The developmental history of the Chordata has been studied far more completely than that of any of the groups so far considered; and the results which have been arrived at are of striking interest and importance. Three main subdivisions of this group can be recognized: (1) the Cephalochorda containing the single genus Amphioxus; (2) the Urochorda or Tunicata; and (3) the Vertebrata[1]. The members of the second and probably of the first of these groups have undergone degeneration, but at the same time the members of the first group especially undergo a less modified development than that of other Chordata.
Cephalochorda.
Our knowledge of the development of Amphioxus is mainly due to Kowalevsky (Nos.1and2). The ripe eggs appear to be dehisced into the branchial or atrial cavity, and to be transported thence through the branchial clefts into the pharynx, and so through the mouth to the exterior. (Kowalevsky,No.1, and Marshall,No.5.)
Illustration: Figure 1Fig. 1. The Segmentation of Amphioxus.(Copied from Kowalevsky.)B. Stage with four equal segments.C. Stage after the four segments have become divided by an equatorial furrow into eight equal segments.D. Stage in which a single layer of cells encloses a central segmentation cavity.E. Somewhat older stage in optical section.sg.segmentation cavity.
Fig. 1. The Segmentation of Amphioxus.(Copied from Kowalevsky.)B. Stage with four equal segments.C. Stage after the four segments have become divided by an equatorial furrow into eight equal segments.D. Stage in which a single layer of cells encloses a central segmentation cavity.E. Somewhat older stage in optical section.sg.segmentation cavity.
When laid the egg is about 0.105mm.in diameter. It is invested by a delicate membrane, and is somewhat opaque owing to the presence of yolk granules, which are however uniformly distributed through it, and proportionately less numerous than in the ova of most Chordata. Impregnation is external and the segmentation is nearly regular (fig. 1). A small segmentation cavity is visible at the stage with four segments, and increases during the remainder of the segmentation; till at the close (fig. 1E) the embryo consists of a blastosphere formed of a single layer of cells enclosing a large segmentation cavity. One side of the blastosphere next becomes invaginated, and during the process the embryo becomes ciliated, and commences to rotate. The cells forming the invaginated layer become gradually more columnar than the remaining cells, and constitute the hypoblast; and a structural distinction between the epiblast and hypoblast is thus established. In the course of the invagination the segmentationcavity becomes gradually obliterated, and the embryo first assumes a cup-shaped form with a wide blastopore, but soon becomes elongated, while the communication of the archenteron, or cavity of invagination, with the exterior is reduced to a small blastopore (fig. 2A), placed at the pole of the long axis which the subsequent development shews to bethe hinder end of the embryo. The blastopore is often known in other Chordata as the anus of Rusconi. Before the invagination is completed the larva throws off the egg-membrane, and commences to lead a free existence.
Illustration: Figure 2Fig. 2. Embryos of Amphioxus.(After Kowalevsky.)The parts in black with white lines are epiblastic; the shaded parts are hypoblastic.A. Gastrula stage in optical section.B. Slightly later stage after the neural platenphas become differentiated, seen as a transparent object from the dorsal side.C. Lateral view of a slightly older larva in optical section.D. Dorsal view of an older larva with the neural canal completely closed except for a small pore (no) in front.E. Older larva seen as a transparent object from the side.bl.blastopore (which becomes in D the neurenteric canal);ne.neurenteric canal;np.neural or medullary plate;no.anterior opening of neural canal;ch.notochord;soI,soII. first and second mesoblastic somites.
Fig. 2. Embryos of Amphioxus.(After Kowalevsky.)The parts in black with white lines are epiblastic; the shaded parts are hypoblastic.A. Gastrula stage in optical section.B. Slightly later stage after the neural platenphas become differentiated, seen as a transparent object from the dorsal side.C. Lateral view of a slightly older larva in optical section.D. Dorsal view of an older larva with the neural canal completely closed except for a small pore (no) in front.E. Older larva seen as a transparent object from the side.bl.blastopore (which becomes in D the neurenteric canal);ne.neurenteric canal;np.neural or medullary plate;no.anterior opening of neural canal;ch.notochord;soI,soII. first and second mesoblastic somites.
