Illustration: Figure 41Fig. 41. Four stages in the development of Petromyzon.(After Owsjannikoff.)c.cephalic extremity;bl.blastopore;op.optic vesicle;au.v.auditory vesicle;br.c.branchial clefts.
Fig. 41. Four stages in the development of Petromyzon.(After Owsjannikoff.)c.cephalic extremity;bl.blastopore;op.optic vesicle;au.v.auditory vesicle;br.c.branchial clefts.
During the above changes in the external form of the embryo, the development of the various organs makes great progress. This is especially the case in the head. The brain becomes distinct from the spinal cord, and the auditory sacks and the optic vesicles of the eye become formed. The branchial region of the mesenteron becomes established, and causes adilatation of the anterior part of the body, and the branchial pouches grow out from the throat. The anus becomes formed, and a neurenteric canal is also established (Scott). The nature of these and other changes will best be understood by a description of the structure of the just-hatched larva. The general appearance of the larva immediately after hatching is shewn infig. 41, D. The body is somewhat curved; the posterior extremity being much dilated with yolk, while the anterior is very thin. All the cells still contain yolk particles, which render the embryo very opaque. The larva only exhibits slow movements, and is not capable of swimming about.
The structure of the head is shewn infigs.42andfig. 43.Fig. 42is a section through a very young larva, whilefig. 43is taken from a larva three days after hatching, and shews the parts with considerably greater detail.
Illustration: Figure 42Fig. 42. Diagrammatic vertical section of a just-hatched larva of Petromyzon.(From Gegenbaur; after Calberla.)o.mouth;o´.olfactory pit;v.septum between stomodæum and mesenteron;h.thyroid involution;n.spinal cord;ch.notochord;c.heart;a.auditory vesicle.
Fig. 42. Diagrammatic vertical section of a just-hatched larva of Petromyzon.(From Gegenbaur; after Calberla.)o.mouth;o´.olfactory pit;v.septum between stomodæum and mesenteron;h.thyroid involution;n.spinal cord;ch.notochord;c.heart;a.auditory vesicle.
On the ventral side of the head is placed the oral opening (fig. 43,m) leading into a large stomodæum which is still without a communication with the mesenteron. Ventrally the stomodæum is prolonged for a considerable distance under the anterior part of the mesenteron. Immediately behind the stomodæum is placed the branchial region of the mesenteron. Laterally it is produced on each side into seven or perhaps eight branchial pouches (fig. 43,br.c), which extend outwards nearly to the skin but are not yet open. Between the successive pouches are placed mesoblastic segments, of the same nature and structure as the walls of the head cavities in the embryos of Elasmobranchs, and like them enclosing a central cavity. Asimilar structure is placed behind the last, and two similar structures in front of the first persistent pouch. This pouch is situated in the same vertical line as the auditory sack (au.v), and would appear therefore to be the hyobranchial cleft; and this identification is confirmed by the fact of two head cavities being present in front of it. At the front end of the branchial region of the mesenteron is placed a thickened ridge of tissue, which, on the opening of the passage between the stomodæum and the mesenteron, forms a partial septum between the two, and is known as the velum (fig. 43,tv).
Illustration: Figure 43Fig. 43. Diagrammatic vertical section through the head of a larva of Petromyzon.The larva had been hatched three days, and was 4.8mm.in length. The optic and auditory vesicles are supposed to be seen through the tissues. The lettertvpointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated.c.h.cerebral hemisphere;th.optic thalamus;in.infundibulum;pn.pineal gland;mb.mid-brain;cb.cerebellum;md.medulla oblongata;au.v.auditory vesicle;op.optic vesicle;ol.olfactory pit;m.mouth;br.c.branchial pouches;th.thyroid involution;v.ao.ventral aorta;ht.ventricle of heart;ch.notochord.
