Ovarian ovum of a Mammal.Fig.30.—Ovarian ovum of a Mammal, (a) magnified and viewed under pressure, (b) burst by increased pressure, with yolk and nucleus escaping: (c) the nucleus more freed from yolk-substance. (FromQuain’s Anatomy, after Allen Thomson.)
Fig.30.—Ovarian ovum of a Mammal, (a) magnified and viewed under pressure, (b) burst by increased pressure, with yolk and nucleus escaping: (c) the nucleus more freed from yolk-substance. (FromQuain’s Anatomy, after Allen Thomson.)
Amœboid movements of young egg-cells.Fig.31.—Amœboid movements of young egg-cells,a, Amœboid ovum ofHydra(from Balfour, after Kleitnenberg);b, early ovum ofToxopneustes variegatus, with pseudopodia-like processes (from Balfour, after Selenka);c, ovum ofToxopneustes lividus, more nearly ripe (from Balfour, Hertwig). A1 to A4, the primitive egg-cell of a Chalk-Sponge (Leuculmis echinus), in four successive conditions of motion. B1 to B8, ditto of a Hermit-Crab (Chondracanthus cornutus), in eight successive stages (after E. von Beneden). C1 to C5, ditto of a Cat, in five successive stages (after Pflüger). D, ditto of Trout; E, of a Hen; F, of Man. The first series is taken from theEncycl. Brit.; the second from Häckel’sEvolution of Man.
Fig.31.—Amœboid movements of young egg-cells,a, Amœboid ovum ofHydra(from Balfour, after Kleitnenberg);b, early ovum ofToxopneustes variegatus, with pseudopodia-like processes (from Balfour, after Selenka);c, ovum ofToxopneustes lividus, more nearly ripe (from Balfour, Hertwig). A1 to A4, the primitive egg-cell of a Chalk-Sponge (Leuculmis echinus), in four successive conditions of motion. B1 to B8, ditto of a Hermit-Crab (Chondracanthus cornutus), in eight successive stages (after E. von Beneden). C1 to C5, ditto of a Cat, in five successive stages (after Pflüger). D, ditto of Trout; E, of a Hen; F, of Man. The first series is taken from theEncycl. Brit.; the second from Häckel’sEvolution of Man.
Human ovum, mature and greatly magnified.Fig.32.—Human ovum, mature and greatly magnified. (After Häckel.)
Fig.32.—Human ovum, mature and greatly magnified. (After Häckel.)
In thus saying that the ova of all animals are, so far as microscopes can reveal,substantiallysimilar, I am of course speaking of the egg-cell proper, and not of what is popularly known as the egg. The egg of a bird, for example, is the egg-cell,plusan enormous aggregation of nutritive material, an egg-shell, and sundry other structures suited to the subsequent development of the egg-cell when separated from the parent’s body. But all these accessories are, from our present point of view, accidental or adventitious. What we have now to understand by the ovum, the egg, or the egg-cell, is the microscopical germ which I have just described. So far then as this germ is concerned, we find that all multicellular organisms begin their existence in the same kind of structure, and that this structure is anatomically indistinguishable from that of the permanent form presented by the lowest, or unicellular organisms. But although anatomically indistinguishable, physiologically they present the sundry peculiarities already mentioned.
Now I have endeavoured to show that none of these peculiarities are such as to exclude—or even so much as to invalidate—the supposition of developmental continuity between the lowest egg-cells and the highest protozoal cells. It remains to show in this place, and on the other hand, that there is no breach of continuity between the lowest and the highest egg-cells; but, on the contrary, that the remarkable uniformity of the complex processes whereby their peculiar characters are exhibited to the histologist, is such as of itself to sustain the doctrine of continuityin a singularly forcible manner. On this account, therefore, and also because the facts will again have to be considered in another connexion when we come to deal with Weismann’s theory of heredity, I will here briefly describe the processes in question.
Polar bodies in the ovum of a star-fish.Fig.33.—Stages in the formation of the polar bodies in the ovum of a star-fish. (After Hertwig.)g.v., germinal vesicle transformed into a spindle-shaped system of fibres;p.′, the first polar body becoming extruded;p.,p., both polar bodies fully extruded;f. pn., female pronucleus, or residue of the germinal vesicle.
Fig.33.—Stages in the formation of the polar bodies in the ovum of a star-fish. (After Hertwig.)g.v., germinal vesicle transformed into a spindle-shaped system of fibres;p.′, the first polar body becoming extruded;p.,p., both polar bodies fully extruded;f. pn., female pronucleus, or residue of the germinal vesicle.
We have already seen that the young egg-cell multiplies itself by simple binary division, after the manner of unicellular organisms in general—thereby indicating, as also by its amœbiform movements, its fundamental identity with such organisms in kind. But, as we have likewise seen, when the ovum ceases to resemble these organisms, by taking on its higher degree of functional capacity, it is no longer able to multiply itself in this manner. On the contrary, its cell-divisions are now of an endogenous character,and result in the formation of many different kinds of cells, in the order required for constructing the multicellular organism to which the whole series of processes eventually give rise. We have now to consider these processesseriatim.
Fertilization of the ovum of an echinoderm.Fig.34.—Fertilization of the ovum of an echinoderm. (FromQuain’s Anatomy, after Selenka.) S, spermatozoön;m. pr., male pronucleus;f. pr., female pronucleus. 1 to 4 correspond to D to G in the next figure.
Fig.34.—Fertilization of the ovum of an echinoderm. (FromQuain’s Anatomy, after Selenka.) S, spermatozoön;m. pr., male pronucleus;f. pr., female pronucleus. 1 to 4 correspond to D to G in the next figure.
