« AnteriorContinuar »
The following four diagrams will convey a clearer idea than mere description of the probable stages of this process:
1. 2. 3. 4.
Diagrams representing the chorization process in the development of the embryo of Alcyonella. The stage represented in 3 has not been actually seen, but seems to be the only one that will supply the missing link.
Towards the opening which leads from without into the central cavity the chorization is incomplete, the membrane as it separates being here still held to the walls of the cavity by irregular transverse bands; these bands check the entire evagination of the membrane, but after a time they disappear, and then the unlining and evagination are perfect. In the interior of the protrusible portion, and before the disappearance of the transverse bands, a polypide is developed (Pl. XI, fig. 30). This appears to take place in a manner quite similar to that by which new polypides are produced by gemmation from the walls of the endocystal cavity in the adult. The gemmation of the first polypide is immediately followed by that of another close beside it (Pl. XI, fig. 31), so that the young polyzoon has now the appearance of a transparent, closed sac, filled with fluid, the posterior part ciliated, the anterior destitute of cilia and partially or entirely pushed back into the posterior by a process of invagination; while the sac carries within it two young polypides, which are suspended from the inner surface of the unciliated portion. The arms of the lophophore in the young polypides are at first but slightly developed, and there is as yet no trace of tentacula (Pl. XI, fig. 30°); these soon present themselves in the form of minute tubercles, at first confined to the body of the lophophore (Pl. XI, fig. 30°), and then extending along the arms, which at the same time acquire increased proportional length; the tentacula gradually elongate themselves and acquire cilia. At the same time, the alimentary canal, represented at first by a single small cavity, hollowed out in the body of the polypide, is undergoing development, and oesophagus, stomach, and intestine begin to be distinguishable. The great retractor muscles have become evident, and the funiculus may be seen extending from the base of the polypide to the walls of the sac (Pl. XI, fig. 31). The polypides have, at first, no communication with the exterior, but at an early stage the tentacular sheath, with the parieto-vaginal bands, had become evident, and the fluid in which the embryo floats within the walls of the ovum is soon afterwards admitted to the lophophore of the young polypides. The parieto-vaginal bands would seem to be drawn out by a process of separation from the walls similar to that just described.
The embryo is still contained within the external membrane of the ovum, which, however, has become much distended, in accordance with the increasing size of the included parts, and at length, giving way, allows the embryo to escape.
The free embryo (Pl. XI, figs. 32, 33,33", 33") now swims actively through the surrounding fluid of the perigastric space. It sometimes remains stationary, with the unciliated portion, in which the polypides are suspended, protruded from the ciliated (Pl. XI, fig. 32); but most frequently this unciliated portion is withdrawn within the ciliated, which is then closed over the aperture (Pl. XI, fig. 33). In this condition the little animal assumes a piriform figure, with the small end corresponding to the aperture. It is thus carried through the surrounding fluid by the vibration of its cilia, performing rapid and elegant motions, always swimming with the broad end foremost, and at the same time revolving gracefully on its axis.
The complete evagination of the unciliated portion is still prevented by the bands already mentioned, but we now find that these bands, which must not be confounded with the permanent parieto-vaginal bands of the adult, have disappeared, and the evagination has become complete (Pl. XI, fig. 34). The unciliated portion is now no longer capable of being withdrawn within the ciliated, with which it has become directly continuous, while the cilia themselves disappear from the ciliated portion, and the entire sac becomes enveloped in an ectocyst, to constitute the cell of the adult polyzoon. The subsequent changes are produced by the gemmation of new polypides, with their proper ectocysts and endocysts.
Plumatella fruticosa presents similar developmental phenomena; the ciliated larva, however, in this species, differs from that just described, in having its polypide single (Pl. XI, fig. 35).
If a specimen of Alcyonella fungosa be cut into small pieces, under water, in the month of July or August, the ciliated embryos will be liberated in abundance, and may be examined with facility.
Gemmation.—The development of gemmae has already been partially traced in the description just given of the larva of Alcyonella; we must now follow it, however, a little more in detail, as it is presented by the buds formed in the adult polyzoon.
With the exception of some peculiar forms of gemmae, to be presently described, these bodies always originate in the endocyst. In Lophopus, Alcyonella, Plumatella, and Fredericella, they occur without any very regular order near the orifice of the cell. In Cristatella the gemmae are produced very regularly from constant points on the sides of the previously existing cells, and the new cells thus produced remaining in apposition with one another, and never becoming extended into branches, constitute several concentric series on the surface of an expanded disc. In Paludicella they also arise with much regularity from fixed points a little below and at each side of the orifice of the previously formed cells; and here, not continuing in apposition, the new cells form branches, which, from the fixed points at which they originate, and the constant angle at which they are given off from the parent-cell, confer upon the whole colony a greater regularity than is met with in the other branched forms of the freshwater Polyzoa.
