Book - Text-Book of the Embryology of Man and Mammals 16-2

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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.

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The Development of the Sensory Organs, Eye, Ear, and Organ of Smell

As the outer germ-layer is the parental tissue of the central nervous system, so also does it form the substratum for the higher sensory organs, the eye, the ear, and the organ of smell. For it furnishes the sensory epithelium, a component which, in comparison with the remaining parts, derived from the mesenchyrna, is, it is true, of very small volume, but, notwithstanding, by far the most important both from a functional and a morphological point of view. Whether a sensory organ is adapted for seeing, hearing, smelling, or tasting depends primarily upon the character of its sensory epithelium, i.e., upon whether it is composed of optic, auditory, olfactory, or gustatory cells. But also morphologically the epithelial part is preeminent, because it is chiefly this which determines the fundamental form of the sensory organs and affords the fixed centre around which the remaining accessory components are arranged. The genetic connection with the outer germ-layer may be most clearly recognised in many Invertebrates, inasmuch as here the sensory organs are permanently located in the epidermis? whereas in Vertebrates, as is well known, they are, for the sake of protection, embedded in deep-lying tissues. I begin with the eye, and then proceed to the organ of hearing and that of smell.

The Development of the Eye

As has already been stated in the description of the brain, the lateral walls of the primary fore-brain (figs. 234, 263) are evaginated and produce the primary optic vesicles (au), which are constricted off more and more and remain in connection with the between-brain by means of a slender stalk only (fig. 204 A st). They possess spacious cavities within, which are connected with the system of brain-ventricles through the narrow canal of the stalk of the optic vesicle. In many Vertebrates, in which the central nervous system is formed as a solid structure, as in the Cyclostom.es and Teleosts, the optic vesicles are also without cavities ; these do not make their appearance until the central nervous system becomes a tube.

Fig. 263. Brain of a human embryo of the third week (Lg). Profile reconstruction, after His. yh, Cerebral vesicle; zh, between-brain vesicle; mh, mid-brain vesicle; kh, nh, vesicles of cerebellum and medulla oblongata ; au, optic vesicle ; gb, auditory vesicle ; tr, infundibulum ; rf, area rhomboidalis ; nb, nuchal flexure ; kb, cephalic flexure.

Since the brain is for a long time separated from the primitive epidermis by only an exceedingly thin sheet of connective tissue, the primary optic vesicles at the time of their evagination either apply themselves directly to the epidermis, as in the case of the Chick, or are separated from it by only a very thin intervening layer, as in Mammals.

Upon each optic vesicle can be distinguished a lateral, a median, an upper and a lower wall. I designate as lateral that surface which reaches the epidermis at the surface of the body, as median the opposite wall joined with the stalk of the optic vesicle, and finally as lower the one which lies on a level with the floor of the between-brain. These designations will be useful in acquainting ourselves with the changes which the form of the optic vesicle undergoes during its imagination, which occurs at two places, namely, at its lateral and lower surfaces. One of the imaginations is connected with the development of the lens, the other with the formation of the vitreous body.

Fig. 264. Two diagrams illustrating the development of the eye.

• A, The primary optic vesicle (au), joined by a hollow

stalk (st) to the between-brain (zh), is invaginated as a result of the development of the lens-pit (Ig).

• B, The lens-pit has become abstricted to form a lens

vesicle (Is). From the optic vesicle has arisen the optic cup with double walls, an inner (ib) and an outer (ab) ; 1st, stalk of the lens ; gl, vitreous body.

The first fundament of the lens appears in the Chick as early as the second day of incubation, in the Rabbit about ten days after the fertilisation of the egg. At the place where the epidermis passes over the surface of the primary optic vesicle (fig. 264 A Ig), it becomes slightly thickened and invaginated into a small pit (lenspit). The pit, by its deepening and by the approximation of its edges until they meet, is converted into a lens-vesicle (fig. 264 B Is], which for a time preserves its connection with its parental substratum, the epidermis, by means of a solid epithelial cord (1st). Upon being constricted off the lens-vesicle naturally pushes the adjacent lateral wall of the optic vesicle before it and folds the latter in against the median wall.

At the same time with the development of the lens, the primary optic vesicle is also invaginated from below along a line which stretches from the epidermis to the attachment of the stalk of the optic vesicle, and is even continued along the latter for some distance (tig. 265 aus). A loop of a blood-vessel from the enveloping connective tissue, embedded in soft, gelatinous substance (gl), here grows against the lower surface of the primary optic vesicle and its stalk, and pushes up before it the lower wall.