Fig. 43. Diagrammatic vertical section through the head of a larva of Petromyzon.The larva had been hatched three days, and was 4.8mm.in length. The optic and auditory vesicles are supposed to be seen through the tissues. The lettertvpointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated.c.h.cerebral hemisphere;th.optic thalamus;in.infundibulum;pn.pineal gland;mb.mid-brain;cb.cerebellum;md.medulla oblongata;au.v.auditory vesicle;op.optic vesicle;ol.olfactory pit;m.mouth;br.c.branchial pouches;th.thyroid involution;v.ao.ventral aorta;ht.ventricle of heart;ch.notochord.
According to Scott (No.87) a hyomandibular pouch forming the eighth pouch is formed in front of the pouch already defined as the hyobranchial. It disappears early and does not acquire gill folds[28]. The tissue forming theline of insertion of the velum appears to me to represent the mandibular arch. The grounds for this view are the following:(1) The structure in question hasexactlythe position usually occupied by the mandibular arch.(2) There is present in late larvæ (about 20 days after hatching) an arterial vessel, continued from the ventral prolongation of the bulbus arteriosus along the insertion of the velum towards the dorsal aorta, which has the relations of a true branchial artery.
On the ventral aspect of the branchial region is placed a sack (figs.42,h, and43,th), which extends from the front end of the branchial region to the fourth cleft. At first it constitutes a groove opening into the throat above (fig. 44), but soon the opening becomes narrowed to a pore placed between the second and third of thepermanentbranchial pouches (fig. 43,th). In Ammocœtes[29]the simple tube becomes divided, and assumes a very complicated form, though still retaining its opening into the branchial region of the throat. In the adult it forms a glandular mass underneath the branchial region of the throat equivalent to the thyroid gland of higher Vertebrates.
On the ventral aspect of the head, and immediately in front of the mouth, is placed the olfactory pit (fig. 43,ol). It is from the firstunpaired, and in just-hatched larvæ simply forms a shallow groove of thickened epiblast at the base of the front of the brain. By the stage represented infig. 43the ventral part of the original groove is prolonged into a pit, extending backwards beneath the brain nearly up to the infundibulum.
Illustration: Figure 44Fig. 44. Diagrammatic transverse sections through the branchial region of a young larva of Petromyzon.(From Gegenbaur; after Calberla.)d.branchial region of throat.
Fig. 44. Diagrammatic transverse sections through the branchial region of a young larva of Petromyzon.(From Gegenbaur; after Calberla.)d.branchial region of throat.
On the side of the head, nearly on a level with the front end of the notochord, is placed the eye (fig. 43,op). It is constituted (figs.45and46) of a very shallow optic cup with a thick outer (retinal) layer, and a thin inner choroid layer. In contact with the retinal layer is placed the lens. The latter is formed as an invagination of theskin; to which it is still attached in the just-hatched larva (fig. 45). The eye only differs at this stage from that of other Vertebrata in its extraordinarily small size, and the rudimentary character of its constituent parts.
The auditory sack is a large vesicle (fig. 43,au.v.), placed at the side of the brain opposite the first persistent branchial pouch.
The brain is formed of the usual vertebrate parts[30], but is characterized by the very slight cranial flexure. The fore-brain consists (fig. 43) of a thalamencephalon (th) and an undivided cerebral rudiment (ch). To the roof of the thalamencephalon is attached a flattened sack (pn) which is probably the pineal gland. The floor is prolonged into an infundibulum (in) which contains a prolongation of the third ventricle. The lateral walls of the cerebral rudiment are much thickened.
Behind the thalamencephalon follows the mid-brain (mb), the sides of which form the optic lobes, and behind this again the hind-brain (md); the front border of the roof of which is thickened to form the cerebellum (cb). The medulla passes without any marked line of demarcation into the spinal cord.
Illustration: Figure 45Fig. 45. Horizontal section through the head of a just-hatched larva of Petromyzon shewing the development of the lens of the eye.th.c.thalamencephalon;op.v.optic vesicle;l.lens of eye;h.c.head cavity.
Fig. 45. Horizontal section through the head of a just-hatched larva of Petromyzon shewing the development of the lens of the eye.th.c.thalamencephalon;op.v.optic vesicle;l.lens of eye;h.c.head cavity.