Most of the steps in the development of the gemma may be traced with considerable facility in Paludicella. In the earliest condition in which I have been able to observe it, the gemma appears here as a minute tubercle, projecting from the external walls of the cell, and filled with a granular parenchyma (Pl. XI, fig. 1). It now becomes elongated (fig. 2), and we soon find it nollowed out into a cavity, which communicates with the interior of the parentcell. The tubercle, with its cavity, increase in size, and the gemma (fig. 3) is now found to consist of an external envelope, continuous with the ectocyst of the parent-cell, and of a thick, fleshy lining, continuous with the endocyst; this internal tunic has numerous large, round, nucleated cells distributed through its substance, and internally it presents an uneven surface. The two tunics of the gemma are to become the ectocyst and endocyst of the future cell. By this time the gemma has become considerably elongated and has acquired a clavate form, and its cavity begins to be cut off from that of the parent-cell by the formation of a septum. We next perceive that a portion of the lining tunic, near the wide extremity of the gemma, projects in the form of a roundish mass into the interior of the young cell (fig. 4). It is the rudimental polypide, and we soon find in it a cavity, which is to become that of the tentacular sheath, within which, when viewed in front, may be seen an oval ring (fig. 5) which is afterwards to become the lophophore of the polypide. This ring is at first quite simple, resembling a mere fold of thickish membrane, but in a short time it presents all round a series of minute tubercles (fig. 6), the rudiments of the future tentacula. When viewed laterally (fig. 7), the lophophore in this stage is seen to be decidedly bilateral, being prolonged towards the side where the rectum is to make its appearance. The central space between the rudimental tentacula is prolonged downwards, constituting the first trace of a pharynx; and immediately below this, the mass of the polypide is hollowed out into an internal cavity, which is to become stomach and intestine. This cavity is at first filled with clear, round bodies, having a high refracting power, but in which I could not trace the double outline of a true cell-wall. The polypide is now seen to be suspended from the wall of the cell by a membranous sac. This sac is closely adherent to the polypide behind the lophophore, but upon the lophophore and tentacula it has become free, and here constitutes the tentacular sheath. Some delicate fibres, the rudiments of the retractor of the polypide, may now be plainly seen extending from just behind the lophophore to the walls of the cell. The body of the polypide at the spot directly opposite to the rudimental crown of tentacula, is also seen to be connected by a short, thick, fleshy mass, with the walls of the cell. Circular fibres may have been already seen in the lining membrane of the cell; these are chiefly collected near its proximal end (fig. 6), and are to become the parietal muscles of the adult. The body of the polypide continues to elongate itself (fig. 8), and we can soon distinguish in its cavity the three regions of oesophagus, stomach, and intestine. The rudimentary parieto-vaginal muscles may also be seen extending from the tentacular sheath to the walls of the cell; the thick, fleshy mass, by which the body of the polypide was connected with the cell-walls, has become lengthened and divided into two chords, the anterior and posterior funiculus; the tentacula continue to increase in length; the lophophore loses its bilaterality and becomes orbicular, and we now (fig. 9) find little wanting to give to the polypide and its cell the form presented by the adult. Up to this period the young polypide has been entirely shut off from all communication with the external water, and its nutrition must have been effected through the general nutrition of the colony; now, however, an opening occurs in the new cell just over the tentacular crown of the polypide, and the last stage of development is entered on. The tentacular crown rapidly acquires its complete form, the inferior extremity of the alimentary canal becomes elongated into the great cul-de-sac of the stomach, the muscles are by this time all formed, and the polypide is capable of exsertion and retraction. It is now no longer dependent for its growth on the general nutrition of the colony, but has become an independent zooid, obtaining its food from without, and submitting it to the action of its own digestive system.
The development of the bud in the species with crescentic lophophore is essentially the same as that just described. The first appearance of the bud in Lophopus is seen near the orifice of the cell, as a minute spherical tubercle (Pl. XI, fig. 10), projecting from the inner surface of the endocyst into the perigastric space. It is evidently composed of a mass of minute cells with a more condensed peripheral layer. It now increases in size, and a clear, oval space becomes apparent towards one side of it (fig. 11). This space soon acquires a triangular figure (fig. 12), and it is now evident that the lophophore has begun to develop itself from its walls, and that the triangular space corresponds to the interval between the two arms of the rudimental lophophore. By this time the young polypide has become removed from the surface of the endocyst of the parent cell, to which it is now only connected by a membranous sac which suspends it in the perigastric space of the adult, and which ultimately becomes the tentacular sheath. At the same time, the alimentary canal begins to appear as a minute cavity in the part of the bud farthest removed from the walls of the cell. This cavity increases in size (figs. 13, 14), and becomes the stomach, while the intestine also shows itself as a tubular process on one side of the bud (fig. 13). The lophophore, which from its earliest appearance is distinctly bilateral, now presents the form of a semicircle, with a bilobed base, and thickened margin; as yet, there is no trace of tentacula. The retractor muscles show themselves as two fasciculi passing off from the base of the lophophore, and the funiculus may also be seen attached to the posterior extremity of the body. The tentacula now make their appearance as minute tubercles upon the thickened margin of the lophophore (fig. 15). The tentacula and arms of the lophophore are then simultaneously elongated (fig. 16), while the three regions of the alimentary canal, oesophagus, stomach, and intestine, are easily distinguishable, and little more is now wanting than an opening through the walls of the parent-cell into the tentacular sheath of the young polypide, to complete the development of the gemma.
