Illustration: Figure 296Fig. 296. 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. 296. 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.
General considerations on the Eye of the Chordata.
There can be but little doubt that the eye of the Tunicata belongs to the same phylum as that of the true Vertebrata, different as the two eyes are. The same may also be said with reference to the degenerate and very rudimentary eye of Amphioxus.
The peculiarity of the eye of all the Chordata consists in the retina being developed from part of the wall of the brain. How is this remarkable feature of the eye of the Chordata to be explained?
Lankester, interpreting the eye in the light of the Tunicata, has made the interesting suggestion[192]“that the original Vertebrate must have been a transparent animal, and had an eye or pair of eyes inside the brain, like that of the Ascidian Tadpole.”
This explanation may possibly be correct, but another explanation appears to me possible, and I am inclined to think that the vertebrate eyes have not been derived from eyes like those of Ascidians, but that the latter is a degenerate form of vertebrate eye.
The fact of the retina being derived from the fore-brain may perhaps be explained in the same way as has already been attempted in the case of the retina of the Crustacea;i.e.by supposing that the eye was evolved simultaneously with the fore part of the brain.
The peculiar processes which occur in the formation of the optic vesicle are more difficult to elucidate; and I can only suggest that the development of a primary optic vesicle, and its conversion into an optic cup, is due to the retinal part of the eye having been involved in the infolding which gave rise to the canal of the central nervous system. The position of the rods and cones on the posterior side of the retina is satisfactorily explained by this hypothesis, because, as may be easily seen from figure 285, the posterior face of the retina is the original external surface of the epidermis, which is infolded in the formation of the brain; so that the rods and cones are, as might be anticipated, situated on what is morphologically the external surface of the epiblast of the retina.
The difficulty of this view arises in attempting to make out how the eye can have continued to be employed during the gradual change of position which the retina must have undergone in being infolded with the brain in the manner suggested. If however the successive steps in this infolding were sufficiently small, it seems to me not impossible that the eye might have continued to be used throughout the whole period of change, and a transparency of the tissues, such as Lankester suggests, may have assisted in rendering this possible.
The difficulty of the eye continuing to be in use when undergoing striking changes in form is also involved in Lankester’s view, in that if, as I suppose, he starts from the eye of the Ascidian Tadpole with its lenses turnedtowardsthe cavity of the brain; it is necessary for him to admit that a fresh lens and other optical parts of the eye became developed on theopposite sideof the eye to the original lens; and it is difficult to understand such a change, unless we can believe that the refractive media on the two sides were in operation simultaneously. It may be noted that the same difficulty is involved in supposing, as I have done, that the eye of the Ascidian Tadpole was developed from that of a Vertebrate. I should however be inclined to suggest that the eye had in this case ceased for a period to be employed; and that it has been re-developed again in some of the larval forms. Its characters in the Tunicata are by no means constant.
Accessory eyes in the Vertebrata.
In addition to the paired eyes of the Vertebrata certain organs are found in the skin of a few Teleostei living in very deep water, which, though clearly not organs of true vision, yet present characters which indicate thatthey may be used in the perception of light. The most important of such organs are those found in Chauliodus, Stomias, etc., the significance of which was first pointed out by Leuckart, while the details of their structure have been recently worked out by Leydig[193]and Ussow. They are distributed not only in the skin, but are also present in the mouth and respiratory cavity, a fact which appears to indicate that their main function must be something else than the perception of light. It has been suggested that they have the function of producing phosphorescence.
Another organ, probably of the same nature, is found on the head of Scopelus.
The organs in Chauliodus are spherical or nearly spherical bodies invested in a special tunic. The larger of them, which alone can have any relation to vision, are covered with pigment except on their outer surface. The interior is filled with two masses, named by Leuckart the lens and vitreous humour. According to Leydig each of them is cellular and receives a nerve, the ultimate destination of which has not however been made out. According to Ussow the anterior mass is structureless, but serves to support a lens, placed in the centre of the eye, and formed of a series of crystalline cones prolonged into fibres, which in the posterior part of the eye diverge and terminate by uniting with the processes of multipolar cells, placed near the pigmented sheath. These cells, together with the fibres of the crystalline cones which pass to them, are held by Ussow to constitute a retina.
Eye of the Mollusca.
