peritoneal folds, and in which originally the primitive kidneys, the ovaries, and the Miillerian ducts are imbedded.
Naturally the round ligament cannot be of influence during this last stage of the descent in the female, because it can exercise a traction only in the direction of the inguinal region, where it is attached. The descent into the true pelvis seems rather to be due to the conversion of the lower region of the Miillerian ducts into the uterus. At any rate, the ovaries are joined to the uterus by means of a firm cord of connective tissue, the ligamentum ovarii.
In rare cases in the female the ovaries can continue to change their position in a manner corresponding to that in the male. They migrate then toward the inguinal region up to the entrance into the processus vaginalis (diverticulum Nuckii); occasionally they here cease to advance, but sometimes they enter farther into the abdominal wall through the inguinal canal ; indeed, as has been observed in several instances, they can pass quite through the wall of the abdomen and at last imbed themselves in the labia majora. The latter then acquire a great similarity to the scrotum of the male.
+++++++++++++++++++++++++++++++++++++++++ Fig. 228. Cross section through the genital cord, after TOOSNEI-X ET LEGAY.
The cross section sho\vs the fusion of the Miillerian ducts (my) ; uft, mesonephric ducts. +++++++++++++++++++++++++++++++++++++++++
The Development of the External Sexual Organs
The section which deals with the urinary and sexual organs is really the most suitable place at which to introduce the development of the external sexual organs, notwithstanding they do not arise from the middle germ -layer, but in part from the outer and in part from the inner germ-layer. In order to give an exhaustive account of them, we must go back to rather early stages of development to the time when in the embryo the Wolffian and Miillerian ducts are established. Having first arisen in the most anterior part of the embryo, they grow backwards to the terminal part of the intestine, and there implant themselves in the allantois. This is, as we have seen in the first part of this text-book (fig. 132, 3 and 4 al), an organ which is produced by evagination of the anterior [ventral] wall of the hind gut. In most Mammals (figs. 134 al and 142 ALO) it attains during embryonic life a quite extraordinary development, for it grows out of the body-cavity, penetrates between the other foatal membranes, and is distended into a large vesicle, which receives the urinary fluid secreted by the embryo. The part of it which lies in the body-cavity remains, on the contrary, narrow. The terminal part of it which receives the Wolffian and Mullerian. ducts is called sinus uroyenitalis (fig. 219 sug and 229 ug), a structure which will often demand our attention in considering the development of the external sexual organs.
The sinus urogenitalis and the hind gut unite to form a short, unpaired region, the cloaca (fig. 229 c), a small depression which opens out at the surface of the body and in very many Vertebrates in the Amphibia, Eeptiles, Birds, and the lowest Mammals, the Monotremes persists throughout life. In the remaining Mammals, however, these structures have only an embryonic existence. In the first case all the eliminationproducts of the body are conducted to the outside through the cloaca, out of the hind intestine the faecal masses, out of the sinus urogenitalis the urinary fluid and the male or female sexual products.
Fig. 229. Diagram of the urogenital organs of a Mammal at an early stage, after ALLEN THOMSON ; from BALFOUR.
The parts are seen chiefly in profile, but the Mullerian and Wolflfian ducts are seen from the front.
3, Ureter ; It, urinary bladder ; 5, urachus ; ot, genital gland (ovary or testis) ; W, left Wolttian body (primitive kidney) ; x, its diaphragmatic ligament ; ic, Wolffian (mesonephric) duct ; m, Mullerian duct ; f/c, genital cord consisting of Wolffian and Miillerian ducts enveloped in a common sheath ; i, rectum ; ug, urogenital sinus ; cp, genital eminence, which becomes the clitoris or penis; Is, genital ridges from which the labia majora or the scrotum are developed.