Up to this stage the larva, although it has acquired a cylindrical elongated form, has only the structure of a simple two-layered gastrula; but the changes which next take placegive rise on the one hand to the formation of the central nervous system, and on the other to the formation of the notochord and mesoblastic somites[2]. The former structure is developed from the epiblast and the two latter from the hypoblast.
The formation of the central nervous system commences with the flattening of the dorsal surface of the embryo. The flattened area forms a plate (fig. 2B andfig. 3A,np), extending backwards to the blastopore, which has in the meantime passed round to the dorsal surface. The sides of the plate become raised as two folds, which are most prominent posteriorly, and meet behind the blastopore, but shade off in front. The two folds next unite dorsally, so as to convert the previous groove into a canal[3]—the neural or medullary canal. They unite first of all over the blastopore, and their line of junction extends from this point forwards (fig. 2C, D, E). There is in this way formed a tube on the floor of which the blastopore opens behind, and which is itself open in front. Finally the medullary canal is formed for the whole length of the embryo. The anterior opening persists however for some time. The communication between the neural and alimentary tracts becomes interrupted when the caudal fin appears and the anus is formed. The neural canal then extends round the end of the notochord to the ventral side, but subsequently retreats to the dorsal side and terminates in a slight dilatation.
In the formation of the medullary canal there are two points deserving notice—viz.(1) the connection with the blastopore; (2) the relation of the walls of the canal to the adjoining epiblast. With reference to the first of these points it is clear that the fact of the blastopore opening on the floor of the neural canal causes a free communication to exist between the archenteron or gastrula cavity and the neural canal; and that, so long as the anterior pore of the neural canal remains open, the archenteron communicates indirectly with the exterior (videfig. 2E). It must not however be supposed (as has been done by some embryologists) that the pore at the front end of the neural canal represents the blastopore carried forwards. It iseven probable that what Kowalevsky describes as the carrying of the blastopore to the dorsal side is really the commencement of the formation of the neural canal, the walls of which are continuous with the lips of the blastopore. This interpretation receives support from the fact that at a later stage, when the neural and alimentary canals become separated, the neural canal extends round the posterior end of the notochord to the ventral side. The embryonic communication between the neural and alimentary canals is common to most Chordata; and the tube connecting them will be called theneurenteric canal. It is always formed in fundamentally the same manner as in Amphioxus. With reference to the second point it is to be noted that Amphioxus is exceptional amongst the Chordata in the fact that, before the closure of the neural groove, the layer of cells which will form the neural tube becomes completely separated from the adjoining epiblast (fig. 3A), and forms a structure which may be spoken of as the medullary plate; and that in the closure of the neural canal the lateral epiblast forms a complete layer above this plate before the plate itself is folded over into a closed canal. This peculiarity will be easily understood from an examination offig. 3A, B and C.
Illustration: Figure 3Fig. 3. Sections of an Amphioxus embryo at three stages.(After Kowalevsky.)A. Section at gastrula stage.B. Section of an embryo slightly younger than that represented in fig. 2 D.C. Section through the anterior part of an embryo at the stage represented in fig. 2 E.np.neural plate;nc.neural canal;mes.archenteron in A and B, and mesenteron in C;ch.notochord;so.mesoblastic somite.
Fig. 3. Sections of an Amphioxus embryo at three stages.(After Kowalevsky.)A. Section at gastrula stage.B. Section of an embryo slightly younger than that represented in fig. 2 D.C. Section through the anterior part of an embryo at the stage represented in fig. 2 E.np.neural plate;nc.neural canal;mes.archenteron in A and B, and mesenteron in C;ch.notochord;so.mesoblastic somite.