(362)N. Bobretzky. “Observations on the development of the Cephalopoda” (Russian).Nachrichten d. kaiserlichen Gesell. d. Freunde der Naturwiss. Anthropolog. Ethnogr. bei d. Universität Moskau.(363)H. Grenacher. “Zur Entwicklungsgeschichte d. Cephalopoden.”Zeit. f. wiss. Zool.,Bd.XXIV. 1874.(364)V. Hensen. “Ueber d. Auge einiger Cephalopoden.”Zeit. f. wiss. Zool.,Vol.XV. 1865.(365)E. R. Lankester. “Observations on the development of the Cephalopoda.”Quart. J. of Micr. Science,Vol.XV. 1875.(366)C. Semper.Ueber Sehorgane von Typus d. Wirbelthieraugen.Wiesbaden, 1877.
Eye of the Arthropoda.
(367)N. Bobretzky.Development of Astacus and Palaemon.Kiew, 1873.(368)A. Dohrn. “Untersuchungen üb. Bau u. Entwicklung d. Arthropoden. Palinurus und Scyllarus.”Zeit. f. wiss. Zool.,Bd.XX. 1870,p.264 et seq.(369)E. Claparède. “Morphologie d. zusammengesetzten Auges bei den Arthropoden.”Zeit. f. wiss. Zool.,Bd.X. 1860.(370)H. Grenacher.Untersuchungen üb. d. Sehorgane d. Arthropoden.Göttingen, 1879.
Vertebrate Eye.
(371)J. Arnold.Beiträge zur Entwicklungsgeschichte des Auges.Heidelberg, 1874.(372)Babuchin. “Beiträge zur Entwicklungsgeschichte des Auges.”Würzburger naturwissenschaftliche Zeitschrift,Bd.8.(373)L. Kessler.Zur Entwicklung d. Auges d. Wirbelthiere.Leipzig, 1877.(374)N. Lieberkühn.Ueber das Auge des Wirbelthierembryo.Cassel, 1872.(375)N. Lieberkühn. “Beiträge z. Anat. d. embryonalen Auges.”Archiv f. Anat. und Phys., 1879.(376)L. Löwe. “Beiträge zur Anatomie des Auges” and “Die Histogenese der Retina.”Archiv f. mikr. Anat.,Vol.XV. 1878.(377)V. Mihalkowics. “Untersuchungen über den Kamm des Vogelauges.”Archiv f. mikr. Anat.,Vol.IX. 1873.(378)W. Müller. “Ueber die Stammesentwickelung des Schorgans der Wirbelthiere.”Festgabe Carl Ludwig.Leipzig, 1874.(379)S. L. Schenk. “Zur Entwickelungsgeschichte des Auges der Fische.”Wiener Sitzungsberichte,Bd.LV. 1867.
Accessory organs of the Vertebrate Eye.
(380)G. Born. “Die Nasenhöhlen u. d. Thränennasengang d. Amphibien.”Morphologisches Jahrbuch,Bd.II. 1876.(381)G. Born. “Die Nasenhöhlen u. d. Thränennasengang d. amnioten Wirbelthiere. I. Lacertilia. II. Aves.”Morphologisches Jahrbuch,Bd.V. 1879.
Eye of the Tunicata.
(382)A. Kowalevsky. “Weitere Studien üb. d. Entwicklung d. einfachen Ascidien.”Archiv f. mikr. Anat.,Vol.VII. 1871.(383)C. Kupffer. “Zur Entwicklung d. einfachen Ascidien.”Archiv f. mikr. Anat.,Vol.VII. 1872.
