The Works of Francis Balfour 3-23
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
- Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol. III. A Treatise on Comparative Embryology 2 (1885)
Chapter XXIII. Excretory Organs
Excretory organs consist of coiled or branched and often ciliated tubes, with an excretory pore opening on the outer surface of the body, and as a rule an internal ciliated orifice placed in the body-cavity. In forms provided with a true vascular system, there is a special development of capillaries around the glandular part of the excretory organs. In many instances the glandular cells of the organs are filled with concretions of uric acid or some similar product of nitrogenous waste.
There is a very great morphological and physiological similarity between almost all the forms of excretory organ found in the animal kingdom, but although there is not a little to be said for holding all these organs to be derived from some common prototype, the attempt to establish definite homologies between them is beset with very great difficulties.
Platyelminthes. Throughout the whole of the Platyelminthes these organs are constructed on a well-defined type, and in the Rotifera excretory organs of a similar form to those of the Platyelminthes are also present.
These organs (Fraipont, No. 513) are more or less distinctly paired, and consist of a system of wide canals, often united into a network, which open on the one hand into a pair of large tubes leading to the exterior, and on the other into fine canals which terminate by ciliated openings, either in spaces between the connective-tissue cells (Platyelminthes), or in the body-cavity (Rotifera). The fine canals open directly into the larger ones, without first uniting into canals of an intermediate size.
The two large tubes open to the exterior, either by means of a median posteriorly placed contractile vesicle, or by a pair of vesicles, which have a ventral and anterior position. The former type is characteristic of the majority of the Trematoda, Cestoda. and Rotifera, and the latter of the Nemertea and some Trematoda. In the Turbellaria the position of the external openings of the system is variable, and in a few Cestoda (Wagner) there are lateral openings on each of the successive proglottides, in addition to the terminal openings. The mode of development of these organs is unfortunately not known.
Mollusca. In the Mollusca there are usually present two independent pairs of excretory organs one found in a certain number of forms during early larval life only 1 , and the other always present in the adult.
The larval excretory organ has been found in the pulmonate Gasteropoda (Gegenbaur, Fol 2 , Rabl), in Teredo (Hatschek), and possibly also in Paludina. It is placed in the anterior region of the body, and opens ventrally on each side, a short way behind the velum. It is purely a larval organ, disappearing before the close of the veliger stage. In the aquatic Pulmonata, where it is best developed, it consists on each side of a V-shaped tube, with a dorsally-placed apex, containing an enlargement of the lumen. There is a ciliated cephalic limb, lined by cells with concretions, and terminating by an internal opening near the eye, and a nonciliated pedal limb opening to the exterior 3 .
Two irreconcilable views are held as to the development of this system. Rabl (Vol. II. No. 268) and Hatschek hold that it is developed in the mesoblast ; and Rabl states that in Planorbis it is formed from the anterior mesoblast cells of the mesoblastic bands. A special mesoblast cell on each side elongates into two processes, the commencing limbs of the future organ. A lumen is developed in this cell, which is continued into each limb, while
1 I leave out of consideration an external renal organ found in many marine Gasteropod larvte, vide Vol. II. p. 280.
2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. Hi. Archiv d. Zool. exfJr. et gener., Vol. VIII.
3 The careful observations of Fol seem to me nearly conclusive in favour of this limb having an external opening, and the statement to the reverse effect on p. 280 of Vol. ii. of this treatise, made on the authority of Rabl and Biitschli, must probably be corrected.
the continuations of the two limbs are formed by perforated mesoblast cells.
According to Fol these organs originate in aquatic Pulmonata as a pair of invaginations of the epiblast, slightly behind the mouth. Each invagination grows in a dorsal direction, and after a time suddenly bends on itself, and grows ventralwards and forwards. It thus acquires its V-shaped form.
In the terrestrial Pulmonata the provisional excretory organs are, according to Fol, formed as epiblastic invaginations, in the same way as those in the aquatic Pulmonata, but have the form of simple non-ciliated sacks, without internal openings.
The permanent renal organ of the Mollusca consists typically of a pair of tubes, although in the majority of the Gasteropoda one of the two tubes is not developed. It is placed considerably behind the provisional renal organ.
Each tube, in its most typical form, opens by a ciliated funnel into the pericardial cavity, and has its external opening at the side of the foot. The pericardial funnel leads into a glandular section of the organ, the lining cells of which are filled with concretions. This section is followed by a ciliated section, from which a narrow duct leads to the exterior.
As to the development of this organ the same divergence of opinion exists as in the case of the provisional renal organ.
Rabl's careful observations on Planorbis (Vol. II. No. 268) tend to shew that it is developed from a mass of mesoblast cells, near the end of the intestine. The mass becomes hollow, and, attaching itself to the epiblast on the left side of the anus, acquires an opening to the exterior. Its internal opening is not established till after the formation of the heart. Fol gives an equally precise account, but states that the first rudiment of the organ arises as a solid mass of epiblast cells. Lankester finds that this organ is developed as a paired invagination of the. epiblast in Pisidium, and Bobretzky also derives it from the epiblast in marine Prosobranchiata. In Cephalopoda on the other hand Bobretzky's observations (I conclude this from his figures) indicate that the excretory sacks of the renal organs are derived from the mesoblast.
Polyzoa. Simple excretory organs, consisting of a pair of ciliated canals, opening between the mouth and the anus, have
been found by Hatschek and Joliet in the Entoproctous Polyzoa, and are developed, according to Hatschek, by whom they were first found in the larva, from the mesoblast
Brachiopoda. One or rarely two (Rhynchonella) pairs of canals, with both peritoneal and external openings, are found in the Brachiopoda. They undoubtedly serve as genital ducts, but from their structure are clearly of the same nature as the excretory organs of the Chaetopoda described below. Their development has not been worked out.
Chaetopoda. Two forms of excretory organ have been met with in the Chaetopoda. The one form is universally or nearly universally present in the adult, and typically consists of a pair of coiled tubes repeated in every segment. Each tube has an internal opening, placed as a rule in the segment in front of that in which the greater part of the organ and the external opening are situated.
There are great variations in the structure of these organs, which cannot be dealt with here. It may be noted however that the internal opening may be absent, and that there may be several internal openings for each organ (Polynoe). In the Capitellidae moreover several pairs of excretory tubes have been shewn by Eisig (No. 512) to be present in each of the posterior segments.
The second form of excretory organ has as yet only been found in the larva of Polygordius, and will be more conveniently dealt with in connection with the development of the excretory system of this form.
There is still considerable doubt as to the mode of formation of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277), from his observations on the Oligochasta, holds that they develop as outgrowths of the epithelial layer covering the posterior side of the dissepiments, and secondarily become connected with the epidermis.
Hatschek finds that in Criodrilus they arise from a continuous linear thickening of the somatic mesoblast, immediately beneath the epidermis, and dorsal to the ventral band of longitudinal muscles. They break up into S-shaped cords, the anterior end of each of which is situated in front of a dissepiment, and is formed at first of a single large cell, while the posterior part is
continued into the segment behind. The cords are covered by a peritoneal lining, which still envelopes them, when in the succeeding stage they are carried into the body-cavity. They subsequently become hollow, and their hinder ends acquire openings to the exterior. The formation of their internal openings has not been followed.
Kleinenberg is inclined to believe that the excretory tubes take their origin from the epiblast, but states that he has not satisfactorily worked out their development.
The observations of Risig (No. 512) on the Capitellidae support Kowalevsky's view that the excretory tubes originate from the lining of the peritoneal cavity.
Hatschek (No. 514) has given a very interesting account of the development of the excretory system in Polygordius.
The excretory system begins to be formed, while the larva is still in the trochospere stage (fig. 383, npli), and consists of a provisional excretory organ, which is placed in front of the future segmented part of the body, and occupies a position very similar to that of the provisional excretory organ found in some Molluscan larvae (vide p. 68 1).
Hatschek, with some shew of reason, holds that the provisional excretory organs of Polygordius are homologous with those of the Mollusca.
In its earliest stage the provisional excretory organ of Polygordius consists of a pair of simple ciliated tubes, FIG. 383. POLYOORDIUS
, . , r 11-1 LARVA. (After Hatschek.)
each with an anterior funnel-like open- m _ moulh . ^ supraKBSO .
ing situated in the midst of the meSO- phageal ganglion ; nph. nephri11 11 . , dion ; ine.p. mesoblastic band;
blast cells, and a posterior external an _ anus 5 oL stomach . opening. The latter is placed immediately in front of what afterwards becomes the segmented region of the embryo. While the larva is still unsegmented, a second internal opening is formed for each tube (fig. 383, np/i) and the two openings so formed may eventually become divided into five (fig. 384 A), all communicating by a single pore with the exterior.
When the posterior region of the embryo becomes segmented,
paired excretory organs are formed in each of the posterior segments, but the account of their development, as given by Hatschek, is so remarkable that I do not think it can be definitely accepted without further confirmation.
From the point of junction of the two main branches of the larval kidney there grows backwards (fig. 384 B), to the hind end of the first segment, a very delicate tube, only indicated by its ciliated lumen, its walls not being differentiated. Near the front end of this tube a funnel, leading into the larval body cavity of the head, is formed, and subsequently the posterior end of the tube acquires an external opening, and the tube distinct walls. The communication with the provisional excretory organ is then lost, and thus the excretory tube of the first segment is established.
The excretory tubes in the second and succeeding segments are formed in the same way as in the first, i.e. by the continuation of the lumen of the hind end of the excretory tube from the preceding segment, and the subsequent separation of this part as a separate tube.
The tube may be continued with a sinuous course through
Y Y Y Y
FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY SYSTEM OF POLYGORDIUS. (After Hatschek.)
several segments without a distinct wall. The external and internal openings of the permanent excretory tubes are thus secondarily acquired. The internal openings communicate with the permanent body-cavity. The development of the perma
nent excretory tubes is diagrammatically represented in fig. 384 C and D.
The provisional excretory organ atrophies during larval life.
If Hatschek's account of the development of the excretory system of Polygordius is correct, it is clear that important secondary modifications must have taken place in it, because his description implies that there sprouts from the anterior excretory organ, while it has its own external opening, a posterior duct, which does not communicate either with the exterior or with the body-cavity! Such a duct could have no function. It is intelligible either (i) that the anterior excretory organ should lead into a longitudinal duct, opening posteriorly ; that then a series of secondary openings into the body-cavity should attach themselves to this, that for each internal opening an external should subsequently arise, and the whole break up into separate tubes ; or (2) that behind an anterior provisional excretory organ a series of secondary independent segmental tubes should be formed. But from Hatschek's account neither of these modes of evolution can be deduced.
Gephyrea. The Gephyrea may have three forms of excretory organs, two of which are found in the adult, and one, similar in position and sometimes also in structure, to the provisional excretory organ of Polygordius, has so far only been found in the larvae of Echiurus and Bonellia.
In all the Gephyrea the so-called 'brown tubes' are apparently homologous with the segmented excretory tubes of Chaetopods. Their main function appears to be the transportation of the generative products to the exterior. There is but a single highly modified tube in Bonellia, forming the oviduct and uterus ; a pair of tubes in the Gephyrea inermia, and two or three pairs in most Gephyrea armata, except Bonellia. Their development has not been studied.
In the Gephyrea armata there is always present a pair of posteriorly placed excretory organs, opening in the adult into the anal extremity of the alimentary tract, and provided with numerous ciliated peritoneal funnels. These organs were stated by Spengel to arise in Bonellia as outgrowths of the gut ; but in Echinrus Hatschek (No. 515) finds that they are developed from the somatic mesoblast of the terminal part of the trunk. They soon become hollow, and after attaching themselves to the epiblast on each side of the anus, acquire external openings. They are not at first provided with peritoneal funnels, but these parts of the organs become developed from a ring of cells at
their inner extremities ; and there is at first but a single funnel for each vesicle. The mode of increase of the funnels has not been observed, nor has it been made out how the organs themselves become attached to the hind-gut.
The provisional excretory organ of Echiurus is developed at an early larval stage, and is functional during the whole of larval life. It at first forms a ciliated tube on each side, placed in front of that part of the larva which becomes the trunk of the adult. It opens to the exterior by a fine pore on the ventral side, immediately in front of one of the mesoblastic bands, and appears to be formed of perforated cells. It terminates internally in a slight swelling, which represents the normal internal ciliated funnel. The primitively simple excretory organ becomes eventually highly complex by the formation of numerous branches, each ending in a slightly swollen extremity. These branches, in the later larval stages, actually form a network, and the inner end of each main branch divides into a bunch of fine tubes. The whole organ resembles in many respects the excretory organ of the Platyelminthes.
In the larva of Bonellia Spengel has described a pair of provisional excretory tubes, opening near the anterior end of the body, which are probably homologous with the provisional excretory organs of Echiurus (vide Vol. II., fig. 162 C, se).
Discophora. As in many of the types already spoken of, permanent and provisional excretory organs may be present in the Discophora. The former are usually segmentally arranged, and resemble in many respects the excretory tubes of the Chaetopoda. They may either be provided with a peritoneal funnel (Nephelis, Clepsine) or have no internal opening (Hirudo).
