[247]I leave out of consideration an external renal organ found in many marine Gasteropod larvæ,videVol.II. p.280.[248]H. Fol, “Etudes sur le dével. d. Mollusques.” Mem.III.Archiv d. Zool. expér. et génér.,Vol.VIII.[249]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 onp.280 ofVol.II.of this treatise, made on the authority of Rabl and Bütschli, must probably be corrected.[250]“On the Structure of the Nephridia of the Medicinal Leech.”Quart. J. of Micr. Science,Vol.XX. 1880.[251]VideF. M. Balfour, “On some points in the Anatomy of Peripatus Capensis.”Quart. J. of Micr. Science,Vol.XIX. 1879.[252]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.[253]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 Müllerian duct; but they appear to atrophy very early, without acquiring a definite attachment to the segmental duct.[254]Acipenser has been investigated by Fürbringer, Salensky, Sedgwick, and also by myself, and Lepidosteus by W. N. Parker and myself.[255]I have not fully proved this point, but have never found more than one opening.[256]Whether the segmental tubes are formed as ingrowths of the peritoneal epithelium, orin situ, could not be determined.[257]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.[258]The foremost extremity of the segmental duct presents, according to Gasser, curious irregularities and an anterior completely isolated portion is often present.[259]Correct figures of the early stages of these structures were first given by Kölliker, 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).[260]These external glomeruli were originally mistaken by me (No.539) for the glomerulus of the pronephros, from their resemblance to the glomerulus of the Amphibian pronephros. Their true meaning was made out by Sedgwick (No.550).[261]This is also called parovarium (His), and Rosenmüller’s organ.[262]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 Kölliker.[263]According to 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.[264]As mentioned in the note onp.729Sedgwick maintains that the anterior segmental tubes of the Chick form an exception to this general statement.[265]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.[266]This suggestion has I believe been made by Fürbringer.[267]The papers of Fürbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs.
[247]I leave out of consideration an external renal organ found in many marine Gasteropod larvæ,videVol.II. p.280.
[248]H. Fol, “Etudes sur le dével. d. Mollusques.” Mem.III.Archiv d. Zool. expér. et génér.,Vol.VIII.
[249]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 onp.280 ofVol.II.of this treatise, made on the authority of Rabl and Bütschli, must probably be corrected.
[250]“On the Structure of the Nephridia of the Medicinal Leech.”Quart. J. of Micr. Science,Vol.XX. 1880.
[251]VideF. M. Balfour, “On some points in the Anatomy of Peripatus Capensis.”Quart. J. of Micr. Science,Vol.XIX. 1879.
[252]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.
[253]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 Müllerian duct; but they appear to atrophy very early, without acquiring a definite attachment to the segmental duct.
[254]Acipenser has been investigated by Fürbringer, Salensky, Sedgwick, and also by myself, and Lepidosteus by W. N. Parker and myself.
[255]I have not fully proved this point, but have never found more than one opening.
[256]Whether the segmental tubes are formed as ingrowths of the peritoneal epithelium, orin situ, could not be determined.
[257]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.
[258]The foremost extremity of the segmental duct presents, according to Gasser, curious irregularities and an anterior completely isolated portion is often present.
[259]Correct figures of the early stages of these structures were first given by Kölliker, 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).
[260]These external glomeruli were originally mistaken by me (No.539) for the glomerulus of the pronephros, from their resemblance to the glomerulus of the Amphibian pronephros. Their true meaning was made out by Sedgwick (No.550).
[261]This is also called parovarium (His), and Rosenmüller’s organ.
[262]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 Kölliker.
[263]According to 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.
[264]As mentioned in the note onp.729Sedgwick maintains that the anterior segmental tubes of the Chick form an exception to this general statement.
[265]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.
[266]This suggestion has I believe been made by Fürbringer.
[267]The papers of Fürbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs.
Generative organs.
The structure and growth of the ovum and spermatozoon were given in the first chapter of this work, but their derivation from the germinal layers was not touched on, and it is this subject with which we are here concerned. If there are any structures whose identity throughout the Metazoa is not open to doubt these structures are the ovum and spermatozoon; and the constancy of their relations to the germinal layers would seem to be a crucial test as to whether the latter have the morphological importance usually attributed to them.