As far as regards the special conditions in Man, the allantois remains in his case very small (fig. 132, 5 al) and possesses a lumen in the region of the body-cavity only, whereas in the umbilical cord and between the remaining foetal membranes only its connectivetissue part, together with the blood-vessels, which shares largely in the development of the placenta, grows further. In the second month its hollow part, lying on the front wall of the abdomen, becomes a spindle-shaped body (fig. 229 4 ). Its middle enlargement becomes the urinary bladder ( 4 ), its upward prolongation, which reaches to the navel, is called urachus ( 5 ), the other end (ug} is the sinus urogenitalis. The urachus degenerates during embryonic life and furnishes a connective-tissue cord, the ligamentum vesico-umbilicale medium, which extends from the apex of the bladder (fig. 219 hbl'} to the navel, and often in the first years after birth still contains an epithelial cord, a remnant of the original epithelial canal.
As is well known, the ureters (figs. 229 3 and 219 hi'} in the adult open close together at the posterior surface of the urinary bladder (229 4 ). In very young embryos this is not the case at first, for the two ureters arise from the posterior part of the mesonephric duct, arid this opens into the sinus urogenitalis. But this condition is soon altered. The ureter splits off from the mesonephric duct, and comes to open independently into the posterior wall of the sinus urogenitalis, from, which it afterwards becomes gradually removed, since its orifice, as it were, creeps higher up on the posterior wall of the bladder. Like the change in the position of the sexual glands, we must also conceive of this shifting as produced by processes of growth in such a way that especially the tract between mesonephric duct and ureter, which is at first small, increases in size, and thereby produces the apparent upward migration of the opening of the ureter.
In the sixth week the cloaca in Man undergoes alterations which are connected with the development of the external sexual organs. The cloacal depression, which in earlier stages (fig. 230 A} appears fissure-like, afterwards becomes (fig. 230 B] surrounded by a ringlike fold, the genital ridge (gw), and there also arises in its anterior portion a growth of connective tissue, which produces the externally protruding genital eminence (gh). Along the lower surface of the latter there is formed at the same time a groove (gr], which extends downward to the cloaca, of which it is, as it were, the continuation.
In the following weeks of development the eminence protrudes si ill more, and thereby becomes converted into the genital member, which is at lirst possessed by both sexes in the same condition; meanwhile the groove (gr) on its under surface becomes deeper, and surrounded, at the right and left, by projecting folds of the skin, the genital folds (gf). (Compare also the diagrams fig. 219 ghi>, yw, cl' and fig. 229 cp, Is, cl.) Alterations follow (fig. 231 M arid W) by which the cloaca is differentiated into two openings, one lying behind the other, the anus (a) and the separate urogenital opening (ug). The deep partition (fig. 229) by which the sinus urogeiiitalis and the rectum are separated from each other begins to grow outward, and at the same time folds also arise on the lateral walls of the cloaca and unite with it. Thus a membrane (fig. 231 d) is developed which separates a posterior opening (), the anus, from an anterior opening, the entrance to the sinus urogeiiitalis (uy). Inasmuch as this partition continues to become thicker up to the end of embryonic life, it finally crowds the two openings far apart and forms between them the perinseum (fig. 231 M* and IF* d). In this way the anus (a) moves entirely out of the territory of the previously mentioned genital ridge (fig. 230 gio).
From the fourth month onward great differences arise in the development of the external sexual parts in male and female embryos.
In the female (fig. 231 IF and W*} the metamorphoses of the originally common embryonic foundations are on the whole only slight ; the genital eminence grows only slowly and becomes the female member, the clitoris (cl). Its anterior end begins to thicken and to be marked off from the remaining part of the body as the glans. By a process of folding in the integument there is developed around it (fig. 231 IF vJi) a kind of foreskin (the prseputiuni clitoridis). The two genital folds (IF gf}, which have bounded the groove on the under surface of the genital knob, take on a more vigorous development in the female than in the male, and are converted into the labia minor a (IF* kscli). The space between them (IF ug), or the sinus urogeiiitalis, which receives the outlet of the urinary bladder and the vagina developed- by the fusion of the Miilleriaii ducts, is called the vestibulum vagince (IF* vv). In the female the genital ridges (IF gw), owing to the deposition of fatty tissue, become very voluminous, and are thus converted into the labia major a (IF* </sch).