First of all the nucleus discharges its polar bodies, as previously mentioned, and in the manner here depicted on the previous page. (Fig. 33.) It will be observed that the nucleus of the ovum, or the germinal vesicle as it is called, gets rid first of one and afterwards of the other polar body by an “indirect,” or karyokinetic, process of division. (Fig. 33.) Extrusion of these bodies from the ovum (or it may be only from the nucleus) having been accomplished, what remains of the nucleus retires from the circumference of the ovum, and is called the female pronucleus. (Fig. 33.f. pn.) The ovum is now ready for fertilization. A similar emission of nuclear substance is said by some goodobservers to take place also from the male germ-cell, or spermatozoön, at or about the close ofitsdevelopment. The theories to which these facts have given rise will be considered in future chapters on Heredity.
Turning now to the mechanism of fertilization, the diagrams (Figs. 34, 35) represent what happens in the case of star-fish.
Fertilization of the ovum of a star-fish.Fig.35.—Fertilization of the ovum of a star-fish. (From theEncycl. Brit.after Fol.) A, spermatozoa in the mucilaginous coat of the ovum; a prominence is rising from the surface of the ovum towards a spermatozoön; B, they have almost met; C, they have met; D, the spermatozoön enters the ovum through a distinct opening; H, the entire ovum, showing extruded polar bodies on its upper surface, and the moving together of the male and female pronuclei; E, F, G, meeting and coalescence of the pronuclei.
Fig.35.—Fertilization of the ovum of a star-fish. (From theEncycl. Brit.after Fol.) A, spermatozoa in the mucilaginous coat of the ovum; a prominence is rising from the surface of the ovum towards a spermatozoön; B, they have almost met; C, they have met; D, the spermatozoön enters the ovum through a distinct opening; H, the entire ovum, showing extruded polar bodies on its upper surface, and the moving together of the male and female pronuclei; E, F, G, meeting and coalescence of the pronuclei.
The sperm-cell, or spermatozoön, is seen in the act of penetrating the ovum. In the first figure it has already pierced the mucilaginous coat of the ovum, the limit of which is represented by a line through which the tail of the spermatozoön is passing: the head of the spermatozoön is just entering the ovum proper. It may be noted that, in the case of many animals, the general protoplasm of the ovum becomes aware, so to speak, of the approach of a spermatozoön, and sends up a process to meet it. (Fig. 35, A, B, C.) Several—or even many—spermatozoa may thus enter the coat of the ovum; but normally only one proceeds further, or right into the substance of the ovum, for thepurpose of effecting fertilization. This spermatozoön, as soon as it enters the periphery of the yolk, or cell-substance proper, sets up a series of remarkable phenomena. First, its own head rapidly increases in size, and takes on the appearance of a cell-nucleus: this is called the male pronucleus. At the same time its tail begins to disappear, and the enlarged head proceeds to make its way directly towards the nucleus of the ovum which, as before stated, is now called the female pronucleus. The latter in its turn moves towards the former, and when the two meet they fuse into one mass, forming a new nucleus. Before the two actually meet, the spermatozoön has lost its tail altogether; and it is noteworthy that during its passage through the protoplasmic cell-contents of the ovum, it appears to exercise upon this protoplasm an attractive influence; for the granules of the latter in its vicinity dispose themselves around it in radiating lines. All these various phenomena are depicted in the above wood-cuts. (Figs. 34, 35.)
Fertilization having been thus effected by fusion of the male and female pronuclei into a single (or new) nucleus, this latter body proceeds to exhibit complicated processes of karyokinesis, which, as before shown, are preliminary to nuclear division in the case of egg-cells. Indeed the karyokinetic process may begin in both the pronuclei before their junction is effected; and, even when their junction is effected, it does not appear that complete fusion of the so-called chromatin elements of the two pronuclei takes place. For the purpose of explaining what this means, and still more for the purpose of giving a general idea of the karyokinetic processes as a whole,I will quote the following description of them, because, for terseness combined with lucidity, it is unsurpassable.
Karyokinesis of a typical tissue-cell (epithelium of Salamander).Fig.36.—Karyokinesis of a typical tissue-cell (epithelium of Salamander). (After Flemming and Klein.) The series from A to I represents the successive stages in the movement of the chromatin fibres during division, excepting G, which represents the “nucleus-spindle” of an egg-cell. A, resting nucleus; D, wreath-form; E, single star, the loops of the wreath being broken; F, separation of the star into two groups of U-shaped fibres; H, diaster or double star; I, completion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are markeda, and correspond to the “equatorial plate";b, achromatin fibres forming the nucleus-spindle;c, granules of the cell-protoplasm forming a “polar star.” Such a polar star is seen at each end of the nucleus-spindle, and is not to be confused with the diaster H, the two ends of which are composed ofchromatin.
Fig.36.—Karyokinesis of a typical tissue-cell (epithelium of Salamander). (After Flemming and Klein.) The series from A to I represents the successive stages in the movement of the chromatin fibres during division, excepting G, which represents the “nucleus-spindle” of an egg-cell. A, resting nucleus; D, wreath-form; E, single star, the loops of the wreath being broken; F, separation of the star into two groups of U-shaped fibres; H, diaster or double star; I, completion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are markeda, and correspond to the “equatorial plate";b, achromatin fibres forming the nucleus-spindle;c, granules of the cell-protoplasm forming a “polar star.” Such a polar star is seen at each end of the nucleus-spindle, and is not to be confused with the diaster H, the two ends of which are composed ofchromatin.