The very fragmentary state of our knowledge of the origin of the generative cells has however prevented this test being so far very generally applied.
Porifera. In the Porifera the researches of Schulze have clearly demonstrated that both the ova and the spermatozoa take their origin from indifferent cells of the general parenchyma, which may be called mesoblastic. The primitive germinal cells of the two sexes are not distinguishable; but a germinal cell by enlarging and becoming spherical gives rise to an ovum; and by subdivision forms a sperm-morula, from the constituent cells of which the spermatozoa are directly developed.
Cœlenterata. The greatest confusion prevails as to the germinal layer from which the male and female products are derived in the Cœlenterata[268].
The following apparent modes of origin of these products have been observed.
(1) The generative products of both sexes originate in the ectoderm (epiblast): Hydra, Cordylophora, Tubularia, all (?) free Gonophores of Hydromedusæ, the Siphonophora, and probably the Ctenophora.
(2) The generative products of both sexes originate in the entoderm (hypoblast): Plumularia and Sertularella, amongst the Hydroids, and the whole of the Acraspeda and Actinozoa.
(3) The male cells are formed in the ectoderm, and the female in the entoderm: Gonothyræa, Campanularia, Hydractinia, Clava.
In view of the somewhat surprising results to which the researches on the origin of the genital products amongst the Cœlenterata have led, it would seem to be necessary either to hold that there is no definite homology between the germinal layers in the different forms of Cœlenterata, or to offer some satisfactory explanation of the behaviour of the genital products, which would not involve the acceptance of the first alternative.
Though it can hardly be said that such an explanation has yet been offered, some observations of Kleinenberg (No.557) undoubtedly point to such an explanation being possible.
Kleinenberg has shewn that in Eudendrium the ova migrate freely from the ectoderm into the endoderm, andvice versa; but he has given strong grounds for thinking thatthey originatein the ectoderm. He has further shewn that the migration in this type is by no means an isolated phenomenon.
Since it is usually only possible to recognise generative elements after they have advanced considerably in development, the mere position of a generative cell, when first observed, can afford, after what Kleinenberg has shewn, no absolute proof of its origin. Thus it is quite possible that there is really only one type of origin for the generative cells in the Cœlenterata.
Kleinenberg has given reasons for thinking that the migration of the ova into the entoderm may have a nutritive object. If this be so, and there are numerous facts which shew that the position of generative cells is often largely influenced by their nutritive requirements, it seems not impossiblethat the endodermal position of the generative organs in the Actinozoa and acraspedote Medusæ may have arisen by a continuously earlier migration of the generative cells from the ectoderm into the endoderm; and that the migration may now take place at so early a period of the development, that we should be justified in formally holding the generative products to be endodermal in origin.
We might perhaps, on this view, formulate the origin of the generative products in the Cœlenterata in the following way:—
Both ova and spermatozoa primitively originated in the ectoderm, but in order to secure a more complete nutrition the cells which give rise to them exhibit in certain groups a tendency to migrate into the endoderm. This migration, which may concern the generative cells of one or of both the sexes, takes place in some cases after the generative cells have become recognisable as such, and very probably in other cases at so early a period that it is impossible to distinguish the generative cells from indifferent embryonic cells.
Very little is known with reference to the origin of the generative cells in the triploblastic Invertebrata.
Chætopoda and Gephyrea. In the Chætopoda and Gephyrea, the germinal cells are always developed in the adult from the epithelial lining of the body cavity; so that their origin from the mesoblast seems fairly established.
If we are justified in holding the body cavity of these forms to be a derivative of the primitive archenteron (videpp.356and357) the generative cells may fairly be held to originate from a layer which corresponds to the endoderm of the Cœlenterata[269].
Chætognatha. In Sagitta the history of the generative cells, which was first worked out by Kowalevsky and Bütschli, has been recently treated with great detail by O. Hertwig[270].
