Illustration: Figure 381Fig. 381. Transverse section through the front part of the head of a young Pristiurus embryo.The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the premandibular and mandibular head-cavities 1ppand 2pp, etc. The section is moreover somewhat oblique from side to side.fb.fore-brain;l.lens of eye;m.mouth;pt.upper end of mouth, forming pituitary involution; 1ao.mandibular aortic arch; 1pp.and 2pp.first and second head-cavities; 1vc.first visceral cleft;V.fifth nerve;aun.auditory nerve;VII.seventh nerve;aa.dorsal aorta;acv.anterior cardinal vein;ch.notochord.
Fig. 381. Transverse section through the front part of the head of a young Pristiurus embryo.The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the premandibular and mandibular head-cavities 1ppand 2pp, etc. The section is moreover somewhat oblique from side to side.fb.fore-brain;l.lens of eye;m.mouth;pt.upper end of mouth, forming pituitary involution; 1ao.mandibular aortic arch; 1pp.and 2pp.first and second head-cavities; 1vc.first visceral cleft;V.fifth nerve;aun.auditory nerve;VII.seventh nerve;aa.dorsal aorta;acv.anterior cardinal vein;ch.notochord.
The front section of the head-cavity grows forward, and soon becomes divided, without the intervention of a visceral cleft, into an anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth involution. The posterior part lies completely within the mandibular arch.
As the rudiments of the successive visceral clefts are formed, the posterior part of the head-cavity becomes divided into successive sections, there being one section for each arch. Thus the whole head-cavity becomes on each side divided into (1) a premandibular section; (2) a mandibular section (videfig. 29 A,pp); (3) a hyoid section; (4) sections in each of the branchial arches.
The first of these divisions forms a space of a considerable size, with epithelial walls of somewhat short columnar cells (fig. 381, 1pp). It is situated close to the eye, and presents a rounded or sometimes a triangular figure in section. The two halves of the cavity are prolonged ventralwards, and meet below the base of the fore-brain. The connection between them appears to last for a considerable time. These two cavities are the only parts of the body-cavity within the head which unite ventrally. The section of the head-cavity just described is so similar to the remaining sections that it must be considered as serially homologous with them.
The next division of the head-cavity, which from its positionmay be called the mandibular cavity, presents a spatulate shape, being dilated dorsally, and produced ventrally into a long thin process parallel to the hyomandibular gill-cleft (fig. 20,pp). Like the previous space it is lined by a short columnar epithelium.
Illustration: Figure 382Fig. 382. Horizontal section through the penultimate visceral arch of an embryo of Pristiurus.ep.epiblast;vc.pouch of hypoblast which will form the walls of a visceral cleft;pp.segment of body-cavity in visceral arch;aa.aortic arch.
Fig. 382. Horizontal section through the penultimate visceral arch of an embryo of Pristiurus.ep.epiblast;vc.pouch of hypoblast which will form the walls of a visceral cleft;pp.segment of body-cavity in visceral arch;aa.aortic arch.
The mandibular aortic arch is situated close to its inner side (fig. 381, 2pp). After becoming separated from the lower part (Marshall), the upper part of the cavity atrophies about the time of the appearance of the external gills. Its lower part also becomes much narrowed, but its walls of columnar cells persist. The outer or somatic wall becomes very thin indeed, the splanchnic wall, on the other hand, thickens and forms a layer of several rows of elongated cells. In each of the remaining arches there is a segment of the original body-cavity fundamentally similar to that in the mandibular arch (fig. 382). A dorsal dilated portion appears, however, to be present in the third or hyoid section alone (fig. 20), and even there disappears very soon, after being segmented off from the lower part (Marshall). The cavities in the posterior parts of the head become much reduced like those in its anterior part, though at rather a later period.
It has been shewn that the divisions of the body-cavity in the head, with the exception of the anterior, early become atrophied,not so however their walls. The cells forming the walls both of the dorsal and ventral sections of these cavities become elongated, and finally become converted into muscles. Their exact history has not been followed in its details, but they almost unquestionably become the musculus constrictor superficialis and musculus interbranchialis[246]; and probably also musculus levator mandibuli and other muscles of the front part of the head.
The anterior cavity close to the eye remains unaltered much longer than the remaining cavities.
Its further history is very interesting. In my original account of this cavity (No.292,p.208) I stated my belief that its walls gave rise to the eye-muscles, and the history of this process has been to some extent worked out by Marshall in his important memoir (No.509).