Bourne 1 has shewn that the cells surrounding the main duct in the medicinal Leech are perforated by a very remarkable network of ductules, and the structure of these organs in the Leech is so peculiar that it is permissible to state with due reserve their homology with the excretory organs of the Chaetopoda.
The excretory tubes of Clepsine are held by Whitman to be developed in the mesoblast.
1 "On the Structure of the Nephridia of the Medicinal Leech." Quart. J. of Micr. Science, Vol. XX. 1880.
There are found in the embryos of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which are not yet fully made out. In Nephelis they appear as one (according to Robin), or (according to Biitschli) as two successive pairs of convoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end of the U forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Fiirbringer is inclined from his own researches to believe that they open laterally. They contain a clear fluid.
In Hirudo, Leuckart has described three similar pairs of organs, the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement, from which a convoluted tube is continued for some distance backwards; the tube then turns forwards again, and after bending again upon itself opens to the exterior. The anterior part is broken up into a kind of labyrinthic network.
The provisional excretory organs of the Leeches cannot be identified with the anterior provisional organs of Polygordius and Echiurus.
Arthropoda. Amongst the Arthropoda Peripatus is the only form with excretory organs of the type of the segmental excretory organs of the Chsetopoda 1 .
These organs are placed at the bases of the feet, in the lateral divisions of the body-cavity, shut off from the main median division of the body-cavity by longitudinal septa of transverse muscles.
Each fully developed organ consists of three parts :
(i) A dilated vesicle opening externally at the base of a foot. (2) A coiled glandular tube connected with this, and subdivided again into several minor divisions. (3) A short terminal portion opening at one extremity into the coiled tube
1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis." Quart. J, of Micr. Science, Vol. XIX. 1879.
EXCRETORY ORGANS. 689
and at the other, as I believe, into the body cavity. This section becomes very conspicuous, in stained preparations, by the intensity with which the nuclei of its walls absorb the colouring matter.
In the majority of the Tracheata the excretory organs have the form of the so-called Malpighian tubes, which always (vide Vol. II.) originate as a pair of outgrowths of the epiblastic proctodaeum. From their mode of development they admit of comparison with the anal vesicles of the Gephyrea, though in the present state of our knowledge this comparison must be regarded as somewhat hypothetical.
The antennary and shell-glands of the Crustacea, and possibly also the so-called dorsal organ of various Crustacean larvae appear to be excretory, and the two former have been regarded by Claus and Grobben as belonging to the same system as the segmental excretory tubes of the Chaetopoda.
Nematoda. Paired excretory tubes, running for the whole length of the body in the so-called lateral line, and opening in front by a common ventral pore, are present in the Nematoda. They do not appear to communicate with the body cavity, and their development has not been studied.
Very little is known with reference either to the structure or development of excretory organs in the Echinodermata and the other Invertebrate types of which no mention has been so far made in this Chapter.
Excretory organs and generative ducts of the Craniata.
Although it would be convenient to separate, if possible, the history of the excretory organs from that of the generative ducts, yet these parts are so closely related in the Vertebrata, in some cases the same duct having at once a generative and a urinary function, that it is not possible to do so.
The excretory organs of the Vertebrata consist of three distinct glandular bodies and of their ducts. These are (i) a small glandular body, usually with one or more ciliated funnels opening into the body cavity, near the opening of which there projects into the body cavity a vascular glomerulus. It is situated very far forwards, and is usually known as the head 44
kidney, though it may perhaps be more suitably called, adopting Lankester's nomenclature, the pronepliros. Its duct, which forms the basis for the generative and urinary ducts, will be called the segmented duct.
(2) The Wolffian body, which may be also called the mesonepJiros. It consists of a series of, at first, segmentally (with a few exceptions) arranged glandular canals (segmental tubes) primitively opening at one extremity by funnel-shaped apertures into the body cavity, and at the other into the segmental duct. This duct becomes in many forms divided longitudinally into two parts, one of which then remains attached to the segmental tubes and forms the Wolffian or mesonepJiric duct, while the other is known as the Milllerian dnct.
(3) The kidney proper or metanephros. This organ is only found in a completely differentiated form in the amniotic Vertebrata. Its duct is an outgrowth from the Wolrfian duct.
The above parts do not coexist in full activity in any living adult member of the Vertebrata, though all of them are found together in certain embryos. They are so intimately connected that they cannot be satisfactorily dealt with separately.
Elasmobranchii. The excretory system of the Elasmobranchii is by no means the most primitive known, but at the same time it forms a convenient starting point for studying the modifications of the system in other groups. The most remarkable peculiarity it presents is the absence of a pronephros. The development of the Elasmobranch excretory system has been mainly studied by Semper and myself.
The first trace of the system makes its appearance as a knob of mesoblast, springing from the intermediate cell-mass near the level of the hind end of the heart (fig. 385 K,pd). This knob is the rudiment of the abdominal opening of the segmental duct, and from it there grows backwards to the level of the anus a solid column of cells, which constitutes the rudiment of the segmental duct itself (fig. 385 B, pd). The knob projects towards the epiblast, and the column connected with it lies between the mesoblast and epiblast. The knob and column do not long remain solid, but the former acquires an opening into the body cavity (fig. 421, sd) continuous with a lumen, which
makes its appearance in the column (fig. 386, sd). The knob forms the only structure which can be regarded as a rudiment of the pronephros.
FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL
The sections illustrate the development of the segmental duct (pd) or primitive duct of the pronephros. In A (the anterior of the two sections) this appears as a solid knob (pd) projecting towards the epiblast. In B is seen a section of the column which has grown backwards from the knob in A.
spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord; X. subnotochordal rod; mp. muscle-plate; mp' . specially developed portion of muscle-plate; ao. dorsal aorta ; pd. segmental duct ; so. somatopleure ; sp. splanchnopleure ; //. body cavity; ep. epiblast; al. alimentary canal.
While the lumen is gradually being formed, the segmental tubes of the mesonephros become established. They appear to arise as differentiations of the parts of the primitive lateral plates of mesoblast, placed between the dorsal end of the body cavity and the muscle-plate (fig. 386, st) 1 , which are usually known as the intermediate cell-masses.
The lumen of the segmental tubes, though at first very small, soon becomes of a considerable size. It appears to be established in the position of the section of the body cavity in the intermediate cell-mass, which at first unites the part of the body cavity in the muscle-plates with the permanent body cavity. The lumen of each tube opens at its lower end into the dorsal part of the body cavity (fig. 386, st}, and each tube curls obliquely
1 In my original account of the development I held these tubes to be invaginations of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of my original statement from his investigations on the chick ; and from a re-examination of my specimens he arrived at the results stated above, and which I am now myself inclined to adopt.
backwards round the inner and dorsal side of the segmental duct, near which it at first ends blindly.
One segmental tube makes its appearance for each somite (fig. 265), commencing with that immediately behind the abdominal opening of the segmental duct, the last tube being situated a few segments behind the anus. Soon after their formation the blind ends of the segmental tubes come in contact with, and open into the segmental duct, and each of them becomes divided into four parts. These are (i) a section carrying the peritoneal opening, known as the peritoneal funnel, (2) a dilated vesicle into which this opens, (3) a coiled tubulus proceeding from (2), and terminating in (4) a wider portion opening into the segmental duct. At the same time, or shortly before this, each segmental duct unites with and opens into one of the horns of the cloaca, and also retires from its primitive position between the epiblast and mesoblast, and assumes a position close to the epithelium lining the body cavity (fig. 380, sd}. The general features of the excretory organs at this period are diagrammatically represented in the woodcut (fig. 387). In this fig. pd is the segmental duct and o its abdominal opening; s.t points to the segmental tubes, the finer details of whose structure are not represented in the diagram. The mesonephros thus forms at this period an elongated gland composed of a series of isolated coiled tubes, one extremity of each of which opens into the body cavity, and the other into the segmental duct, which forms the only duct of the system, and communicates at its front end with the body cavity, and behind with the cloaca.
FIG. 386. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN
sp.c. spinal canal; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; nip, muscle-plate ; nip', inner layer of muscle-plate already converted into muscles ; Vr, rudiment of vertebral body ; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; v. subintestinal vein ; p.o. primitive generative cells.
EXCRETORY ORGANS. 693
The next important change concerns the segmental duct, which becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second duct in the male. The manner in which this takes place is diagrammatically represented in fig. 387 by the clear line x, and in transverse section in figs. 388 and 389. The resulting ducts are (i) the Wolffian duct or mesonephric duct (wd\ dorsally, which remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Miillerian duct in the female, and the rudiments of this duct in the male. In the
FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN
pd. segmental duct. It opens at o into the body cavity and at its other extremity into the cloaca; x. line along which the division appears which separates the segmental duct into the Wolffian duct above and the Miillerian duct below; s.t. segmental tubes. They open at one end into the body cavity, and at the other into the segmental duct.
female the formation of these ducts takes place (fig. 389) by a nearly solid rod of cells being gradually split off from the ventral side of all but the foremost part of the original segmental duct. This nearly solid cord is the Miillerian duct (pd}. A very small portion of the lumen of the original segmental duct is perhaps continued into it, but in any case it very soon acquires a wide lumen (fig. 389 A). The anterior part of the segmental duct is not divided, but remains continuous with the Mullerian duct, of which its anterior pore forms the permanent peritoneal opening 1 (fig. 387). The remainder of the segmental duct (after the loss of its anterior section, and the part split off from its ventral side) forms the Wolffian duct. The process of formation of these ducts in the male differs from that in the female chiefly
1 Five or six segmental tubes belong to the region of the undivided anterior part of the segmental duct, which forms the front end of the Mullerian duct ; but they appear to atrophy very early, without acquiring a definite attachment to the segmental duct.
in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Miillerian duct, being shorter,
FIG. 389. FOUR SECTIONS THROUGH THE ANTERIOR I'ART OF THE SEGMENTAL DUCT OF A FEMALE EMBRYO OF SCYLLIUM CANICULA.
The figure shews how the segmental duct becomes split into the Wolffian or mesonephric duct above, and Miillerian duct or oviduct below.
wd. Wolffian or mesonephric duct; od. Miillerian duct or oviduct ; sd. segmental duct.
FIG. 388. DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A
SCYLLIUM EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MlJLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.
me. medullary canal; mp. muscle-plate; ch. notochord; ao. aorta; cav. cardinal vein; st. segmental tube. On the left side the section passes through the opening of a segmental tube into the body cavity. On the right this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through; iv.d. Wolffian duct; m.d. Miillerian duct. The section is taken through the point where the segmental duct and Wolffian duct have just become separate; gr. the germinal ridge with the thickened germinal epithelium ; /. liver ; i. intestine with spiral valve.
and in the column of cells with which it is continuous being from the first incomplete.
The segmental tubes of the mesonephros undergo further important changes. The vesicle at the termination of each peritoneal funnel sends a bud forwards towards the preceding tubulus, which joins the fourth section of it close to the opening
into the Wolffian duct (fig. 390, px). The remainder of the vesicle becomes converted into a Malpighian body (mg}.
By the first of these changes 10^-4 M @W>f a tube is established connecting each pair of segments of the mesonephros, and though this tube is in part aborted (or only represented by a fibrous band) in the anterior part of the excretory organs in the adult, and most probably in the hinder part, yet it seems almost certain that the secondary and tertiary Malpighian bodies of the majority of segments are developed from its persisting blind end. Each of these
FIG. 390. LONGITUDINAL VERTICAL SECTION THROUGH PART OF THE MESONEPHROS OF AN EMBRYO OF SCYLLIUM.
The figure contains two examples of the budding of the vesicle of a segmental tube (which forms a Malpighian body in its own segment) to unite with the tubulus in the preceding segment close to its opening into the Wolffian (mesonephric) duct.
ge. epithelium of body-cavity; st. peritoneal funnel of segmental tube with its peritoneal opening; mg. Malpighian body; px. bud from Malphigian body uniting with preceding segment.
secondary and tertiary Malpighian bodies is connected with a convoluted tubulus (fig. 391, a.mg), which is also developed from the tube connecting each pair of segmental tubes, and therefore falls into the primary tubulus close to its junction with the
FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF A NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT. The figure shews a fibrous band passing from the primary to the secondary Malpighian bodies in two segments, which is the remains of the outgrowth from the primary Malpighian body.
sf.o. peritoneal funnel; p. ing. primary Malpighian body; a.mg. accessory Malpighian body; w.d. mesonephric (Wolffian) duct.
segmental duct. Owing to the formation of the accessory tubuli the segments of the mesonephros acquire a compound character.
The third section of each tubulus becomes by continuous growth, especially in the hinder segments, very bulky and convoluted.