[182]O. and R. Hertwig.Das Nervensystem u. Sinnesorgane d. Medusen.Leipzig, 1878.[183]VideHensen,Zeit. f. wiss. Zool.Bd.XV.[184]“Devel. of Cephalopoda.”Q. J. Micro. Scien.1875,p.44.[185]Ueber Sehorgane von Typus d. Wirbelthieraugen, etc., Wiesbaden, 1877, andArchiv f. mikr. Anat.Vol.XIV.pp.118-122.[186]VideHensen (No.364) and S. J. Hickson, “The Eye of Pecten,”Quart. J. of Micr. Science,Vol.XX. 1880.[187]O. Hertwig. “Die Chætognathen.”Jenaische Zeitschrift,Vol.XIV.1880.[188]The eye of Peripatus is similar neither to the eye of the Arthropoda, nor to that of the Chætopoda, but resembles much more closely the Molluscan eye. The hypodermis and cuticle form together a highly convex cornea, within which is a large optic chamber, the posterior wall of which is formed by the retina. The optic chamber would appear to contain a structureless lens, but it is possible that what I regard as a lens may, on fuller investigation, turn out to be only a coagulum.[189]There would appear to be some confusion as to the nomenclature of these parts in Bobretzky’s account.[190]It appears to me possible that Lieberkühn may be right in stating that the epithelium of Descemet’s membrane grows in between the lens and the epiblast before the formation of the cornea proper, and that Kessler’s account, given above, may on this point require correction. From the structure of the eye in the Ammocœte it seems probable that Descemet’s membrane is continuous with the choroid.[191]The most detailed account is that of W. Müller (No.377).[192]Degeneration, London, 1880,p.49.[193]F. Leydig. “Ueber Nebenaugen d. Chauliodus Sloani.”Archiv f. Anat. und Phys., 1879. M. Ussow. “Ueb. d. Bau d. augenähnlichen Flicken einiger Knochenfische.”Bul. d. la Soc. d. Naturalistes de Moscou,Vol.LIV. 1879.Videfor general description and further literature, Günther,The Study of Fishes, Edinburgh, 1880.
[182]O. and R. Hertwig.Das Nervensystem u. Sinnesorgane d. Medusen.Leipzig, 1878.
[183]VideHensen,Zeit. f. wiss. Zool.Bd.XV.
[184]“Devel. of Cephalopoda.”Q. J. Micro. Scien.1875,p.44.
[185]Ueber Sehorgane von Typus d. Wirbelthieraugen, etc., Wiesbaden, 1877, andArchiv f. mikr. Anat.Vol.XIV.pp.118-122.
[186]VideHensen (No.364) and S. J. Hickson, “The Eye of Pecten,”Quart. J. of Micr. Science,Vol.XX. 1880.
[187]O. Hertwig. “Die Chætognathen.”Jenaische Zeitschrift,Vol.XIV.1880.
[188]The eye of Peripatus is similar neither to the eye of the Arthropoda, nor to that of the Chætopoda, but resembles much more closely the Molluscan eye. The hypodermis and cuticle form together a highly convex cornea, within which is a large optic chamber, the posterior wall of which is formed by the retina. The optic chamber would appear to contain a structureless lens, but it is possible that what I regard as a lens may, on fuller investigation, turn out to be only a coagulum.
[189]There would appear to be some confusion as to the nomenclature of these parts in Bobretzky’s account.
[190]It appears to me possible that Lieberkühn may be right in stating that the epithelium of Descemet’s membrane grows in between the lens and the epiblast before the formation of the cornea proper, and that Kessler’s account, given above, may on this point require correction. From the structure of the eye in the Ammocœte it seems probable that Descemet’s membrane is continuous with the choroid.
[191]The most detailed account is that of W. Müller (No.377).
[192]Degeneration, London, 1880,p.49.
[193]F. Leydig. “Ueber Nebenaugen d. Chauliodus Sloani.”Archiv f. Anat. und Phys., 1879. M. Ussow. “Ueb. d. Bau d. augenähnlichen Flicken einiger Knochenfische.”Bul. d. la Soc. d. Naturalistes de Moscou,Vol.LIV. 1879.Videfor general description and further literature, Günther,The Study of Fishes, Edinburgh, 1880.
Auditory Organs.
A great variety of organs, very widely distributed amongst aquatic forms, and also found, though less universally, in land forms, are usually classed together as auditory organs.
In the case of all aquatic forms, or of forms which have directly inherited their auditory organs from aquatic forms, these organs are built upon a common type; although in the majority of instances the auditory organs of the several groups have no genetic relations. All the organs have their origin in specialized portions of the epidermis. Some of the cells of a special region become provided at their free extremities with peculiar hairs, known as auditory hairs; while in other cells concretions, known as otoliths, are formed, which appear often to be sufficiently free to be acted upon by vibrations of the surrounding medium, and to be so placed as to be able in their turn to transmit their vibrations to the cells with auditory hairs[194]. The auditory regions of the epidermis are usually shut off from the surface in special sacks.
The actual function of these organs is no doubt correctly described, in the majority of instances, as being auditory; but it appears to me very possible that in some cases their function may be to enable the animals provided with them to detect the presence of other animals in their neighbourhood, through theundulatory movements in the water, caused by the swimming of the latter.
