-Amphioxus._
-Amphioxus._
Section 1. We find in Amphioxus the essential vertebrate features reduced to their simplest expression and, in addition, somewhat distorted. There are wide differences from that vertebrate plan with which the reader may now be considered familiar. There are no limbs. There is an unbroken fin along the median dorsal line and coming round along the ventral middle line for about half the animal's length. But two lowly vertebrates, the hag-fish and lamprey, have no limbs and a continuous fin. There is, as we shall see more clearly, a structure, the respiratoryatrium, not apparently represented in the true vertebrate types, at least in their adult stages. There is no distinct heart, only a debateable brain, quite without the typical division into three primary vesicles, no skull, no structures whatever of cartilage or bone, no genitalducts, no kidneys at all resembling those of the vertebrata, no pancreas, no spleen; apparently no sympathetic chain, nopairedsense organs, eyes, ears, or nasal sacs, in all of which points we have striking differences from all true vertebrata; and such a characteristic vertebrate peculiarity as the pineal gland we can only say is represented very doubtfully by the eye spot.
Section 2. The vertebral column is devoid of vertebrae; it is throughout life a rod of gelatinous tissue, thenotochord(Figure 1, n.c.), surrounded by a cellular sheath. Such a rod is precursor to the vertebral column in the true vertebrates, but, except in such lowly forms as the lamprey, is usually replaced, partially (e.g., dog-fish) or wholly (as in the rabbit) byat firstcartilaginous vertebrae whose bodies are derived from its sheath. Further, while in all true vertebrata the notochord of the developing young reaches anteriorly at most to the mid-brain, and is there at its termination enclosed by the middle portion of the skull, inAmphioxusit reaches far in front of the anterior extremity of the nervous system, to the end of the animal's body.* On this account the following classification is sometimes made of those animals which have a notochord:--
-Chordata_ (=Vertebrata, as used by Lankester).1. Having the notochord reaching in front of the brain.Cephalochorda=Amphioxus.2. Having the notochord reaching anteriorly to the mid-brain, a brain of three primary vesicles and a skull.Craniata= all "true vertebrata": fishes, amphibia, reptiles, birds, and mammals (Vertebrataof Balfour).3. Having the notochord confined to the tail.Urochorda= the ascidians, or sea-squirts, certain forms of life only recently recognised as relatives of the vertebrata.
1. Having the notochord reaching in front of the brain.Cephalochorda=Amphioxus.2. Having the notochord reaching anteriorly to the mid-brain, a brain of three primary vesicles and a skull.Craniata= all "true vertebrata": fishes, amphibia, reptiles, birds, and mammals (Vertebrataof Balfour).3. Having the notochord confined to the tail.Urochorda= the ascidians, or sea-squirts, certain forms of life only recently recognised as relatives of the vertebrata.
* The anterior end of the notochord in the developing rabbit or dog lies where the middle of the basisphenoid bone is destined to be.
Section 3. Figure 1,Sheet 19, shows the general anatomy ofAmphioxus. We recognise four important points of resemblance to the earlier phases of the higher and the permanent structure of the lower members of thevertebrata, and it is these that justify the inclusion of amphioxus in this volume. In the first place there is the--
-Notochord_.In the next, justabove it(at s.c.) we find---A DorsalTubularNervous Axis_,the spinal cord. Thirdly, the pharynx (ph.) is perforated by---Respiratory Slits_,though these, instead of being straight slashes, are modified from aU-shape [slant very much forward and are much more numerous than in any true vertebrate.]. -And-, Fourthly, there is, as we shall see, a---Vertebrate Type of Circulation_.[And finally the body-wall muscles are divided into--][-Myomers_.]
Section 4. Thealimentary canalofAmphioxuscommences with an "oral cavity," not represented in our vertebrata, surrounded by a number ofcirri, or tentacles, supported by a horny substance which seems to bechitin, a common skeletal material among invertebrates. Avelum(v.) forms a curtain, perforated by the mouth and by two smallerhyoidean apertures, between the oral cavity and the pharynx (ph.). "Pharynx" is here used in a wider sense than in the true vertebrata; it reaches back close to the liver, and is therefore equivalent to pharynx + oesophagus + a portion or all of the stomach. The [so-called] hyoidean apertures are not equivalent to the similarly-named parts of the vertebrata. Behind the pharynx the intestine (int.) runs straight out to theanus(an.), which opens not in the middle line, as one might expect, but in the left side! The liver lies usually on the creature's right, and instead of being a compact gland, is simply bag-like.