The generative cells appear during the gastrula stage, as two large cells with conspicuous nuclei, which are placed in the hypoblast lining the archenteron, at the pole opposite the blastopore. These cells soon divide, and at the same time pass out of the hypoblast, and enter the archenteric cavity (fig. 408A,ge). The division into four cells, which is not satisfactorily represented in my diagram, takes place in such a way that twocells are placed nearer the median line, and two externally. The two inner cells form the eventual testes, and the outer the ovaries,one half of each primitive cell thus forming an ovary, and the other a testis.
Illustration: Figure 408Fig. 408. Three stages in the development of Sagitta.(A and C after Bütschli, and B after Kowalevsky.)The three embryos are represented in the same positions.A. Represents the gastrula stage.B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.C. Represents a later stage, in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore is closed.m.mouth;al.alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnic mesoblast;so.somatic mesoblast;ge.generative organs.
Fig. 408. Three stages in the development of Sagitta.(A and C after Bütschli, and B after Kowalevsky.)The three embryos are represented in the same positions.A. Represents the gastrula stage.B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.C. Represents a later stage, in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore is closed.m.mouth;al.alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnic mesoblast;so.somatic mesoblast;ge.generative organs.
Illustration: Figure 409Fig. 409. Two views of a late embryo of Sagitta.A, from the dorsal surface. B, from the side. (After Bütschli.)m.mouth;al.alimentary canal;v.g.ventral ganglion (thickening of epiblast);ep.epiblast;c.pv.cephalic section of body cavity;so.somatopleure;sp.splanchnopleure;ge.generative organs.
Fig. 409. Two views of a late embryo of Sagitta.A, from the dorsal surface. B, from the side. (After Bütschli.)m.mouth;al.alimentary canal;v.g.ventral ganglion (thickening of epiblast);ep.epiblast;c.pv.cephalic section of body cavity;so.somatopleure;sp.splanchnopleure;ge.generative organs.
When the archenteric cavity is divided into a median alimentary tract, and two lateral sections forming the body cavity, the generative organs are placed in the common vestibule into which both the body cavity and alimentary cavity at first open (fig. 408).
The generative organs long retain their character as simple cells. Eventually (fig. 409) the two ovaries travel forwards, and apply themselves to the body walls, while the two testes also become separated by a backward prolongation of the median alimentary tract.
On the formation of the transverse septum dividing the tail from the body, the ovarian cells lie immediately in front of this septum, and the testicular cells in the region behind it.
Polyzoa. In Pedicellina amongst the entoproctous Polyzoa Hatschek finds that the generative organs originate from a pair of specially large mesoblast cells, situated in the space between the stomach and the floor of the vestibule. The two cells undergo changes, which have an obvious resemblance to those of the generative cells of the Chætognatha. They become surrounded by an investment of mesoblast cells, and divide so as to form two masses. Each of these masses at a later period separates into an anterior and a posterior part. The former becomes the ovary, the latter the testis.
Nematoda. In the Nematoda the generative organs are derived from the division of a single cell which would appear to be mesoblastic[271].
Insecta. The generative cells have been observed at a very early embryonic stage in several insect forms (Vol.II. p.404), but the observations so far recorded with reference to them do not enable us to determine with certainty from which of the germinal layers they are derived.
Crustacea. In Moina, one of the Cladocera, Grobben[272]has shewn that the generative organs are derived from a single cell, which becomes differentiated during the segmentation. This cell, which is in close contiguity with the cells from which both the mesoblast and hypoblast originate, subsequently divides;but at the gastrula stage, and after the mesoblast has become formed, the cells it gives rise toare enclosed in the epiblast, and do not migrate inwards till a later stage. The products of the division of the generative cell subsequently divide into two masses. It is not possible to assign the generative cell of Moina to a definite germinal layer. Grobben, however, thinks that it originates from the division of a cell, the remainder of which gives rise to the hypoblast.
Chordata. In the Vertebrata, the primitive generative cells (often known as primitive ova) are early distinguishable, being imbedded amongst the cells of two linear streaks of peritoneal epithelium, placed on the dorsal side of the body cavity, one on each side of the mesentery (figs.405C and410,po). They appear to be derived from the epithelial cells amongst which they lie; and are characterized by containing a large granular nucleus, surrounded by a considerable body of protoplasm. The peritoneal epithelium in which they are placed is known as thegerminal epithelium.