Auditory organs with the above characters, sometimes freely open to the external medium, but more often closed, are found in various Cœlenterata, Vermes and Crustacea, and universally or all but universally in the Mollusca and Vertebrata.
In many terrestrial Insects a different type of auditory organ has been met with, consisting of a portion of the integument modified to form a tympanum or drum, and supported at its edge by a chitinous ring. The vibrations set up in the membranous tympanum stimulate terminal nerve organs at the ends of chitinous processes, placed in a cavity bounded externally by the tympanic membrane.
The tympanum of Amphibia and Amniota is an accessory organ added, in terrestrial Vertebrata, to an organ of hearing primitively adapted to an aquatic mode of life; and it is interesting to notice the presence of a more or less similar membrane in the two great groups of terrestrial forms,i.e.terrestrial Vertebrata and Insecta.
Nothing is known with reference to the mode of development or evolution of the tympanic type of auditory organ found in Insects, and, except in the case of Vertebrates, but little is known with reference to the development of what may be called the vesicular type of auditory organ found in aquatic forms. Some very interesting facts with reference to the evolution of such organs have however been brought to light by the brothers Hertwig in their investigations on the Cœlenterata; and I propose to commence my account of the development of the auditory organs in the animal kingdom by a short statement of the results of their researches.
Cœlenterata. Three distinct types of auditory organ have been recognised in the Medusæ; two of them resulting from the differentiation of a tentacle-like organ, and one from ectoderm cells on the under surface of the velum. We may commence with the latter as the simplest. It is found in the Medusæ known as the Vesiculata. The least differentiated form of this organ, so far discovered, is present in Mitrotrocha, Tiaropsis and other genera. It has the form of an open pit; and a series of such organs are situated along the attached edgeof the velum with their apertures directed downwards. The majority of the cells lining the outer,i.e.peripheral side of the pit, contain an otolith, while a row of the cells on the inner,i.e.central side, are modified as auditory cells. The auditory cells are somewhat strap-shaped, their inner ends being continuous with the fibres of the lower nerve-ring, and their free ends being provided with bent auditory hairs, which lie in contact with the convex surfaces of the cells containing the otoliths.
Illustration: Figure 297Fig. 297. Auditory vesicle of Phialidium after treatment with dilute osmic acid.(From Lankester; after O. and R. Hertwig.)d1. epithelium of the upper surface of the velum;d2. epithelium of the under surface of the velum;r.circular canal at the edge of the velum;nr1. upper nerve-ring;h.auditory cells;hh.auditory hairs;np.nervous cushion formed of a prolongation of the lower nerve-ring. Close to the nerve-ring is seen a cell, shewn as black, containing an otolith.
Fig. 297. Auditory vesicle of Phialidium after treatment with dilute osmic acid.(From Lankester; after O. and R. Hertwig.)d1. epithelium of the upper surface of the velum;d2. epithelium of the under surface of the velum;r.circular canal at the edge of the velum;nr1. upper nerve-ring;h.auditory cells;hh.auditory hairs;np.nervous cushion formed of a prolongation of the lower nerve-ring. Close to the nerve-ring is seen a cell, shewn as black, containing an otolith.
By the conversion of such open pits into closed sacks a more complicated type of auditory organ, which is present in many of the Vesiculata,viz.Æquorea, Octorchis, Phialidium,&c., is produced. A closed vesicle of this type is shewn infig. 297. Such organs form projections on the upper surface of the velum. They are covered by a layer of the epithelium (d1) of the upper surface of the velum, but the lining of the vesicle (d2) is derived from what was originally part of the epithelium of the lower surface of the velum, homologous with that lining the open pits in the type already described. The general arrangement of the cells lining such vesicles is the same as that of the cells lining the open pits.
A second type of auditory organ, found in the Trachymedusæ, appears in its simplest condition as a modified tentacle.It is formed of a basal portion, covered by auditory cells with long stiff auditory hairs, supporting at its apex a club-shaped body, attached to it by a delicate stalk. An endodermal axis is continued through the whole structure, and in one or more of the endoderm cells of the club-shaped body otoliths are always present. The tails of the auditory cells are directly continued into the upper nerve-ring.
In more complicated forms of this organ the tentacle becomes enclosed in a kind of cup, by a wall-like upgrowth of the surrounding parts (fig. 298); and in some forms,e.g.Geryonia, by the closure of the cup, the whole structure takes the form of a completely closed vesicle, in the cavity of which the original tentacle forms an otolith-bearing projection.