The general character of a slightly developed segment of the mesonephros at its full growth may be gathered from fig. 391. It commences with (i) a peritoneal opening, somewhat oval in form (st.d) and leading directly into (2) a narrow tube, the segmental tube, which takes a more or less oblique course backwards, and, passing superficially to the Wolffian duct (w.d}, opens into (3) a Malpighian body (p.mg) at the anterior extremity of an isolated coil of glandular tubuli. This coil forms the third section of each segment, and starts from the Malpighian body. It consists of a considerable number of rather definite convolutions, and after uniting with tubuli from one, two, or more (according to the size of the segment) accessory Malpighian bodies (a.mg) smaller than the one into which the segmental tube falls, eventually opens by (4) a narrowish collecting tube into the Wolffian duct at the posterior end of the segment. Each segment is probably completely isolated from the adjoining segments, and never has more than one peritoneal funnel and one communication with the Wolffian duct.
Up to this time there has been no distinction between the anterior and posterior tubuli of the mesonephros, which alike open into the Wolffian duct. The collecting tubes of a considerable number of the hindermost tubuli (ten or eleven in Scyllium canicula), either in some species elongate, overlap, while at the same time their openings travel backward so that they eventually open by apertures (not usually so numerous as the separate tubes), on nearly the same level, into the hindermost section of the Wolffian duct in the female, or into the urinogenital cloaca, formed by the coalesced terminal parts of the Wolffian ducts, in the male; or in other species become modified, by a peculiar process of splitting from the Wolnian duct, so as to pour their secretion into a single duct on each side, which opens in a position corresponding with the numerous ducts of the other species (fig. 392). In both cases the modified posterior kidney-segments are probably equivalent to the per
EXCRETORY ORGANS. 697
manent kidney or metanephros of the amniotic Vertebrates, and for this reason the numerous collecting tubes or single collecting tube, as the case may be, will be spoken of as ureters. The anterior tubuli of the primitive excretory organ retain their early relation to the Wolffian duct, and form the permanent Wolffian body or mesonephros.
The originally separate terminal extremities of the Wolffian ducts always coalesce, and form a urinal cloaca, opening by a single aperture, situated at the extremity of the median papilla behind the anus. Some of the peritoneal openings of the segmental tubes in Scyllium, or in other cases all the openings, become obliterated.
In the male the anterior segmental tubes undergo remarkable modifications, and become connected with the testes. Branches appear to grow from the first three or four or more of them (though probably not from their peritoneal openings), which pass to the base of the testis, and there uniting into a longitudinal canal, form a network, and receive the secretion of the testicular ampullae (fig. 393, nf). These ducts, the vasa efferent ia, carry the semen to the Wolffian body, but before opening into the tubuli of this body they unite into a canal known as the longitudinal canal of the Wolffian body (l.c\ from which pass off ducts equal in number to the vasa efferentia, each of which normally ends in a Malpighian corpuscle. From the Malpighian corpuscles so connected there spring the convoluted tubuli, forming the generative segments of the Wolffian body, along which the semen is conveyed to the Wolffian duct (v.d). The Wolffian duct itself becomes much contorted and acts as vas deferens.
Figs. 392 and 393 are diagrammatic representations of the chief constituents of the adult urinogenital organs in the two sexes. In the adult female (fig. 392), there are present the following parts :
(1) The oviduct or Mullerian duct (m.d) split off from the segmental duct of the kidneys. Each oviduct opens at its anterior extremity into the body cavity, and behind the two oviducts have independent communications with the general cloaca.
(2) The mesonephric ducts (w.d), the other product of the
segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anterior persisting segment of the mesonephros on each side, and unite behind to
FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS
IN AN ADULT FEMALE ELASMOBRANCH.
m.d. Miillerian duct; w.d. Wolffian duct; s.t. segmental tubes; five of them are represented with openings into the body cavity, the posterior segmental tubes form the mesonephros ; ov. ovary.
open by a common papilla into the cloaca. The mesonephric duct receives the secretion of the anterior tubuli of the primitive mesonephros.
(3) The ureter which carries off the secretion of the kidney proper or metanephros. It is represented in my diagram in its most rare and differentiated condition as a single duct connected with the posterior segmental tubes.
(4) The segmental tubes (.$-./) some of which retain their
FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS
IN AN ADULT MALE ELASMOBRANCH.
m.d. rudiment of Miillerian duct; w.d. Wolffian duct, marked vd in front and serving as vas deferens; s.t. segmental tubes; two of them are represented with openings into the body cavity; d. ureter; /. testis; nt. canal at the base of the testis; VE, vasa efferentia; Ic. longitudinal canal of the Wolffian body.
EXCRETORY ORGANS. 699
original openings into the body cavity, and others are without them. They are divided into two groups, an anterior forming the mesonephros or Wolffian body, which pours its secretion into the Wolffian duct ; and a posterior group forming a gland which is probably equivalent to the kidney proper of amniotic Craniata, and is connected with the ureter.
In the male the following parts are present (fig. 393):
(1) The Mlillerian duct (m.d], consisting of a small rudiment attached to the liver, representing the foremost end of the oviduct of the female.
(2) The mesonephric duct (w.d] which precisely corresponds to the mesonephric duct of the female, but, in addition to serving as the duct of the Wolffian body, also acts as a vas deferens (vd}. In the adult male its foremost part has a very tortuous course.
(3) The ureter (d\ which has the same fundamental constitution as in the female.
(4) The segmental tubes (s.t). The posterior tubes have the same arrangement in both sexes, but in the male modifications take place in connection with the anterior tubes to fit them to act as transporters of the semen.
Connected with the anterior tubes there are present (i) the vasa efferentia (VE], united on the one hand with (2) the central canal in the base of the testis (/), and on the other with the longitudinal canal of the Wolffian body (/<?). From the latter are seen passing off the successive tubuli of the anterior segments of the Wolffian body, in connection with which Malpighian bodies are typically present, though not represented in my diagram.
Apart from the absence of the pronephros the points which deserve notice in the Elasmobranch excretory system are (i) The splitting of the segmental duct into Wolffian (mesonephric) and Mullerian ducts. (2) The connection of the former with the mesonephros, and of the latter with the abdominal opening of the segmental duct which represents the pronephros of other types. (3) The fact that the Mullerian duct serves as oviduct, and the Wolffian duct as vas deferens. (4) The differentiation of a posterior section of the mesonephros into a special gland foreshadowing the metanephros of the Amniota.
Cyclostomata. The development of the excretory system amongst the Cyclostomata has only been studied in Petromyzon (Miiller, Furbringer, and Scott).
The first part of the system developed is the segmental duct. It appears in the embryo of about 14 days (Scott) as a solid cord of cells, differentiated from the somatic mesoblast near the dorsal end of the body cavity. This cord is at first placed immediately below the epiblast, and grows backwards by a continuous process of differentiation of fresh mesoblast cells. It soon acquires a lumen, and joins the cloacal section of the alimentary tract before the close of foetal life. Before this communication is established, the front end of the duct sends a process towards the body cavity, the blind end of which acquires a ciliated opening into the latter. A series of about four or five successively formed outgrowths from the duct, one behind the other, give rise to as many ciliated funnels opening into the body cavity, and each communicating by a more or less elongated tube with the segmental duct. These funnels, which have a metameric arrangement, constitute the pronephros, the whole of which is situated in the pericardial region of the body cavity.
On the inner side of the peritoneal openings of each pronephros there is formed a vascular glomerulus, projecting into the body cavity, and covered by peritoneal epithelium. For a considerable period the pronephros constitutes the sole functional part of the excretory system.
A mesonephros is formed (Furbringer) relatively late in larval life, as a segmentally arranged series of solid cords, derived from the peritoneal epithelium. These cords constitute the rudiments of the segmental tubes. They are present for a considerable portion of the body cavity, extending backwards from a point shortly behind the pronephros. They soon separate from the peritoneal epithelium, become hollowed out into canals, and join the segmental duct. At their blind extremity (that originally connected with the peritoneal epithelium) a Malpighian body is formed.
The pronephros is only a provisional excretory organ, the atrophy of which commences during larval life, and is nearly completed when the Ammoccete has reached 180 mm. in length.
EXCRETORY ORGANS. 70 1
Further changes take place in connection with the excretory system on the conversion of the Ammoccete into the adult.
The segmental ducts in the adult fall into a common urinogenital cloaca, which opens on a papilla behind the anus. This cloaca also communicates by two apertures (abdominal pores) with the body cavity. The generative products are carried into the cloaca by these pores ; so that their transportation outwards is not performed by any part of the primitive urinary system. The urinogenital cloaca is formed by the separation of the portion of the primitive cloaca containing the openings of the segmental ducts from that connected with the alimentary tract.
The mesonephros of the Ammoccete undergoes at the metamorphosis complete atrophy, and is physiologically replaced by a posterior series of segmental tubes, opening into the hindermost portion of the segmental duct (Schneider).
In Myxine the excretory system consists (i) of a highly developed pronephros with a bunch of ciliated peritoneal funnels opening into the pericardial section of the body cavity. The coiled and branched tubes of which the pronephros is composed open on the ventral side of the anterior portion of the segmental duct, which in old individuals is cut off from the posterior section of the duct. On the dorsal side of the portion of the segmental duct belonging to the pronephros there are present a small number of diverticula, terminating in glomeruli : they are probably to be regarded as anterior segmental tubes. (2) Of a mesonephros, which commences a considerable distance behind the pronephros, and is formed of straight extremely simple segmental tubes opening into the segmental duct (fig. 385).
The excretory system of Myxine clearly retains the characters of the system as it exists in the larva of Petromyzon.
Teleostei. In most Teleostei the pronephros and mesonephros coexist through life, and their products are carried off by a duct, the nature of which is somewhat doubtful, but which is probably homologous with the mesonephric duct of other types.
The system commences in the embryo (Rosenberg, Oellacher, Gotte, Furbringer) with the formation of a groove-like fold of the somatic layer of peritoneal epithelium, which becomes gradually constricted into a canal; the process of constriction commencing in the middle and extending in both directions. The canal does not however close anteriorly, but remains open to the body cavity, thus giving rise to a funnel equivalent to the pronephric funnels of Petromyzon and Myxine. On the inner side of this
funnel there is formed a glomerulus, projecting into the body
cavity ; and at the same time that
this is being formed the anterior end
of the canal becomes elongated and
convoluted. The above structures
constitute a pronephros, while the
posterior part of the primitive canal
forms the segmental duct.
The portion of the body cavity with the glomerulus and peritoneal funnel of the pronephros (fig. 395, po) soon becomes completely isolated from the remainder, so as to form a closed cavity (gl). The development of the mesonephros does not take place till long after that of the pronephros. The segmental tubes which form it are stated by Fiirbringer to arise from solid ingrowths of peritoneal epithelium, developed successively from before backwards, but Sedgwick informs me that they arise as differentiations of the mesoblastic cells near the peritoneal epithelium. They soon become hollow, and unite with the segmental duct. Malpighian bodies are developed on their median portions. They grow very greatly in length, and become much convoluted, but the details of this process have not been followed out.
The foremost segmental tubes are situated close behind the pronephros, while the hindermost are in many cases developed in the post-anal continuations of the body cavity. The pronephros appears to form the swollen cephalic portion of the kidney of the adult, and the mesonephros the remainder ; the so-called caudal portion, where present, being derived (?) from the postanal segmental tubes.
In some cases the cephalic portion of the kidneys is absent
FIG. 394. PORTIONS OF THE MESONEPHROS OF MYXINE. (From Gegenbaur; after J. Miiller.)
a. segmental duct ; b. segmental tube; c. glomerulus ; d. afferent, e. efferent artery.
B represents a portion of A highly magnified.
EXCRETORY ORGANS. 703
in the adult, which probably implies the atrophy of the pronephros ; in other instances the cephalic portion of the kidneys is the only part developed. Its relation to the embryonic proncphros requires however further elucidation.
In the adult the ducts in the lower part of the kidneys lie as a rule on their outer borders, and almost invariably open into a
FIG. 395. SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT PARTS TEN DAYS BEFORE HATCHING.
pr.n. pronephros ; po. opening of pronephros into the isolated portion of the body cavity containing the glomerulus ; gl. glomerulus ; ao. aorta ; ch. notochord ; x. subnotochordal rod ; al. alimentary tract.
urinary bladder, which usually opens in its turn on the urinogenital papilla immediately behind the genital pore, but in a few instances there is a common urinogenital pore.
In most Osseous Fish there are true generative ducts continuous with the investment of the generative organs. It appears to me most probable, from the analogy of Lepidostcus, to be described in the next section, that these ducts are split off from the primitive segmental duct, and correspond with the Miillerian ducts of Elasmobranchii, etc. ; though on this point we have at present no positive embryological evidence (vide general considerations at the end of the Chapter). In the female Salmon and the male and female Eel the generative products are carried to the exterior by abdominal pores. It is possible that this may represent a primitive condition, though it
is more probably a case of degeneration, as is indicated by the presence of ducts in the male Salmon and in forms nearly allied to the Salmonidae.
The coexistence of abdominal pores and generative ducts in Mormyrus appears to me to demonstrate that the generative ducts in Teleostei cannot be derived from the coalescence of the investment of the generative organs with the abdominal pores.