Section 5. Thecirculationis peculiarly reduced(Figure 2). The cardiac aorta (c.ao.) lies along the ventral side of the pharynx, and sends branches up along the complete bars between the gill slits. There is no -distinct- heart, but the whole of the cardiac aorta is contractile, and at the bases of the aortic arches that run up the bars there arecontractile dilatationsthat assist in the propulsion of the blood. Dorsal to the pharynx, as in fishes, there is a pair ofdorsal aorta(d.ao.) that unite above the liver (compare the frog, for instance), and thence run backward as a median dorsal aorta (d.ao.'). A portal vein (p.v.) bring blood back from the intestine (and apparently from the whole posterior portion of the animal) to the liver. Thence hepatic veins (hep.) take it to the cardiac aorta.
{Lines from First Edition only.}-When we remember that in the embryonic vertebrate the heart is at first a straight tube, this circulation appears even more strikingly vertebrate in its character than before.-
Section 6. Thecoelom, or body cavity, ofAmphioxuslies, of course, as in the vertebrata, between the intestinal wall and the body walls, and, just as in the vertebrata, it is largely reduced where gill slits occur. But matters are rather complicated by the presence of anatrial cavityround the pharynx, which is not certainly represented in the vertebrata, and which the student is at first apt to call the body cavity, although it is entirely distinct and different from that space. The mutual relation of the two will become apparent after a study of Figures 10, 11, 12 (Sheet 21). Figure 10 gives diagrammatically a section of a very young stage ofAmphioxus; P is the pharynx portion of the alimentary canal, coe. is the coelom surrounding it at this stage here as elsewhere; mt.c. are certain lymph spaces, themetapleural canals, between which a small invagination (i.e., a pushing-in), at., of the outer epidermis occurs; n.c. is the notochord, and s.c. the spinal cord. The gill slits, by which P. communicates with the exterior, are not shown. NextFigure 11shows the invagination (at.) pushing its way in, and cut off from the exterior by a meeting of the body wall below. Note that at. is a portion of the animal's exterior thus embraced by its body, and that its lining is therefore of the same material as the external integument. InFigure 12,at. is developing upward, so that the true body hangs into it. Now imagine the gill slits perforated, as shown by the double-headed arrow inFigure 12. Figure 3, onSheet 20, is a less diagrammatic representation of a cross-section of the pharyngeal region (videFigure 1,Sheet 19). The student should compare Figure 3,Sheet 20, and Figure 12,Sheet 21. The atrium and metapleural canals are easily recognised in both. In Figure 3 the coelom is much cut up by the gill slits, and we have remaining of it (a) thedorsal coelomic canals(d.c.c.) and (b) thebranchial canals(br.c.) in the bars between the slits. The atrial cavity remains open to the exterior at one point, the atrial pore (at.p.).
Section 7. The method of examining cross-sections is an extremely convenient one in the study of such a type asAmphioxus. The student should very carefully go over and copy the six sections onSheet 20, comparing Figure 1 as he goes. He should do this before reading what follows. One little matter must be borne in mind. These figures are merely intended to convey the great structural ideas, and they are considerably simplified; they must not be regarded as a substitute for the examination of microscopic sections. [He will notice a number of rounded masses from the body wall. The] -For instance, the body-wall- muscles ofAmphioxusare arranged in bundles bent sharply in an arrow shape, the point forward. -A number of these bundles are cut in any one section, and so the even shading of our diagrams, if they professed to be anything more than diagrams, should be broken up into masses.- These -bundles, we may mention-, are calledmyomeres, and they are indicated inFigure 1by lines pointing acutely forward. [Several are consequently cut in any transverse section (Sheet 20), and these are the rounded masses he sees.] Similarmyomeres, similarly situated, are found in fish, behind the head, and, less obviously, they occur with diminishing importance as the scale of the vertebrata is ascended.