The formation of the mesoblastic somites commences, at about the same time as that of the neural canal, as a pair of hollow outgrowths of the walls of the archenteron. Theseoutgrowths, which are shewn in surface view infig. 2B and D,so, and in section infig. 3B and C,so, arise near the front end of the body and gradually extend backwards as wing-like diverticula of the archenteric cavity. As they grow backwards their dorsal part becomes divided by transverse constrictions into cubical bodies (fig. 2D and E), which, with the exception of the foremost, soon cease to open into what may now be called the mesenteron, and form the mesoblastic somites. Each mesoblastic somite, after its separation from the mesenteron, is constituted of two layers, an inner one—the splanchnic—and an outer—the somatic, and a cavity between the two which was originally continuous with the cavity of the mesenteron. Eventually the dorsal parts of the outgrowths become separated from the ventral, and form the muscle-plates, while their cavities atrophy. The cavity of the ventral part, which is not divided into separate sections by the above described constrictions, remains as the true body cavity. The ventral part of the inner layer of the mesoblastic outgrowths gives rise to the muscular and connective tissue layers of the alimentary tract, and the dorsal part to a section of the voluntary muscular system. The ventral part of the outer layer gives rise to the somatic mesoblast, and the dorsal to a section of the voluntary muscular system. The anterior mesoblastic somite long retains its communication with the mesenteron, and was described by Max Schultze, and also at first by Kowalevsky, as a glandular organ. While the mesoblastic somites are becoming formed the dorsal wall of the mesenteron develops a median longitudinal fold (fig. 3B,ch), which is gradually separated off from before backwards as a rod (fig. 3C,ch), underlying the central nervous system. This rod is the notochord. After the separation of those parts the remainder of the hypoblast forms the wall of the mesenteron.
With the formation of the central nervous system, the mesoblastic somites, the notochord, and the alimentary tract the main systems of organs are established, and it merely remains briefly to describe the general changes of form which accompany the growth of the larva into the adult. By the time the larva is but twenty-four hours old there are formed about seventeen mesoblastic somites. The body, during the period in whichthese are being formed, remains cylindrical, but shortly afterwards it becomes pointed at both ends, and the caudal fin appears. The fine cilia covering the larva also become replaced by long cilia, one to each cell. The mesenteron is still completely closed, but on the right side of the body, at the level of the front end of the mesenteron, the hypoblast and epiblast now grow together, and a perforation becomes formed through their point of contact, which becomes the mouth. The anus is probably formed about the same time if not somewhat earlier[4].
Illustration: Figure 4Fig. 4. Sections through two advanced embryos of Amphioxus to shew the formation of the peribranchial cavity.(After Kowalevsky.)In A are seen two folds of the body wall with a prolongation of the body cavity. In B the two folds have coalesced ventrally, forming a cavity into which a branchial cleft is seen to open.mes.mesenteron;br.c.branchial cavity;pp.body cavity.
Fig. 4. Sections through two advanced embryos of Amphioxus to shew the formation of the peribranchial cavity.(After Kowalevsky.)In A are seen two folds of the body wall with a prolongation of the body cavity. In B the two folds have coalesced ventrally, forming a cavity into which a branchial cleft is seen to open.mes.mesenteron;br.c.branchial cavity;pp.body cavity.
Of the subsequent changes the two most important are (1) the formation of the gill slits or clefts; (2) the formation of the peribranchial or atrial cavity.
The formation of the gill slits is, according to Kowalevsky’s description, so peculiar that one is almost tempted to suppose that his observations were made on pathological specimens. The following is his account of the process. Shortly after the formation of the mouth there appears on the ventral line a coalescence between the epiblast and hypoblast. Here an opening is formed, and a visceral cleft is thus established, which passes to the left side,viz.the side opposite the mouth. A second and apparently a third slit are formed in the same way. The stages immediately following were not observed, but in the next stage twelve slits were present, no longer however on the left side, but in the median ventral line. There now appears on the side opposite the mouth, and the same therefore as that originally occupied by the first three clefts, a series of fresh clefts, which in theirgrowth push the original clefts over to the same side as the mouth. Each of the fresh clefts becomes divided into two, which form the permanent clefts of their side.
The gill slits at first open freely to the exterior, but during their formation two lateral folds of the body wall, containing a prolongation of the body cavity, make their appearance (fig. 4A), and grow downwards over the gill clefts, and finally meet and coalesce along the ventral line, leaving a widish cavity between themselves and the body wall. Into this cavity, which is lined by epiblast, the gill clefts open (fig. 4B,br.c). This cavity—which forms a true peribranchial cavity—is completely closed in front, but owing to the folds not uniting completely behind it remains in communication with the exterior by an opening known as the atrial or abdominal pore.
The vascular system of Amphioxus appears at about the same time as the first visceral clefts.
Bibliography.