The corresponding fundaments pass through much more essential metamorphoses in the male (fig. 231 M and J/*). By an extraordinarily vigorous growth in length the genital eminence is converted into the male member, or the penis, which corresponds to the clitoris of the female. Like the latter, it possesses an anterior knob-like enlargement, the glans (M gp}, which is embraced by a fold of the skin, the prseputium (M* vh}. The sinus urogenitalis, which in the female remains short and broad as the vestibulum vagina?, is in the male converted by a process of fusion into a long narrow canal, the urinary tube or urethra. This results from the fact that the furrow on the under surface of the genital protuberance ( M gr) becomes elongated during the development of the latter and at the same time deeper, and that the sexual folds (gf] bordering it protrude farther, coming into immediate contact along their edges (M*} as early as the fourth month, and begin to fuse together.
+++++++++++++++++++++++++++++++++++++++++ Figs. 230 and 231. Six stages in the development of the external sexual organs in the male and the female, after the ECKER-ZIEGLER wax models. Fig. 230 A and B. Two stages in which a difference of the sexes is not yet to be recognised, B from an embryo S weeks old. Fig. 231. The two stages 3/and M* exhibit the metamorphosis of the original fundament in the male in embryos 2J and 3 months old respectively. The stages W and IF* present the metamorphosis in the female (2 and 4J months). The same designations are used for all of the figures. he, Posterior paired extremity ; do, cloaca ; gh, genital eminence ; (if, genital fold ; gr, genital groove ; gw, genital ridges ; gp, glans penis ; cl, clitoris ; d, perinaeum ; , anus ; v.y, entrance to sinus urogenitalis or vestibulum vaginae ; vv, vestibulum vaginas ; rh, foreskin (prepuce) ; Its, scrotum ; d <& r, raphe perinei and scroti ; gsch, Jabia ruajora ; ksch, labia niinora.
The posterior end of the urethra early (second month) undergoes changes by which the prostata (fig. 222 pr] is formed. The walls become greatly thickened, acquire non-striate muscular tissue, and constitute a ring-like ridge, into which evaginations from the epithelium of the tube penetrate, and by their branchings furnish the glandular portions of the organ. On its posterior wall are found, as is well known, the openings (dej) of the vasa deferentia, and between them the sinus prostaticus or uterus masculinus (urn), produced by the fusion of the Miillerian ducts.
The genital ridges (fig. 231 M gw), which in woman become the labia majora, also undergo a fusion in man. They surround the root of the penis and then fuse in the median plane, where the place of union is indicated afterwards by the so-called raphe scroti (M*'r). Into the scrotum (M* hs) thus formed the testes, toward the end of embryonic life, migrate, as previously described.
From the fact that originally the external sexual parts are constituted exactly alike in both sexes, it is evident why, with a derangement of the normal course of development, forms come into existence in which it is sometimes extremely difficult to determine whether one lins to do with male or female external parts. These are cases which in earlier times were erroneously designated as hermaphroditism. There are two ways in which they may arise. They are either to lie referred to the fact that in a female the process of development lias proceeded further than normally (i.e., as in the male), or that in a male the process of development has suffered an early interruption, and thereby led to formations which are similar to the female genital parts.
As far as regards the first kind of malformations, the genital eminence in the female occasionally assumes such a, size and form that it resembles in every particular the male organ. The resemblance may become even greater, when the ovaries migrate into the inguinal region instead of the true pelvis, pass through the wall of the abdomen, and become imbedded in the labia majora. In consequence of this the latter lie upon the root of the large clitoris and simulate a kind of scrotum.
The malformations which have given occasion for the assumption of hermaphroditism are of more frequent occurrence in the male. They are attributable to the fact that the processes of fusion which normally take place are interrupted. We then have a genital member, which ordinarily is rudimentary, along the under side of which there runs only a furrow instead of the urethra, a malformation which is designated as kijpospadias. With this morphological deficiency may be united, secondly, an arrest of the normal descent of the testes. The latter remain in the body-cavity, and the genital ridges thus acquire a great similarity to the labia majora of the female.