Section 8. If we compare thenervous systemof amphioxus with that of any vertebrate, we find at once a number of striking differences. In the first place, the skeletal covering of it, the cranium and the neural arches of vertebrae, are represented only by a greatly simplified connective tissue. In the next, a simple and slight anterior dilatation alone represents the brain. A patch of black pigment anterior to this (e.s.) may or may not be what its name implies aneye-spot. There is aciliated funnel, c.f. (Figure 1,Sheet 19), openingon the left side, which has been assumed to be olfactory in its functions, and in the mouth chamber aciliated pit(c.p.), which may, or may not, be an organ of taste. The ventral fissure of the spinal cord is absent. The dorsal nerves are without ganglia, anddo not come off in pairs, but alternately, one to the left, then one to the right, one to the left, one to the right, and so on. The ventral nerves are very short, more numerous than the dorsal, and never unite with these latter to form mixed nerves.
The student will observe that here, just as in the case of the ciliated funnel and anus, theAmphioxusisnot strictly symmetrical, buttwisted, as it were, and so departs from the general rule of at least external bilateral symmetry obtaining among the vertebrates. It habitually lies on one side in the mud of the sea bottom, and it is probable that this external asymmetry is due to this habit, so that too much classificatory importance must not be attached to it. The soles and other related fish, for instance, are twisted and asymmetrical, through a similar specific habit, to such an extent that both eyes lie on one side of the animal.
Section 9. Nokidneyon the vertebrate pattern is found, but the following structures have, among others, been suggested as renal organs:--
(a) Certain canals, thebrown tubes of Lankester(b.t.L., Figure 2,Sheet 19), a pair of pigmented tubes opening into the atrium at the hind end of the pharynx, lying forward along by the dorsal coelomic canals, and having an internal opening also.(b) Certain tubuli described by Weiss as situated in a series along the upper corners of the atrial cavity, and communicating, after the fashion, of the "nephridia" of the earthworm, with the coelom and with the exterior (or, rather, with that portion of the animal's exterior enclosed in by the atrial wall; compareSection 6).(c) The general epithelial lining of the atrium.
Thereproductive organs(Figure 4,Sheet 20, g.) are masses of cells situated in an isolated part of the coelom in the atrial folds, and, having no ducts, their contents must escape into the atrium by rupture of the body-wall. Thence they escape either by gill-slits, pharynx and mouth, or, more generally, through the atrial pore. The animals, like all the vertebrata, are dioecious, i.e., male or female.
Section 10. Theendostyle(end.), inFigures 3 and 4, is a ciliated path or groove on the under side of the pharynx, which is generally supposed to represent the thyroid gland of vertebrates. The vertebrate thyroid, early in development, is certainly an open and long narrow groove in the ventral side of the pharynx. Thehyper-pharyngeal groove(h.p.) has been in the past compared to the pituitary body, but there is little doubt now that this structure is represented by the ciliated pit.
Section 11. The student is advised to revise this chapter before proceeding, and to schedule carefully the anatomical features under the headings of (1.) distinctly vertebrate characters, (2.) characters contrasting with the normal vertebrate structure, (3.) facts of doubtful import, with the suggestions given in the text written against them.
Section 12. The development of amphioxus, studied completely, is at once one of the most alluring and difficult tasks in the way of the zoologist; but certain of its earlier and most obvious fasts may very conveniently be taken into consideration now.
Section 13. The phenomena of the extrusion of polar bodies and fertilization are treated of later, and will, therefore, not be considered now. We will start our description with an egg-cell, which has escaped, of course, since there are no genital ducts, by rupture of the parent, has been fertilized by the male element, and is about to develop into a young amphioxus. It is simply a single cell, with some power of amoeboid motion, a single nucleus and nucleolus; and in amphioxus its protoplasm is clear and transparent. Frequently ova are loaded with granules of food store (yolk), which enable the young animal to go far with its development before it is hatched and has to begin fending for itself. Such an ovum as that of our present type, however being devoid of such yolk (alecithal= without yolk), necessitates a very early start in life, and, for reasons too complicated to state fully here, the development in such a case is considered particularly instructive and primitive by zoologists.