Illustration: Figure 298Fig. 298. Auditory organ of Rhopalonema.(From Lankester; after O. and R. Hertwig.)The organ consists of a modified tentacle (hk) with auditory cells and concretions, partially enclosed in a cup.
Fig. 298. Auditory organ of Rhopalonema.(From Lankester; after O. and R. Hertwig.)The organ consists of a modified tentacle (hk) with auditory cells and concretions, partially enclosed in a cup.
The auditory organs found in the Acraspedote Medusæ approach in many respects to the type of organ found in the Trachymedusæ. They consist of tentacular organs placed in grooves on the under surface of the disc. They have a swollen extremity, and are provided with an endodermal axis for half the length of which there is a diverticulum of the gastrovascular canal system. The terminal portion of the endoderm is solid, and contains calcareous concretions. The ectodermal cells at the base of these organs have the form of auditory cells.
Mollusca. Auditory vesicles are found in almost all Mollusca on the ventral side of the body in close juxtaposition to the pedal ganglia. Except possibly in some Cephalopods, thesevesicles are closed. They are provided with free otoliths, supported by the cilia of the walls of the sack, but in addition some of the cells of the sack are provided with stiff auditory hairs.
In many forms these sacks have been observed to originate by an invagination of the epiblast of the foot (Paludina,Nassa,Heteropoda,Limax,Clio, Cephalopoda and Lamellibranchiata). In other instances (some Pteropods, Lymnæus,&c.) they appear, by a secondary modification in the development, to originate by a differentiation of a solid mass of epiblast.
According to Fol the otocysts in Gasteropods are formed by cells of the wall of the auditory sacks; and the same appears to hold good for Cephalopoda (Grenacher)[195]shewing that free otoliths have in these instances originated from otoliths originally placed in cells.
Crustacea. In the decapodous Crustacea organs, which have been experimentally proved to be true organs of hearing, are usually present on the basal joint of the anterior antennæ. They may have (Hensen,No.384) the form either of closed or of open sacks, lined by an invagination of the epidermis. They are provided with chitinous auditory hairs and free otoliths. In the case of the open sacks the otoliths appear to be simply stones transported into the interior of the sacks, but in the closed sacks the otoliths, though free, are no doubt developed within the sacks.
The Schizopods, which, as mentioned in the last chapter, are remarkable as containing a genus (Euphausia) with abnormally situated eyes, distinguish themselves again with reference to their auditory organs, in that another genus (Mysis) is characterized by the presence of a pair of auditory sacks in the inner plates of the tail. These sacks have curved auditory hairs supporting an otolith at their extremity.
The development of the auditory organs in the Crustacea has not been investigated.
The Vertebrata. The Cephalochorda are without organs of hearing, and the auditory organ of the Urochorda is constructed on a special type of its own. The primitive auditory organs of the true Vertebrata have the same fundamental characters as those of the majority of aquatic invertebrate forms. They consist of a vesicle, formed by the invagination of a patch of epiblast, and usually shut off from the exterior, but occasionally (Elasmobranchii)remaining open. The walls of this vesicle are always much complicated and otoliths of various forms are present in its cavity. To this vesicle accessory structures, derived from the walls of the hyomandibular cleft, are added in the majority of terrestrial Vertebrata.
The development of the true auditory vesicle will be considered separately from that of the accessory structures derived from the hyomandibular cleft.
Illustration: Figure 299Fig. 299. Section through the head of an Elasmobranch embryo, at the level of the auditory involution.aup.auditory pit;aun.ganglion of auditory nerve;iv.v.roof of fourth ventricle;a.c.v.anterior cardinal vein;aa.aorta;I.aa.aortic trunk of mandibular arch;pp.head cavity of mandibular arch;Ivc.alimentary pouch which will form the first visceral cleft;Th.rudiment of thyroid body.
Fig. 299. Section through the head of an Elasmobranch embryo, at the level of the auditory involution.aup.auditory pit;aun.ganglion of auditory nerve;iv.v.roof of fourth ventricle;a.c.v.anterior cardinal vein;aa.aorta;I.aa.aortic trunk of mandibular arch;pp.head cavity of mandibular arch;Ivc.alimentary pouch which will form the first visceral cleft;Th.rudiment of thyroid body.