III. The Development of the Suprarenal Bodies.
The discussion of the suprarenal bodies best follows that of the urogenital system. For, aside from the fact that the suprarenal bodies and the genito -urinary organs are in all Vertebrates very closely connected spatially, they also appear to stand in very close relation to each other in the history of their development. At least the recent investigations of WELDON, JANOSIK, and MIHALKOVICS point that way, and are perhaps also sufficient to suggest the direction of the physiological research by which one can acquire an explanation concerning the ever problematic function of these bodies.
As is well known, there are to be distinguished in the suprarenal bodies two different substances, which in Mammals are described, according to their mutual relations, as medulla and cortex. Most investigators ascribe to them a double origin. BALFOUR, BRAUN, KOLLIKER, and MITSUKURI make the medulla arise from the ganglionic fundaments of the sympathetic nerve-trunk (Grenzstrang), it is for this reason that in many text-books the suprarenal bodies are treated of in connection with the sympathetic, but GOTTSCHAU and JANOSIK controvert this ; they maintain that only certain ganglionic cells and nerve-fibres grow in from the sympathetic, but that the real medullary cells arise by a metamorphosis of cortical cells. It appears to me from the existing investigations that the question is not ready for discussion.
There are also two different interpretations concerning the development of the cortical substance. J>ALFOUR, BRAUN, BRUNN, and MrrSUKURI derive it from accumulations of connective-tissue cells, which are formed at the anterior portion of the primitive kidney along the course of the inferior vena cava and the cardinal veins. According to JANOSIK, WELDON, and MIIIALKOVICS, on the contrary, the cellaccumulations are either directly or indirectly formative products of the epithelium of the body-cavity. I say " direct or indirect " because in details the results of the three investigators named differ somewhat. According to JANOSIK and MIHALKOVICS, it is the germinal epithelium in the anterior portion of the genital ridge that furnishes by its proliferation the material for the suprarenal body. MIIIALKOVICS therefore calls it " a detached part of the sexually undifferentiated genital gland, which consequently remains at a primitive stage of development." WELDON, on the contrary, brings the suprarenal body into relation with the most anterior part of the primitive kidney. According to his representation, which appears to me to deserve especial consideration, and from which indeed other researches will have to begin, the sexual cords of the primitive kidney are concerned in the formation of the suprarenal bodies. When, at the head-end of the kidney, they sprout out of the epithelium of the Malpighian glomerulus in the manner previously (p. 383) described, they divide into two branches. One of these grows ventrally into the fundament of the sexual gland, the other turns dorsally and spreads out in the vicinity of the vena cava.
Moreover, even MIHALKOVICS describes a connection of the sexual cords with the fundament of the suprarenal body at certain places, but makes both arise from proliferations of the epithelium of the body-cavity. The connection is subsequently destroyed by the interpolation of blood-vessels.
For the solution of the still pending questions most is to be expected from the investigation of non-amniotic animals.
During its development the suprarenal body is for a time of very considerable size. In Mammals it temporarily covers the much smaller kidney, as in the human embryo of the eighth week represented in fig. 220, in which a,t the left the suprarenal body (mi) is to be seen in its normal position, whereas on the right it has been removed to disclose the kidney (n). Afterwards its growth does not keep pace with that of the kidney ; however at birth (fig. 208), when it already rests upon the latter (n) as a crescentic body (rm), it still is larger in comparison with the kidney than it is in the adult.
During its development some small portions of the fundament of the suprarenal cortex appear sometimes to detach themselves and to remain in. the vicinity of the sexual organs, in whose migrations they participate. Thus, indeed, are to be explained the accessory suprarenal bodies observed by MARCHAND at the margin of the broad ligament.
1. The following structures are to be interpreted as formative products of the middle germ-layer : the epithelium of the body-cavity (of the pericardium, of the thoracic and abdominal cavities, of the cavity of the scrotum), the whole of the transversely striped, voluntary musculature, the seminal cells and ova, the epithelium of the sexual glands, of the kidneys and their outlets, and the cortical cords of the suprarenal bodies.
The Development of the Musculature
2. The musculature of the trunk is developed exclusively from the cell -layer of the primitive segments that abuts upon the chorda and neural tube, which by the formation of muscle-fibrilla? is converted into a muscle-plate.