Researches, chiefly due to Flemming, have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of “achromatin,” not deeply stained by re-agents, ramifying in which is a reticulum of “chromatin” consisting of fibres which readily take a deep stain. (Fig. 36, A). Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in the arrangement of its fibres first in theform of a wreath (D), and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appearance of the achromatin substance between the two groups of chromatin loops is observable (H). In some cases (especially egg-cells) this striated arrangement of the achromatin is then termed a “nucleus-spindle,” and the group of chromatin loops (G,a) is known as “the equatorial plate.” At each end of the nucleus-spindle in these cases there is often seen a star consisting of granules belonging to the general protoplasm of the cell (G,c). These are known as “polar stars.” After the separation of the two sets of loops (H) the protoplasm of the general substance of the cell becomes constricted, and division occurs, so as to include a group of chromatin loops in each of the two fission products. Each of these then rearranges itself together with the associated chromatin into a nucleus such as was present in the mother cell to commence with (I)[13].
Researches, chiefly due to Flemming, have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of “achromatin,” not deeply stained by re-agents, ramifying in which is a reticulum of “chromatin” consisting of fibres which readily take a deep stain. (Fig. 36, A). Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in the arrangement of its fibres first in theform of a wreath (D), and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appearance of the achromatin substance between the two groups of chromatin loops is observable (H). In some cases (especially egg-cells) this striated arrangement of the achromatin is then termed a “nucleus-spindle,” and the group of chromatin loops (G,a) is known as “the equatorial plate.” At each end of the nucleus-spindle in these cases there is often seen a star consisting of granules belonging to the general protoplasm of the cell (G,c). These are known as “polar stars.” After the separation of the two sets of loops (H) the protoplasm of the general substance of the cell becomes constricted, and division occurs, so as to include a group of chromatin loops in each of the two fission products. Each of these then rearranges itself together with the associated chromatin into a nucleus such as was present in the mother cell to commence with (I)[13].
Since the above was published, however, further progress has been made. In particular it has been found that the chromatin fibres pass from phase D to phase F by a process of longitudinal splitting (Fig. 37g,h; Fig. 38, VI, VII)—which is a point of great importance for Weismann’s theory of heredity,—and that the protoplasm outside the nucleus seems to take as important a part in the karyokinetic process as does the nuclear substance. For the so-called “attraction-spheres” (Fig. 38 IIa, III, IIIa, VIII to XII), which were at first supposed to be of subordinate importance in the process as a whole, are now known to take an exceedingly active part in it (see especially IX to XI). Lastly, it may be added that there is agrowing consensus of authoritative opinion, that the chromatin fibres are the seats of the material of heredity, or, in other words, that they contain those essential elements of the cell which endow the daughter-cells with their distinctive characters. Therefore, where the parent-cell is an ovum, it follows from this view that all hereditary qualities of the future organism are potentially present in the ultra-microscopical structure of the chromatin fibres.
Successive changes in the nucleus of an epithelium cell, preparatory to division of the cell.Fig.37.—Study of successive changes taking place in the nucleus of an epithelium cell, preparatory to division of the cell. (FromQuain’s Anatomy, after Flemming.)a, resting cell, showing the nuclear network;b, first stage of division, the chromatoplasm transformed into a skein of closely contorted filaments;ctof, further stages in the growth and looping arrangement of the filaments;g, stellate phase, or aster;h, completion of the splitting of the filaments, already begun infandg;i,j,k, successive stages in separation of the filaments into two groups;l, the final result of this (diaster);mtoq, stages in the division of the whole cell into two, showing increasing contortion of the filaments, until they reach the resting stage atq.
Fig.37.—Study of successive changes taking place in the nucleus of an epithelium cell, preparatory to division of the cell. (FromQuain’s Anatomy, after Flemming.)a, resting cell, showing the nuclear network;b, first stage of division, the chromatoplasm transformed into a skein of closely contorted filaments;ctof, further stages in the growth and looping arrangement of the filaments;g, stellate phase, or aster;h, completion of the splitting of the filaments, already begun infandg;i,j,k, successive stages in separation of the filaments into two groups;l, the final result of this (diaster);mtoq, stages in the division of the whole cell into two, showing increasing contortion of the filaments, until they reach the resting stage atq.
Formation and conjugation of the pronuclei in Ascaris megalocephala.Fig.38.—Formation and conjugation of the pronuclei inAscaris megalocephala. (FromQuain’s Anatomy, after E. von Beneden.)f, female pronucleus;m, male pronucleus;p, one of the polar bodies.I. The second polar body has just been extruded; both male and female pronuclei contain two chromatin particles; those of the male pronucleus are becoming transformed into a skein. II. The chromatin in both pronuclei now forms into a skein.IIa. The skeins are more distinct. Two attraction (or protoplasmic) spheres, each with a central particle united with a small spindle of achromatic fibres, have made their appearance in the general substance of the egg close to the mutually approaching pronuclei. The male pronucleus has the remains of the body of the spermatozoön adhering to it.III. Only the female pronucleus is shown in this figure. The skein is contracted and thickened. The attraction-spheres are near one side of the ovum, and are connected with its periphery by a cone of fibres forming a polar circle,p.c.;e.c., equatorial circle.IIIa. The pronuclei have come into contact, and the spindle-system is now arranged across their common axis.IV. Contraction of the skein, and formation of two U-or V-shaped chromatin fibres in each pronucleus.V. The V-shaped chromatin filaments are now quite distinct: the male and female pronuclei are in close contact.