The histological differentiation of the brain has already proceeded to some extent; and it has in the main the same character as the spinal cord. Before the larva has been hatched very long a lateral investment of white matter is present throughout. The notochord (ch) is continued forwards in the head to the hinder border of the infundibulum. It is slightly flexed anteriorly.
From the hinder border of the auditory region to the end of the branchial region the mesoblast is dorsally divided intomyotomes, which nearly, though not quite, correspond in number with the branchial pouches.
The growth of the myotomes would seem, as might be anticipated from their independent innervation, not to be related to that of the branchial pouches, so that there is a want of correspondence between these parts, the extent of which varies at different periods of life. The relation between the two in an old larva is shewn infig. 47.
Illustration: Figure 46Fig. 46. Eye of a larva of Petromyzon nine days after hatching.l.lens;r.retina.The section passes through one side of the lens.
Fig. 46. Eye of a larva of Petromyzon nine days after hatching.l.lens;r.retina.The section passes through one side of the lens.
The head of the larva of Petromyzon differs very strikingly in general appearance from that of the normal Vertebrata. This is at once shewn by a comparison offig. 43withfig. 29. The most important difference between the two is due to the absence of a pronounced cranial flexure in Petromyzon; an absence which is in its turn probably caused by the small development of the fore-brain.
The stomodæum of Petromyzon is surprisingly large, and its size and structure in this type militate against the view of some embryologists that the stomodæum originated from the coalescence of a pair of branchial pouches.
In the region of the trunk there is present an uninterrupted dorsal fin continuous with a ventral fin round the end of the tail.
There is a well-developed body cavity, which is especially dilated in front, in the part which afterwards becomes the pericardium. In this region is placed the nearly straight heart, divided into an auricle and ventricle (figs.42and43), the latter continued forwards into a bulbus arteriosus.
The myotomes are now very numerous (about 57, including those of the head, in a three days’ larva). They are separated by septa, but do not fill up the whole space between the septa, and have a peculiar wavy outline. The notochord is provided with a distinct sheath, and below it is placed a subnotochordal rod.
The alimentary canal consists of a narrow anterior section free from yolk, and a posterior region, the walls of which arelargely swollen with yolk. The anterior section corresponds to the region of the œsophagus and stomach, but exhibits no distinction of parts. Immediately behind this point the alimentary canal dilates considerably, and on the ventral side is placed the opening of a single large sack, which forms the commencement of the liver. The walls of the hepatic sack are posteriorly united to the yolk-cells. At the region where the hepatic sack opens into the alimentary tract the latter dilates considerably.
The posterior part of the alimentary tract still constitutes a kind of yolk-sack, the ventral wall being enormously thick and formed of several layers of yolk-cells. The dorsal wall is very thin.
The excretory system is composed of two segmental ducts, each connected in front with a well-developed pronephros (head-kidney), with about five ciliated funnels opening into the pericardial region of the body cavity. The segmental ducts in the larvæ open behind into the cloacal section of the alimentary tract.
Illustration: Figure 47Fig. 47. Head of a larva of Petromyzon six weeks old.(Altered from Max Schultze.)au.v.auditory vesicle;op.optic vesicle;ol.olfactory pit;ul.upper lip;ll.lower lip;or.p.papillæ at side of mouth;v.velum;br.s.extra branchial skeleton; 1-7. branchial clefts.
Fig. 47. Head of a larva of Petromyzon six weeks old.(Altered from Max Schultze.)au.v.auditory vesicle;op.optic vesicle;ol.olfactory pit;ul.upper lip;ll.lower lip;or.p.papillæ at side of mouth;v.velum;br.s.extra branchial skeleton; 1-7. branchial clefts.
The development of the larva takes place with considerable rapidity. The yolk becomes absorbed and the larva becomes accordingly more transparent. It generally lies upon its side, and resembles in general appearance and habit a minute Amphioxus. It is soon able to swim with vigour, but usually, unless disturbed, is during the day quite quiescent, and chooses bypreference the darkest situations. It soon straightens out, and, with the disappearance of the yolk, the tail becomes narrower than the head. A large caudal fin becomes developed.