In all Vertebrata the development of the auditory vesicle commences with the formation of a thickened patch of epiblast, at the side of the hind-brain, on the level of the second visceral cleft. This patch soon becomes invaginated in the form of a pit (fig. 299,aup), to the inner side of which the ganglion of the auditory nerve (aun), which as shewn in a previous chapter is primitively a branch of the seventh nerve, closely applies itself.
In those Vertebrata (viz.Teleostei, Lepidosteus and Amphibia) in which the epiblast is early divided into a nervous and epidermic stratum, the auditory pit arises as an invagination of the nervous stratum only, and the mouth of the auditory pit is always closed (fig. 300) by the epidermic stratum of the skin. Since the opening of the pit is retained through life in Elasmobranchii the closed form of pit in the above forms is clearly secondary.
In Teleostei the auditory pit arises as a solid invagination of the epiblast.
The mouth of the auditory vesicle gradually narrows, and in most forms soon becomes closed, though in Elasmobranchii it remains permanently open. In any case the vesicle is gradually removed from the surface, remaining connected with it by an elongated duct, either opening on the dorsal aspect of the head (Elasmobranchii), or ending blindly close beneath the skin.
Illustration: Figure 300Fig. 300. Section through the head of a Lepidosteus embryo on the sixth day after impregnation.au.v.auditory vesicle;au.n.auditory nerve;ch.notochord;hy.hypoblast.
Fig. 300. Section through the head of a Lepidosteus embryo on the sixth day after impregnation.au.v.auditory vesicle;au.n.auditory nerve;ch.notochord;hy.hypoblast.
Illustration: Figure 301Fig. 301. Section through the hind-brain of a Chick at the end of the third day of incubation.IV.fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle.Ch.notochord;CV.anterior cardinal vein;CC.involuted auditory vesicle (CCpoints to the end which will form the cochlear canal);RL.recessus labyrinthi (remains of passage connecting the vesicle with the exterior);hy.hypoblast lining the alimentary canal;AO.,AO.A.aorta, and aortic arch.
Fig. 301. Section through the hind-brain of a Chick at the end of the third day of incubation.IV.fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle.Ch.notochord;CV.anterior cardinal vein;CC.involuted auditory vesicle (CCpoints to the end which will form the cochlear canal);RL.recessus labyrinthi (remains of passage connecting the vesicle with the exterior);hy.hypoblast lining the alimentary canal;AO.,AO.A.aorta, and aortic arch.
In all Vertebrata the auditory vesicle undergoes furtherchanges of a complicated kind. In the Cyclostomata these changes are less complicated than in other forms, though whether this is due to degeneration, or to the retention of a primitive state of the auditory organ, is not known. In the Lamprey the auditory vesicle is formed in the usual way by an invaginationof the epiblast, which soon becomes vesicular, and for a considerable period retains a simple character. As pointed out by Max Schultze, a number of otoliths appears in the vesicle during larval life, and, although such otoliths are stated by J. Müller to be absent both in the full-grown Ammocœte and in the adult, they have since been found by Ketel (No.387). The formation of the two semicircular canals has not been investigated.
In all the higher Vertebrates the changes of the auditory sacks are more complicated. The ventral end of the sack is produced into a short process (fig. 301,CC); while at the dorsal end there is the canal-like prolongation of the lumen of the sack (RL), derived from the duct which primitively opened to the exterior, and which in most cases persists as a blind diverticulum of the auditory sack, known as the recessus labyrinthi or aqueductus vestibuli. The parts thus indicated give rise to the whole of the membranous labyrinth of the ear. The main body of the vesicle becomes the utriculus and semicircular canals, while the ventral process forms the sacculus hemisphericus and cochlear canal.
The growth of these parts has been most fully studied in Mammalia, where they reach their greatest complexity, and it will be convenient to describe their development in this group, pointing out how they present, during some of the stages in their growth, a form permanently retained in lower types.
The auditory vesicle in Mammalia is at first nearly spherical, and is imbedded in the mesoblast at the side of the hind-brain. It soon becomes triangular in section, with the apex of the triangle pointing inwards and downwards. This apex gradually elongates to form the rudiment of the cochlear canal and sacculus hemisphericus (fig. 302,CC). At the same time the recessus labyrinthi (R.L) becomes distinctly marked, and the outer wall of the main body of the vesicle grows out into two protuberances, which form the rudiments of the vertical semicircular canals (V.B). In the lower forms (fig. 305) the cochlear process of the vestibule hardly reaches a higher stage of development than that found at this stage in Mammalia.