Fig.38.—Formation and conjugation of the pronuclei inAscaris megalocephala. (FromQuain’s Anatomy, after E. von Beneden.)f, female pronucleus;m, male pronucleus;p, one of the polar bodies.I. The second polar body has just been extruded; both male and female pronuclei contain two chromatin particles; those of the male pronucleus are becoming transformed into a skein. II. The chromatin in both pronuclei now forms into a skein.IIa. The skeins are more distinct. Two attraction (or protoplasmic) spheres, each with a central particle united with a small spindle of achromatic fibres, have made their appearance in the general substance of the egg close to the mutually approaching pronuclei. The male pronucleus has the remains of the body of the spermatozoön adhering to it.III. Only the female pronucleus is shown in this figure. The skein is contracted and thickened. The attraction-spheres are near one side of the ovum, and are connected with its periphery by a cone of fibres forming a polar circle,p.c.;e.c., equatorial circle.IIIa. The pronuclei have come into contact, and the spindle-system is now arranged across their common axis.IV. Contraction of the skein, and formation of two U-or V-shaped chromatin fibres in each pronucleus.V. The V-shaped chromatin filaments are now quite distinct: the male and female pronuclei are in close contact.
Fig.38.—Formation and conjugation of the pronuclei inAscaris megalocephala. (FromQuain’s Anatomy, after E. von Beneden.)f, female pronucleus;m, male pronucleus;p, one of the polar bodies.
I. The second polar body has just been extruded; both male and female pronuclei contain two chromatin particles; those of the male pronucleus are becoming transformed into a skein. II. The chromatin in both pronuclei now forms into a skein.
IIa. The skeins are more distinct. Two attraction (or protoplasmic) spheres, each with a central particle united with a small spindle of achromatic fibres, have made their appearance in the general substance of the egg close to the mutually approaching pronuclei. The male pronucleus has the remains of the body of the spermatozoön adhering to it.
III. Only the female pronucleus is shown in this figure. The skein is contracted and thickened. The attraction-spheres are near one side of the ovum, and are connected with its periphery by a cone of fibres forming a polar circle,p.c.;e.c., equatorial circle.
IIIa. The pronuclei have come into contact, and the spindle-system is now arranged across their common axis.
IV. Contraction of the skein, and formation of two U-or V-shaped chromatin fibres in each pronucleus.
V. The V-shaped chromatin filaments are now quite distinct: the male and female pronuclei are in close contact.
Previous figure continued.VI., VII. The V-shaped filaments are splitting longitudinally; their structure of fine granules of chromatin is apparent in VII., which is more highly magnified. The conjugation of the pronuclei is apparently complete in VII. The attraction-spheres and achromatic spindle, although present, are not depicted in IV., V., VI., and VII.VIII. Equatorial arrangement of the four chromatin loops in the middle of the now segmenting ovum: the achromatic substance forming a spindle-shaped system of granules with fibres radiating from the poles of the spindle (attraction-spheres); the chromatin forms an equatorial plate. (Compare Fig. 36 G.)IX. Shows diagrammatically the commencing separation of the chromatin fibres of the conjugated nuclei, and the system of fibres radiating from the attraction-spheres. (Compare again Fig. 36 G.)p.c., polar circle;e.c., equatorial circle;c.c., central particle.X. Further separation of the chromatin filaments. Each of the central particles of the attraction-spheres has divided into two.XI. The chromatin fibres are becoming developed into the skeins of the two daughter-nuclei. These are still united by fibres of achromatin. The general protoplasm of the ovum is becoming divided.XII. The two daughter-nuclei exhibit a chromatin network. Each of the attraction-spheres has divided into two, which are joined by fibres of achromatin, and connected with the periphery of the cell in the same way as in the original or parent sphere, III.
VI., VII. The V-shaped filaments are splitting longitudinally; their structure of fine granules of chromatin is apparent in VII., which is more highly magnified. The conjugation of the pronuclei is apparently complete in VII. The attraction-spheres and achromatic spindle, although present, are not depicted in IV., V., VI., and VII.VIII. Equatorial arrangement of the four chromatin loops in the middle of the now segmenting ovum: the achromatic substance forming a spindle-shaped system of granules with fibres radiating from the poles of the spindle (attraction-spheres); the chromatin forms an equatorial plate. (Compare Fig. 36 G.)IX. Shows diagrammatically the commencing separation of the chromatin fibres of the conjugated nuclei, and the system of fibres radiating from the attraction-spheres. (Compare again Fig. 36 G.)p.c., polar circle;e.c., equatorial circle;c.c., central particle.X. Further separation of the chromatin filaments. Each of the central particles of the attraction-spheres has divided into two.XI. The chromatin fibres are becoming developed into the skeins of the two daughter-nuclei. These are still united by fibres of achromatin. The general protoplasm of the ovum is becoming divided.XII. The two daughter-nuclei exhibit a chromatin network. Each of the attraction-spheres has divided into two, which are joined by fibres of achromatin, and connected with the periphery of the cell in the same way as in the original or parent sphere, III.
VI., VII. The V-shaped filaments are splitting longitudinally; their structure of fine granules of chromatin is apparent in VII., which is more highly magnified. The conjugation of the pronuclei is apparently complete in VII. The attraction-spheres and achromatic spindle, although present, are not depicted in IV., V., VI., and VII.
VIII. Equatorial arrangement of the four chromatin loops in the middle of the now segmenting ovum: the achromatic substance forming a spindle-shaped system of granules with fibres radiating from the poles of the spindle (attraction-spheres); the chromatin forms an equatorial plate. (Compare Fig. 36 G.)
IX. Shows diagrammatically the commencing separation of the chromatin fibres of the conjugated nuclei, and the system of fibres radiating from the attraction-spheres. (Compare again Fig. 36 G.)p.c., polar circle;e.c., equatorial circle;c.c., central particle.
X. Further separation of the chromatin filaments. Each of the central particles of the attraction-spheres has divided into two.
XI. The chromatin fibres are becoming developed into the skeins of the two daughter-nuclei. These are still united by fibres of achromatin. The general protoplasm of the ovum is becoming divided.