Marshall finds that the ventral portion of this cavity, where its two halves meet, becomes separated from the remainder. The eventual fate of this part has not however been followed. Each dorsal section acquires a cup-like form, investing the posterior and inner surface of the eye. The cells of its outer wall subsequently give rise to three sets of muscles. The middle of these, partly also derived from the inner walls of the cup, becomes the rectus internus of the eye, the dorsal set forms the rectus superior, and the ventral the rectus inferior. The obliquus inferior appears also to be in part developed from the walls of this cavity.
Marshall brings evidence to shew that the rectus externus (as might be anticipated from its nerve supply) has no connection with the walls of the premandibular head-cavity, and finds that it arises close to the position originally occupied by the second and third cavities. Marshall has not satisfactorily made out the mode of development of the obliquus superior.
The walls of the cavities, whose history has just been recorded, have definite relations with the cranial nerves, an account of which has already been given atp.461.
Head-cavities, in the main similar to those of Elasmobranchii, have been found in the embryo of Petromyzon (fig. 45,hc), the Newt (Osborn and Scott), and various Reptilia (Parker).
Bibliography.
(507)G. M. Humphry. “Muscles in Vertebrate Animals.”Journ. of Anat. and Phys.,Vol.VI. 1872.(508)J. Müller. “Vergleichende Anatomie d. Myxinoiden. Part I. Osteologie u. Myologie.”Akad. Wiss., Berlin, 1834.(509)A. M. Marshall. “On the head cavities and associated nerves of Elasmobranchs.”Quart. J. of Micr. Science,Vol.XXI. 1881.(510)A. Schneider. “Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere.”Sitz. d. Oberhessischen Gesellschaft, 1873.(511)A. Schneider.Beiträge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.Berlin, 1879.Videalso Götte (No.296), Kölliker (No.298), Balfour (No.292), Huxley, etc.
[242]If recent statements of Metschnikoff are to be trusted, the Echinodermata must be added to these groups. The amœboid cells stated in the first volume of this treatise to form the muscles in this group, on the authority of Selenka, give rise, according to Metschnikoff, only to the cutis, while the same naturalist states the epithelial cells of the vaso-peritoneal vesicles are provided with muscular tails.[243]O. and R. Hertwig,Die Cœlomtheorie.Jena, 1881.[244]Ehrlich, “Ueber den peripher. Theil d. Urwirbel.”Archiv f. mikr. Anat.,Vol.XI.[245]The brothers Hertwig have recently maintained that only the inner layer of the muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demonstrate the incorrectness of this view, and in Acipenser (videfig. 57,mp) the two layers of the muscle-plate retain their original relations after the cells of both of them have become converted into muscles.[246]VideVetter, “Die Kiemen und Kiefermusculatur d. Fische.”Jenaische Zeitschrift,Vol.VII.
[242]If recent statements of Metschnikoff are to be trusted, the Echinodermata must be added to these groups. The amœboid cells stated in the first volume of this treatise to form the muscles in this group, on the authority of Selenka, give rise, according to Metschnikoff, only to the cutis, while the same naturalist states the epithelial cells of the vaso-peritoneal vesicles are provided with muscular tails.
[243]O. and R. Hertwig,Die Cœlomtheorie.Jena, 1881.
[244]Ehrlich, “Ueber den peripher. Theil d. Urwirbel.”Archiv f. mikr. Anat.,Vol.XI.
[245]The brothers Hertwig have recently maintained that only the inner layer of the muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demonstrate the incorrectness of this view, and in Acipenser (videfig. 57,mp) the two layers of the muscle-plate retain their original relations after the cells of both of them have become converted into muscles.
[246]VideVetter, “Die Kiemen und Kiefermusculatur d. Fische.”Jenaische Zeitschrift,Vol.VII.
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 medianposteriorlyplaced contractile vesicle, or by a pair of vesicles, which have a ventral andanteriorposition. 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[247], and the other always present in the adult.
The larval excretory organ has been found in the pulmonate Gasteropoda (Gegenbaur, Fol[248], 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 non-ciliated pedal limb opening to the exterior[249].
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, whilethe 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, havebeen 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 Chætopoda described below. Their development has not been worked out.
Chætopoda. Two forms of excretory organ have been met with in the Chætopoda. 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 beseveral internal openings for each organ(Polynoe). In the Capitellidæ 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 Chætopoda. Kowalevsky (No.277), from his observations on the Oligochæta, 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 iscontinued 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 Eisig (No.512) on the Capitellidæ 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.