When the larva is about twenty days old, it bears in most anatomical features a close resemblance to an Ammocœtes; though the histological differences between my oldest larva (29 days) and even very young Ammocœtes are considerable.
The mouth undergoes important changes. The upper lip becomes much more prominent, forming of itself the anterior end of the body (fig. 47,ul). The opening of the nasal pit is in this way relatively thrown back, and at the same time is caused to assumea dorsal position. This will be at once understood by a comparison offig. 43withfig. 47. On the inner side of the oral cavity a ring of papillæ is formed (fig. 47,or.p). Dorsally these papillæ are continued forward as a linear streak on the under side of the upper lip. A communication between the oral cavity and the branchial sack is very soon established.
The gill pouches gradually become enlarged; but it is some time before their small external openings are established. Their walls, which are entirely lined by hypoblast, become raised in folds, forming the branchial lamellæ. The walls of the head cavities between them become resolved into the contractors and dilators of the branchial sacks. The extra-branchial basketwork becomes established very early (it is present in the larva of 6 millimetres, about 9 days after hatching) and is shewn in an older larva infig. 47,br.s. It is not so complicated in these young larvæ as in the Ammocœtes, but in Max Schultze’s figure, which I have reproduced, the dorsal elements of the system are omitted. On the dorsal wall of the branchial region a ciliated ridge is formed, which may be homologous with the ridge on the dorsal wall of the branchial sack of Ascidians. It has been described by Schneider in Ammocœtes.
With reference to the remainder of the alimentary canal there is but little to notice. The primitive hepatic diverticulum rapidly sprouts out and forms a tubular gland. The opening into the duodenum changes from a ventral to a lateral or even dorsal position. The duct leads into a gall-bladder imbedded in the substance of the liver. Ventrally the liver is united with the abdominal wall, but laterally passages are left by which the pericardial and body cavities continue to communicate.
The greater part of the yolk becomes employed in the formation of the intestinal wall. This part of the intestine in a nine days’ larva (67mm.) has the form of a cylindrical tube with very thick columnar cells entirely filled with yolk particles. The dorsal wall is no longer appreciably thinner than the ventral. In the later stages the cells of this part of the intestine become gradually less columnar as the yolk is absorbed.
The fate of the yolk-cells in the Lamprey is different from that in most other Vertebrata with an equally large amount of yolk. They no doubtsupply nutriment for the growth of the embryo, and although in the anterior part of the intestine they become to some extent enclosed in the alimentary tract and break up, yet in the posterior part they become wholly transformed into the regular epithelium of the intestine.
On the ninth day a slight fold filled with mesoblastic tissue is visible on the dorsal wall of the intestine. This fold appears to travel towards the ventral side; at any rate a similar but better-marked fold is visible in a ventro-lateral position at a slightly later period. This fold is the commencement of the fold which in the adult makes a half spiral, and is no doubt equivalent to the spiral valve of Elasmobranchs and Ganoids. It contains a prolongation of the cœliac artery, which constitutes at first the vitelline artery.
The nervous system does not undergo during the early larval period changes which require a description.
The opening of the olfactory sack becomes narrowed and ciliated (fig. 47,ol). It is carried by the process already mentioned to the dorsal surface of the head. The lumen of the sack is well developed; and lies in contact with the base of the fore part of the brain.
The vascular system presents no very remarkable features. The heart is two-chambered and straight. The ventricle is continued forwards as a bulbus arteriosus, which divides into two arteries at the thyroid body. From the bulbus and its continuations eight branches are given off to the gills; and, as mentioned above, a vessel, probably of the same nature, is given off in the region of the velum. The blood from the branchial sacks is collected into the dorsal aorta. Some of it is transmitted to the head, but the greater part flows backwards under the notochord.
The venous system consists of the usual anterior and posterior cardinal veins which unite on each side into a ductus Cuvieri, and of a great subintestinal vessel of the same nature as that in embryo Elasmobranchs, which persists however in the adult. It breaks up into capillaries in the liver, and constitutes therefore the portal vein. From the liver the blood is brought by the hepatic vein into the sinus venosus. In addition to these vessels there is a remarkable unpaired sub-branchial vein, which brings back the blood directly to the heart from the ventral part of the branchial region.