(1)A. Kowalevsky.“Entwicklungsgeschichte des Amphioxus lanceolatus.”Mém. Acad. Impér. des Sciences deStPétersbourg, SeriesVII.Tom.XI. 1867.(2)A. Kowalevsky. “Weitere Studien über die Entwicklungsgeschichte des Amphioxus lanceolatus.”Archiv f. mikr. Anat.,Vol.XIII. 1877.(3)Leuckart u. Pagenstecher. “Untersuchungen über niedere Seethiere.”Müller’s Archiv, 1858.(4)Max Schultze. “Beobachtung junger Exemplare von Amphioxus.”Zeit. f. wiss. Zool.,Bd.III. 1851.(5)A. M. Marshall. “On the mode of Oviposition of Amphioxus.”Jour. of Anat. and Phys.,Vol.X. 1876.
[1]The term Vertebrata is often used to include the Cephalochorda. It is in many ways convenient to restrict its use to the forms which have at any rate some indications of vertebræ; a restriction which has the further convenience of restoring to the term its original limitations. In the first volume of this work the term Craniata was used for the forms which I now propose to call Vertebrata.[2]The protovertebræ of most embryologists will be spoken of as mesoblastic somites.[3]The details of this process are spoken of below.[4]The lateral position of the mouth in the embryo Amphioxus has been regarded as proving that the mouth represents a branchial cleft, but the general asymmetry of the organs is such that no great stress can, I think, be laid on the position of the mouth.
[1]The term Vertebrata is often used to include the Cephalochorda. It is in many ways convenient to restrict its use to the forms which have at any rate some indications of vertebræ; a restriction which has the further convenience of restoring to the term its original limitations. In the first volume of this work the term Craniata was used for the forms which I now propose to call Vertebrata.
[2]The protovertebræ of most embryologists will be spoken of as mesoblastic somites.
[3]The details of this process are spoken of below.
[4]The lateral position of the mouth in the embryo Amphioxus has been regarded as proving that the mouth represents a branchial cleft, but the general asymmetry of the organs is such that no great stress can, I think, be laid on the position of the mouth.
In the Solitaria, except Cynthia, the eggs are generally laid, and impregnation is effected sometimes before and sometimes after the eggs have left the atrial cavity. In Cynthia and most Caducichordata development takes place within the body of the parent, and in the Salpidæ a vascular connection is established between the parent and the single fœtus, forming a structure physiologically comparable with the Mammalian placenta.
Solitaria.The development of the Solitary Ascidians has been more fully studied than that of the other groups, and appears moreover to be the least modified. It has been to a great extent elucidated by the splendid researches of Kowalevsky (Nos.18and20), whose statements have been in the main followed in the account below. Their truth seems to me to be established, in spite of the scepticism they have met with in some quarters, by the closeness of their correspondence with the developmental phenomena in Amphioxus.
The type most fully investigated by Kowalevsky is Ascidia (Phallusia) mammillata; and the following description must be taken as more especially applying to this type.
The segmentation is complete and regular. A small segmentation cavity appears fairly early, and is surrounded, according to Kowalevsky, by a single layer of cells, though on this point Kupffer (No.27) and Giard (No.11) are at variance with him.
Illustration: Figure 5Fig. 5. Transverse section through the front end of an embryo of Phallusia mammillata.(After Kowalevsky.)The embryo is slightly younger than that represented in fig. 8III.mg.medullary groove;al.alimentary tract.
Fig. 5. Transverse section through the front end of an embryo of Phallusia mammillata.(After Kowalevsky.)The embryo is slightly younger than that represented in fig. 8III.mg.medullary groove;al.alimentary tract.
The segmentation is followed by an invagination of nearly the same character as in Amphioxus. The blastosphere resulting from the segmentation first becomes flattened on one side, and the cells on the flatter side become more columnar (fig. 8I.). Very shortly a cup-shaped form is assumed, the concavity of which is lined by the more columnar cells. The mouth of the cup or blastopore next becomes narrowed; while at the same time the embryo becomes oval. The blastopore is situated not quite at a pole of the oval but in a position which subsequent development shews tobe on the dorsal side close to the posterior end of the embryo. The long axis of the oval corresponds with the long axis of the embryo. At this stage the embryo consists of two layers; a columnar hypoblast lining the central cavity or archenteron, and a thinner epiblastic layer. The dorsal side of the embryo next becomes flattened (fig. 8II.), and the epiblast covering it is shortly afterwards marked by an axial groove continued forwards from the blastopore to near the front end of the body (fig. 5,mg). This is the medullary groove, and it soon becomes converted into a closed canal—the medullary or neural canal—below the external skin (fig. 6,n.c). The closure is effected by the folds on each side of the furrow meeting and coalescing dorsally. The original medullary folds fall into one another behind the blastopore so that the blastopore is situated on thefloor of the groove, and, on the conversion of the groove into a canal, the blastopore connects the canal with the archenteric cavity, and forms a short neurenteric canal. The closure of the medullary canal commences at the blastopore and is thence continued forwards, the anterior end of the canal remaining open. The above processes are represented in longitudinal section infig. 8III,n. When the neural canal is completed for its whole length, it still communicates by a terminal pore with the exterior. In the relation of the medullary canal to the blastopore, as well as in the closure of the medullary groove from behind forwards, the Solitary Ascidians agree closely with Amphioxus.