Ganoidei. The true excretory gland of the adult Ganoidei resembles on the whole that of Teleostei, consisting of an elongated band on each side the mesonephros an anterior dilatation of which probably represents the pronephros.
There is in both sexes a Mullerian duct, provided, except in Lepidosteus, with an abdominal funnel, which is however situated relatively very far back in the abdominal cavity. The Mullerian ducts appear to serve as generative canals in both sexes. In Lepidosteus they are continuous with the investment of the generative glands, and thus a relation between the generative ducts and glands, very similar to that in Teleostei, is brought about.
Posteriorly the Mullerian ducts and the ducts of the mesonephros remain united. The common duct so formed on each side is clearly the primitive segmental duct. It receives the secretion of a certain number of the posterior mesonephric tubules, and usually unites with its fellow to form a kind of bladder, opening by a single pore into the cloaca, behind the anus. The duct which receives the secretion of the anterior mesonephric tubules is the true mesonephric or Wolffian duct.
The development of the excretory system, which has been partially worked out in Acipenscr and Lepidosteus 1 , is on the whole very similar to that in the Teleostei. The first portion of the system to
FIG. 396. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION.
me. medullary cord ; ms. mesoblast ; sg. segmental duct ; ch. notochord ; .r. subnotochordal rod; hy. hypoblast.
1 Acipenser has been investigated by Fiirbringer, Salensky, Sedgwick, and also by myself, and Lepidosteus by W. N. Parker and myself.
be formed is the segmental duct. In Lepidosteus this duct is formed as a groove-like invagination of the somatic peritoneal epithelium, precisely as in Teleostei, and shortly afterwards forms a duct lying between the mesoblast and the epiblast (fig. 396, sg}. In Acipenser (Salensky) however it is formed as
FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER
EMBRYO. (After Salensky.)
Rf. medullary groove ; Alp. medullary plate ; Wg. segmental duct ; Ch. notochord ; En. hypoblast ; Sgp. mesoblastic somite ; Sp. parietal part of mesoblastic plate.
a solid ridge of the somatic mesoblast, as in Petromyzon and Elasmobranchii (fig. 397, Wg).
In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed. This gland is provided with but one 1 peritoneal opening, which together with the glomerulus belonging to it becomes encapsuled in a special section of the body cavity. The opening of the pronephros of Acipenser into this cavity is shewn in fig. ^<^>,pr.n. At this early stage of Acipenser (larva of 5 mm.) I could find no glomerulus.
The mesonephros is formed some distance behind, and some time after the pronephros, both in Acipenser and Lepidosteus, so that in the larvae of both these genera the pronephros is for a considerable period the only excretory organ. In Lepidosteus especially the development of the mesonephros occurs very late.
The development of the mesonephros has not been worked out in Lepidosteus, but in Acipenser the anterior segmental tubes become first established as (I believe) solid cords of cells, attached at one extremity to the peritoneal epithelium on each
1 I have not fully proved this point, but have never found more than one opening.
side of the insertion of the mesentery, and extending upwards and outwards round the segmental duct 1 . The posterior segmental tubes arise later than the anterior, and (as far as can be determined from the sections in my possession) they are formed independently of the peritoneal epithelium, on the dorsal side of the segmental duct.
In later stages (larvae of 7 10 mm.) the anterior segmental tubes gradually lose their attachment to the peritoneal epithelium. The extremity near the peritoneal epithelium forms a Malpighian body, and the other end unites with the segmental duct. At a still later stage wide peritoneal funnels are es
FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A
LARVA OF ACIPENSER 5 MM. IN LENGTH.
st. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnotochordal rod; pr.n. pronephros ; ao. aorta; mf. muscle-plate formed of large cells, the outer parts of which are differentiated into contractile fibres ; sp.c. spinal cord ; b.c. body cavity.
tablished, for at any rate a considerable number of the tubes, leading from the body cavity to the Malpighian bodies. These
1 Whether the segmental tubes are formed as ingrowths of the peritoneal epithelium, or in situ, could not be determined.
EXCRETORY ORGANS. 707
funnels have been noticed by Furbringer, Salensky and myself, but their mode of development has not, so far as I know, been made out. The funnels appear to be no longer present in the adult. The development of the Mullerian ducts has not been worked out.
Dipnoi. The excretory system of the Dipnoi is only known in the adult, but though in some respects intermediate in character between that of the Ganoidei and Amphibia, it resembles that of the Ganoidei in the important feature of the Mullerian ducts serving as genital ducts in both sexes.
Amphibia. In Amphibia (Gotte, Furbringer) the development of the excretory system commences, as in Teleostei, by the formation of the segmental duct from a groove formed by a fold of the somatic layer of the peritoneal epithelium, near the dorsal border of the body cavity (fig. 399, u). The anterior end of the groove is placed immediately behind the branchial region. Its posterior part soon becomes converted into a canal by a constriction which commences a short way from the front end of the groove, and thence extends backwards. This canal at first ends blindly close to the cloaca, into which however it soon opens.
The anterior open part of the groove in front of the constriction (fig. 399, n] becomes differentiated into a longitudinal duct, which remains in open communication with the body cavity by two (many Urodela) three (many Anura) or four (Cceciliidae) canals. This constitutes the dorsal part of the pronephros. The ventral part of the gland is formed from the section of the duct immediately behind the longitudinal canal. This part grows in length, and, assuming an S-shaped curvature, becomes placed on the ventral side of the first formed part of the pronephros. By continuous growth in a limited space the convolutions of the canal of the pronephros become more numerous, and the complexity of the gland is further increased by the outgrowth of blindly ending diverticula.
At the root of the mesentery, opposite the peritoneal openings of the pronephros, a longitudinal fold, lined by peritoneal epithelium, and attached by a narrow band of tissue, makes its appearance. It soon becomes highly vascular, and constitutes a glomerulus homologous with that in Petromyzon and Teleostei.
The section of the body cavity which contains the openings of the pronephros and the glomerulus, becomes dilated, and then temporarily shut off from the remainder. At a later period it forms a special though not completely isolated compartment. For a long time the pronephros and its duct form the only excretory organs of larval Amphibia. Eventually however the formation of the mesonephros commences, and is followed by the atrophy of the pronephros. The mesonephros is composed, as in other types, of a series of segmental tubes, but these, except in Cceciliidae, no longer correspond in number with the myotomes, but are in all instances more numerous. Moreover, in the posterior part of the mesonephros in the Urodeles, and through the whole length of the gland in other types, secondary and tertiary segmental tubes are formed in addition to the primary tubes.
FIG. 399. TRANSVERSE SECTION THROUGH A VERY YOUNG TADPOLE OF BOMBINATOR AT THE LEVEL OF THE ANTERIOR END OF THE YOLK-SACK. (After
a. fold of epiblast continuous with the dorsal fin; is", neural cord; m. lateral muscle; as 1 . outer layer of muscle-plate; s. lateral plate of mesoblast ; b. mesentery ; u. open end of the segmental duct, which forms the pronephros ; f. alimentary tract ; f. ventral diverticulum which becomes the liver; e. junction of yolk cells and hypoblast cells ; d. yolk cells.
The development of the mesonephros commences in Salamandra (Fiirbringer) with the formation of a series of solid cords, which in the anterior myotomes spring from the peritoneal epithelium on the inner side of the segmental duct, but posteriorly arise independently of this epithelium in the adjoining mesoblast. Sedgwick informs me that in the
Frog the segmental tubes are throughout developed in the mesoblast, independently of the peritoneal epithelium. These cords next become detached from the peritoneal epithelium (in so far as they are primitively united to it), and after first assuming a vesicular form, grow out into coiled tubes, with a median limb the blind end of which assists in forming a Malpighian body, and a lateral limb which comes in contact with and opens into the segmental duct, and an intermediate portion connecting the two. At the junction of the median with the intermediate portion, and therefore at the neck of the Malpighian body, a canal grows out in a ventral direction, which meets the
EXCRETORY ORGANS. 709
peritoneal epithelium, and then develops a funnel-shaped opening into the body cavity, which subsequently becomes ciliated. In this way the peritoneal funnels which are present in the adult are established.
The median and lateral sections of the segmental tubes become highly convoluted, and the separate tubes soon come into such close proximity that their primitive distinctness is lost.
The first fully developed segmental tube is formed in Salamandra maculata in about the sixth myotome behind the pronephros. But in the region between the two structures rudimentary segmental tubes are developed.
The number of primary segmental tubes in the separate myotomes of Salamandra is as follows :
In the 6th myotome (i.e. the first with a true
segmental tube) 12 segmental tubes
yth roth myotome 23
IIth ... 34
I2th 3 4 or 4 5
I3th y> 45
1 3th i6th 56
It thus appears that the segmental tubes are not only more numerous than the myotomes, but that the number in each myotome increases from before backwards. In the case of Salamandra there are formed in the region of the posterior (10 16) myotomes secondary, tertiary, etc. segmental tubes out of independent solid cords, which arise in the mesoblast dorsally to the tubes already established.
The secondary segmental tubes appear to develop out of these cords exactly in the same way as the primary ones, except that they do not join the segmental duct directly, but unite with the primary segmental tubes shortly before the junction of the latter with the segmental duct. In this way compound segmental tubes are established with a common collecting tube, but with numerous Malpighian bodies and ciliated peritoneal openings. The difference in the mode of origin of these compound tubes and of those in Elasmobranchii is very striking.
The later stages in the development of the segmental tubes have not been studied in the other Amphibian types.
In Cceciliidas the earliest stages are not known, but the tubes present in the adult (Spengel) a truly segmental arrangement, and in the young each of them is single, and provided with only a single peritoneal funnel. In the adult however many of the segmental organs become compound, and may have as many as twenty funnels, etc. Both simple and compound segmental tubes occur in all parts of the mesonephros, and are arranged in no definite order.
In the Anura (Spengel) all the segmental tubes are compound, and an enormous number of peritoneal funnels are present on the ventral surface, but it has not yet been definitely determined into what part of the segmental tubes they open.
Before dealing with the further changes of the Wolffian body it is necessary to return to the segmental duct, which, at the time when the pronephros is undergoing atrophy, becomes split into a dorsal Wolffian and ventral Mullerian duct. The process in Salamandra (Fiirbringer) has much the same character as in Elasmobranchii, the Mullerian duct being formed by the gradual separation, from before backwards, of a solid row of cells from the ventral side of the segmental duct, the remainder of the duct constituting the Wolffian duct. During the formation of the Mullerian duct its anterior part becomes hollow, and attaching itself in front to the peritoneal epithelium acquires an opening into the body cavity. The process of hollowing is continued backwards pari passu with the splitting of the segmental duct. In the female the process is continued till the Mullerian duct opens, close to the Wolffian duct, into the cloaca. In the male the duct usually ends blindly. It is important to notice that the abdominal opening of the Mullerian duct in the Amphibia (Salamandra) is a formation independent of the pronephros, and placed slightly behind it ; and that the undivided anterior part of the segmental duct (with the pronephros) is not, as in Elasmobranchii, united with the Mullerian duct, but remains connected with the Wolffian duct.
The development of the Mullerian duct has not been satisfactorily studied in other forms besides Salamandra. In Cceciliidae its abdominal opening is on a level with the anterior end of the Wolffian body. In other forms it is usually placed very far forwards, close to the root of the lungs (except in Proteus and Batrachoseps, where it is placed somewhat further back), and some distance in front of the Wolffian body.
The Mullerian duct is always well developed in the female, and serves as oviduct. In the male it does not (except possibly in Alytes) assist in the transportation of the genital products, and is always more or less rudimentary, and in Anura may be completely absent.
After the formation of the Mullerian duct, the Wolffian duct remains as the excretory channel for the Wolffian body, and, till the atrophy of the pronephros, for this gland also. Its anterior section, in front of the Wolffian body, undergoes a more or less complete atrophy.
The further changes of the excretory system concern (i) the junction in the male of the anterior part of the Wolffian body with the testis ; (2) certain changes in the collecting tubes of the
posterior part of the mesonephros. The first of these processes results in the division of the Wolffian body into a sexual and a non-sexual part, and in Salamandra and other Urodeles the division corresponds with the distribution of the simple and compound segmental tubes.
Since the development of the canals connecting the testes with the sexual part of the Wolffian body has not been in all points satisfactorily elucidated, it will be convenient to commence with a description of the adult arrangement of the parts (fig. 400 B). In most instances a non-segmental system of canals the vasa effcrentia (ve) coming from the testis, fall into a canal known as the longitudinal canal of the Wolffian body, from which there pass off transverse canals, which fall into, and are equal in number to, the primary Malpighian bodies of the sexual part of the gland. The spermatozoa, brought to the Malpighian bodies, are thence transported along the segmental tubes to the Wolffian duct, and so to the exterior. The system of canals connecting the testis with the Malpighian bodies is known as the testicular network. The number of segmental tubes connected with the testis varies very greatly. In Siredon there are as many as from 30 32 (Spengel).