Section 14. The first thing to be seen in the developing cell is a deepening circular groove (Figure 1,Sheet 21), which divides the ovum into two parts. Another groove then cuts at right angles to this subdividing the two into four (Figure 2). Another groove, at right angles to both the former, follows, making the four eight (Figure 3). And so subdivision goes on. The whole process is calledsegmentationorcleavage.
Section 15. At the end of segmentation we get ahollowsphere of small cells, the cells separating from one another centrally and enclosing a cavity as the process proceeds. This is theblastosphere, shown diagrammatically inFigure 4, and of which an internal view, rather truer to the facts of the case as regards shape, is given asFigure 5. The central cavity is thesegmentation cavity(s.c.).
Section 16.Invaginationfollows (Figure 6). In this process a portion of the blastosphere wall is the tucked into the rest, as indicated by the arrow, so that a two-layered sack is formed. The space ar. is thearchenteron, the primordial intestine, and its mouth is called, theblastopore(bp.). The outer layer of this double-walled sac is called theepiblast. For the present we will give the inner lining no special term. The young amphioxus has, at this stage, which is called thegastrulastage, a curious parallelism with such a lowly form as theHydraof our ditches. This latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, orarchenteron, and its mouth is sometimes regarded as being ablastopore. All animals that have little yolk, and start early in life for themselves, pass through agastrulastage, substantially the same as this of amphioxus.
Section 17. The anus is perforated later near the region occupied at this stage by the blastopore. Hence the anterior end of the future amphioxus, the head end, is pointing towards the Figure 6, and the letters ep. are marked on the side which will be dorsal.
Section 18.Figure 7 i.is a dorsal view of the gastrula at a somewhat later stage, and here indications of distinctly vertebrate relationships already appear.Figure 7 ii. is a cross-section, its position, being shown by cross-lines in 7 i. and 6. Note first that the epiblast along the mid-dorsal line is sinking in to form what is called theneural plate(n.p.), and simultaneously on either side of it rise the neural folds (n.f.). Now, atFigure 8, a slightly later stage is represented, and at 9 i. the inturned part is separated from the general external epiblast as the spinal cord. The remainder of the epiblast constitutes the epidermis.
Section 19. Reverting toFigure 7 ii., along the dorsal side of the archenteron a thickening of its wall appears, and is gradually pinched off from it to form a cellular rod, lying along under the nervous axis and above the intestine. This is thenotochord(compareFigures 8 and 9).
Section 20. Finally, we note two series of buds of cells, one on either side of the archenteron inFigure 7 ii. In8these buds have become hollow vesicles, growing out from it, thecoelomic pouches. They are further developed in9; and in9 ii., which is a diagrammatic figure, they are indicated by dotted lines. They finally appear to (? entirely) obliterate the segmentation cavity-- they certainly do so throughout the body; and their cavities are in time cut off from themesenteron, by the gradual constriction of their openings. In this way the coelom (body cavity) arises as a series of hollow "archenteric" outgrowths, and ms. becomes the alimentary canal. mt.c., the metapleural canals, probably arise subsequently to, and independently of, the general coelomic space, by a splitting in the body-wall substance.
Section 21. Hence, in considering the structure of amphioxus, we have three series of cells from which its tissues are developed:--
1. Theepiblast.2. Walls of the coelomic pouches, which form (a) an inner lining to the epiblast, (b) an outer coating to the hypoblast, and (c) the mesentery (m.), by which the intestine is supported. This is themesoblast.3. The lining of the mesenteron, orhypoblast.
From the epiblast the epidermis (not the dermis), the nervous system (including the nerves), and the sensory part of all sense organs are derived. From the mesoblast the muscles, the dermis genital and excretory organs, circulatory fluid and apparatus, any skeletal structures; and all connective tissue are derived. The mass of the body is thus evidently made of mesoblast. The hypoblast is theliningof the intestine and of the glands which open into it; and the material of the notochord is also regarded, as hypoblast.