Illustration: Figure 6Fig. 6. Transverse optical section of the tail of an embryo of Phallusia mammillata.(After Kowalevsky.)The section is from an embryo of the same age as fig. 8IV.ch.notochord;n.c.neural canal;me.mesoblast;al.hypoblast of tail.
Fig. 6. Transverse optical section of the tail of an embryo of Phallusia mammillata.(After Kowalevsky.)The section is from an embryo of the same age as fig. 8IV.ch.notochord;n.c.neural canal;me.mesoblast;al.hypoblast of tail.
The cells of the dorsal wall of the archenteron immediately adjoining the front and sides of the blastopore have in the meantime assumed a somewhat different character from the remaining cells of the archenteron, and give rise to a body which, when viewed from the dorsal surface, has somewhat the form of a horseshoe. This body was first observed by Metschnikoff. On the elongation of the embryo and the narrowing of the blastopore the cells forming this body arrange themselves as a broad linear cord, two cells wide, underlying about the posterior half of the neural canal (fig. 7,ch). They form the rudiment of the notochord, which, as in Amphioxus, is derived from the dorsal wall of the archenteron. They are seen in longitudinal section infig. 8II. andIII.ch.
With the formation of the notochord the body of the embryo becomes divided into two distinct regions—a posterior region where the notochord is present, and an anterior region into which it is not prolonged. These two regions correspond with the tail and the trunk of the embryo at a slightly later stage. The section of the archenteric cavity in the trunk dilates and constitutes the permanent mesenteron (figs.7,al, and8III. andIV.dd). It soon becomes shut off from the slit-like posteriorpart of the archenteron. The nervous system in this part also dilates and forms what may be called the cephalic swelling (fig. 8IV.), and the pore at its anterior extremity gradually narrows and finally disappears. In the region of the tail we have seen that the dorsal wall of the archenteron becomes converted into the notochord, which immediately underlies the posterior part of the medullary canal, and soon becomes an elongated cord formed of a single or double row of flattened cells. The lateral walls of the archenteron (fig. 7,me) in the tail become converted into elongated cells arranged longitudinally, which form powerful lateral muscles (fig. 8IV.m). After the formation of the notochord and of the lateral muscles there remains of the archenteron in the tail only the ventral wall, which according to Kowalevsky forms a simple cord of cells (fig. 6,al). It is however not always present, or else has escaped the attention of other observers. It is stated by Kowalevsky to be eventually transformed into blood corpuscles. The neurenteric canal leads at first into the narrow space between the above structures, which is the remnant of the posterior part of the lumen of the archenteron. Soon both the neurenteric canal and the caudal remnant of the archenteron become obliterated.
Illustration: Figure 7Fig. 7. Optical section of an embryo of Phallusia mammillata.(After Kowalevsky.)The embryo is of the same age as fig. 8III, but is seen in longitudinal horizontal section.al.alimentary tract in anterior part of body;ch.notochord;me.mesoblast.
Fig. 7. Optical section of an embryo of Phallusia mammillata.(After Kowalevsky.)The embryo is of the same age as fig. 8III, but is seen in longitudinal horizontal section.al.alimentary tract in anterior part of body;ch.notochord;me.mesoblast.
During the above changes the tail becomes considerably elongated and, owing to the larva being still in the egg-shell, is bent over to the ventral side of the trunk.