Illustration: Figure 410Fig. 410. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal cord;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. 410. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal cord;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.
It is at first impossible to distinguish the germinal cells which will become ova from those which will become spermatozoa.
The former however remain within the peritoneal epithelium (fig. 411), and become converted into ova in a manner more particularly described inVol.II.pp.54-59.
The history of the primitive germinal cells in the male has not been so adequately worked out as in the female.
The fullest history of them is that given by Semper (No.559) for the Elasmobranchii, the general accuracy of which I can fully support;though with reference to certain stages in the history further researches are still required[273].
In Elasmobranchii the male germinal cells, instead of remaining in the germinal epithelium, migrate into the adjacent stroma, accompanied I believe by some of the indifferent epithelial cells. Here they increase in number, and give rise to masses of variable form, composed partly of true germinal cells, and partly of smaller cells with deeply staining nuclei, which are, I believe, derived from the germinal epithelium.
Illustration: Figure 411Transverse section through the ovary of a young embryo of scyllium canicula, to shew the primitive germinal cells(po)lying in the germinal epithelium on the outer side of the ovarian ridge.
Transverse section through the ovary of a young embryo of scyllium canicula, to shew the primitive germinal cells(po)lying in the germinal epithelium on the outer side of the ovarian ridge.
These masses next break up into ampullæ, mainly formed of germinal cells, and each provided with a central lumen; and these ampullæ attach themselves to tubes derived from the smaller cells, which are in their turn continuous with the testicular network. The spermatozoa are developed from the cells forming the walls of the primitive ampullæ; but the process of their formation does not concern us in this chapter.
In the Reptilia Braun has traced the passage of the primitive germinal cells into the testicular tubes, and I am able to confirm his observations on this point: he has not however traced their further history.
In Mammalia the evidence of the origin of the spermatospores from the germinal epithelium is not quite complete, but there can be but little doubt of its occurrence[274].
In Amphioxus Langerhans has shewn that the ova and spermatozoa are derived from similar germinal cells, which may be compared to the germinal epithelium of the Vertebrata. These cells are however segmentally arranged as separate masses (videVol.II. p.54).
Bibliography.
(554)G. Balbiani.Leçons s. la génération des Vertébrés.Paris, 1879.(555)F. M. Balfour. “On the structure and development of the Vertebrate ovary.”Quart. J. of Micr. Science,Vol.XVIII.(556)E. van Beneden. “De la distinction originelle du tecticule et de l'ovaire, etc.”Bull. Ac. roy. belgique,Vol.XXXVII. 1874.(557)N. Kleinenberg. “Ueb. d. Entstehung d. Eier b. Eudendrium.”Zeit. f. wiss. Zool.,Vol.XXXV. 1881.(558)H. Ludwig. “Ueb. d. Eibildung im Theirreiche.”Arbeit. a. d. zool.-zoot. Instit. Würzburg,Vol.I. 1874.(559)C. Semper. “Das Urogenitalsystem d. Plagiostomen, etc.”Arbeit. a. d. zool.-zoot. Instit. Würzburg,Vol.II. 1875.(560)A. Weismann. “Zur Frage nach dem Ursprung d. Geschlechtszellen bei den Hydroiden.”Zool. Anzeiger,No.55, 1880.
Videalso O. and R. Hertwig (No.271), Kölliker (No.298), etc.
Genital ducts.
The development and evolution of the generative ducts is as yet very incompletely worked out, but even in the light of our present knowledge a comparative review of this subject brings to light features of considerable interest, and displays a fruitful field for future research.
In the Cœlenterata there are no generative ducts.
In the Hydromedusæ and Siphonophora the generative products are liberated by being dehisced directly into the surrounding medium; while in the Acraspeda, the Actinozoa and the Ctenophora, they are dehisced into parts of the gastrovascular system, and carried to the exterior through the mouth.
The arrangement in the latter forms indicates the origin ofthe methods of transportation of the genital products to the exterior in many of the higher types.