3. The muscle-plate enlarges dorsally and ventrally, where it becomes continuous (zone of growth) with the outer (lateral) epithelial layer of the primitive segment, and spreads itself out over the neural tube above and into the walls of the abdomen below.
4. The original musculature consists of segments of longitudinal fibres (myomeres), Avhich are separated from one another by connective-tissue partitions (ligamenta intermuscularia).
5. The musculature causes the first segmentation of the body of Vertebrates into equivalent successive parts or metamera.
6. Buds grow out from the muscle-plates (Selachians) into the fundaments of the limbs, and thus furnish the foundation for the whole musculature of the extremities.
7. In the head-region of Vertebrates the musculature is developed not only out of the primitive segments, the number of which in Selachians amounts to nine, but also out of that part of the middle germ-layer which corresponds to the lateral plates of the trunk, and which is divided up by the formation of the visceral clefts into separate visceral-arch cords, which in the Selachians are provided with cavities.
8. From the primitive segments of the head arc formed the muscles of the eyes, and from the visceral-arch cords the masticatory muscles, the muscles of the hyoid arch and also those of the small bones of the ear (?).
The Development of the Urogenital System
9. The first fundament of the urogenital system is the same in both sexes: it consists of (1) three pairs of canals the mesonephric duct, the Miillerian duct, and the ureter; (2) four pairs of glands the pro-, rneso-, and metanephros and the sexual gland, which at first is indifferent.
10. The mesonephric duct arises in its most anterior part out of a groove-like evagination or a ridge-like thickening of the parietal middle layer ; posteriorly it detaches itself from its parental tissues, fuses with the neighboring outer germ-layer, and thereby forms at first a short, tubular communication between the coslom and the surface of the body.
11. The mesonephric duct is gradually converted into a long canal, inasmuch as it grows backward 011 the outer germ-layer, which forms a thickened ridge, until it opens out into the cloaca (terminal part of the hind intestine).
12. The pronephros (head-kidney) is developed at the anterior part of the mesonephric duct in the following manner : the duct, upon being constricted off from the parietal middle layer, remains in connection with the latter at several places, and the resulting cords of connection grow out into long pronephric tubules, at the inner openings of which an intraperitoneal vascular glomerulus is established out of the wall of the body-cavity.
13. Behind the pronephros the mesonephros (primitive kidney) arises thus : when the primitive segments are constricted off from the lateral plates, segmentally arranged cellular tubes or cords (nephrotome) are formed, which communicate at one of their ends with the body- cavity and at their other ends put themselves into connection with the laterally situated mesonephric duct and become the mesonephric tubules. (Development of Malpignian corpuscles, of secondary and tertiary mesonephric tubules and the glomeration of the latter.) 14. In the higher Vertebrates the development of the primitive kidney is to a certain extent abbreviated, in so far as the separate cords of cells which arise at the constricting off of the primitive segments lie very close together and constitute an apparently undifferentiated cell-mass (the middle plate or the mesonephric blastema), out of which the mesonephric tubules subsequentlyvvhen they become clearly distinguishable appear to have been differentiated.
15. In a part of the non-amniotic Vertebrates (some Selachians, Amphibians) the primitive kidney remains in open communication with the body-cavity by means of numerous ciliate funnels (nephrostomes), whereas in all Amniota the mesonephric tubules early surrender their genetically established connection with the bodycavity through the disappearance of the ciliate funnels.
16. The permanent kidney (metanephros) is the latest to be formed and takes its origin from two separate parts : (a) From an evagination of the end of the mesonephric duct, which furnishes the ureters, the pelvis of the kidney, and the straight urinary tubules (in other words, the efferent apparatus) ; (&) .From a renal blastema, which represents a backward prolongation of the mesonephric blastema, has the same origin as the latter, and is converted into the tortuous urinary tubules with the Malpighiaii corpuscles (therefore the secretory part of the kidney).