The larva at this stage is represented in a side view infig. 8IV.The epidermis is formed throughout of a single layer of cells. In the trunk the mesenteron is shewn atddand the dilated part of the nervous system, no longer communicating with the exterior, atn. In the tail the notochord is shewn atch, the muscles atm, and the solid remnant of the ventral wallof the archenteron atdd´. The delicate continuation of the neural canal in the tail is seen above the notochord atn. An optical section of the tail is shewn infig. 6. It is worthy of notice that the notochord and muscles are formed in the same manner as in Amphioxus, except that the process is somewhat simplified. The mode of disappearance of the archenteric cavity in the tail, by the employment of the whole of its walls in the formation of various organs, is so peculiar, that I feel some hesitation in accepting Kowalevsky’s statements on this head[6].
Illustration: Figure 8Fig. 8. Various stages in the development of Phallusia mammillata.(From Huxley; after Kowalevsky.)The embryos are represented in longitudinal vertical section.I.Commencing gastrula stage.fh.segmentation cavity.II.Late gastrula stage with flattened dorsal surface.eo.blastopore;ch.notochord;dd.hypoblast.III.A more advanced embryo with a partially-formed neural tube.ch.anddd.as before;n.neural tube;c.epiblast.IV.Older embryo in which the formation of the neural tube is completed.dd.hypoblast enclosing persistent section of alimentary tract;dd´.hypoblast in the tail;m.muscles.V.Larva just hatched. The end of the tail is not represented.a.eye;gb.dilated extremity of neural tube with otolith projecting into it;Rg.anterior swelling of the spinal division of the neural tube;f. anterior pore of neural tube;Rm.posterior part of neural tube;o.mouth;Chs.notochord;kl.atrial invagination;dd.branchial region of alimentary tract; d. commencement of œsophagus and stomach;dd´.hypoblast in the tail;m.muscles;hp.papilla for attachment.VI.Body and anterior part of the tail of a two days’ larva.klm.atrial aperture; en. endostyle;ks.branchial sack;1ks. 2ks.branchial slits;bb.branchial vessel between them;ch.axial portion of notochord;chs.peripheral layer of cells. Other reference letters as before.
Fig. 8. Various stages in the development of Phallusia mammillata.(From Huxley; after Kowalevsky.)The embryos are represented in longitudinal vertical section.I.Commencing gastrula stage.fh.segmentation cavity.II.Late gastrula stage with flattened dorsal surface.eo.blastopore;ch.notochord;dd.hypoblast.III.A more advanced embryo with a partially-formed neural tube.ch.anddd.as before;n.neural tube;c.epiblast.IV.Older embryo in which the formation of the neural tube is completed.dd.hypoblast enclosing persistent section of alimentary tract;dd´.hypoblast in the tail;m.muscles.V.Larva just hatched. The end of the tail is not represented.a.eye;gb.dilated extremity of neural tube with otolith projecting into it;Rg.anterior swelling of the spinal division of the neural tube;f. anterior pore of neural tube;Rm.posterior part of neural tube;o.mouth;Chs.notochord;kl.atrial invagination;dd.branchial region of alimentary tract; d. commencement of œsophagus and stomach;dd´.hypoblast in the tail;m.muscles;hp.papilla for attachment.VI.Body and anterior part of the tail of a two days’ larva.klm.atrial aperture; en. endostyle;ks.branchial sack;1ks. 2ks.branchial slits;bb.branchial vessel between them;ch.axial portion of notochord;chs.peripheral layer of cells. Other reference letters as before.
The larva continues to grow in length, and the tail becomes further curled round the ventral side of the body within the egg-membrane. Before the tail has nearly reached its full length the test becomes formed as a cuticular deposit of the epiblast cells (O. Hertwig,No.13, Semper,No.37). It appears first in the tail and gradually extends till it forms a complete investment round both tail and trunk, and is at first totally devoid of cells. Shortly after the establishment of the test there grow out from the anterior end of the body three peculiar papillæ, developed as simple thickenings of the epidermis. At a later stage, after the hatching of the larva, these papillæ develop glands at their extremities, secreting a kind of glutinous fluid[7]. After these papillæ have become formed cells first make their appearance in the test; and there is simultaneously formed a fresh inner cuticular layer of the test, to which at first the cells are confined, though subsequently they are found in the outer layer also. On the appearance of cells in the test the latter must be regarded as a form, though a very abnormal one, of connective tissue. When the tail of the larva has reached a very considerable length the egg-membrane bursts, and the larva becomes free. The hatching takes place in Asc. canina about 48-60 hours after impregnation. The free larva (fig. 8V.) has a swollen trunk, and a very long tail, which soon becomesstraightened out. It has a striking resemblance to a tadpole (videfig. 10).