It has been already pointed out that the body cavity in a very large number of forms is probably derived from parts of a gastrovascular system like that of the Actinozoa.
When the part of the gastrovascular system into which the generative products were dehisced became, on giving rise to the body cavity, shut off from the exterior, it would be essential that some mode of transportation outwards of the generative products should be constituted.
In some instances simple pores (probably already existing at the time of the establishment of a closed body cavity) become the generative ducts. Such seems probably to have been the case in the Chætognatha (Sagitta) and in the primitive Chordata.
In the latter forms the generative products are sometimes dehisced into the peritoneal cavity, and thence transported by the abdominal pores to the exterior (Cyclostomata and some Teleostei,videp. 626). In Amphioxus they pass by dehiscence into the atrial cavity, and thence through the gill slits and by the mouth, or by the abdominal pore (?) to the exterior. The arrangement in Amphioxus and the Teleostei is probably secondary, as possibly also is that in the Cyclostomata; so that the primitive mode of exit of the generative products in the Chordata is still uncertain. It is highly improbable that the generative ducts of the Tunicata are primitive structures.
A better established and more frequent mode of exit of the generative products when dehisced into the body cavity is by means of the excretory organs. The generative products pass from the body cavity into the open peritoneal funnels of such organs, and thence through their ducts to the exterior. This mode of exit of the generative products is characteristic of the Chætopoda, the Gephyrea, the Brachiopoda and the Vertebrata, and probably also of the Mollusca. It is moreover quite possible that it occurs in the Polyzoa, some of the Arthropoda, the Platyelminthes and some other types.
The simple segmental excretory organs of the Polychæta, the Gephyrea and the Brachiopoda serve as generative canals, and in many instances they exhibit no modification, or but a very slight one, in connection with their secondary generativefunction; while in other instances,e.g.Bonellia, such modification is very considerable.
The generative ducts of the Oligochæta are probably derived from excretory organs. In the Terricola ordinary excretory organs are present in the generative segments in addition to the generative ducts, while in the Limicola generative ducts alone are present in the adult, but before their development excretory organs of the usual type are found, which undergo atrophy on the appearance of the generative ducts (Vedjovsky).
From the analogy of the splitting of the segmental duct of the Vertebrata into the Müllerian and Wolffian ducts, as a result of a combined generative and excretory function (videp.728), it seems probable that in the generative segments of the Oligochæta the excretory organs had at first both an excretory and a generative function, and that, as a secondary result of this double function, each of them has become split into two parts, a generative and an excretory. The generative part has undergone in all forms great modifications. The excretory parts remain unmodified in the Earthworms (Terricola), but completely abort on the development of the generative ducts in the Limicola. An explanation may probably be given of the peculiar arrangements of the generative ducts in Saccocirrus amongst the Polychæta (videMarion and Bobretzky), analogous to that just offered for the Oligochæta.
The very interesting modifications produced in the excretory organs of the Vertebrata by their serving as generative ducts were fully described in the last chapter; and with reference to this part of our subject it is only necessary to call attention to the case of Lepidosteus and the Teleostei.
In Lepidosteus the Müllerian duct appears to have become attached to the generative organs, so that the generative products, instead of falling directly into the body cavity and thence entering the open end of a peritoneal funnel of the excretory organs, pass directly into the Müllerian duct without entering the body cavity. In most Teleostei the modification is more complete, in that the generative ducts in the adult have no obvious connection with the excretory organs.
The transportation of the male products to the exterior in all the higher Vertebrata, without passing into the body cavity, is in principle similar to the arrangement in Lepidosteus.
The above instances of the peritoneal funnels of an excretory organ becoming continuous with the generative glands, render it highly probable that there may be similar instances amongst the Invertebrata.
As has been already pointed out by Gegenbaur there are many features in the structure of the genital ducts in the more primitive Mollusca, which point to their having been derived from the excretory organs. In several Lamellibranchiata[275](Spondylus, Lima, Pecten) the generative ducts open into the excretory organs (organ of Bojanus), so that the generative products have to pass through the excretory organ on their way to the exterior. In other Lamellibranchiata the genital and excretory organs open on a common papilla, and in the remaining types they are placed close together.