Section 22.Figure 9 ii.shows all the essential points of the structure of amphioxus. Epiblast is indicated by a line of dashes, mesoblast by dots, and hypoblast, dark or black. The true mouth is formed late by a tucking-in of epiblast, thestomodaeum(s.d.), which meets and fuses with the hypoblast, and is then perforated. The position of this mouth is at the velum. The formation of the atrium has been described. The metapleural folds run forward in front of thevelum, as theepipleurs(ep. inSections 1 and 2), and form anoral hood(b.c.), around which the tentacles appear, and which is evidently not equivalent to the vertebrate mouth cavity, but in front of and outside it. The anus is formed by a tucking in, theproctodaeum, similar to the stomodaeum.
Section 23. The formation of the respiratory slits is complicated, and difficult to describe, but, since investigators have still to render its meaning apparent, it need not detain the elementary student.*
* See Balfour'sEmbryology, Volume 2, and Quarterly Journal of Microscopical Science March, 1891.
-Development._
-Development._
Section 1. We have now to consider how the body of the frog is built up out of the egg cell, but previously to doing so we must revert to the reproductive organs of our type.
Section 2. In the testes of themaleis found an intricate network of tubuli, the lining of which is, of course, an epithelium. The cells of this epithelium have their internal borders differentiated intospermatozoa, which, at a subsequent stage, are liberated. A spermatozoon (Figure 3,Sheet 13, sp.) is a rod-shaped cell containing a nucleus; in fact, consisting chiefly of nucleus, with a tail, the flagellum, which is vibratile, and forces the spermatozoon, forward by its lashing. The spermatozoa float in a fluid which is the joint product of the testes, anterior part of the kidney, and perhaps the prostate glands.
Section 3. In the ovary, theovaare formed, and grow to a considerable size. They are nucleated cells, the nucleus going by the special name of thegerminal vesicleand the nucleolus thegerminal spot. The ova prey upon the adjacent cells as they develop. The protoplasm of the ovum, except at that part of the surface where the germinal vesicle lies, is packed with a great amount of food material, the yolk granules. This yolk is non-living inert matter. An ovum such as this, in which the protoplasm is concentrated towards one pole, is calledtelolecithal.
Section 4. After the ovum has finished its growth, and elaborated the yolk within itself, a peculiar change occurs in the small area free from yolk-- theanimal pole, in which the germinal vesicle lies. This germinal vesicle divides, and one moiety is budded off from the ovum. The ovum has, in fact, undergone cell division into a very large cell containing most of its substance, and a small protoplasmic pimple surrounding half of its nucleus. The disproportion is so great between the two cells, that the phenomenon does not at first suggest the idea of cell division, and it is usually described as theextrusion of the first polar body. There follows a second and similar small cell, behind the first, thesecond polar body. Since the nucleus of the ovum has divided twice, it is evident that the nucleus remaining now in the ovum is a quarter of the original nucleus. Very little protoplasm is given off with the polar bodies; they play no further part in development, but simply drop off and disappear. Not only in the frog's ovum, but in all vertebrata, two polar bodies are given off in this way before the sexual process occurs. Their exact meaning has been widely discussed. It is fairly evident that some material is removed from the nucleus, which would be detrimental to further developments, and the point debated is what is the precise nature of thisexcretedmaterial. This burning question we can scarcely deal with here.
Section 5. But here we may point out that in all cells thefunction of the nucleusappears to be to determine growth and division. It is the centre of directive energy in the cell.
Section 6. Fertilization is effected by a spermatozoon meeting with the ovum. It fuses with it, its nucleus becoming themale pro-nucleus. This and the female pro-nucleus, left after the extrusion of the polar cells, move towards each other, and unite to form thefirst segmentation nucleus.
Section 7. The ovum next begins to divide. A furrow cutting deeper and deeper divides it into two; another follows at right angles to this, making the two four, and another equatorial furrow cuts off the animal pole from the yolk orvegetative pole. (SeeSheet 22, Figures 1, 2, and 3.) And sosegmentation(=cleavage) proceeds, and, at last, a hollow sphere, theblastosphere(Figure 4) is formed, with a segmentation cavity (s.c.). But, because of the presence of the yolk at the vegetative pole of ovum, and of the mechanical resistance it offers to the force of segmentation, the protoplasm there is not nearly so finely divided-- the cells, that is to say, are much larger than at the animal pole. The blastosphere of the frog is like what the blastosphere of amphioxus would be, if the future hypoblast cells were enormously larger through their protoplasm being diluted with yolk.