XII. The two daughter-nuclei exhibit a chromatin network. Each of the attraction-spheres has divided into two, which are joined by fibres of achromatin, and connected with the periphery of the cell in the same way as in the original or parent sphere, III.
As I shall have more to say about these processes in the next volume, when we shall see the important part which they bear in Weismann’s theory of heredity, it is with a double purpose that I here introduce these yet further illustrations of them upon a somewhat larger scale. The present purpose is merely that of showing, more clearly than hitherto, the great complexity of these processes on the one hand, and, on the other, the general similarity which they display in egg-cells and in tissue-cells. But as in relation to this purpose the illustrations speak for themselves, I may now pass on at once to the history of embryonic development, which follows fertilization of the ovum.
We have seen that when the new nucleus of the fertilized ovum (which is formed by a coalescence of the male pronucleus with the female) has completed its karyokinetic processes, it is divided into two equal parts; that these are disposed at opposite poles of the ovum; and that the whole contents of the ovum are thereupon likewise divided into two equal parts, with the result that there are now two nucleated cells within the spherical wall of the ovum where before there had only been one. Moreover, we have also seen that aprecisely similar series of events repeat themselves in each of these two cells, thus giving rise to four cells (see Fig. 29). It must now be added that such duplication is continued time after time, as shown in the accompanying illustrations (Figs. 39, 40).
Segmentation of ovum.Fig.39.—Segmentation of ovum. (After Häckel.) Successive stages are marked by the letters A, B, C. D represents several stages in advance of C.
Fig.39.—Segmentation of ovum. (After Häckel.) Successive stages are marked by the letters A, B, C. D represents several stages in advance of C.
AllThe contents of an ovum in an advanced stage of segmentation.Fig.40.—The contents of an ovum in an advanced stage of segmentation, drawn in perspective. (After Häckel.)this, it will be noticed, is a case of cell-multiplication, which differs from that which takes place in the unicellular organisms only in its beinginvariablypreceded (as far as we know) by karyokinesis, and in the resulting cells being all confined within a common envelope, and so in not being free to separate. Nevertheless, from what has already been said, it will also be noticed that this feature makes all the difference between a Metazoön and a Protozoön; so that already the ovum presents the distinguishing character of a Metazoön.
I have dealt thus at considerable length upon the processes whereby the originally unicellular ovum and spermatozoön become converted into the multicellular germ, because I do not know of any other exposition of the argument from Embryology where this, the first stage of the argument, has been adequately treated. Yet it is evident that the fact of all the processes above described being so similar in the case of sexual (or metazoal) reproduction among the innumerable organisms where it occurs, constitutes in itself a strong argument in favour of evolution. For the mechanism of fertilization, and all the processes which even thus far we have seen to follow therefrom, are hereby shown to be not only highly complex, but likewise highly specialized. Therefore, the remarkable similarity which they present throughout the whole animal kingdom—not to speak of the vegetable—is expressive of organic continuity, rather than of absolute discontinuity in every case, as the theory of special creation must necessarily suppose. And it is evident that this argument is strong in proportion to the uniformity, the specialization, and the complexity of the processes in question.
Having occupied so much space with supplying what appear to me the deficiencies in previous expositions of the argument from Embryology, I can now afford to take only a very general view of the more important features of this argument as they are successively furnished by all the later stages of individual development. But this is of little consequence, seeing that from the point at which we have now arrived previous expositions of the argument are both good and numerous. The following then is to be regarded as a mere sketchOf the evidences of phyletic (or ancestral) evolution, which are so abundantly furnished by all the subsequent phases of ontogenetic (or individual) evolution.
Formation of the gastrula of Amphioxus.Fig.41.—Formation of the gastrula ofAmphioxus. (After Kowalevsky.) A, wall of the ovum, composed of a single layer of cells; B, a stage in the process of gastrulation; C, completion of the process; S, original or segmentation cavity of ovum;al, alimentary cavity of gastrula;ect, outer layer of cells;ent, inner layer of cells;b, orifice, constituting the mouth in permanent forms.
Fig.41.—Formation of the gastrula ofAmphioxus. (After Kowalevsky.) A, wall of the ovum, composed of a single layer of cells; B, a stage in the process of gastrulation; C, completion of the process; S, original or segmentation cavity of ovum;al, alimentary cavity of gastrula;ect, outer layer of cells;ent, inner layer of cells;b, orifice, constituting the mouth in permanent forms.
The multicellular body which is formed by the series of segmentations above described is at first a sphere of cells (Fig. 40). Soon, however, a watery fluid gathers in the centre, and progressively pushes the cells towards the circumference, until they there constitute a single layer. The ovum, therefore, is now in the form of a hollow sphere containing fluid, confined within a continuous wall of cells (Fig. 41 A). The next thing that happens is a pitting in of one portion of the sphere (B). The pit becomes deeper and deeper, until there is a complete invagination of this part of the sphere—the cells which constitute it being progressivelypushed inwards until they come into contact with those at the opposite pole of the ovum. Consequently, instead of a hollow sphere of cells, the ovum now becomes an open sac, the walls of which are composed of a double layer of cells (C). The ovum is now what has been called a gastrula; and it is of importance to observe that probably all the Metazoa pass throughthis stage. At any rate it has been found to occur in all the main divisions of the animal kingdom, as a glance at the accompanying figures will serve to show (Fig. 42)[14]. Moreover many of the lower kinds of Metazoa never pass beyond it; but are all their lives nothing else than gastrulæ, wherein the orifice becomes the mouth of the animal, the internal or invaginated layer of cells the stomach, and the outer layer the skin. So that if we take a child’s india-rubber ball, of the hollowkind with a hole in it, and push in one side with our fingers till internal contact is established all round, by then holding the indented side downwards we should get a very fair anatomical model of a gastræa form, such as is presented by the adult condition of many of the most primitive Metazoa—especially the lowerCœlenterata. The preceding figures represent twoother such forms in nature, the first locomotive and transitory, the second fixed and permanent (Figs. 43, 44).