In the Cephalopoda again the peculiar relations of the generative organs to their ducts point to the latter having primitively had a different, probably an excretory, function. The glands are not continuous with the ducts, but are placed in special capsules from which the ducts proceed. The genital products are dehisced into these capsules and thence pass into the ducts.
In the Gasteropoda the genital gland is directly continuous with its duct, and the latter, especially in the Pulmonata and Opisthobranchiata, assumes such a complicated form that its origin from the excretory organ would hardly have been suspected. The fact however that its opening is placed near that of the excretory organ points to its being homologous with the generative ducts of the more primitive types.
In the Discophora, where the generative ducts are continuous with the glands, the structure both of the generative glands and ducts points to the latter having originated from excretory organs.
It seems, as already mentioned, very possible that there are other types in which the generative ducts are derived from the excretory organs. In the Arthropoda for instance the generative ducts, where provided with anteriorly placed openings, as in the Crustacea, Arachnida and the Chilognathous Myriapoda, the Pœcilopoda, etc., may possibly be of this nature, but the data for deciding this point are so scanty that it is not at present possible to do more than frame conjectures.
The ontogeny of the generative ducts of the Nematoda andthe Insecta appears to point to their having originated independently of the excretory organs.
In the Nematoda the generative organs of both sexes originate from a single cell (Schneider,Vol.I.No.390).
This cell elongates and its nuclei multiply. After assuming a somewhat columnar form, it divides into (1) a superficial investing layer, and (2) an axial portion.
In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the further development the two ends of the column become the blind ends of the ovary, and the axial tissue they contain forms the germinal tissue of nucleated protoplasm. The superficial layer gives rise to the epithelium of the uterus and oviduct. The germinal tissue, which is originally continuous, is interrupted in the middle part (where the superficial layer gives rise to the uterus and oviduct), and is confined to the two blind extremities of the tube.
In the male the superficial layer, which gives rise to the epithelium of the vas deferens, is only formed at the hinder end of the original column. In other respects the development takes place as in the female.
In the Insecta again the evidence, though somewhat conflicting, indicates that the generative ducts arise very much as in Nematodes, from the same primitive mass as the generative organs. In both of these types it would seem probable that the generative organs were primitively placed in the body cavity, and attached to the epidermis, through a pore in which their products passed out; and that, acquiring a tubular form, the peripheral part of the gland gave rise to a duct, the remainder constituting the true generative gland. It is quite possible that the generative ducts of such forms as the Platyelminthes may have had a similar origin to those in Insecta and Nematoda, but from the analogy of the Mollusca there is nearly as much to be said for regarding them as modified excretory organs.
In the Echinodermata nothing is unfortunately known as to the ontogeny of the generative organs and ducts. The structure of these organs in the adult would however seem to indicate that the most primitive type of echinoderm generative organ consists of a blind sack, projecting into the body cavity, and opening bya pore to the exterior. The sack is lined by an epithelium, continuous with the epidermis, the cells of which give rise to the ova or spermatozoa. The duct of these organs is obviously hardly differentiated from the gland; and the whole structure might easily be derived from the type of generative organ characteristic of the Hydromedusæ, where the generative cells are developed from special areas of the ectoderm, and, when ripe, pass directly into the surrounding medium.
If this suggestion is correct we may suppose that the generative ducts of the Echinodermata have a different origin to those of the majority of[276]the remaining triploblastica.
Their ducts have been evolved in forms in which the generative products continued to be liberated directly to the exterior, as in the Hydromedusæ; while those of other types have been evolved in forms in which the generative products were first transported, as in the Actinozoa, into the gastrovascular canals[277].