Section 8. The next phase of development has an equally curious resemblance to and difference from what occurs in the case of the ova of animals which do not contain yolk. In such types (e.g., amphioxus) a part of the blastosphere wall is tucked into the rest, and a gastrula formed by this process of invagination. In the frog (Figure 5) there is a tucking-in, but the part that should liewithinthe gastrula, the yolk-containing cells, are far larger than the epiblast (ep.) which should, form the outer layer of cells. Hence theepiblastcan only by continual growth accommodate what it must embrace, and the process of tucking-in is accompanied by one of growth of the epiblast, as shown by the unbarbed arrow, over the yolk. This stage is called the gastrula stage; ar. is the cavity of the gastrula, thearchenteron; b.p. is its opening or blastopore. Such a gastrula, formed mainly by overgrowth of the epiblast, is called anepibolicgastrula, as distinguished from theinvaginategastrula of amphioxus. The difference is evidently entirely due to the presence of yolk, and the consequent modification of invagination in the former case.
Section 9. Comparing the two gastrulas, it is not difficult to see that if we imagine theventralwall of the archenteron of amphioxus to have its cells enormously enlarged through the mixing of yolk with their protoplasm, we should have a gastrula essentially like that of the frog.
Section 10.Figure 6shows a slightly later ovum thanFigure 5, seen from the dorsal side. b.p. is the blastopore. In front of that appears a groove, theneural groove, bordered on either side by a ridge, theneural fold(n.f.). This is seen in section inFigure 7; s.c. is the neural groove; n.f., as before, the neural fold. The neural folds ultimately bend over and meet above, so that s.c. becomes a canal, and is finally separated from the epiblast to form the spinal cord. Below the neural groove a thickening of the dorsal wall of the archenteron appears, and is pinched off to form a longitudinal rod, the precursor of the vertebral column, thenotochord, shown inFigure 7(n.c.), as imperfectly pinched off.
Section 11. Simultaneously, on either side of the notochord appear a series ofsolidmasses of cells, derived mainly by cell division from the cells of the wall of the archenteron, and filling up and obliterating the segmentation cavity. These masses increase in number by the addition of fresh ones behind, during development, and are visible in the dorsal view as brick-like masses, themesoblastic somitesorproto-vertebrae(Figure 6, i., ii., iii.). InFigure 7, these masses are indicated by dotting. In such a primitive type as amphioxus these mesoblastic -somites- [masses] contain a cavity, destined to be the future body cavity, from the first. In the frog, the cavity is not at first apparent; the mesoblast at first seems quite solid, but subsequently what is called thesplitting of the mesoblast occurs, and the body cavity (b.c. inFigure 7) appears. The outer mesoblast, lying immediately under the epiblast, constitutes the substance of thesomatopleur, and from it will be formed the dermis, the muscles of the body wall, almost all the cartilage and bone of the skeleton, the substance of the limbs, the kidneys, genital organs, heart and bloodvessels, and, in short, everything between the dermis and thecoelom, except the nervous system and nerves, and the notochord. The inner mesoblast, the mass of thesplanchnopleur, will form the muscle and connective tissue of the wall of the alimentary canal, and the binding substance of the liver and other glands that open into the canal.
Section 12.Figure 8is one which we reproduce, with the necessary changes in each plate of embryological figures given in this book, so that the student will find it a convenient, one for the purpose of comparison. The lines of dashes, in all cases, signify -epiblast- [hypoblast] , the unbroken black line is -hypoblast-, [epiblast] dotting showsmesoblast, and the shaded rod (n.c.) is the notochord. c.s. is the spinal cord; br.1, br.2, br.3 are the three primary vesicles which constitute the brain, and which form fore, mid, and hind brain respectively. I. is the intestine and Y. the yolk cells that at this early stage constitute its ventral wall.