Gastrulation.Fig.42.—Gastrulation. A, Gastrula of a Zoophyte (Gastrophysema). (After Häckel.) B, Gastrula of a Worm (Sagitta). (After Kowalevsky.) C, Gastrula of an Echinoderm (Uraster). (After A. Agassiz.) D, Gastrula of an Arthropod (Nauplius). (After Häckel.) E, Gastrula of a Mollusk (Limnæus). (After Rabl.) F, Gastrula of a Vertebrate (Amphioxus). (After Kowalevsky.) In all,d, indicates the intestinal cavity;o, the primitive mouth;s, the cleavage-cavity;i, the endoderm, or intestinal layer;e, the ectoderm or skin-layer.
Fig.42.—Gastrulation. A, Gastrula of a Zoophyte (Gastrophysema). (After Häckel.) B, Gastrula of a Worm (Sagitta). (After Kowalevsky.) C, Gastrula of an Echinoderm (Uraster). (After A. Agassiz.) D, Gastrula of an Arthropod (Nauplius). (After Häckel.) E, Gastrula of a Mollusk (Limnæus). (After Rabl.) F, Gastrula of a Vertebrate (Amphioxus). (After Kowalevsky.) In all,d, indicates the intestinal cavity;o, the primitive mouth;s, the cleavage-cavity;i, the endoderm, or intestinal layer;e, the ectoderm or skin-layer.
Gastrula of a Chalk Sponge.Fig.43.—Gastrula of a Chalk Sponge. (After Häckel.) A, External view. B, Longitudinal section.g, digestive cavities;o, mouth;i, endoderm;e, ectoderm.
Fig.43.—Gastrula of a Chalk Sponge. (After Häckel.) A, External view. B, Longitudinal section.g, digestive cavities;o, mouth;i, endoderm;e, ectoderm.
Prophysema primordiale, an extant gastræa-form.Fig.44.—Prophysema primordiale, an extant gastræa-form. (After Häckel.) (A). External view of the whole animal, attached by its foot to seaweed. (B). Longitudinal section of the same. The digestive cavity (d) opens at its upper end in the mouth (m). Among the cells of the endoderm (g) lie amœboid egg-cells of large size (e). The ectoderm (h) is encrusted with grains of sand, above the sponge spicules.
Fig.44.—Prophysema primordiale, an extant gastræa-form. (After Häckel.) (A). External view of the whole animal, attached by its foot to seaweed. (B). Longitudinal section of the same. The digestive cavity (d) opens at its upper end in the mouth (m). Among the cells of the endoderm (g) lie amœboid egg-cells of large size (e). The ectoderm (h) is encrusted with grains of sand, above the sponge spicules.
Here, then, we leave the lower forms of Metazoa in their condition of permanent gastrulæ. They differ from the transitory stage of other Metazoa only in being enormously larger (owing to greatly furthergrowth, without any furtherdevelopmentas to matters of fundamental importance), and in having sundry tentacles and other organs added later on to meet their special requirements. The point to remember is, that in all cases a gastrula is an open sac composed of two layers of cells—the outer layer being called the ectoderm, and the inner the endoderm. They have also been called the animal layer and the vegetative layer, because it is the outer layer (ectoderm) that gives rise to all the organs of sensation and movement—viz. the skin, the nervous system, and the muscular system; while it is the inner layer (endoderm) that gives rise to all the organs of nutrition and reproduction. It is desirable only further to explain that gastrulation does not take place in all the Metazoa after exactly the same plan. In different lines of descent various and often considerable modifications of the original and most simple plan have been introduced; but I will not burden the present exposition by describing these modifications[15]. It is enough for us that they always end in the formation of the two primary layers of ectoderm and endoderm.
The next stage of differentiation is common to all the Metazoa, except those lowest forms which, as weHave just seen, remain permanently as large gastrulæ, with sundry specialized additions in the way of tentacles, &c. This stage of differentiation consists in the formation of either a pouch or an additional layer between the ectoderm and the endoderm, which is called the mesoderm. It is probably in most cases derived from the endoderm, but the exact mode of its derivation is still somewhat obscure. sometimes it has the appearance of itself constituting two layers; but it is needless to go into these details; for in any case the ultimate result is the same—viz. that of converting the metazoön into the form of a tube, the walls of which are composed of concentric layers of cells. The outermost layer afterwards gives rise to the epidermis with its various appendages, and also to the central nervous system with its organs of special sense. The median layer gives rise to the voluntary muscles, bones, cartilages, &c., the nutritive systems of the blood, the chyle, the lymph, and the muscular tube of the intestine. lastly, the innermost layer developes into the epithelium lining of the intestine, with its various appendages of liver, lungs, intestinal glands, &c.
I have just said that this three or four layered stage is shared by all the Metazoa, except those very lowest forms—such as sponges and jelly-fish—which do not pass on to it. But from this point the developmental histories of all the main branches of the Metazoa diverge—the Vermes, the Echinodermata, the Mollusca, the Articulata, and the Vertebrata, each taking a different road in their subsequent evolution. I will therefore confine attention to only one of these several roads or methods, namely, that which isfollowed by the Vertebrata—observing merely that, if space permitted, the same principles of progressive though diverging histories of evolution would equally well admit of being traced in all the other sub-kingdoms which have just been named.