[268]E. van Beneden (No.556) was the first to discover a different origin for the generative products of the two sexes in Hydractinia, and his observations have led to numerous subsequent researches on the subject. For a summary of the observations on the HydroidsvideWeismann (No.560).[269]The Hertwigs (No.271) state that in their opinion the generative cells arise from the lining of the body cavity in all the forms whose body cavity is a product of the archenteron. We do not know anything of the embryonic development of the generative organs in the Echinodermata, but the adult position of the generative organs in this group is very unfavourable to the Hertwigs’ view.[270]O. Hertwig,Die Chætognathen. Jena, 1880.[271]VideVol.II. p.374; also Götte,Zool. Anzeiger,No.80,p.189.[272]C. Grobben. “Die Entwick. d. Moina rectirostris.”Arbeit. a. d. zool. Instit. Wien.Vol.II.1879.[273]Balbiani (No.554) has also recently dealt with this subject, but I cannot bring my own observations into accord with his as to the structure of the Elasmobranch testis.[274]An entirely different view of the origin of the sperm cells has been adopted by Balbiani, for which the reader is referred to his Memoir (No.554).[275]For a summary of the facts on this subjectvideBronn,Klassen u. Ordnungen d. Thierreichs,Vol.III. p.404.[276]It would be interesting to have further information about Balanoglossus.[277]These views fit in very well with those already put forward in ChapterXIII.on the affinities of the Echinodermata.
[268]E. van Beneden (No.556) was the first to discover a different origin for the generative products of the two sexes in Hydractinia, and his observations have led to numerous subsequent researches on the subject. For a summary of the observations on the HydroidsvideWeismann (No.560).
[269]The Hertwigs (No.271) state that in their opinion the generative cells arise from the lining of the body cavity in all the forms whose body cavity is a product of the archenteron. We do not know anything of the embryonic development of the generative organs in the Echinodermata, but the adult position of the generative organs in this group is very unfavourable to the Hertwigs’ view.
[270]O. Hertwig,Die Chætognathen. Jena, 1880.
[271]VideVol.II. p.374; also Götte,Zool. Anzeiger,No.80,p.189.
[272]C. Grobben. “Die Entwick. d. Moina rectirostris.”Arbeit. a. d. zool. Instit. Wien.Vol.II.1879.
[273]Balbiani (No.554) has also recently dealt with this subject, but I cannot bring my own observations into accord with his as to the structure of the Elasmobranch testis.
[274]An entirely different view of the origin of the sperm cells has been adopted by Balbiani, for which the reader is referred to his Memoir (No.554).
[275]For a summary of the facts on this subjectvideBronn,Klassen u. Ordnungen d. Thierreichs,Vol.III. p.404.
[276]It would be interesting to have further information about Balanoglossus.
[277]These views fit in very well with those already put forward in ChapterXIII.on the affinities of the Echinodermata.
The alimentary canal in the Chordata is always formed of three sections, analogous to those so universally present in the Invertebrata. These sections are (1) the mesenteron lined by hypoblast; (2) the stomodæum or mouth lined by epiblast, and (3) the proctodæum or anal section lined like the stomodæum by epiblast.
Mesenteron.
The early development of the epithelial wall of the mesenteron has already been described (ChapterXI.). It forms at first a simple hypoblastic tube extending from near the front end of the body, where it terminates blindly, to the hinder extremity where it is united with the neural tube by the neurenteric canal (fig. 420,ne). It often remains for a long time widely open in the middle towards the yolk-sack.
It has already been shewn that from the dorsal wall of the mesenteron the notochord is separated off nearly at the same time as the lateral plates of mesoblast (pp. 292-300).
The subnotochordal rod. At a period slightly subsequent to the formation of the notochord, and before any important differentiations in the mesenteron have become apparent, a remarkable rod-like body, which was first discovered by Götte, becomes split off from the dorsal wall of the alimentary tract in all the Ichthyopsida. This body, which has a purely provisional existence, is known as the subnotochordal rod.
It develops in Elasmobranch embryos in two sections, one situated in the head, and the other in the trunk.
The section in the trunk is the first to appear. The wall of the alimentary canal becomes thickened along the median dorsal line (fig. 412,x), or else produced into a ridge into which there penetrates a narrow prolongation of the lumen of the alimentary canal. In either case the cells at the extreme summit become gradually constricted off as a rod, which lies immediately dorsal to the alimentary tract, and ventral to the notochord (fig. 413,x).