Section 13.Figure 9gives a similar diagram of a later stage, but here the blastopore is closed. An epiblastic tucking-in at st., thestomodaeumpre-figures the mouth; pr., theproctodaeum, is a similar posterior invagination which will become the anus. Y., the yolk, is evidently much absorbed.Figure 10is a young tadpole, seen from the side. The still unabsorbed yolk in the ventral wall of the mesentery gives the creature a big belly. Its mouth is suctorial at this stage, and behind it is a sucker (s.) by which the larvae attach themselves to floating reeds and wood, as shown in the three black figures below.
Section 14. We may now consider the development of the different organs slightly more in detail, though much of this has already been approached. Thenervous system, before the closure of the neural groove, has three anterior dilatations, the fore-, mid-, and hind-brains, the first of which gives rise by hollow outgrowths to two pairs of lateral structures, the hemispheres and the optic vesicles. The latter give rise to the retina and optic nerve as described in {Development}Section 40.
Section 15. The hypoblasticnotochordis early embraced by a mesoblastic sheath derived from the protovertebrae. This becomes truly cartilaginous, and at regular intervals is alternately thicker and thinner, compressing the notochord at the thicker parts. Hence the notochord has a beaded form within this, at first, continuous cartilaginous sheath. This sheath is soon cut into a series of vertebral bodies by jointings appearing through the points where the cartilage is thickest and the notochord most constricted. Hence what remains of the notochord lieswithinthe vertebral bodies in the frog; while in a cartilaginous fish, such as the dog-fish, or in the embryonic rabbit, the lines of separation appear where the notochord is thickest, and it comes to liebetweenhollow-faced vertebrae. Cartilaginous neural arches and spines, formed outside the notochordal sheath, enclose the spinal cord in an arcade. The final phase is ossification. As the tadpole approaches the frog stage the vertebral column in the tail is rapidly absorbed, and its vestiges appear in the adult as the urostyle.
Section 16. The development of the skull is entirely dissimilar to that of the vertebral column. It is shown on Figures 1 and 8,Sheet 14; and in the section devoted to the frog's skull a very complete account of the process is given. The process of ossification is described under the histology of the Rabbit.
Section 17. The origin of thecirculatory and respiratory organsis of especial interest in the frog. In the tadpole we have essentially the necessities and organization of the fish; in the adult frog we have a clear exposition of the structure of pigeon and rabbit. The tadpole has, at first, a straight tubular heart, burrowed out in somatic mesoblast, and produced forward into atruncus arteriosus. From this arise four afferent branchial arteries, running up along the sides of the four branchial arches, and supplying gills. They unite above on either side in paired hyper-branchial arteries, which meet behind dorsal to the liver, to form a median dorsal aorta. Internal and external carotid arteries supply the head. These four afferent branchial arches are equivalent to the first four of the five vessels of the dog-fish. At first, the paired gills are three in number,external, and tree-like, covered by epiblast (Figures 10 and 11, e.g.), and not to be compared to fish gills in structure, or in fact -with- [to] any other gills within the limits of the vertebrata. Subsequently (hypoblastic)internal gills(int.g.,Figure 12), strictly homologous with the gills of a fish, appear. Then a flap of skin outside the hyoid arch grows back to cover over the gills; this is theoperculum(op. in Figures 11 and 12,Sheet 22), and it finally encloses them in agill chamber, open only by a pore on the left, which resembles in structure and physiological meaning, but differs evidently very widely in development, from the amphioxus atrium. At this time, the lungs are developing as paired hollow outgrowths on the ventral side of the throat (Figure 12, L.). As the limbs develop, and the tail dwindles, the gill chamber is obliterated. The capillary interruptions of the gills on the branchial arches (aortic arches) are also obliterated. Thecarotid glandoccupies the position of the first of these in the adult. The front branchial arch here, as in all higher vertebrata, becomes the carotid arch; the lingual represents the base of a pre-branchial vessel; the second branchial becomes the aortic arch. Thefourthloses its connection with the dorsal aorta, and sends a branch to the developing lung, which becomes thepulmonaryartery. The third disappears. A somewhat different account to this is still found in some text-books of the fate of this third branchial arch. Balfour would appear to have been of opinion that it gave rise to the cutaneous artery, and that the third and fourth vessels coalesced to form the pulmocutaneous, the fourth arch moving forward so as to arise from the base of the third; and most elementary works follow him. This opinion was strengthened by the fact that in the higher types (reptiles, birds, and mammals) no fourth branchial arch was observed, and the apparent third, becomes the pulmonary. But it has since been shown that a transitory third arch appears and disappears in these types.