In consequence of the two invaginations the optic vesicle acquires the form of a beaker or cup, the foot of which is represented by its stalk (Sn). But the optic cup, as we can from this time forward designate the structure, exhibits two peculiarities. First, it has, as it were, a defect (fig. 265 aus) in its lower wall ; for there runs along the latter from the margin of the broad opening which embraces the lens (/) to the attachment of the stalk (Sn) a fissure (aus), which is caused by the development of the vitreous body (gl) and bears the name fcetal optic fissure [or choroid fissure\. At first it is rather wide, but then becomes narrower and narrower by the approximation of its edges and finally closed altogether. Secondly, the optic cup, like the toy called the cup of Tantalus, is provided with double walls, which are continuous with each other along the edge of the front opening and also along the fissure. They will henceforth be designated as inner (figs. 264 B and 265 ib) and outer (ab) layers; the former is the invaginated, the latter the uninvaginatecl part of the primary optic vesicle.

Fig. 265. Plastic representation of the optic cup with lens and vitreous body.

o.b, Outer wall of the cup ; lb, its inner wall ; h, cavity between the two walls, which later disappears entirely ; Sn, fundament of the optic nerve. (Stalk of the optic vesicle with a furrow on its lower surface.) aus, Optic [choroid] fissure ; yl, vitreous body ; I, lens.

At the beginning of the infolding the two layers are separated by a broad space (k), which leads into the third ventricle through the stalk of the vesicle (/Sn) ; but afterwards the space becomes reduced proportionally to the increase in the size of the vitreous body.

Finally outer and inner layers come to lie in close contact (fig. 2G(> pi and ?). The fundaments of the lens (le and If) and the vitreous body (g) constitute the contents of the cup. The vitreous body fills the bottom of the cup, the lens its opening.

Fig. 266. Section through the optic fundament of an embryo Mouse, after KESSLER.

j>i, Pigmented epithelium of the eye (outer lamella of the optic cup, or secondary optic vesicle) ; r, retina (inner lamella of the optic cup) ; /M, marginal zone of the optic cup, which forms the pars ciliaris et iridis retinas ; y, vitreous body with blood-vessels ; tv, tunica vasculosa lentis ; bk, blood-corpuscles ; ch, choroidea ; If, lens-fibres ; le, lens-epithelium ; I' ', zone of the lens-fibre nuclei ; It, fundament of the cornea ; he, external corneal epithelium.

In the process of invagination the stalk of the optic vesicle has also changed its form. Originally it is a small tube with an epithelial wall, but afterwards it becomes an inverted trough with double walls, inasmuch as its lower surface participates in the invagination caused by that growth of connective tissue which toward the periphery furnishes the vitreous body. Later the edges of the trough bend together and fuse with each other. In this way the connective tissue cord, with the arteria centralis retina?, which traverses it, is enclosed within the stalk, which is now a quite compact structure.

Finally the tissue of the intermediate layer, apart from its producing the vitreous body, takes a further active share in the development of the whole eye, inasmuch as that portion of it which is adjacent to the optic cup is differentiated into the choroid membrane (tig. 266 ch) and the sclerotica of the eye.

After having thus delineated briefly the source of the most important components of the eye, it will be my purpose in what follows to pursue in detail the development of each part separately. I shall begin with the lens and vitreous body, then pass to the optic cup, and at that point add an account of the formation of the choroid membrane and the sclerotica, as well as the optic nerve ; in a final section I shall treat of the organs that are accessory to the optic cup the eye-lids, the lachrymal glands and their ducts.

The Development of the Lens

When the lens- vesicle has been completely constricted off from the primitive epidermis (fig. 264 B Is), it possesses a thick wall, which is composed of two or three layers of epithelial cells, and encloses a cavity that in Birds is partially filled with fluid, but in the case of Mammals by a mass of small cells. The mass of cells is the result of a proliferation of the most superficial flattened sheet of the primitive epidermis ; it is without importance in the further development a transient mass, that soon degenerates and is absorbed when the lensfibres are developed. (ARNOLD, MIHALKOVICS, GOTTSCHAU, KORANYI.) Externally the epithelial vesicle is sharply limited by a thin membrane, which is afterwards thickened into the capsule of the lens (capsula lentis). There are two opposing views in regard to its development. According to one, the capsule is a cuticular structure, that is to say, a structure secreted by the cells of the lens at their bast's ; according to the other view it is the product of a connectivetissue layer, to be described more fully hereafter, enveloping the lens-vesicle.