Illustration: Figure 383Fig. 383. Polygordius larva.(After Hatschek.)m.mouth;sg.supraœsophageal ganglion;nph.nephridion;me.p.mesoblastic band;an.anus;ol.stomach.
Fig. 383. Polygordius larva.(After Hatschek.)m.mouth;sg.supraœsophageal ganglion;nph.nephridion;me.p.mesoblastic band;an.anus;ol.stomach.
The excretory system begins to be formed, while the larva is still in the trochospere stage (fig. 383,nph), 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 larvæ (videp. 681).
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, each with an anterior funnel-like opening situated in the midst of the mesoblast cells, and a posterior external 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,nph) and the two openings so formed may eventually become divided into five (fig. 384A), 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. 384B), 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.
Illustration: Figure 384Fig. 384. Diagram illustrating the development of the excretory system of Polygordius.(After Hatschek.)
Fig. 384. Diagram illustrating the development of the excretory system of Polygordius.(After Hatschek.)
The tube may be continued with a sinuous course through 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 permanentexcretory tubes is diagrammatically represented infig. 384C 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 (1) 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 larvæ of Echiurus and Bonellia.
In all the Gephyrea the so-called ‘brown tubes’ are apparently homologous with the segmented excretory tubes of Chætopods. 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 inEchiurusHatschek (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 attheir 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 hindgut.
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 ofBonelliaSpengel 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 (videVol.II.,fig. 162C,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 Chætopoda. They may either be provided with a peritoneal funnel (Nephelis, Clepsine) or have no internal opening (Hirudo).
Bourne[250]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 Chætopoda.
The excretory tubes of Clepsine are held by Whitman to be developed in the mesoblast.
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 Bütschli) 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. Fürbringer 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 ArthropodaPeripatusis the only form with excretory organs of the type of the segmental excretory organs of the Chætopoda[251].
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:
(1) 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 tubeand 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 (videVol.II.) originate as a pair of outgrowths of the epiblastic proctodæum. 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 larvæ 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 Chætopoda.
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 (1) 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-kidney,though it may perhaps be more suitably called, adopting Lankester’s nomenclature, thepronephros. Its duct, which forms the basis for the generative and urinary ducts, will be called thesegmental duct.
(2) The Wolffian body, which may be also called themesonephros. 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 theWolffian or mesonephric duct, while the other is known as theMüllerian duct.
(3) The kidney proper ormetanephros. This organ is only found in a completely differentiated form in the amniotic Vertebrata. Its duct is an outgrowth from the Wolffian 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. 385A,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. 385B,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, whichmakes its appearance in the column (fig. 386,sd). The knob forms the only structure which can be regarded as a rudiment of the pronephros.
Illustration: Figure 385Fig. 385. Two sections of a Pristiurus embryo with three visceral clefts.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;mc.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;pp.body cavity;ep.epiblast;al.alimentary canal.
Fig. 385. Two sections of a Pristiurus embryo with three visceral clefts.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;mc.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;pp.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)[252], 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 obliquelybackwards round the inner and dorsal side of the segmental duct, near which it at first ends blindly.
Illustration: Figure 386Fig. 386. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal canal;W.white matter of spinal cord;pr.posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;mp.muscle-plate;mp´.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.
Fig. 386. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal canal;W.white matter of spinal cord;pr.posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;mp.muscle-plate;mp´.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.
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 (1) 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.pdis the segmental duct andoits abdominal opening;s.tpoints 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.
Illustration: Figure 387Fig. 387. Diagram of the primitive condition of the kidney in an Elasmobranch embryo.pd.segmental duct. It opens atointo 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 Müllerian duct below;s.t.segmental tubes. They open at one end into the body cavity, and at the other into the segmental duct.
Fig. 387. Diagram of the primitive condition of the kidney in an Elasmobranch embryo.pd.segmental duct. It opens atointo 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 Müllerian duct below;s.t.segmental tubes. They open at one end into the body cavity, and at the other into the segmental duct.
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 infig. 387by the clear linex, and in transverse section infigs.388and389. The resulting ducts are (1) the Wolffian duct or mesonephric duct (wd), dorsally, which remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Müllerian duct in the female, and the rudiments of this duct in the male. In the 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 Müllerian duct (od). 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. 389A). The anterior part of the segmental duct is not divided, but remains continuous with the Müllerian duct, of which its anterior pore forms the permanent peritoneal opening[253](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 chieflyin the fact of the anterior undivided part of the segmental duct, which forms the front end of the Müllerian duct, being shorter, and in the column of cells with which it is continuous being from the first incomplete.