In the free larval condition the Ascidians have in many respects a higher organization than in the adult state. It is accordingly convenient to divide the subsequent development into two periods, the first embracing the stages from the condition represented infig. 8V.up to the full development of the free larva, and the second the period from the full development of the larva to the attainment of the fixed adult condition.
Growth and Structure of the free larva.
The nervous system.The nervous system was left as a closed tube consisting of a dilated anterior division, and a narrow posterior one. The former may be spoken of as the brain, and the latter as the spinal cord; although the homologies of these two parts are quite uncertain. The anterior part of the spinal cord lying within the trunk dilates somewhat (fig. 8V. andVI.Rg) and there may thus be distinguished a trunk and a caudal section of the spinal cord.
Illustration: Figure 9Fig. 9. Larva of Ascidia mentula.(From Gegenbaur; after Kupffer.) Only the anterior part of the tail is represented.N´. anterior swelling of neural tube;N.anterior swelling of spinal portion of neural tube;n.hinder part of neural tube;ch.notochord;K.branchial region of alimentary tract;d.œsophageal and gastric region of alimentary tract;O.eye;a.otolith;o.mouth;s.papilla for attachment.
Fig. 9. Larva of Ascidia mentula.(From Gegenbaur; after Kupffer.) Only the anterior part of the tail is represented.N´. anterior swelling of neural tube;N.anterior swelling of spinal portion of neural tube;n.hinder part of neural tube;ch.notochord;K.branchial region of alimentary tract;d.œsophageal and gastric region of alimentary tract;O.eye;a.otolith;o.mouth;s.papilla for attachment.
The original single vesicle of the brain becomes divided by the time the larva is hatched into two sections (fig. 9)—(1) an anterior vesicle with, for the most part, thin walls, in whichunpaired auditory and optic organs make their appearance, and (2) a posterior nearly solid cephalic ganglion, through which there passes a narrow continuation of the central canal of the nervous system. This ganglion consists of a dorsal section formed of distinct cells, and a ventral section formed of a punctated material with nuclei. The auditory organ[8]consists of a ‘crista acustica’ (fig. 9), in the form of a slight prominence of columnar cells on the ventral side of the anterior cerebral vesicle; to the summit of which a spherical otolith is attached by fine hairs. In the crista is a cavity containing clear fluid. The dorsal half of the otolith is pigmented: the ventral half is without pigment. The crista is developedin situ, but the otolith is formed from a single cell on the dorsal side of the cerebral vesicle, which forms a projection into the cavity of the vesicle, and then travels (in a manner not clearly made out) round the right side of the vesicle till it comes to the crista; to which it is at first attached by a narrow pedicle. The fully developed eye (figs.8VI.and9,O) consists of a cup-shaped retina, which forms a prominence slightly on the right side of the posterior part of the dorsal wall of the anterior cerebral vesicle, and of refractive media. The retina is formed of columnar cells, the inner ends of which are imbedded in pigment. The refractive media of the eye are directed towards the cavity of the cerebral vesicle, and consist of a biconvex lens and a meniscus. Half the lens is imbedded in the cavity of the retina and surrounded by the pigment, and the other half is turned toward a concavo-convex meniscus which corresponds in position with the cornea. The development of the meniscus and lens is unknown, but the retina is formed (fig. 8V.a) as an outgrowth of the wall of the brain. At the inner ends of the cells of this outgrowth a deposit of pigment appears.
The trunk section of the spinal cord (fig. 9,N) is separated by a sharp constriction from the brain. It is formed of a superficial layer of longitudinal nervous fibres, and a central core of ganglion cells. The layer of fibres diminishes in thickness towards the tail, and finally ceases to be visible. Kupffer detected three pairs of nerves passing off from the spinal cord tothe muscles of the tail. The foremost of these arises at the boundary between the trunk and the tail, and the two others at regular intervals behind this point.