In later stages considerable differences arise in the development of the anterior and posterior walls (fig. 266). In the region of the anterior wall the epithelium (le) becomes more and more flattened ; the cylindrical cells are converted into cubical elements, which are preserved throughout life in a single layer and constitute the so-called lens-epithelium in the lens of the adult (fig. 266 le). In the posterior wall, on the contrary, the cells increase greatly in length (fig. 266) and grow out into long fibres, which form a protuberance projecting into the cavity of the vesicle. The fibres stand perpendicular to the posterior wall, are longest in its middle, become shorter towards the equator of the lens (figs. 266, 267 Z), and finally appear as ordinary cylindrical cells; these in turn become still shorter and are continuous with the cubical cells (le) of the lens-epithelium. In this way there exists at the equator a zone of transition between the fibrous portion and the epithelial part of the lens.

Fig. 267. Part of a section through the fundament of the eye of an embryo Mouse. Somewhat older stage than that shown in fig. 266. After KESSLER.

A part of the lens, the rim of the optic cup, the cornea, and the anterior chamber of the eye. pi, 1'igmented epithelium of the eye ; r, retina ; rz, marginal zone of the optic cup; y, bloodvessels of the vitreous body in the vascular capsule of the lens ; tv, tunica vasculosa lentis ; x, connection of the latter with the choroid membrane of the eye ; I', transition of the lensepithelium into the lens-fibres ; Ic, lens-epithelium ; k, anterior chamber of the eye ; d, Descemet's membrane ; //, cornea ; he, corneal epithelium.

The next change consists in the elongation of the fibres until their anterior ends have reached the epithelium (fig. 267). Consequently the vesicle has now become a solid structure, which, as the lens-core, furnishes the foundation of the lens of the adult.

The further increase in the size of the lens is an appositional growth. Around the core first formed arise new lens-fibres, which are arranged parallel to the surface of the organ and are united into coats. These lie in layers one over another, which in macerated lenses may be detached like the coats of an onion. All fibres (fig. 268 If, If") extend from the anterior to the posterior surface, where their ends meet one another along regular lines, which in the embryo and the new-born animal have the form of two three-rayed figures, the so-called stars of the lens (fig. 268 vst and list). These exhibit the peculiarity that the rays of the anterior face alternate with those of the posterior face, so that the three rays of one star halve the spaces between the three rays of the other.

In the adult the figure becomes more complicated, because lateral rays arise on each of the three chief rays.

How have the newly deposited fibres been formed ? Their origin is ultimately to be referred to the lens-epithelium of the front surface of the organ. In these cells figures of nuclear division can frequently be observed even in late stages of development. The cells which result from division serve to replace those which grow out into lens-fibres, and are placed upon the already formed layers. The new formation takes place only at the equator of the lens (fig. 267) in the zone of transition (I'} previously described, where, in the adult as well as in the recently born animal, the cubical epithelial cells gradually merge into cylindrical and fibrous elements, as one can convince himself from any properly directed section.

In the adult, as is well known, there exist no special provisions for the nutrition of the lens, which, after attaining full size, is not much altered, and certainly undergoes only a slight metastasis. With the embryo it is otherwise. Here a more active growth necessitates a special apparatus for nutrition. This is furnished in Mammals by the tunica vasculosa lentis (figs. 266, 267 tv}. By this is understood a highly vascular connective -tissue membrane, which envelops the outer surface of the capsule of the lens on all sides. In Man it is already distinctly developed as early as the second month. Its vessels are derived from those of the vitreous body. Consequently on the posterior wall of the lens they are large. These, resolved into numerous fine branches, bend around the equator of the lens, and run toward the middle of the anterior surface, where they form terminal loops, and also unite with blood-vessels of the choroid membrane (fig. 267 x).

Fig. 268. Diagram of the arrangement of the lens-fibres.

One sees the opposite positions of the anterior (vsl) and the posterior (/<*<) stars of the lens. if, Course of the lens-fibres on the anterior surface of the lens and their termination at the anterior star of the lens ; If", continuation of the same fibres Co the posterior star on the posterior surface.

Separate parts of the nourishing membrane of the lens, having been discovered at different times by various investigators, have received special names, as membrana pupillaris, m. capsulo-pupillaris, m. capsularis. The first to be observed was the membrana pupillaris, the part of the vascular membrane which is situated behind the pupil on the anterior surface of the lens. It was the most easily found, because occasionally it persists even after birth as a fine membrane closing the pupil, and producing atresia pupillw congenita. Later it was found that the membrana pupillaris is also continued laterally from the pupil on the anterior face of the lens, and this part was called membrana capsulo-pupillaris. Finally it was discovered that the blood-vessels are spread out on the posterior wall of the lens the membrana capsularis. It is superfluous to retain all these names, and most suitable to speak of a nutritive membrane of tlie lens, or a membrana vasculosa lentis.