17. The fundaments of the kidney, which have arisen far back in the body, rapidly increase in size and undergo a change of position by moving farther forward by the side of the primitive kidneys, whereby the ureter becomes wholly detached from the mesonephric duct and moves to the posterior [dorsal] surface of the allantois, the future urinary bladder.
18. In the non-amniotic Vertebrates the mesonephros also gives rise by a process of fission to the Miillerian duct, which runs parallel with it.
19. In the Amniota the relation of the Miillerian duct to the mesonephric duct is still uncertain, because the front end of the former is established by a groove-like depression of the epithelial investment on the lateral face of the mesonephros, while concerning the remaining part it is still undetermined whether it grows backwards independently or is constricted off from the mesonephric duct.
20. The sexual glands proceed from two fundaments : (a) From a germinal epithelium, a modified part of the epithelium of the body-cavity, located on the median face of the primitive kidney ; From the sexual cords, which grow out toward the germinal epithelium from the adjacent part of the primitive kidney (in Reptiles and Birds from the epithelium of Malpighian glomeruli).
21. The specific components of the sexual glands, the eggs and seminal cells, arise from the germinal epithelium (with its primitive ova and primitive seminal cells).
22. In the female there arise, in consequence of a process of mutual intergrowth on the part of the germinal epithelium and the subjacent stroma, the tubes of PFLUGER and egg-balls (or nests), and out of these finally egg-follicles, containing each a single ovum ; in the male there are formed, in consequence of a similar process, seminal ampullae (Selachians, some Amphibia) or seminal tubules (tubuli seminiferi) with their seminal mother-cells.
23. The sexual cords of the primitive kidney participate in the composition of the medullary substance of the ovary as medullary cords ; in the testis they unite with the seminal ampullae or seminal tubules and furnish the tubuli recti and the rete testis, consequently the initial part of the outlet for the semen.
24. The ovarian follicles are composed of a centrally located ovum, an envelope of follicular cells, and a vascular connective-tissue capsule (theca folliculi).
25. In Mammals the ovarian follicle is converted into a Graafian follicle by an increase in the number of follicular cells and by their secreting between them a follicular fluid. (Discus proligerus, membrana granulosa.) 26. The Graafian follicles, after the elimination of the mature ova into the abdominal cavity, become the so-called yellow bodies in the following manner : blood flows out of the ruptured blood-vessels into their cavities, and both the follicular cells left behind and the connective-tissue capsule undergo proliferation accompanied by an emigration of white blood-corpuscles (true and false corpora lutea).
27. The yellow bodies subsequently cause by their scar-like shrivelling the cicatriculae and callosities on the surface of old ovaries.
28. The canals and glands of the urogenital system, which are at first established in the same form in both sexes, are afterwards differently employed in the male and female and undergo a partial degeneration.
29. In the male the mesonephric duct becomes the vas deferens, in the female it becomes rudimentary (GARTNER'S duct, in many Mammals).
30. The Miillerian duct assumes in the male no function, and only inconspicuous remnants of it are left at its ends (hydatid of the epiclidymis and sinus prostaticus or uterus masculinus) ; in the female it becomes the efferent apparatus of the ovary, the anterior part the oviduct, the posterior part the uterus and vagina, the latter resulting from the fusion of the ducts of the opposite sides of the body as far as they are enclosed in the genital cord.
31. In the male the anterior portion of the primitive kidney (mesonephros) having united with the seminal tubules by means of the sexual cords persists as the epiclidymis ; the remainder degenerates into the paradidymis. In the female both parts degenerate into epoophoron and paroophoron, which correspond respectively to the epididymis and paradidymis of the male.
32. The sexual glands, which are originally established in the lumbar region, gradually move with their outlets downward toward the pelvic cavity. (Descensus testiculorum et ovariorum. Oblique course of the spermatic arteries and veins.)
33. In the migration of the sexual glands a role appears to be played by the inguinal ligament, which passes from the primitive kidney underneath the peritoneum to the inguinal region, penetrates through the wall of the abdomen, and ends in the skin of the genital ridges that surround the cloaca. (Gubernaculum Hunteri in the male ; round ligament and ligamentum ovarii of the female.)