Metamorphosis. The larva just described does not grow directly into the adult, but first becomes a larval form, known as Ammocœtes, which was supposed to be a distinct species till Aug. Müller (No.80) made the brilliant discovery of its nature.
The Ammocœtes does not differ to any marked extent from the larva just described. The histological elements become more differentiated, and a few organs reach a fuller development.
The branchial skeleton becomes more developed, and capsules for the olfactory sack and auditory sacks are established.
The olfactory sack is nearly divided into two by a ventral septum. The eye (fig. 48) is much more fully developed, but lies a long way below the surface. The optic cup forms a deep pit, in the mouth of which is placed the lens. The retinal layers are well developed (cf.Langerhans), and the outer layer of the optic cup or layer of retinal pigment (rp) contains numerous pigment granules, especially on its dorsal side. At the edge of the optic cup the two layers fall into each other. They constitute the commencement of the pigment layer of the iris; but at this stage they are not pigmented. The mesoblast of the iris is hardly differentiated. The lens (l) has the normal structure of the embryonic lens of Vertebrata. The inner wall is thick and doubly convex, while the outer wall, which will form the anterior epithelium, is very thin. There is a large space between the lens and the retina containing the vitreous humour (v.h). There is no aqueous humour, and the tissues in front of the lens bear but little resemblance to those in higher Vertebrata. The cornea is represented by (1) the epidermis (ep); (2) the dermis (d.c); (3) the subdermal connective tissue (s.d.c) which passes without any sharp line of demarcation into the dermis; (4) a thick membrane continuous with the choroid which represents Descemet’s membrane. The subdermal connective tissue is continued as an investment round the whole eye. There is no specially differentiated sclerotic, and a choroid is only imperfectly indicated[31]. The peculiar features of the eye of the young larva of the Ammocœtes are probably due to degeneration.
Illustration: Figure 48Fig. 48. Eye of an Ammocœtes lying beneath the skin.ep.epidermis;d.c.dermal connective tissue continuous with the subdermal connective tissue (s.d.c), which is also shaded. There is no definite boundary to this tissue where it surrounds the eye.m.muscles;dm.membrane of Descemet;l.lens;v.h.vitreous humour;r.retina;rp.retinal pigment.
Fig. 48. Eye of an Ammocœtes lying beneath the skin.ep.epidermis;d.c.dermal connective tissue continuous with the subdermal connective tissue (s.d.c), which is also shaded. There is no definite boundary to this tissue where it surrounds the eye.m.muscles;dm.membrane of Descemet;l.lens;v.h.vitreous humour;r.retina;rp.retinal pigment.
In the brain the two cerebral hemispheres lie one on each side of the anterior end of the thalamencephalon. There are well-defined olfactory lobes, and two distinct olfactory nerves are present.
The excretory system has undergone great changes. A series of segmental tubes, which first appear in a larva of about 9mm.,becomes established behind the pronephros, and in an Ammocœtes of 65mm.the pronephros has begun to atrophy. The generative organs are formed in a larva of about 35mm.Shortly before the metamorphosis the portion of the cloaca into which the segmental tubes open becomes separated off as a distinct urinogenital sinus, the walls of which become perforated by the two abdominal pores.
The Ammocœtes of Petromyzon Planeri lives in the mud in streams. Without undergoing any marked changes in structure it gradually grows larger, and after three or four years undergoes a metamorphosis. The full-grown larva may be as large or even larger than the adult. The metamorphosis takes place from August till January. The breeding season sets in during the second half of April; and shortly after depositing its generative products the Lamprey dies. The changes which take place in the metamorphosis are of a most striking kind.
The dome-shaped mouth of the larva is replaced (fig. 47) by a more definitely suctorial mouth with horny cuticular teeth (fig. 49). The eyes appear on the surface; and the dorsal fin becomes more prominent, and is divided into two parts.
Illustration: Figure 49Fig. 49. Mouth of Petromyzon marinus with its horny teeth.(From Gegenbaur; after Heckel and Kner.)