In all the fresh-water phylactolaematous Polyzoa, bodies of a very peculiar nature occur at certain seasons, lying loose in the perigastric space. To these bodies, for reasons to be presently mentioned, I propose to give the name of Statoblasts. From the earliest period that the fresh-water Polyzoa became an object of study, the statoblasts attracted the attention of observers. Their form is not exactly the same in the different species, but they may be generally described as lenticular bodies, varying, according to the species, from an orbicular to an elongated oval figure, and enclosed in a horny shell, which consists of two concavo-convex discs united by their margins, where they are further strengthened by a ring which runs round the entire margin, and is of a different structure from the disc (Pl. I, figs. 3–8; Pl. IV, figs. 4, 5; &c.) In Fredericella alone is the marginal ring so little developed as to be scarcely apparent. In all, one side would seem to be slightly more convex than the other.
In all cases, except Cristatella and Pectinatella, the statoblast is destitute of any further appendage; in these two genera, however, it is furnished, when mature, with hooked spines, which, at least in Cristatella, spring alternately from the two sides just within the annulus (Pl. I, figs. 4–7), being generally more numerous and better developed on the more convex side; they thence pass outwards over the annulus, and project in short rays beyond the margin. The disc in all the species is of a deep brown colour, and would seem to be composed of a single layer of hexagonal cells, whose external walls in most cases slightly project beyond the surface of the disc, and thus give to the latter an elegantly mammillated condition. In some cases, however, the cellular condition of one or both discs is very obscure. The annulus is also composed of cells, which here occur in several layers; these cells are also for the most part larger than those of the disc and of a different colour; they are filled with air, giving to the annulus a light spongy texture, and act as a float by which the statoblast, when set free, is kept near the surface of the surrounding water. When the statoblasts are placed under circumstances favouring their development, they open by the separation from one another of the two faces, and there then escapes from them a young polyzoon already in an advanced stage of development, and in all essential points resembling the adult individual in whose cell the statoblasts were produced (Pl. XI, figs. 42, 43). The statoblasts have been always viewed and described as the eggs of the polyzoon in whose cell they occur, a very natural mistake, and one the more excusable as the true ovary had not yet been detected. Into this error I fell myself;” but I have now become convinced that they are a peculiar form of bud, and must on no account be confounded with genuine ova. They are produced in the funiculus, from which they are evidently developed as buds, and may generally be seen in various stages of growth, arranged upon this chord like beads on a necklace, being younger as they approach the distal extremities of the funiculus (Pl. III, fig. 7; W, fig. 5, 2 ; XI, fig. 36). In Lophopus, I have succeeded in following them through various stages of their early development. Their first appearance here is in the form of a little swelling upon the funiculus, consisting of a mass of minute cells, surrounded by a denser layer, which is continuous with the surface of the funiculus. The swelling now increases in size, and assumes a more regularly oval form, while its contents appear more uniformly granular, and are plainly seen to be composed of two masses in close apposition with one another (fig. 37). We next find that the two masses have lost their distinctness, and become fused together, and the whole contents now appear to be composed of minute cells confined by an external, common, transparent membrane, which is itself manifestly cellular (fig. 38). This cellular condition of the contents must not be confounded with a true segmentation. The whole body now assumes a more lenticular form, and within the external envelope two other investments begin to show themselves. One of these (the more internal) extends over the whole surface of the cellular contents, but the other is confined to the margin of the lenticular mass which it embraces in the form of a ring (fig. 39). No manifest structure, beyond a simply granular one, can be as yet detected in either of these last-formed envelopes; but the ring is soon seen to be composed of distinct cells (fig. 40), which present a bright central nucleus-like point, and a number of concentric layers, which remind us of the secondary deposits in certain vegetable cells. Up to this point the investments are all colourless, and nearly transparent; but we now find that the internal envelope and annulus become more and more opaque, while the former assumes a deep brown colour, and the latter becomes yellow. They have both acquired a horny consistence,