The longitudinal canal of the Wolffian body is in rare instances (Spelerpes, etc.) absent, where the sexual part of the Wolffian body is slightly developed. In the Urodela the testes are united with the anterior part of the Wolffian body. In the Cceciliidas the junction takes place in an homologous part of the Wolffian body, but, owing to the development of the anterior segmental tubes, which are rudimentary in the Urodela, it is situated some way behind the front end. Amongst the Anura the connection of the testis with the tubules of the Wolffian body is subject to considerable variations. In Bufo cinereus the normal Urodele type is preserved, and in Bombinator the same arrangement is found in a rudimentary condition, in that there are transverse trunks from the longitudinal canal of the Wolffian body, which end blindly, while the semen is carried into the Wolffian duct by canals in front of the Wolffian body. In Alytes and Discoglossus the semen is carried away by a similar direct continuation of the longitudinal canal in front of the Wolffian body, but there are no rudimentary transverse canals passing into the Wolffian body, as in Bombinator. In Rana the transverse ducts which pass off from the longitudinal canal of the Wolffian body, after dilating to form (?) rudimentary Malpighian bodies, enter directly into the collecting tubes near their opening into the Wolffian duct.
In most Urodeles the peritoneal openings connected with the primary generative Malpighian bodies atrophy, but in Spelerpes they persist. In the Cceciliidie they also remain in the adult state.
With reference to the development of these parts little is known except that the testicular network grows out from the primary Malpighian bodies, and becomes united with the testis. Embryological evidence, as well as the fact of the persistence of the peritoneal funnels of the generative region in the adults of some forms, proves that the testicular network is not developed from the peritoneal funnels.
Rudiments of the testicular network are found in the female Cceciliidae and in the females of many Urodela (Salamandra, Triton). These rudiments may in their fullest development consist of a longitudinal canal and of transverse canals passing from this to the Malpighian bodies, together with some branches passing into the mesovarium.
Amongst the Urodela the collecting tubes of the hinder non-sexual part of the Wolffian body, which probably represents a rudimentary metanephros, undergo in the male sex a change similar to that which they usually undergo in Elasmobranchii. Their points of junction with the Wolffian duct are carried back to the hindermost end of the duct (fig. 400 B), and the collecting tubes themselves unite together into one or more short ducts (ureters) before joining the Wolffian duct.
In Batrachoseps only the first collecting tube becomes split off in this way ; and it forms a single elongated ureter which receives all the collecting tubes of the posterior segmental tubes. In the female and in the male of Proteus, Menobranchus, and Siren the collecting tubes retain their primitive transverse course and open laterally into the Wolffian duct. In rare cases (Ellipsoglossus, Spengel} the ureters open directly into the cloaca.
The urinary bladder of the Amphibia is an outgrowth of the ventral wall of the cloacal section of the alimentary tract, and is homologous with the allantois of the amniotic Vertebrata.
The subjoined diagram (fig. 400) of the urogenital system of Triton illustrates the more important points of the preceding description.
In the female (A) the following parts are present :
(1) The Mullerian duct or oviduct (od) derived from the splitting of the segmental duct.
(2) The Wolffian duct (sug) constituting the portion of the segmental duct left after the formation of the Mullerian duct.
(3) The mesonephros (r), divided into an anterior sexual part
connected with a rudimentary testicular network, and a posterior part. The collecting tubes from both parts fall transversely into the Wolffian duct.
(4) The ovary (ov).
(5) The rudimentary testicular network.
In the male (B) the following parts are present :
(1) The functionless though fairly developed Miillerian duct (;).
(2) The Wolffian duct (sug).
(3) The mesonephros (r) divided into a true sexual part, through the segmental tubes of which the semen passes, and a non-sexual part. The collecting tubes of the latter do not enter the Wolffian duct directly, but bend obliquely backwards and only fall into it close to its cloacal aperture, after uniting to form one or two primary tubes (ureters).
(4) The testicular network (ve) consisting of (i) transverse ducts from the testes, falling into (2) the longitudinal canal of the Wolffian body, from which (3) transverse canals are again given off to the Malpighian bodies.
Amniota. The amniotic Vertebrata agree, so far as is known, very closely amongst themselves in the formation of the urinogenital system.
The most characteristic feature of the system is the full development of a metanephros, which constitutes the functional kidney on the atrophy of the mesonephros or Wolffian body, which is a purely embryonic organ. The first part of the system to develop is a duct, which is usually spoken of as the Wolffian duct, but which is really the homologue of the seg
FIG. 400. DIAGRAM OF THE URINOGENITAL SYSTEM OF TRITON. (From Gegenbaur ; after Spengel.)
A. Female. B. Male. r. mesonephros, on the surface of which numerous peritoneal funnels are visible ; sug. mesonephric or Wolffian duct; od. oviduct (Miillerian duct); in. Miillerian duct of male ; ve. vasa efferentia of testis ; t. testis ; ov. ovary ; up. urinogenital pore.
mental duct. It apparently develops in all the Amniota nearly on the Elasmobranch type, as a solid rod, primarily derived from the somatic mesoblast of the intermediate cell mass (fig. 401 W.d}\
The first trace of it is visible in an embryo Chick with eight somites, as a ridge projecting from the intermediate cell mass towards the epiblast in the region of the seventh somite. In the course of further development it continues to constitute such a ridge as far as the eleventh somite (Sedgwick), but from this point it grows backwards in the space between the epiblast and mesoblast In an embryo with fourteen somites a small lumen has appeared in its middle part and in front it is connected with rudimentary Wolffian tubules, which develop in continuity with it (Sedgwick). In the succeeding stages the lumen of the duct gradually extends backwards and forwards, and the duct itself also passes inwards relatively to the epiblast (fig. 402). Its hindend elongates till it comes into connection with, and opens into, the cloacal section of the hind-gut' 2 .
It might have been anticipated that, as in the lower types, the anterior end of the segmental duct would either open into the body cavity, or come into connection with a pronephros. Neither of these occurrences takes place, though in some types (the Fowl) a structure, which is probably the rudiment of a pronephros, is developed ; it does not however appear till a later stage, and is then unconnected with the segmental duct. The next part of the system to appear is the mesonephros or Wolffian body.
This is formed in all Amniota as a series of segmental tubes, which in Lacertilia (Braun) correspond with the myotomes, but in Birds and Mammalia are more numerous.
In Reptilia (Braun, No. 542), the mesonephric tubes develop as segmentally-arranged masses on the inner side of the Wolffian duct, and appear to be at first united with the peritoneal epithelium. Each mass soon becomes an oval vesicle, probably opening for a very short period into the
1 Dansky and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as developing from a groove opening to the peritoneal cavity, which subsequently becomes constricted into a duct. I have never met with specimens such as those figured by these authors.
2 The foremost extremity of the segmental duct presents, according to Gasser, curious irregularities and an anterior completely isolated portion is often present.
peritoneal cavity by a peritoneal funnel. The vesicles become very early detached from the peritoneal epithelium, and lateral outgrowths from them give rise to the main parts of the segmental tubes, which soon unite with the segmental duct.
In Birds the development of the segmental tubes is more complicated 1 .
The tubules of the Wolffian body are derived from the intermediate cell mass, shewn in fig. 401, between the upper end of the body cavity and the
FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN
EMBRYO CHICK OF 45 HOURS.
M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ; p.p. pleuroperitoneal cavity ; ch. notochord ; op. boundary of area opaca; v. bloodvessel.
muscle-plate. In the Chick the mode of development of this mass into the segmental tubules is different in the regions in front of and behind about the sixteenth segment. In front of about the sixteenth segment the intermediate cell mass becomes detached from the peritoneal epithelium at certain points, remaining attached to it at other points, there being several such to each segment. The parts of the intermediate cell mass attached to the peritoneal epithelium become converted into S-shaped cords (fig. 402, st] which soon unite with the segmental duct (wd}. Into the commencement of each of these cords the lumen of the body cavity is for a short distance prolonged, so that this part constitutes a rudimentary peritoneal funnel.
1 Correct figures of the early stages of these structures were first given by Kolliker, but the correct interpretation of them and the first satisfactory account of the development of the excretory organs of Birds was given by Sedgwick (No. 549).
In the Duck the attachment of the intermediate cell mass to the peritoneal epithelium is prolonged further back than in the Chick.
In the foremost segmental tubes, which never reach a very complete development, the peritoneal funnels widen considerably, while at the same time they acquire a distinct lumen. The section of the tube adjoining the wide peritoneal funnel becomes partially invaginated by the formation of a glomerulus, and this glomerulus soon grows to such an extent as to project through the peritoneal funnel, the neck of which it completely fills, into the body cavity (fig. 403, gl). There is thus formed a series of free peritoneal glomeruli belonging to the anterior Wolfnan tubuli 1 . These tubuli become however early aborted.
In the case of the remaining tubules developed from the S-shaped cords the attachment to the peritoneal epithelium is very soon lost. The cords acquire a lumen, and open into the segmental duct. Their blind extremities constitute the rudiments of Malpighian bodies.
FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH
ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.
am. amnion ; so. somatopleure ; sp. splanchnopleure ; ivd. Wolffian duct ; st. segmental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c. spinal cord ; ch. notochord ; ao. aorta ; hy. hypoblast.
1 These external glomeruli were originally mistaken by me (No. 539) for the glomeralus of the pronephros, from their resemblance to the glomerulus of the Amphibian pronephros. Their true meaning was made out by Sedgwick (No. 550).
In the posterior part of the Wolffian body of the Chick the intermediate cell mass becomes very early detached from the peritoneal epithelium, and at a considerably later period breaks up into oval vesicles similar to those of the Reptilia, which form the rudiments of the segmental tubes.
Secondary and tertiary segmental tubules are formed in the Chick, on the dorsal side of the primary tubules, as direct differentiations of the mesoblast. They open independently into the Wolffian duct.
In Mammalia the segmental tubules (Egli) are formed as solid masses in the same situation as in Birds and Reptiles. It is not known whether they are united with the peritoneal epithelium. They soon become oval vesicles, which develop into complete tubules in the manner already indicated.
After the establishment of the Wolffian body there is formed in both sexes in all the Amniota a duct, which in the female becomes the oviduct, but which is functionless and disappears more or less completely in the male. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Mullerian duct of
FIG. 403. SECTION THROUGH THE EXTERNAL GLOMERULUS OF ONE OF THE ANTERIOR SEGMENTAL TUBES OF AN EMBRYO CHICK OF ABOUT IOO H.
gl. glomerulus ; ge. peritoneal epithelium ; Wd. Wolffian duct ; ao. aorta ; me. mesentery. The segmental tube, and the connection between the external and internal parts of the glomerulus are not shewn in this figure.
FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH GIVE RISE TO THE ANTERIOR PART OF THE MULLERIAN DUCT (PRONEPHROS). (After Balfour and Sedgwick. )
A is the nth section of the series. B i 5th
C i8th ,, ,,
gri. second groove ; gr$. third groove ; ri. second ridge ; wit. Wolffian duct.
the Ichthyopsida. In connection with its anterior extremity certain structures have been found in the Fowl, which are probably, on grounds to be hereafter stated, homologous with the pronephros (Balfour and Sedgwick).
The pronephros, as I shall call it, consists of a slightly convoluted longitudinal canal with three or more peritoneal openings. In the earliest condition, it consists of three successive open involutions of the peritoneal epithelium, connected together by more or less well-defined ridge-like thickenings of the epithelium. It takes its origin from the layer of thickened peritoneal epithelium situated near the dorsal angle of the body cavity, and is situated some considerable distance behind the front end of the Wolfifian duct.
In a slightly later stage the ridges connecting the grooves become partially constricted off from the peritoneal epithelium,
FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times. m. mesentery; Z. somatopleure ; a', portion of the germinal epithelium from which the involution (2) to form the pronephros (anterior part of Miillerian duct) takes place; a. thickened portion of the germinal epithelium in which the primitive germinal cells C and o are lying ; E. modified mesoblast which will form the stroma of the ovary ; WK. Wolffian body ; y. Wolffian duct.
EXCRETORY ORGANS. 719
and develop a lumen. The condition of the structure at this stage is illustrated by fig. 404, representing three transverse sections through two grooves, and through the ridge connecting them.
The pronephros may in fact now be described as a slightly convoluted duct, opening into the body cavity by three groovelike apertures, and continuous behind with the rudiment of the true Miillerian duct.
The stage just described is that of the fullest development of the pronephros. In it, as in all the previous stages, there appear to be only three main openings into the body cavity ; but in some sections there are indications of the possible presence of one or two additional rudimentary grooves.
In an embryo not very much older than the one last described the pronephros atrophies as such, its two posterior openings vanishing, and its anterior opening remaining as the permanent opening of the Miillerian duct.
The pronephros is an extremely transitory structure, and its development and atrophy are completed between the QOth and i2Oth hours of incubation.