Fig. 49. Mouth of Petromyzon marinus with its horny teeth.(From Gegenbaur; after Heckel and Kner.)
Besides these obvious external changes very great modifications are effected in almost all the organs, which may be very briefly enumerated.
1. Very profound changes take place in the skeleton. An elaborate system of cartilages is developed in connection with the mouth; the cranium itself undergoes important modifications; and neural arches become formed.
2. Considerable changes are effected in the gill pouches, and, according to Schneider, whose statements must however be received with some caution, the branchial sack becomes detached posteriorly from the œsophagus, the œsophagus then sends forwards a prolongation above the branchial sack which is at first solid. This prolongation forms the anterior part of theœsophagus of the adult, and joins the primitive oral cavity at the velum. The so-called bronchus of the adult is thus the whole branchial region of the Ammocœtes, and the anterior part of the œsophagus of the adult is an entirely new formation.
3. The posterior part of the alimentary tract of the Ammocœtes undergoes partial atrophy. The gall-bladder of the liver is absorbed; and the liver itself ceases to communicate with the intestine.
4. The eye undergoes important changes in that it travels to the surface, and acquires all the characters of the normal vertebrate eye.
5. The brain becomes relatively larger but more compact, and the optic lobes (corpora bigemina) become more distinct.
6. The pericardial cavity becomes completely separated from the body cavity, and a distinct pericardium is formed.
7. The mesonephros of the larva disappears, and a fresh posterior part is formed.
Myxine. The ovum of Myxine when ready to be laid is inclosed, as shewn by Allen Thomson[32], in an oval horny shell in many respects similar to that of Elasmobranchii; from its ends there project a number of trumpet-shaped tubular processes, which no doubt serve to attach it to marine objects. No observations have been made on the development.
Bibliography.
(77)E. Calberla. “Der Befruchtungsvorgang beim Petromyzon Planeri.”Zeit. f. wiss. Zool.,Vol.XXX. 1877.(78)E. Calberla. “Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda dorsalis d. Teleostier u. d. Petromyzonten.”Morpholog. Jahrbuch,Vol.III.1877.(79)C. Kupffer u. B. Benecke.Der Vorgang d. Befruchtung am Ei d. Neunaugen.Königsberg, 1878.(80)Aug. Müller. “Ueber die Entwicklung d. Neunaugen.” Müller’sArchiv, 1856.(81)Aug. Müller.Beobachtungen üb. d. Befruchtungserscheinungen im Ei d. Neunaugen.Königsberg, 1864.(82)W. Müller. “Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen.”Jenaische Zeitschrift,Vol.IX. 1875.(83)Ph. Owsjannikoff.“Die Entwick. von d. Flussneunaugen.” Vorläuf. Mittheilung.Mélanges Biologiques tirés du Bulletin de l'Acad. Imp. St Pétersbourg,Vol.VII. 1870.(84)Ph. Owsjannikoff.On the development of Petromyzon fluviatilis(Russian).(85)Anton Schneider.Beiträge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.Quarto. Berlin, 1879.(86)M. S. Schultze. “Die Entwickl. v. Petromyzon Planeri.” Gekrönte Preisschrift. Haarlem, 1856.(87)W. B. Scott. “Vorläufige Mittheilung üb. d. Entwicklungsgeschichte d. Petromyzonten.”Zoologischer Anzeiger,Nos.63 and 64.III.Jahrg. 1880.