The parts of the auditory labyrinth thus established soon increase in distinctness (fig. 303); the cochlear canal (CC) becomes longer and curved; its inner and concave surface beinglined by a thick layer of columnar epiblast. The recessus labyrinthi also increases in length, and just below the point where the bulgings to form the vertical semicircular canals are situated, there is formed a fresh protuberance for the horizontal semicircular canal. At the same time the central parts of the walls of the flat bulgings of the vertical canals grow together, obliterating this part of the lumen, but leaving a canal round the periphery; and, on the absorption of their central parts, each of the original simple bulgings of the wall of the vesicle becomes converted into a true semicircular canal, opening at its two extremities into the auditory vesicle. The vertical canals are first established and then the horizontal canal.
Illustration: Figure 302Fig. 302. Transverse section of the head of a fœtal Sheep (16mm.in length) in the region of the hind-brain.(After Böttcher.)HB.the hind-brain.The section is somewhat oblique, hence while on the right side the connections of the recessus vestibuliR.L., and of the commencing vertical semicircular canalV.B., and of the ductus cochlearisCC., with the cavity of the primary otic vesicle are seen; on the left side, only the extreme end of the ductus cochlearisCC, and of the semicircular canalV.B.are shewn.Lying close to the inner side of the otic vesicle is seen the cochlear ganglionGC; on the left side the auditory nerveGand its connectionNwith the hind-brain are also shewn.Below the otic vesicle on either side lies the jugular vein.
Fig. 302. Transverse section of the head of a fœtal Sheep (16mm.in length) in the region of the hind-brain.(After Böttcher.)HB.the hind-brain.The section is somewhat oblique, hence while on the right side the connections of the recessus vestibuliR.L., and of the commencing vertical semicircular canalV.B., and of the ductus cochlearisCC., with the cavity of the primary otic vesicle are seen; on the left side, only the extreme end of the ductus cochlearisCC, and of the semicircular canalV.B.are shewn.Lying close to the inner side of the otic vesicle is seen the cochlear ganglionGC; on the left side the auditory nerveGand its connectionNwith the hind-brain are also shewn.Below the otic vesicle on either side lies the jugular vein.
Shortly after the formation of the rudiment of the horizontal semicircular canal a slight protuberance becomes apparent on the inner commencement of the cochlear canal. A constriction arises on each side of the protuberance, converting it into a prominent hemispherical projection, the sacculus hemisphericus (fig. 304,S.R).
Illustration: Figure 303Fig. 303. Section of the head of a fœtal Sheep 20mm.in length.(After Böttcher.)R.V.recessus labyrinthi;V.B.vertical semicircular canal;H.B.horizontal semicircular canal;C.C.cochlear canal;G.cochlear ganglion.
Fig. 303. Section of the head of a fœtal Sheep 20mm.in length.(After Böttcher.)R.V.recessus labyrinthi;V.B.vertical semicircular canal;H.B.horizontal semicircular canal;C.C.cochlear canal;G.cochlear ganglion.
The constrictions are so deep that the sacculus is only connected with the cochlear canal on the one hand, and with the general cavity of the auditory vesicle on the other, by, in each case, a narrow though short canal.
The former of these canals (fig. 304,b) is known as the canalis reuniens. At this stage we may call the remaining cavity of the original otic vesicle, into which all the above parts open, the utriculus.
Soon after the formation of the sacculus hemisphericus, thecochlear canal and the semicircular canals become invested with cartilage. The recessus labyrinthi remains however still enclosed in undifferentiated mesoblast.
Between the cartilage and the parts which it surrounds there remains a certain amount of indifferent connective tissue, which is more abundant around the cochlear canal than around the semicircular canals.
As soon as they have acquired a distinct connective-tissue coat, the semicircular canals begin to be dilated at one of their terminations to form the ampullæ. At about the same time a constriction appears opposite the mouth of the recessus labyrinthi, which causes its opening to be divided into two branches—one towards the utriculus and the other towards the sacculus hemisphericus; and the relations of the parts become so altered that communication between the sacculus and utriculus can only take place through the mouth of the recessus labyrinthi (fig. 305).
When the cochlear canal has come to consist of two and a half coils, the thickened epithelium which lines the lower surface of the canal forms a double ridge from which the organ of Corti is subsequently developed. Above the ridge there appears a delicate cuticular membrane, the membrane of Corti or membrana tectoria.