The position of the pronephros in relation to the Wolffian body is shewn in fig. 405, which probably passes through a region between two of the peritoneal openings. As long as the pronephros persists, the Mullerian duct consists merely of a very
FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour
In A the terminal portion of the duct is quite distinct ; in B it has united with the walls of the Wolffian duct.
md. Mullerian duct ; Wd. Wolffian duct.
small rudiment, continuous with the hindermost of the three peritoneal openings, and its solid extremity appears to unite with the walls of the Wolffian duct.
After the atrophy of the pronephros, the Miillerian duct commences to grow rapidly, and for the first part of its course it appears to be split off as a solid rod from the outer or ventral wall of the Wolffian duct (fig. 406). Into this rod the lumen, present in its front part, subsequently extends. Its mode of development in front is thus precisely similar to that of the Miillerian duct in Elasmobranchii and Amphibia.
This mode of development only occurs however in the anterior part of the duct. In the posterior part of its course its growing point lies in a bay formed by the outer walls of the Wolffian duct, but does not become definitely attached to that duct. It seems however possible that, although not actually split off from the walls of the Wolrfian duct, it may grow backwards from cells derived from that duct.
The Miillerian duct finally reaches the cloaca though it does not in the female for a long time open into it, and in the male never does so.
The mode of growth of the Miillerian duct in the posterior part of its course will best be understood from the following description quoted from the paper by Sedgwick and myself.
"A few sections before its termination the Miillerian duct appears as a well-defined oval duct lying in contact with the wall of the Wolffian duct on the one hand and the germinal epithelium on the other. Gradually, however, as we pass backwards, the Miillerian duct dilates ; the external wall of the Wolffian duct adjoining it becomes greatly thickened and pushed in in its middle part, so as almost to touch the opposite wall of the duct, and so form a bay in which the Miillerian duct lies. As soon as the Miillerian duct has come to lie in this bay its walls lose their previous distinctness of outline, and the cells composing them assume a curious vacuolated appearance. No well-defined line of separation can any longer be traced between the walls of the Wolffian duct and those of the Miillerian, but between the two is a narrow clear space traversed by an irregular network of fibres, in some of the meshes of which nuclei are present.
The Miillerian duct may be traced in this condition for a considerable number of sections, the peculiar features above described becoming more and more marked as its termination is approached. It continues to dilate and attains a maximum size in the section or so before it disappears. A lumen may be observed in it up to its very end, but is usually irregular in outline and frequently traversed by strands of protoplasm. The Miillerian
EXCRETORY ORGANS. 721
duct finally terminates quite suddenly, and in the section immediately behind its termination the Wolffian duct assumes its normal appearance, and the part of its outer wall on the level of the Miillerian duct conies into contact with the germinal epithelium."
Before describing the development of the Mullerian duct in other Amniotic types it will be well to say a few words as to the identifications above adopted. The identification of the duct, usually called the Wolffian duct, with the segmental duct (exclusive of the pronephros) appears to be morphologically justified for the following reasons : (i) that it gives rise to part of the Mullerian duct as well as to the duct of the Wolffian body ; behaving in this respect precisely as does the segmental duct of Elasmobranchii and Amphibia. (2) That it serves as the duct for the Wolffian body, before the Mullerian duct originates from it. (3) That it develops in a manner strikingly similar to that of the segmental duct of various lower forms.
With reference to the pronephros it is obvious that the organ identified as such is in many respects similar to the pronephros of the Amphibia. Both consist of a somewhat convoluted longitudinal canal, with a certain number of peritoneal openings ;
The main difficulties in the homology are :
(1) the fact that the pronephros in the Bird is not united with the segmental duct ;
(2) the fact that it is situated behind the front end of the Wolffian body. It is to be remembered in connection with the first of these difficulties
that in the formation of the Mullerian duct in Elasmobranchii the anterior undivided extremity of the primitive segmental duct, with the peritoneal opening, which probably represents the pronephros, is attached to the Mullerian duct, and not to the Wolffian duct ; though in Amphibia the reverse is the case. To explain the discontinuity of the pronephros with the segmental duct it is only necessary to suppose that the segmental duct and pronephros, which in the Ichthyopsida develop as a single formation, develop in the Bird as two independent structures a far from extravagant supposition, considering that the pronephros in the Bird is undoubtedly quite functionless.
With reference to the posterior position of the pronephros it is only necessary to remark that a change in position might easily take place after the acquirement of an independent development, and that the shifting is probably correlated with a shifting of the abdominal opening of the Mullerian duct.
The pronephros has only been observed in Birds, and is very possibly not developed in other Amniota. The Mullerian duct is also usually stated to develop as a groove of the peritoneal epithelium, shewn in the Lizard in fig. 354, md., which is continued backward as a primitively solid rod in the space between B. ill. 46
AM N IOTA.
the Wolffian duct and peritoneal epithelium, without becoming attached to the Wolffian duct.
On the formation of the Miillerian duct, the duct of the mesonephros becomes the true mesonephric or Wolffian duct.
After these changes have taken place a new organ of great importance makes its appearance. This organ is the permanent kidney, or metanephros.
Metanephros. The mode of development of the metanephros has as yet only been satisfactorily elucidated in the Chick (Sedgwick, No. 549). The ureter and the collecting tubes of the kidney are developed from a dorsal outgrowth of the hinder part of the Wolffian duct. The outgrowth from the Wolffian duct grows forwards, and extends along the outer side of a mass of mesoblastic tissue which lies mainly behind, but somewhat overlaps the dorsal aspect of the Wolffian body.
This mass of mesoblastic cells may be called the metanephric blastema. Sedgwick, of the accuracy of whose account I have satisfied myself, has shewn that in the Chick it is derived from the intermediate cell mass of the region of about the thirty-first to the thirty-fourth somite. It is at first continuous with, and indistinguishable in structure from, the portion of the intermediate cell mass of the region immediately in front of it, which breaks up into Wolffian tubules. The metanephric blastema remains however quite passive during the formation of the Wolffian tubules in the adjoining blastema ; and on the formation of the ureter breaks off from the Wolffian body in front, and, growing forwards and dorsalwards, places itself on the inner side of the ureter in the position just described.
In the subsequent development of the kidney collecting tubes grow out from the ureter, and become continuous with masses of cells of the metanephric blastema, which then differentiate themselves into the kidney tubules.
The process just described appears to me to prove that the kidney of the A mniota is a specially differentiated posterior section of the primitive mesonephros.
According to the view of Remak and Kolliker the outgrowths from the ureter give rise to the whole of the tubuli uriniferi and the capsules of the Malpighian bodies, the mesoblast around them forming blood-vessels, etc. On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in
EXCRETORY ORGANS. 723
accordance with the account given above, that the outgrowths of the ureter form only the collecting tubes, and that the secreting tubuli, etc. are formed in situ in the adjacent mesoblast.
Braun (No. 542) has arrived at the conclusion that in the Lacertilia the tissue, out of which the tubuli of the metanephros are formed, is derived from irregular solid ingrowths of the peritoneal epithelium, in a region behind the Wolffian body, but in a position corresponding to that in which the segmental tubes take their origin. These ingrowths, after separating from the peritoneal epithelium, unite together to form a cord into which the ureter sends the lateral outgrowths already described. These outgrowths unite with secreting tubuli and Malpighian bodies, formed in situ. In Lacertilia the blastema of the kidney extends into a postanal region. Braun's account of the origin of the metanephric blastema does not appear to me to be satisfactorily demonstrated.
The ureter does not long remain attached to the Wolffian duct, but its opening is gradually carried back, till (in the Chick between the 6th and 8th day) it opens independently into the cloaca.
Of the further changes in the excretory system the most important is the atrophy of the greater part of the Wolffian body, and the conversion of the Wolffian duct in the male sex into the vas deferens, as in Amphibia and the Elasmobranchii.
The mode of connection of the testis with the Wolffian duct is very remarkable, but may be derived from the primitive arrangement characteristic of Elasmobranchii and Amphibia.
In the structures connecting the testis with the Wolffian body two parts have to be distinguished, (i) that equivalent to the testicular network of the lower types, (2) that derived from the segmental tubes. The former is probably to be found in peculiar outgrowths from the Malpighian bodies at the base of the testes.
These were first discovered by Braun in Reptilia, and consist in this group of a series of outgrowths from the primary (?) Malpighian bodies along the base of the testis : they unite to form an interrupted cord in the substance of the testis, from which the testicular tubuli (with the exception of the seminiferous cells) are subsequently differentiated. These outgrowths, with the exception of the first two or three, become detached from the Malpighian bodies. Outgrowths similar to those in the male are found in the female, but subsequently atrophy.
Outgrowths homologous with those found by Braun have
been detected by myself (No. 555) in Mammals. It is not certain to what parts of the testicular tubuli they give rise, but they probably form at any rate the vasa recta and rete vasculosum.
In Mammals they also occur in the female, and give rise to cords of tissue in the ovary, which may persist through life.
The comparison of the tubuli, formed out of these structures, with the Elasmobranch and Amphibian testicular network is justified in that both originate as outgrowths from the primary Malpighian bodies, and thence extend into the testis, and come into connection with the true seminiferous stroma.
As in the lower types the semen is transported from the testicular network to the Wolffian duct by parts of the glandular tubes of the Wolffian body. In the case of Reptilia the anterior two or three segmental tubes in the region of the testis probably have this function. In the case of Mammalia the vasa efferentia, i.e. the coni vasculosi, appear, according to the usually accepted view, to be of this nature, though Banks and other investigators believe that they are independently developed structures. Further investigations on this point are required. In Birds a connection between the Wolffian body and the testis appears to be established as in the other types. The Wolffian duct itself becomes, in the males of all Amniota, the vas deferens and the convoluted canal of the epididymis the latter structure (except the head) being entirely derived from the Wolffian duct.
In the female the Wolffian duct atrophies more or less completely.
In Snakes (Braun) the posterior part remains as a functionless canal, commencing at the ovary, and opening into the cloaca. In the Gecko (Braun) it remains as a small canal joining the ureter ; in Blindworms a considerable part of the canal is left, and in Lacerta (Braun) only interrupted portions.
In Mammalia the middle part of the duct, known as Gaertner's canal, persists in the females of some monkeys, of the pig and of many ruminants.
The Wolffian body atrophies nearly completely in both sexes ; though, as described above, part of it opposite the testis persists as the head of the epididymis. The posterior part of the gland from the level of the testis may be called the sexual part of the gland, the anterior part forming the non-sexual part.
EXCRETORY ORGANS. 725
The latter, i.e. the anterior part, is first absorbed ; and in some Reptilia the posterior part, extending from the region of the genital glands to the permanent kidney, persists till into the second year.
Various remnants of the Wolffian body are found in the adults of both sexes in different types. The most constant of them is perhaps the part in the female equivalent to the head of the epididymis and to parts also of the coiled tube of the epididymis, which may be called, with Waldeyer, the epoophoron 1 . This is found in Reptiles, Birds and Mammals ; though in a very rudimentary form in the first-named group. Remnants of the anterior non-sexual part of the Wolffian bodies have been called by Waldeyer parepididymis in the male, and paroophoron in the female. Such remnants are not (Braun) found in Reptilia, but are stated to be found in both male and female Birds, as a small organ consisting of blindly ending tubes with yellow pigment. In some male Mammals (including Man) a parepididymis is found on the upper side of the testis. It is usually known as the organ of Giraldes.
The Mlillerian duct forms, as has been stated, the oviduct in the female. The two ducts originally open independently into the cloaca, but in the Mammalia a subsequent modification of this arrangement occurs, which is dealt with in a separate section. In Birds the right oviduct atrophies, a vestige being sometimes left. In the male the Miillerian ducts atrophy more or less completely.
In most Reptiles and in Birds the atrophy of the Miillerian ducts is complete in the male, but in Lacerta and Anguis a rudiment of the anterior part has been detected by Leydig as a convoluted canal. In the Rabbit (Kolliker) 2 and probably other Mammals the whole of the ducts probably disappears, but in some Mammals, e.g. Man, the lower fused ends of the Miillerian ducts give rise to a pocket opening into the urethra, known as the uterus masculinus ; and in other cases, e.g. the Beaver and the Ass, the rudiments are more considerable, and may be continued into horns homologous with the horns of the uterus (Weber).
The hydatid of Morgani in the male is supposed (Waldeyer) to represent the abdominal opening of the Fallopian tube in the female, and therefore to be a remnant of the Miillerian duct.
Changes in the lower parts of the urinogenital ducts in the Amniota.
The genital cord. In the Monodelphia the lower part of the Wolffian ducts becomes enveloped in both sexes in a special
1 This is also called parovarium (His), and Rosenmiiller's organ.
2 Weber (No. 553) states that a uterus masculinus is present in the Rabbit, but his account is by no means satisfactory, and its presence is distinctly denied by Kolliker.
cord of tissue, known as -the genital cord (fig. 407, gc), within the lower part of which the MUllerian ducts are also enclosed. In the male the MUllerian ducts in this cord atrophy, except at their distal end where they unite to form the uterus masculinus. The Wolffian ducts, after becoming the vasa deferentia, remain for some time enclosed in the common cord, but afterwards separate from each other. The seminal vesicles are outgrowths of the vasa deferentia.