[23]The following classification of the Cyclostomata is employed in the present chapter:I. Hyperoartiaex. Petromyzon.II. Hyperotretaex. Myxine, Bdellostoma.[24]The present chapter is in the main founded upon observations which I was able to make in the spring of 1880 upon the development of Petromyzon Planeri. Mr Scott very kindly looked over my proof-sheets and made a number of valuable suggestions, and also sent me an early copy of his preliminary note (No.87), which I have been able to make use of in correcting my proof-sheets.[25]Der Ursprung d. Wirbelthiere, etc. Leipzig, 1875.[26]Artificial impregnation may be effected without difficulty by squeezing out into the same vessel the ova and spermatozoa of a ripe female and male. The fertilized eggs are easily reared. Petromyzon Planeri breeds during the second half of April.[27]In Calberla’s figure, shewing the development of the notochord, the limits of mesoblast and hypoblast are wrongly indicated.[28]Scott informs me that he has been unable to find the hyomandibular pouch in larvæ larger than 4.8mm.My material of the stages when it should be present is somewhat scanty, but I have as yet, very likely owing to the imperfection of my material, been unable to find Scott’s hyomandibular pouch either in my sections or surface-views. Huxley describes this pouch as present in the form of a cleft in later stages; I have failed to find his cleft also. The vessel interpreted below as the branchial artery of the mandibular arch was only imperfectly investigated by me, and I was not sure of my interpretations about it. Scott however informs me by letter that it is undoubtedly present.[29]Schneider (No.85) states that in the full-grown Ammocœtes the opening is situated between the third and fourth pouches. This is certainly not true for the young larva.[30]Max Schultze’s statements as to the structure and histology of the brain are very inadequate in the present state of our knowledge.[31]Langerhansloc. cit.describes the eye of the Ammocœtes in some respects very differently from the above. Very probably his description applies to an older Ammocœtes. The most important points of difference appear to be (1) that the vitreous humour is all but obliterated; (2) that the iris is much better developed.[32]Cyclopædia of Anat. and Phys.Article ‘Ovum.’
[23]The following classification of the Cyclostomata is employed in the present chapter:I. Hyperoartiaex. Petromyzon.II. Hyperotretaex. Myxine, Bdellostoma.
[24]The present chapter is in the main founded upon observations which I was able to make in the spring of 1880 upon the development of Petromyzon Planeri. Mr Scott very kindly looked over my proof-sheets and made a number of valuable suggestions, and also sent me an early copy of his preliminary note (No.87), which I have been able to make use of in correcting my proof-sheets.
[25]Der Ursprung d. Wirbelthiere, etc. Leipzig, 1875.
[26]Artificial impregnation may be effected without difficulty by squeezing out into the same vessel the ova and spermatozoa of a ripe female and male. The fertilized eggs are easily reared. Petromyzon Planeri breeds during the second half of April.
[27]In Calberla’s figure, shewing the development of the notochord, the limits of mesoblast and hypoblast are wrongly indicated.
[28]Scott informs me that he has been unable to find the hyomandibular pouch in larvæ larger than 4.8mm.My material of the stages when it should be present is somewhat scanty, but I have as yet, very likely owing to the imperfection of my material, been unable to find Scott’s hyomandibular pouch either in my sections or surface-views. Huxley describes this pouch as present in the form of a cleft in later stages; I have failed to find his cleft also. The vessel interpreted below as the branchial artery of the mandibular arch was only imperfectly investigated by me, and I was not sure of my interpretations about it. Scott however informs me by letter that it is undoubtedly present.
[29]Schneider (No.85) states that in the full-grown Ammocœtes the opening is situated between the third and fourth pouches. This is certainly not true for the young larva.
[30]Max Schultze’s statements as to the structure and histology of the brain are very inadequate in the present state of our knowledge.
[31]Langerhansloc. cit.describes the eye of the Ammocœtes in some respects very differently from the above. Very probably his description applies to an older Ammocœtes. The most important points of difference appear to be (1) that the vitreous humour is all but obliterated; (2) that the iris is much better developed.
[32]Cyclopædia of Anat. and Phys.Article ‘Ovum.’
It is only within quite recent times that any investigations have been made on the embryology of this heterogeneous, but primitive group of fishes. Much still remains to be done, but we now know the main outlines of the development of Acipenser and Lepidosteus, which are representatives of the two important subdivisions of the Ganoids. Both types have a complete segmentation, but Lepidosteus presents in its development some striking approximations to the Teleostei. I have placed at the end of the chapter a few remarks with reference to the affinities indicated by the embryology.
Acipenser[34].