Ideal primitive vertebrate, seen from the left side.Fig.45.—Ideal primitive vertebrate, seen from the left side. (After Häckel.)na, nose;au, eye;g, ear;md, mouth;ks, gill-openings;x, notochord;mr, spinal tube;kg, gill-vessels;k, gill-intestine;hz, heart;ms, muscles;ma, stomach;v, intestinal vein;c, body-cavity;a, aorta;l, liver;d, small intestine;e, ovary;h, testes;n, kidney canal;af, anus;lh, true or leather-skin;oh, outer-skin (epidermis);f, skin-fold, acting as a fin.
Fig.45.—Ideal primitive vertebrate, seen from the left side. (After Häckel.)na, nose;au, eye;g, ear;md, mouth;ks, gill-openings;x, notochord;mr, spinal tube;kg, gill-vessels;k, gill-intestine;hz, heart;ms, muscles;ma, stomach;v, intestinal vein;c, body-cavity;a, aorta;l, liver;d, small intestine;e, ovary;h, testes;n, kidney canal;af, anus;lh, true or leather-skin;oh, outer-skin (epidermis);f, skin-fold, acting as a fin.
In order to trace these principles in the case of the Vertebrata, it is desirable first of all to obtain an idea of the anatomical features which most essentially distinguish the sub-kingdom as a whole.Fig. 46.—The same in transverse section through the ovaries; lettering as in the preceding Fig.Fig.46.—The same in transverse section through the ovaries; lettering as in the preceding Fig.The following, then, is what may be termed the ideal plan of vertebrate organization, as given by Prof. Häckel. First, occupying the major axis of body we perceive the primitive vertebral column. The parts lying above this axis are those which have been developed from the ectoderm and mesoderm—viz. voluntary muscles, central nervous system, and organs of special sense. The parts lying below this axis are for the most part those which have been developed from the endoderm—namely,the digestive tract with its glandular appendages, the circulating system and the respiratory system. In transverse section, therefore, the ideal vertebrate consists of a solid axis, with a small tube occupied by the nervous system above, and a large tube, or body-cavity, below. This body-cavity contains the viscera, breathing organs, and heart, with its prolongations into the main blood-vessels of the organism. Lastly, on either side of the central axis are to be found large masses of muscle—two on the dorsal and two on the ventral. As yet, however, there are no limbs, nor even any bony skeleton, for the primitive vertebral column is hitherto unossified cartilage. This ideal animal, therefore, is to all appearance as much like a worm as a fish, and swims by means of a lateral undulation of its whole body, assisted, perhaps, by a dorsal fin formed out of skin.
Amphioxus lanceolatus.Fig. 47.—Amphioxus lanceolatus. (After Häckel.)a, anus;au, eye;b, ventral muscles;c, body-cavity;ch, notochord;d, intestine;doanddu, dorsal and ventral walls of intestine;f, fin-seam;h, skin;k, gills;ka, gill-artery;lb, liver;lv, liver-vein;m 1, brain-bladder;m 2, spinal marrow;mg, stomach;o, mouth;p, ventral pore;r, dorsal muscle;s, tail-fin;t, aorta;v, intestinal vein;x, boundary between gill-intestine and stomach-intestine;y, hypobranchial groove.
Fig. 47.—Amphioxus lanceolatus. (After Häckel.)a, anus;au, eye;b, ventral muscles;c, body-cavity;ch, notochord;d, intestine;doanddu, dorsal and ventral walls of intestine;f, fin-seam;h, skin;k, gills;ka, gill-artery;lb, liver;lv, liver-vein;m 1, brain-bladder;m 2, spinal marrow;mg, stomach;o, mouth;p, ventral pore;r, dorsal muscle;s, tail-fin;t, aorta;v, intestinal vein;x, boundary between gill-intestine and stomach-intestine;y, hypobranchial groove.
Now I should not have presented this ideal representation of a primitive vertebrate—for I have very little faith in the “scientific use of the imagination” where it aspires to discharge the functions of a Creator in the manufacture of archetypal forms—I say I should not have presented this ideal representative of a primitive vertebrate, were it not that the ideal is actually realized in a still existing animal. For there still survives what must be an immensely archaic form of vertebrate, whose anatomy is almost identical with that of the imaginary type which has just beendescribed. I allude, of course, toAmphioxus, which is by far the most primitive or generalized type of vertebrated animal hitherto discovered. Indeed, we may say that this remarkable creature is almost as nearly allied to a worm as it is to a fish. For it has no specialized head, and therefore no skull, brain, or jaws: it is destitute alike of limbs, of a centralized heart, of developed liver, kidneys, and, in short, of most of the organs which belong to the other Vertebrata. It presents, however, a rudimentary backbone, in the form of what is called a notochord. Now a primitive dorsal axis of this kind occurs at a very early period of embryonic life in all vertebrated animals; but, with the exception ofAmphioxus, in all other existing Vertebrata this structure is not itself destined to become the permanent or bony vertebral column. On the contrary, it gives way to, or is replaced by, this permanent bony structure at a later stage of development. Consequently, it is very suggestive that so distinctively embryonic a structure as this temporary cartilaginous axis of all the other known Vertebrata should be found actually persisting to the present day as the permanent axis ofAmphioxus. In many other respects, likewise, the early embryonic history of other Vertebrata refers us to the permanent condition ofAmphioxus. In particular, we must notice that the wall of the neck is always perforated by what inAmphioxusare the gill-openings, and that the blood-vessels as they proceed from the heart are always distributed in the form of what are called gill-arches, adapted to convey the blood round or through the gills for the purpose of aeration. In all existing fish and other gill-breathing Vertebrata, thisarrangement is permanent. It is likewise met with in a peculiar kind of worm, calledBalanoglossus—a creature so peculiar, indeed, that it has been constituted by Gegenbaur a class all by itself. We can see by the wood-cuts that it presents a series of gill-slits, like the homologous parts of the fishes with which it is compared—i. e. fishes of a comparatively low type of organization, which dates from a time before the development of external gills. (Figs. 48, 49, 50.) Now, as I have already said, these gill-slitsare supported internally by the gill-arches, or the blood-vessels which convey the blood to be oxygenized in the branchial apparatus (see below, Figs. 51, 52, 53); and the whole arrangement is developed from the anterior part of the intestine—as is likewise the respiratory mechanism of all the gill-breathing Vertebrata. That so close a parallel to this peculiar mechanism should be met with in a worm, is a strong additional piece of evidence pointing to the derivation of the Vertebrata from the Vermes.