In the female the Wolffian ducts within the genital cord atrophy, though rudiments of them are for a long time visible or even permanently persistent. The lower parts of the MUllerian ducts unite to form the vagina and body of the uterus. The junction commences in the middle and extends forwards and backwards ; the stage with a median junction being retained permanently in Marsupials.
The urinogenital sinus and external generative organs. In all the Amniota, there open at first into the common cloaca the alimentary canal dorsally, the allantois ventrally, and the Wolffian and MUllerian ducts and ureters laterally. In Reptilia and Aves the embryonic condition is retained. In both groups the allantois serves as an embryonic urinary bladder, but while it atrophies in Aves, its stalk dilates to form a permanent urinary bladder in Reptilia. In Mammalia the dorsal part of the cloaca with the alimentary tract becomes first of all partially constricted off from the ventral, which then forms a urinogenital sinus (fig. 407, ug). In the course of development the urinogenital sinus becomes, in all Mammalia but the Ornithodelphia, completely separated from the intestinal cloaca, and the two parts obtain separate external openings. The ureters (fig. 407, 3) open higher up than the other ducts into the stalk of the allantois which dilates to form the bladder (4). The stalk connecting the bladder with the ventral wall of the body constitutes the urachus, and loses its lumen before the close of embryonic life. The part of the stalk of the allantois below the openings of the ureters narrows to form the urethra, which opens together with the Wolffian and MUllerian ducts into the urinogenital cloaca.
In front of the urinogenital cloaca there is formed a genital prominence (fig. 407, cp), with a groove continued from the
EXCRETORY ORGANS. 727
urinogenital opening ; and on each side a genital fold (&). In the male the sides of the groove on the prominence coalesce together, embracing between them the opening of the urinogenital cloaca ; and the prominence itself gives rise to the penis,
FIG. 407. DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT AN EARLY STAGE. (After Allen Thomson ; from Quain's Anatomy.)
The parts are seen chiefly in profile, but the Miillerian and Wolffian ducts are seen from the front.
3. ureter; 4. urinary bladder ; 5. urachus; of. genital ridge (ovary or testis) ; W. left Wolffian body ; x. part at apex from which coni vasculosi are afterwards developed ; w. Wolffian duct ; m. Miillerian duct ; gc. genital cord consisting of Wolffian and Mullerian ducts bound up in a common sheath ; i. rectum ; ug. urinogenital sinus ; cp. elevation which becomes the clitoris or penis ; Is. ridge from which the labia majora or scrotum are developed.
along which the common urinogenital passage is continued. The two genital folds unite from behind forwards to form the scrotum.
In the female the groove on the genital prominence gradually disappears, and the prominence remains as the clitoris, which is therefore the homologue of the penis : the two genital folds form the labia majora. The urethra and vagina open independently into the common urinogenital sinus.
728 GENERAL CONCLUSIONS.
General conclusions and Summary.
Pronephros. Sedgwick has pointed out that the pronephros is always present in types with a larval development, and either absent or imperfectly developed in those types which undergo the greater part of their development within the egg. Thus it is practically absent in the embryos of Elasmobranchii and the Amniota, but present in the larvae of all other forms.
This coincidence, on the principles already laid down in a previous chapter on larval forms, affords a strong presumption that the pronephros is an ancestral organ ; and, coupled with the fact that it is the first part of the excretory system to be developed, and often the sole excretory organ for a considerable period, points to the conclusion that the pronephros and its duct the segmental duct are the most primitive parts of the Vertebrate excretory system. This conclusion coincides with that arrived at by Gegenbaur and Fiirbringer.
The duct of the pronephros is always developed prior to the gland, and there are two types according to which its development may take place. It may either be formed by the closing in of a continuous groove of the somatic peritoneal epithelium (Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of cells derived from the somatic mesoblast, which grows backwards between the epiblast and the mesoblast (Petromyzon, Elasmobranchii, and the Amniota).
It is quite certain that the second of these processes is not a true record of the evolution of 'the duct, and though it is more possible that the process observable in Amphibia and the Teleostei may afford some indications of the manner in which the duct was established, this cannot be regarded as by any means certain.
The mode of development of the pronephros itself is apparently partly dependent on that of its duct. In Petromyzon, where the duct does not at first communicate with the body cavity, the pronephros is formed as a series of outgrowths from the duct, which meet the peritoneal epithelium and open into the body cavity ; but in other instances it is derived from the anterior open end of the groove which gives rise to the segmental duct. The open end of this groove may either remain single
EXCRETORY ORGANS. 729
(Teleostci, Ganoidei) or be divided into two, three or more apertures (Amphibia). The main part of the gland in either case is formed by convolutions of the tube connected with the peritoneal funnel or funnels. The peritoneal funnels of the pronephros appear to be segmentally arranged.
The pronephros is distinguished from the mesonephros by developmental as well as structural features. The most important of the former is the fact that the glandular tubules of which it is formed are always outgrowths of the segmental duct ; while in the mesonephros they are always or almost always 1 formed independently of the duct.
The chief structural peculiarity of the pronephros is the absence from it of Malpighian bodies with the same relations as those in the meso- and metanephros; unless the structures found in Myxine are to be regarded as such. Functionally the place of such Malpighian bodies is taken by the vascular peritoneal ridge spoken of in the previous pages as the glomerulus.
That this body is really related functionally to the pronephros appears to be indicated (i) by its constant occurrence with the pronephros and its position opposite the peritoneal openings of this body ; (2) by its atrophy at the same time as the pronephros ; (3) by its enclosure together with the pronephridian stoma in a special compartment of the body-cavity in Teleostei and Ganoids, and its partial enclosure in such a compartment in Amphibia.
The pronephros atrophies more or less completely in most types, though it probably persists for life in the Teleostei and Ganoids, and in some members of the former group it perhaps forms the sole adult organ of excretion.
The cause of its atrophy may perhaps be related to the fact that it is situated in the pericardial region of the body-cavity, the dorsal part of which is aborted on the formation of a closed pericardium ; and its preservation in Teleostei and Ganoids may on this view be due to the fact that in these types its peritoneal funnel and its glomerulus are early isolated in a special cavity.
Mesonephros. The mesonephros is in all instances composed of a series of tubules (segmental tubes) which are developed independently of the segmental duct. Each tubule is
1 According t.o Sedgwick some of the anterior segmental tubes of Aves form an exception to the general rule that there is no outgrowth from the segmental or metanephric duct to meet the segmental tubes.
730 GENERAL CONCLUSIONS.
typically formed of (i) a peritoneal funnel opening into (2) a Malpighian body, from which there proceeds (3) a coiled glandular tube, finally opening by (4) a collecting tube into the segmental duct, which constitutes the primitive duct for the mesonephros as well as for the pronephros.
The development of the mesonephridian tubules is subject to considerable variations.
(1) They may be formed as differentiations of the intermediate cell mass, and be from the first provided with a lumen, opening into the body-cavity, and directly derived from the section of the body-cavity present in the intermediate cell mass; the peritoneal funnels often persisting for life (Elasmobranchii).
(2) They may be formed as solid cords either attached to or independent of the peritoneal epithelium, which after first becoming independent of the peritoneal epithelium subsequently send downwards a process, which unites with it and forms a peritoneal funnel, which may or may not persist (Acipenser, Amphibia).
(3) They may be formed as in the last case, but acquire no secondary connection with the peritoneal epithelium (Teleostei, Amniota). In connection with the original attachment to the peritoneal epithelium, a true peritoneal funnel may however be developed (Aves, Lacertilia).
Physiological considerations appear to shew that of these three methods of development the first is the most primitive. The development of the tubes as solid cords can hardly be primary.
A question which has to be answered in reference to the segmental tubes is that of the homology of the secondarily developed peritoneal openings of Amphibia, with the primary openings of the Elasmobranchii. It is on the one hand difficult to understand why, if the openings are homologous in the two types, the original peritoneal attachment should be obliterated in Amphibia, only to be shortly afterwards reacquired. On the other hand it is still more difficult to understand what physiological gain there could be, on the assumption of the non-homology of the openings, in the replacement of the primary opening by a secondary opening exactly similar to it. Considering the great variations in development which occur in undoubtedly homologous parts I incline to the view that the openings in the two types are homologous.
In the majority of the lower Vertebrata the mesonephric tubes have at first a segmental arrangement, and this is no doubt the primitive condition. The coexistence of two, three, or more of them in a single segment in Amphibia, Aves and Mammalia has recently been shewn, by an interesting discovery of Eisig, to have a parallel amongst Chaetopods, in the coexistence of several segmental organs in a single segment in some of the Capitellidae.
In connection with the segmental features of the mesonephros it is perhaps worth recalling the fact that in Elasmobranchii as well as other types there are traces of segmental tubes in some of the postanal segments. In the case of all the segmental tubes a Malpighian body becomes established close to the extremity of the tube adjoining the peritoneal opening, or in an homologous position in tubes without such an opening. The opposite extremity of the tube always becomes attached to the segmental duct.
In many of the segments of the mesonephros, especially in the hinder ones, secondary and tertiary tubes become developed in certain types, which join the collecting canals of the primary tubes, and are provided, like the primary tubes, with Malpighian bodies at their blind extremities.
There can it appears to me be little or no doubt that the secondary tubes in the different types are homodynamous if not homologous. Under these circumstances it is surprising to find in what different ways they take their origin. In Elasmobranchii a bud sprouts out from the Malpighian body of one segment, and joins the collecting tube of the preceding segment, and subsequently, becoming detached from the Malpighian body from which it sprouted, forms a fresh secondary Malpighian body at its blind extremity. Thus the secondary tubes of one segment are formed as buds from the segment behind. In Amphibia (Salamandra) and Aves the secondary tubes develop independently in the mesoblast. These great differences in development are important in reference to the homology of the metanephros or permanent kidney, which is discussed below.
Before leaving the mesonephros it may be worth while putting forward some hypothetical suggestions as to its origin and relation to the pro
732 GENERAL CONCLUSIONS.
nephros, leaving however the difficult questions as to the homology of the segmental tubes with the segmental organs of Chastopods for subsequent discussion.
It is a peculiarity in the development of the segmental tubes that they at first end blindly, though they subsequently grow till they meet the segmental duct with which they unite directly, without the latter sending out any offshoot to meet them 1 . It is difficult to believe that peritoneal infundibula ending blindly and unprovided with some external orifice can have had an excretory function, and we are therefore rather driven to suppose that the peritoneal infundibula which become the segmental tubes were either from the first provided each with an orifice opening to the exterior, or were united with the segmental duct. If they were from the first provided with external openings we may suppose that they became secondarily attached to the duct of the pronephros (segmental duct), and then lost their external openings, no trace of these structures being left, even in the ontogeny of the system. It would appear to me more probable that the pronephros, with its duct opening into the cloaca, was the only excretory organ of the unsegmented ancestors of the Chordata, and that, on the elongation of the trunk and its subsequent segmentation, a series of metameric segmental tubes became evolved opening into the segmental duct, each tube being in a sort of way serially homologous with the primitive pronephros. With the segmentation of the trunk the latter structure itself may have acquired the more or less definite metameric arrangement of its parts.
Another possible view is that the segmental tubes may be modified derivatives of posterior lateral branches of the pronephros, which may at first have extended for the whole length of the body-cavity. If there is any truth in this hypothesis it is necessary to suppose that, when the unsegmented ancestor of the Chordata became segmented, the posterior branches of the primitive excretory organ became segmentally arranged, and that, in accordance with the change thus gradually introduced in them, the time of their development became deferred, so as to accord to a certain extent with the time of formation of the segments to which they belonged. The change in their mode of development which would be thereby introduced is certainly not greater than that which has taken place in the case of segmental tubes, which, having originally developed on the Elasmobranch type, have come to develop as they do in the posterior part of the mesonephros of Salamandra, Birds, etc.
Genital ducts. So far the origin and development of the excretory organs have been considered without reference to the modifications introduced by the excretory passages coming to serve as generative ducts. Such an unmodified state of the
1 As mentioned in the note on p. 729 Sedgwick maintains that the anterior segmental tubes of the Chick form an exception to this general statement.
EXCRETORY ORGANS. 733
excretory organs is perhaps found permanently in Cyclostomata 1 and transitorily in the embryos of most forms.
At first the generative products seem to have been discharged freely into the body-cavity, and transported to the exterior by the abdominal pores (vide p. 626).
The secondary relations of the excretory ducts to the generative organs seem to have been introduced by an opening connected with the pronephridian extremity of the segmental duct having acquired the function of admitting the generative products into it, and of carrying them outwards ; so that primitively the segmental duct must have served as efferent duct both for the generative products and the pronepJiric secretion (just as the Wolffian duct still does for the testicular products and secretion of the Wolffian body in Elasmobranchii and Amphibia).