Section 18. The origin of therenal organ and ducthas very considerable controversial interest.* In Figure 13,Sheet 22, a diagrammatic cross-section, of an embryo is shown. I. is the intestine, coe. the coelom, s.c. the spinal cord; n.c. the notochord, surrounded by n.s., the notochordal sheath, ao. is the dorsal aorta. In the masses of somatic mesoblast on either side, a longitudinal canal appears, which, in the torpedo, a fish related to the dog-fish, and in the rabbit, and possibly in all other cases, is epiblastic in origin. This is thesegmental duct, which persists, apparently, as the Wolffian duct (W.D.). Ventral to this appears a parallel canal, the Mullerian duct (M.D.), which is often described as being split off from the segmental duct, but which is, very probably, an independent structure in the frog. A number of tubuli, at first metamerically arranged, now appear, each opening, on the one hand, into the coelom by a ciliated mouth, thenephrostome(n.s.), and on the other into the segmental duct. These tubuli are thesegmental tubesornephridia. There grows out from the aorta, towards each, a bunch, of bloodvessels, theglomerulus(compareSection 62, Rabbit). These tubuli ultimately become, in part, the renal tubuli, so that the primitive kidney stretches, at first, along the length of the body cavity from the region, of the gill-slits backward. The anterior part of the kidney, called thepronephros, disappears in the later larval stages. Internal to the kidney on either side there has appeared a longitudinal ridge, the genital ridge (g.r.), which gives rise to testes or ovary, as the case may be.
* In the discussion whether the vertebrata have arisen from some ancestral type, like the earthworm, metamerically segmented, and of fairly high organization, or from a much lower form, possibly even from a coelenterate. Such a discussion is entirely outside the scope of the book, though its mention is necessary to explain the importance given to these organs.
Section 19. The student should now compare the figures onSheet 17. In the male, tubular connections are established between the testes and the middle part of the primitive kidney (mesonephros). These connections are thevasa efferentia(v.e.), and the mesonephros is now equivalent to the epididymis of the rabbit. The Wolffian duct is the urogenital duct of the adult, and the Mullerian duct is entirely absorbed, or remains, more or less, in exceptional cases.
In the female, the Mullerian duct increases greatly in length-- so that at sexual maturity its white coils appear thicker and longer than the intestine-- and becomes the oviduct; the Wolffian duct is the ureter, and the mesonephros is not perverted in function from its primary renal duty.
Section 20. Tabulating these facts--
In the adult male:Pronephros disappears.The Mullerian duct (? = pronephric duct) disappears.Mesonephros = Epididymis; its duct, the urogenital.Metanephros and duct, not clearly marked off fromMesonephros.(Compare Dog-fish,Section 19.)In the adult female:Pronephros disappears.The Mullerian duct, the oviduct.Mesonephros and Metanephros, the kidney, and their unseparated ducts, the ureters.
Pronephros disappears.The Mullerian duct (? = pronephric duct) disappears.Mesonephros = Epididymis; its duct, the urogenital.Metanephros and duct, not clearly marked off fromMesonephros.(Compare Dog-fish,Section 19.)
Pronephros disappears.The Mullerian duct, the oviduct.Mesonephros and Metanephros, the kidney, and their unseparated ducts, the ureters.
Section 21.Hermaphrodism(i.e., cases of common sex) is occasionally found among frogs; the testis produces ova in places, and the Mullerian duct is retained and functional. The ciliated nephrostomata remain open to a late stage of development in the frog, and in many amphibia throughout life. Their connection with the renal tubuli is, however, lost.
Section 22. The alimentary canal is, at first, a straight tube. Its disproportionate increase in length throws it into a spiral in the tadpole (int.Figure 11), and accounts for its coiling in the frog. The liver and other digestive glands are first formed, like the lungs, as hollow outgrowths, and their lining is therefore hypoblastic. The greatest relative length of intestine is found in the tadpole, which, being a purely vegetable feeder, must needs effect the maximum amount of preparatory change in its food.