The epithelial walls of the utricle, the recessus labyrinthi, the semicircular canals, and the cochlear canal constitute together the highly complicated product of the original auditory vesicle. The whole structure forms a closed cavity, the various parts of which are in free communication. In the adult the fluid present in this cavity is known as the endolymph.
In the mesoblast lying between these parts and the cartilage, which at this period envelopes them, lymphatic spaces become established, which are partially developed in the Sauropsida, but become in Mammals very important structures.
They consist in Mammals partly of a space surrounding the utricle and semicircular canals, and partly of two very definite channels, which largely embrace between them the cochlear canal. The latter channels form the scala vestibuli on the upper side of the cochlear canal and the scala tympani on the lower. The scala vestibuli is in free communication with the lymphatic cavity surrounding the vestibule, and opens at the apex of the cochleainto the scala tympani. The latter ends blindly at the fenestra rotunda.
The fluid contained in the two scalæ, and in the remaining lymphatic cavities of the auditory labyrinth, is known as perilymph.
Illustration: Figure 304Fig. 304. Section through the internal ear of an embryonic Sheep 28mm.in length.(After Böttcher.)D.M.dura mater;R.V.recessus labyrinthi;H.V.B.posterior vertical semicircular canal;U.utriculus;H.B.horizontal semicircular canal;b.canalis reuniens;a.constriction by means of which the sacculus hemisphericusS.R.is formed;f.narrowed opening between sacculus hemisphericus and utriculus;C.C.cochlea;C.C´.lumen of cochlea;K.K.cartilaginous capsule of cochlea;K.B.basilar plate;Ch.notochord.
Fig. 304. Section through the internal ear of an embryonic Sheep 28mm.in length.(After Böttcher.)D.M.dura mater;R.V.recessus labyrinthi;H.V.B.posterior vertical semicircular canal;U.utriculus;H.B.horizontal semicircular canal;b.canalis reuniens;a.constriction by means of which the sacculus hemisphericusS.R.is formed;f.narrowed opening between sacculus hemisphericus and utriculus;C.C.cochlea;C.C´.lumen of cochlea;K.K.cartilaginous capsule of cochlea;K.B.basilar plate;Ch.notochord.
The cavities just spoken of are formed by an absorption ofparts of the embryonic mucous tissue between the perichondrium and the walls of the membranous labyrinth.
The scala vestibuli is formed before the scala tympani, and both scalæ begin to be developed at the basal end of the cochlea: the cavity of each is continually being carried forwards towards the apex of the cochlear canal by a progressive absorption of the mesoblast. At first both scalæ are somewhat narrow, but they soon increase in size and distinctness.
The cochlear canal, which is often known as the scala media of the cochlea, becomes compressed on the formation of the scalæ so as to be triangular in section, with the base of the triangle outwards. This base is only separated from the surrounding cartilage by a narrow strip of firm mesoblast, which becomes the stria vascularis, etc. At the angle opposite the base the canal is joined to the cartilage by a narrow isthmus of firm material, which contains nerves and vessels. This isthmus subsequently forms the lamina spiralis, separating the scala vestibuli from the scala tympani.
The scala vestibuli lies on the upper border of the cochlear canal, and is separated from it by a very thin layer of mesoblast, bordered on the cochlear aspect by flat epiblast cells. This membrane is called the membrane of Reissner. The scala tympani is separated from the cochlear canal by a thicker sheet of mesoblast, called the basilar membrane, which supports the organ of Corti and the epithelium adjoining it. The upper extremity of the cochlear canal ends in a blind extremity called the cupola, to which the two scalæ do not for some time extend. This condition is permanent in Birds, where the cupola is represented by a structure known as the lagena (fig. 305, II.L). Subsequently the two scalæ join at the extremity of the cochlear canal; the point of the cupola still however remains in contact with the bone, which has now replaced the cartilage, but at a still later period the scala vestibuli, growing further round, separates the cupola from the adjoining osseous tissue.
The ossification around the internal ear is at first confined to the cartilage, but afterwards extends into the thick periosteum between the cartilage and the internal ear, and thus eventually makes its way into the lamina spiralis, etc.
The organ of Corti. In Mammalia there is formed from theepithelium of the cochlear canal a very remarkable organ known as the organ of Corti, the development of which is of sufficient importance to merit a brief description. A short account of this organ in the adult state may facilitate the understanding of its development.