Balanoglossus.Fig. 48.—Balanoglossus. (After A. Agassiz.)r, proboscis;h, collar;k, gill-slits;d, digestive posterior intestine;v, intestinal vessel;a, anus.
A large Sea-lamprey (Petromyzon marinus)Fig. 49.—A large Sea-lamprey (Petromyzon marinus), much reduced in size. (After Cuvier and Häckel.) A series of seven gill-slits are visible.
Adult Shark (Carcharias melanopterus).Fig. 50—Adult Shark (Carcharias melanopterus). (After Cuvier and Häckel.)
Heart and gill-arches of a fish.Fig. 51.—Diagram of heart and gill-arches of a fish. (After Owen.)One gill-arch, with branchial fringe attached.Fig. 52.—One gill-arch, with branchial fringe attached. (After Owen.) H, Heart.Heart and gill-arches in a lizard.Fig. 53.—Diagram of heart and gill-arches in a lizard. (After Owen.) The gill-arches,a a' a'', andb b' b'', are called aortic arches in air-breathing vertebrata.
Heart and gill-arches of a fish.Fig. 51.—Diagram of heart and gill-arches of a fish. (After Owen.)
Fig. 51.—Diagram of heart and gill-arches of a fish. (After Owen.)
One gill-arch, with branchial fringe attached.Fig. 52.—One gill-arch, with branchial fringe attached. (After Owen.) H, Heart.
Fig. 52.—One gill-arch, with branchial fringe attached. (After Owen.) H, Heart.
Heart and gill-arches in a lizard.Fig. 53.—Diagram of heart and gill-arches in a lizard. (After Owen.) The gill-arches,a a' a'', andb b' b'', are called aortic arches in air-breathing vertebrata.
Fig. 53.—Diagram of heart and gill-arches in a lizard. (After Owen.) The gill-arches,a a' a'', andb b' b'', are called aortic arches in air-breathing vertebrata.
Well, I have just said that in all the gill-breathing Vertebrata, this mechanism of gill-slits and vascular gill-arches in the front part of the intestinal tract is permanent. But in the air-breathing Vertebrata such an arrangement would obviously be of no use. Consequently, the gill-slits in the sides of the neck (see Figs. 16 and 57, 58), and the gill-arches of the large blood-vessels (Figs. 54, 55, 56), are here exhibited only as transitory phases of development. But as such they occur in all air-breathing Vertebrata. And, as if to make the homologies as striking as possible, at the time when the gill-slits and the gill-arches are developed in the embryonic young of air-breathingVertebrata, the heart is constructed upon the fish-like type. That is to say, it is placed far forwards, and, from having been a simple tube as in Worms, is now divided into two chambers, as in Fish. Later on it becomes progressively pushed further back between the developing lungs, while it progressively acquires the three cavities distinctive of Amphibia, and finally the four cavities belonging only to the complete double circulation of Birds and Mammals. Moreover, it has now been satisfactorily shown that the lungs of air-breathing Vertebrata, which are thus destined to supersede the function of gills, are themselves the modified swim-bladder or float, which belongs to Fish. Consequently, all these progressive modifications in the important organs of circulation and respiration in the air-breathing Vertebrata, together make up as complete a history of their aquatic pedigree as it would be possible for the most exacting critic to require.
Ideal diagram of primitive gill- or aortic-arches.Fig. 54.—Ideal diagram, of primitive gill- or aortic-arches. (After Rathke.) H, outline of heart. The arrows show the course of the blood.The same, modified for a bird.Fig. 55.—The same, modified for a bird. (After Le Conte.) The dark lines show the aortic arches which persist. A, aorta;p, pulmonary arches; SC, S'C', sub-clavian; C, C', carotids.The same, modified for a mammal.Fig. 56.—The same, modified for a mammal. (After Le Conte.)
Ideal diagram of primitive gill- or aortic-arches.Fig. 54.—Ideal diagram, of primitive gill- or aortic-arches. (After Rathke.) H, outline of heart. The arrows show the course of the blood.
Fig. 54.—Ideal diagram, of primitive gill- or aortic-arches. (After Rathke.) H, outline of heart. The arrows show the course of the blood.
The same, modified for a bird.Fig. 55.—The same, modified for a bird. (After Le Conte.) The dark lines show the aortic arches which persist. A, aorta;p, pulmonary arches; SC, S'C', sub-clavian; C, C', carotids.
Fig. 55.—The same, modified for a bird. (After Le Conte.) The dark lines show the aortic arches which persist. A, aorta;p, pulmonary arches; SC, S'C', sub-clavian; C, C', carotids.
The same, modified for a mammal.Fig. 56.—The same, modified for a mammal. (After Le Conte.)
Fig. 56.—The same, modified for a mammal. (After Le Conte.)
A series of embryos of the classes of vertebrated animals below the MammaliaFig. 57.—A series of embryos at three comparable and progressive stages of development (marked I, II, III), representing each of the classes of vertebrated animals below the Mammalia (After Häckel.)
Fig. 57.—A series of embryos at three comparable and progressive stages of development (marked I, II, III), representing each of the classes of vertebrated animals below the Mammalia (After Häckel.)