34. The testis is received some time before birth into the scrotum, an appendage of the body-cavity ; the scrotum owes its origin to the fact that the peritoneum forms an evagination (processus vaginalis peritonei) through the wall of the abdomen into the genital ridges, and that afterwards the evagination is completely cut off from the body- cavity by the closure of the inguinal canal.
35. The layers of the scrotum or the envelopes of the testes correspond, in accordance with their development, to the separate layers of the body-wall, as is shown in the following comparative summary : Envelopes of the Testes. Wall of the Abdomen.
Scrotum with tunica dartos. Skin of the abdomen.
COOPER'S fascia. Superficial abdominal fascia.
Tunica vaginalis commnnis with Muscle-layer and fascia trans cremaster. versa abdominis.
Tunica vaginalis propria (parietal Peritoneum.
and visceral layers).
36. The external sexual organs are developed in man and woman from the same kinds of fundaments in the neighborhood of the cloaca.
37. The term cloaca is applied to a depression at the hinder end of the embryo, into which open the hind gut and the allantois, after the latter has received on the posterior face of its attenuated terminal part, the sinus urogenitalis the closely approximated Miillerian and mesonephric ducts.
38. The cloaca becomes divided by projecting folds, which unite to form the perinseum, into an anterior [ventral] and posterior [dorsal] portion, of which the former is the prolongation of the sinus urogenitalis, the latter the prolongation of the intestine (anus).
39. At the anterior margin of the cloaca, or, after completed separation, at the anterior rim of the sinus urogenitalis, there is found in both sexes the genital eminence, which bears along its under surface a groove flanked by the two genital folds ; the eminence, together with the opening lying under it (cloaca or sinus urogenitalis), is embraced by the genital ridges.
40. In the female the genital eminence remains small and becomes the clitoris, the genital folds become the labia minora, the genital ridges the labia majora; the sinus urogenitalis remains short and broad and represents the vestibulum, which receives the vagina (the end of the Miillerian ducts) and the external orifice of the allantois or urinary bladder, the female urethra.
41. In the male the genital eminence grows out to a great length as the male organ ; the genital folds close on their under surface to form a narrow canal, which appears as a prolongation of the narrow sinus urogenitalis, together with the latter is designated as the male urethra, and receives at its beginning the vas deferens and the uterus masculinus ; the two genital ridges, which increase in size for the reception of the testes, surround the roots of the male organ and unite to form the scrotum.
42. The following table gives a brief survey (1) of the comparable parts of the outer and inner sexual organs of the male and female, and (2) of their derivation from indifferent fundaments of the urogenital system in Mammals :
Male sexual parts.
Seminal ampulla; and seminal tubules.
() Epicliclymis with vete testis and tiibuli recti. (b) Paradidymis.
The common form from ivhich both arise.
(a) Anterior part with the sexual cords (sexual part). (&) Posterior part (the real mesonephric part).
Female sexual parts.
Ovarian follicle, Graafian follicle.
() Epoophoron with medullary cords of the ovary. (b) Paroophoron.
V.i- clef evens with seminal vesicles.
Kidney and uretev.
Hyclatid of epididyinis. Sinus prostaticus. (Utevus niasciilinus.) Gnbevnaculnm Hnnteri.
Male urethra (pavs pvostatica et membrauacea).
Pars cavevnosa nrethrpe.
The common form from which lioth arise.
Mesonepliric duct. Kidney and ureter.
Inguinal ligament of primitive kidney.
Genital eminence. ,, folds, ridges.
GARTNER'S canal, in some Mammals.
Kidney and ureter.
< )viduct and firnbviaj. Uterus and vagina.
Round ligament and lig. ovarii.
Clitoris. Labia minora. ,, majora.
The Development of the Suprarenal Bodies
43. The most anterior part of the mesonephros appears to share in the development of the suprarenal bodies, since lateral branches sprout out from the sexual cords, become detached, and are converted .into the peculiar cellular cords of the cortical substance.
44. The suprarenal bodies in the embryo for a time exceed in size the kidneys.
(1) Development of the Musculature.
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