The mesoblast and muscular system.It has already been stated that the lateral walls of the archenteron in the tail give rise to muscular cells. These cells lie about three abreast, and appear not to increase in number; so that with the growth of the tail they grow enormously in length, and eventually become imperfectly striated. The mesoblast cells at the hinder end of the trunk, close to its junction with the tail, do not become converted into muscle cells, but give rise to blood corpuscles; and the axial remnant of the archenteron undergoes a similar fate. According to Kowalevsky the heart is formed during larval life as an elongated closed sack on the right side of the endostyle.
The notochord.The notochord was left as a rod formed of a single row of cells, or in As. canina and some other forms of two rows, extending from just within the border of the trunk to the end of the tail.
According to Kowalevsky, Kupffer, Giard, etc. the notochord undergoes a further development which finds its only complete parallel amongst Chordata in the doubtful case of Amphioxus.
There appear between the cells peculiar, highly refractive discs (fig. 8V.Chs). These become larger and larger, and finally, after pushing the remnants of the cells with their nuclei to the sides, coalesce together to form a continuous axis of hyaline substance. The remnants of the cells with their nuclei form a sheath round the hyaline axis (fig. 8VI.ch.). Whether the axis is to be regarded as formed of an intercellular substance, or of a differentiation of parts of the cells is still doubtful. Kupffer inclines to the latter view: the analogy of the notochord of higher types appears to me to tell in favour of the former one.
The alimentary tract.The anterior part of the primitive archenteron alone retains a lumen, and from this part the whole of the permanent alimentary tract (mesenteron) becomes developed. The anterior part of it grows upwards, and before hatching an involution of the epiblast on the dorsal side, just in front of the anterior extremity of the nervous system, meets and opens into this upgrowth, and gives rise to the permanent mouth (fig. 8V.o).
Kowalevsky states that a pore is formed at the front end of the nervous tube leading into the mouth (fig. 8V. andVI.f) which eventually gives rise to the ciliated sack, which lies in the adult at the junction between the mouth and the branchial sack. Kupffer however was unable to find this opening; but Kowalevsky’s observations are confirmed by those of Salensky on Salpa.
From the hinder end of the alimentary sack an outgrowth directed dorsalwards makes its appearance (figs.8V.and9,d), from which the œsophagus, stomach and intestine become developed. It at first ends blindly. The remainder of the primitive alimentary sack gives rise to the branchial sack of the adult. Just after the larva has become hatched, the outgrowth to form the stomach and œsophagus, etc. bends ventralwards and to the right, and then turns again in a dorsal and left direction till it comes close to the dorsal surface, somewhat to the left of and close to the hinder end of the trunk. The first ventral loop of this part gives rise to the œsophagus, which opens into the stomach; from this again the dorsally directed intestine passes off.
On the ventral wall of the branchial sack there is formed a narrow fold with thickened walls, which forms the endostyle. It ends anteriorly at the stomodæum and posteriorly at the point where the solid remnant of the archenteron in the tail was primitively continuous with the branchial sack. The whole of the alimentary wall is formed of a single layer of hypoblast cells.
A most important organ connected with the alimentary system still remains to be dealt with,viz.the atrial or peribranchial cavity. The first rudiments of it appear at about the time of hatching, in the form of a pair of dorsal epiblastic involutions (fig. 8V.kl), at the level of the junction between the brain and the spinal cord. These involutions grow inwards, and meet corresponding outgrowths of the branchial sack, with which they fuse. At the junction between them is formed an elongated ciliated slit, leading from the branchial sack into the atrial cavity of each side. The slits so formed are the first pair of branchial clefts. Behind the first pair of branchial clefts a second pair is formed during larval life by a second outgrowth of the branchial sack meeting the epiblastic atrial involutions (fig. 8VI.1ksand2ks). The intestine at first ends blindly closeto the left atrial involution, but the anus becomes eventually formed by an opening being established between the left atrial involution and the intestine.
During the above described processes the test remains quite intact, and is not perforated at the oral or the atrial openings.
The retrogressive metamorphosis of the larva.
The development of the adult from the larva is, as has already been stated, in the main a retrogressive metamorphosis. The stages in this metamorphosis are diagrammatically shewn infigs.10and11. It commences with the attachment of the larva (fig. 10A) which takes place by one of the three papillæ. Simultaneously with the attachment the larval tail undergoes a complete atrophy (fig. 10B), so that nothing is left of it but a mass of fatty cells situated close to the point of the previous insertion of the tail in the trunk.