The freshly laid ovum is 2mm.in diameter and is invested by a two-layered shell, covered by a cellular layer derived from the follicle[35]. The segmentation, though complete, approachesthe meroblastic type more nearly than the segmentation of the frog’s egg. The first furrow appears at the formative pole, at which the germinal vesicle was situated. The earlier phases of the segmentation are like those of meroblastic ova, in that the furrows only penetrate for a certain distance into the egg. Eight vertical furrows appear before the first equatorial furrow; which is somewhat irregular, and situated close to the formative pole.
In the later stages the vertical furrows extend through the whole egg, and a segmentation cavity appears between the small and the large spheres. The segmentation is thus in the main similar to that of a frog, from which it diverges in the fact that there is a greater difference in size between the small and the large segments.
Illustration: Figure 50Fig. 50. Embryos of Acipenser viewed from the dorsal surface.(After Salensky.)A. Stage before the appearance of the mesoblastic somites.B. Stage with five somites.Mg.medullary groove;bl.p.blastopore;s.d.segmental duct;Fb.fore-brain;Hb.hind-brain;m.s.mesoblastic somite.
Fig. 50. Embryos of Acipenser viewed from the dorsal surface.(After Salensky.)A. Stage before the appearance of the mesoblastic somites.B. Stage with five somites.Mg.medullary groove;bl.p.blastopore;s.d.segmental duct;Fb.fore-brain;Hb.hind-brain;m.s.mesoblastic somite.
In the final stages of the segmentation the cells become distinctly divided into two layers. A layer of small cells is placed at the formative pole, and constitutes the epiblast. The cells composing it are divided, like those of Teleostei, etc., into a superficial epidermic and a deeper nervous layer. The remaining cells constitute the primitive hypoblast (the eventual hypoblast and mesoblast); they form a great mass of yolk-cells at the lower pole, and also spread along the roof of the segmentation cavity, on the inner side of the epiblast.
A process of unsymmetrical invagination now takes place, which is in its essential features exactly similar to that in thefrog or the lamprey, and I must refer the reader for the details of the process to the chapter on the Amphibia. The edge of the cap of epiblast forms an equatorial line. For the greater extent of this line the epiblast cells grow over the hypoblast, as in an epibolic gastrula, but for a small arc they are inflected. At the inflected edge an invagination of cells takes place, underneath the epiblast, towards the segmentation cavity, and gives rise to the dorsal wall of the mesenteron and the main part of the dorsal mesoblast. The slit below the invaginated layer gradually dilates to form the alimentary cavity; the ventral wall of which is at first formed of yolk-cells. The epiblast along the line of the invaginated cells soon becomes thickened, and forms a medullary plate, which is not very distinct in surface views. The cephalic extremity of this plate, which is furthest removed from the edge, dilates, and the medullary plate then assumes a spatula form (fig. 50A,Mg).
By the continued extension of the epiblast the uncovered part of the hypoblast has in the meantime become reduced to a small circular pore—the blastopore—and in surface views of the embryo has the form represented infig. 50A,bl.p. The invagination of the mesenteron has in the meantime extended very far forwards, and the segmentation cavity has become obliterated. The lip of the blastopore has moreover become inflected for its whole circumference.
The invaginated cells forming the dorsal wall of the mesenteron soon become divided into a pigmented hypoblastic epithelium adjoining the lumen of the mesenteron (fig. 51,En) and a mesoblastic layer (Sgp), between the hypoblast and the epiblast. The mesoblast is divided into two plates, between which is placed the notochord[36](Ch).
With the completion of the medullary plate and the germinal layers, the first embryonic period may be considered to come to a close. The second period ends with the hatching of the embryo. During it the rudiments of the greater number of organs make their appearance. The general form of the embryo during this period is shewn infigs.50B and52A and B.
One of the first changes to take place is the conversion of themedullary plate into the medullary canal. This, as shewn infig. 51, is effected in the usual vertebrate fashion, by the establishment of a medullary groove which is then converted into a closed canal by the folding over of the sides.
The uncovered patch of yolk in the blastoporic area soon becomes closed over; and on the formation of the medullary canal the usual neurenteric canal becomes established.