The opening by which the generative products entered the segmental duct can hardly have been specially developed for this purpose, but must almost certainly have been one of the peritoneal openings of the pronephros. As a consequence (by a process of natural selection) of the segmental duct having both a generative and a urinary function, a further differentiation took place, by which that duct became split into two a ventral Mullerian duct and a dorsal Wolffian duct.
The Mullerian duct was probably continuous with one or more of the abdominal openings of the pronephros which served as generative pores. At first the segmental duct was probably split longitudinally into two equal portions, and this mode of splitting is exceptionally retained in some Elasmobranchii ; but the generative function of the Mullerian duct gradually impressed itself more and more upon the embryonic development, so that, in the course of time, the Mullerian duct developed less and less at the expense of the Wolffian duct. This process appears partly to have taken place in Elasmobranchii, and still more in Amphibia, the Amphibia offering in this respect a less primitive condition than the Elasmobranchii ; while in Aves it has been carried even further, and it seems possible that in some Amniota the Mullerian and segmental
1 It is by no means certain that the transportation outwards of the genital products by the abdominal pores in the Cyclostomata may not be the result of degeneration.
734 GENERAL CONCLUSIONS.
ducts may actually develop independently, as they do exceptionally in individual specimens of Salamandra (Fiirbringer). The abdominal opening no doubt also became specialised. At first it is quite possible that more than one pronephric abdominal funnel may have served for the entrance of the generative products ; this function being, no doubt, eventually restricted to one of them.
Three different types of development of the abdominal opening of the Mullerian duct have been observed.
In Amphibia (Salamandra) the permanent opening of the Mullerian duct is formed independently, some way behind the pronephros.
In Elasmobranchii the original opening of the segmental duct forms the permanent opening of the Mullerian duct, and no true pronephros appears to be formed.
In Birds the anterior of the three openings of the rudimentary pronephros remains as the permanent opening of the Mullerian duct.
These three modes of development very probably represent specialisations of the primitive state along three different lines. In Amphibia the specialisation of the opening appears to have gone so far that it no longer has any relation to the pronephros. It was probably originally one of the posterior openings of this gland.
In Elasmobranchii, on the other hand, the functional opening is formed at a period when we should expect the pronephros to develop. This state is very possibly the result of a differentiation by which the pronephros gradually ceased to become developed, but one of its peritoneal openings remained as the abdominal aperture of the Mullerian duct. Aves, finally, appear to have become differentiated along a third line ; since in their ancestors the anterior (?) pore of the head-kidney appears to have become specialised as the permanent opening of the Mullerian duct.
The Mullerian duct is usually formed in a more or less complete manner in both sexes. In Ganoids, where the separation between it and the Wolffian duct is not completed to the cloaca, and in the Dipnoi, it probably serves to carry off the generative products of both sexes. In other cases however only the female
products pass out by it, and the partial or complete formation of the Mullerian duct in the male in these cases needs to be explained. This may be done either by supposing the Ganoid arrangement to have been the primitive one in the ancestors of the other forms, or, by supposing characters acquired primitively by the female to have become inherited by both sexes.
It is a question whether the nature of the generative ducts of Teleostei can be explained by comparison with those of Ganoids. The fact that the Mullerian ducts of the Teleostean Ganoid Lepidosteus attach themselves to the generative organs, and thus acquire a resemblance to the generative ducts of Teleostei, affords a powerful argument in favour of the view that the generative ducts of both sexes in the Teleostei are modified Mullerian ducts. Embryology can however alone definitely settle this question.
In the Elasmobranchii, Amphibia, and Amniota the male products are carried off by the Wolffian duct, and they are transported to this duct, not by open peritoneal funnels of the mesonephros, but by a network of ducts which sprout either from a certain number of the Malpighian bodies opposite the testis (Amphibia, Amniota), or from the stalks connecting the Malpighian bodies with the open funnels (Elasmobranchii). After traversing this network the semen passes (except in certain Anura) through a variable number of the segmental tubes directly to the Wolffian duct. The extent of the connection of the testis with the Wolffian body is subject to great variations, but it is usually more or less in the anterior region. Rudiments of the testicular network have in many cases become inherited by the female.
The origin of the connection between the testis and Wolffian body is still very obscure. It would be easy to understand how the testicular products, after falling into the body-cavity, might be taken up by the open extremities of some of the peritoneal funnels, and how such open funnels might have groove-like prolongations along the mesorchium, which might eventually be converted into ducts. Ontogeny does not however altogether favour this view of the origin of the testicular network. It seems to me nevertheless the most probable view which has yet been put forward.
The mode of transportation of the semen by means of the mesonephric tubules is so peculiar as to render it highly improbable that it was twice acquired, it becomes therefore necessary to suppose that the Amphibia and
736 GENERAL CONCLUSIONS.
Amniota inherited this mode of transportation of the semen from the same ancestors as the Elasmobranchii. It is remarkable therefore that in the Ganoidei and Dipnoi this arrangement is not found.
Either (i) the arrangement (found in the Ganoidei and Dipnoi) of the Miillerian duct serving for both sexes is the primitive arrangement, and the Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary condition, which has originated at a stage in the evolution of the Vertebrata when some of the segmental tubes had begun to serve as the efferent ducts of the testis, and has resulted in consequence of a degeneration of the latter structures. Although the second alternative is the more easy to reconcile with the affinities of the Ganoid and Elasmobranch types, as indicated by the other features of their organization, I am still inclined to accept the former ; and consider that the incomplete splitting of the segmental duct in Ganoidei is a strong argument in favour of this view.
Metanephros. With the employment of the Wolffian duct to transport the semen there seems to be correlated (i) a tendency of the posterior segmental tubes to have a duct of their own, in which the seminal and urinary fluids cannot become mixed, and (2) a tendency on the part of the anterior segmental tubes to lose their excretory function. The posterior segmental tubes, when connected in this way with a more or less specialised duct, have been regarded in the preceding pages as constituting a metanephros.
This differentiation is hardly marked in the Anura, but is well developed in the Urodela and in the Elasmobranchii ; and in the latter group has become inherited by both sexes. In the Amniota it culminates, according to the view independently arrived at by Semper and myself, (i) in the formation of a completely distinct metanephros in both sexes, formed however, as shewn by Sedgwick, from the same blastema as the Wolffian body, and (2) in the atrophy in the adult of the whole Wolffian body, except the part uniting the testis and the Wolffian duct.
The homology between the posterior metanephridian section of the Wolffian body, in Elasmobranchii and Urodela, and the kidney of the Amniota, is only in my opinion a general one, i.e. in both cases a common cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more or less similar differentiation of parts.
Fiirbringer has urged against Semper's and my view that no satisfactory proof of it has yet been offered. This proof has however, since Fiirbringer wrote his paper, been supplied by Sedgwick's observations. The development of the kidney in the Amniota is no doubt a direct as opposed to a phylogenetic development ; and the substitution of a direct for
EXCRETORY ORGANS. 737
a phylogenetic development has most probably been rendered possible by the fact that the anterior part of the mesonephros continued all the while to be unaffected and to remain as the main excretory organ during foetal life.
The most serious difficulty urged by Fiirbringer against the homology is the fact that the ureter of the metanephros develops on a type of its own, which is quite distinct from the mode of development of the ureters of the metanephros of the Ichthyopsidan forms. It is however quite possible, though far from certain, that the ureter of Amniota may be a special formation confined to that group, and this fact would in no wise militate against the homology I have been attempting to establish.
Comparison of the Excretory organs of the Chordata and Invertebrata.
The structural characters and development of the various forms of excretory organs described in the preceding pages do not appear to me to be sufficiently distinctive to render it possible to establish homologies between these organs on a satisfactory basis, except in closely related groups.
The excretory organs of the Platyelminthes are in many respects similar to the provisional excretory organ of the trochosphere of Polygordius and the Gephyrea on the one hand, and to the Vertebrate pronephros on the other ; and the Platyelminth excretory organ with an anterior opening might be regarded as having given origin to the trochosphere organ, while that with a posterior opening may have done so for the Vertebrate pronephros 1 .
Hatschek has compared the provisional trochosphere excretory organ of Polygordius to the Vertebrate pronephros, and the posterior Chastopod segmental tubes to the mesonephric tubes ; the latter homology having been already suggested independently by both Semper and myself. With reference to the comparison of the pronephros with the provisional excretory organ of Polygordius there are two serious difficulties :
(1) The pronephric (segmental) duct opens directly into the cloaca, while the duct of the provisional trochosphere excretory organ opens anteriorly, and directly to the exterior.
(2) The pronephros is situated within the segmented region of the trunk, and has a more or less distinct metameric arrangement of its parts ; while the provisional trochosphere organ is placed in front of the segmented region of the trunk, and is in no way segmented.
The comparison of the mesonephric tubules with the segmental excretory organs of the Chaetopoda, though not impossible, cannot be satisfactorily admitted till some light has been thrown upon the loss of the supposed external openings of the tubes, and the origin of their secondary connection with the segmental duct.
1 This suggestion has I believe been made by Fiirbringer. B. III. 47
Confining our attention to the Invertebrata it appears to me fairly clear that Hatschek is justified in holding the provisional trochosphere excretory organs of Polygordius, Echiurus and the Mollusca to be homologous. The atrophy of all these larval organs may perhaps be due to the presence of a well-developed trunk region in the adult (absent in the larva), in which excretory organs, probably serially homologous with those present in the anterior part of the larva, became developed. The excretory organs in the trunk were probably more conveniently situated than those in the head, and the atrophy of the latter in the adult state was therefore brought about, while the trunk organs became sufficiently enlarged to serve as the sole excretory organs.
BIBLIOGRAPHY OF THE EXCRETORY ORGANS. Invertebrata.
(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool. Stat. z. Neapel, Vol. I. 1879.
(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Trematodes et d. Cesto'ides." Archives de Biologic, Vol. I. 1880.
(514) B. Hatschek. "Studien lib. Entwick. d. Anneliden." Arbeit, a. d. zool. Instit. Wien, Vol. I. 1878.
(515) B. Hatschek. "Ueber Entwick. von Echiurus," etc. Arbeit, a. d. zool. Instit. Wien, Vol. in. 1880.
EXCRETORY ORGANS OF VERTEBRATA. General.
(516) F. M. Balfour. "On the origin and history of the urinogenital organs of Vertebrates." yournal of Anat. and Phys., Vol. X. 1876.
(517) Max. Furbringer 1 . "Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.
(518) H. Meek el. Zur Morphol. d. Hani- u. Geschlechtnverkz.d. Wirbelthiere, etc. Halle, 1848.
(519) Joh. Miiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.
(520) H. Rathke. " Beobachtungen u. Betrachtungen u. d. Entwicklung d. Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in Dantzig, Bd. I. 1825.
(521) C. Semper 1 . "Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. iibrigen Wirbelthiere." Arb. a. d. zool.-zoot. Instit. Wurzburg, Vol. II. 1875 (522) W. Waldeyer 1 . Eierstock u. Ei. Leipzig, 1870.
1 The papers of Furbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs.
(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anat., Vol. XI. 1875.
Vide also Semper (No. 521) and Balfour (No. 292).
(524) J. Miiller. " Untersuchungen ii. d. Eingeweide d. Fische." Abh. d. k. Ak. Wiss. Berlin, 1845.
(525) W. Miiller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa." Jenaische Zeitschrift, Vol. VII. 1873.
(526) W. Miiller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaische Zeitschri/t, Vol. IX. 1875.
(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.
(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol. Jahrbuch, Vol. vn. 1881.
(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d. k. k. Akad. Wiss. Wien, Vol. n. 1850.
(530) A. Rosenberg. Untersuchungen iib. die Entivicklung d. Teleostierniere. Dorpat, 1867.
Vide also Oellacher (No. 72).
(531) F. H. Bidder. Vergleichend-anatomische u. histologische Untersitchungen ii. die mdnnlichen Geschleehts- und Harnwerkzeuge d. nackten Amphibien. Dorpat, 1846.
(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 95.
(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.
(534) F. Leydig. Anatomie d. Amphibien u. Reptilien. Berlin, 1853.
(535) F. Leydig. Lehrbuch d. Hisiologie. Hamm, 1857.
(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien." Sitz. d. naturfor. Gesellsch. Leipzig, 1875.
(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d. zool.- zoot. Instil. Wiirzburg. Vol. III. 1876.
(538) VonWittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit. f. wiss. Zool., Vol. IV.
Vide also Gotte (No. 296).
(539) F. M. Balfour and A. Sedgwick. "On the existence of a head -kidney in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. xix. 1878.
(540 ) Banks. On the Wolffian bodies of the fatus and their remains in the adult. Edinburgh, 1864.
(541) Th. Bornhaupt. Untersuchungen iib. die Entwicklung d. Urogenitalsystems beim Hiihnchen. Inaug. Diss. Riga, 1867.
(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. iv. 1877.
(543) J. Dansky u. J. Kostenitsch. "Ueb. d. Entwick. d. Keimblatter u. d. WolfFschen Ganges im Hiihnerei." Mini. Acad. Imp. Petersbourg, vn. Series, Vol. xxvil. 1880.
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Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297), Kolliker (No. 298).