This vascular membrane attains its greatest development in the seventh month, after which it begins to degenerate. Ordinarily it has entirely disappeared before birth ; only in exceptional cases do some parts of it persist. Toward the end of embryonic life, moreover, the chief growth of the lens itself has ceased. For according to weighings carried on by the anatomist HUSCHKE, it has a weight of 123 milligrammes in the new-born child, and 190 milligrammes in the adult, so that the total increase which the organ undergoes during life amounts to only 67 milligrammes.

The Development of the Vitreous Body

The question of the development of the vascular membrane of the lens leads to that of the vitreous body. As was previously mentioned, there grows out from the embryonic connective tissue a process with a vascular loop, which makes its way into the primary optic vesicle and its stalk (fig. 265). The vascular loop then begins to send out new lateral branches ; likewise the connective-tissue matrix, which is at first scanty, increases greatly and is characterised by its extraordinarily slight consistency and its large proportion of water (figs. 266, 267 g). There are also to be found in it here and there isolated stellate connective -tissue cells ; but these disappear later, and in their place occur migratory cells (leucocytes), which are assumed to be immigrated white blood-corpuscles.

There are two opposing views regarding the nature and development of the vitreous body. According to KESSLER we have to do, not with a genuine connective substance, but with a transudation,a fluid, which has been secreted from the vascular loops ; the cells are from, the beginning simply immigrated white blood -corpuscles KOLLIKER, SCHWALBE, and other investigators, on the contrary, regard the vitreous body as a genuine connective substance. According to SCHWALBE'S definition, to which I adhere, it consists of an exceedingly watery connective tissue, whose fixed cells have early disappeared, but whose interfibrillar substance infiltrated with water is traversed by migratory cells. The vitreous body is afterwards surrounded by a structureless membrane, the membrana hyaloidea, which, according to some investigators, belongs to the retina, although, according to the researches of SCHWALBE, this view is not admissible.

The vitreous body, which in the adult is quite destitute of bloodvessels, is bountifully supplied with them in the embryo. They come from the arteria centralis retince, the branch of the ophthalmic artery that runs along the axis of the optic nerve.

The arteria centralis retinae is prolonged from the papilla of the optic nerve as a branch which is designated as the arteria hyaloidea. This, resolved into several branches, runs forward through the vitreous body to the posterior surface of the lens, where its numerous terminal ramifications spread out in the tunica vasculosa, and at the equator pass over on to the anterior face of the lens. During the last months of embryonic life the vessels of the vitreous body, together with the nutritive membrane of the lens, undergo degeneration ; they entirely disappear, with the exception of a rudiment of the chief stem, which runs forward from the entrance of the optic nerve to the anterior surface of the vitreous body, and during the degeneration is converted into a canal filled with fluid, the canalis liyaloideus.

The Development of the Secondary Optic Cup and the, Goats of the Eye

The optic cup is further metamorphosed at the same time with the layer of mesenchyma which envelops it, and which furnishes the middle and outer tunics of the eye, so that it seems to be desirable to treat of both at the same time. I begin with the stage represented in figures 266 and 269. The optic cup still possesses at this time a broad opening, in which the lens (le) is embraced. The latter is either separated from the epidermis by only an exceedingly thin sheet of mesenchyma, as in the Mammals (fig. 266), or its anterior face is in immediate contact with the epidermis, as in the Chick (fig. 269). In the beginning, therefore, there is no separate fundament for the cornea between lens and epidermis ; moreover, both the anterior chamber of the eye and the iris are wanting.

The fundament of the cornea is derived from the surrounding mesenchyma, which, as a richly cellular tissue, envelops the eyeball. In the Chick (fig. 269), as early as the fourth day, it grows in between the epidermis and the front surface of the lens as a thin sheet (bi}. At first this sheet is structureless, then numerous mesenchymatic cells migrate into it from, the margin and become the corneal corpuscles. These produce the cornea! fibres in the same way that embryonic connective-tissue cells do the connectivetissue fibres, while the structureless sheet in part goes to form the cementing substance between them, and in part is preserved on the anterior and posterior walls as thin layers destitute of cells ; these layers, undergoing chemical metamorphosis, become respectively the membrana elastica anterior and the membrane of DESCEMET.

Fig. 269. Section through the anterior portion of the fundament of the eye in an embryo Chick on the fifth day of incubation, after KESSLEE. he, Corneal epithelium ; h, lens-epithelium ; h, structureless sheet of the corneal fundament ; li, embryonic connective stibstance, which envelops the optic cup and, penetrating between lensepithelium (/c) and corneal epithelium (Ac), furnishes the fundament of the cornea ; ah, outer, ib, inner layer of the secondary optic cup.

The internal endothelium of the cornea is developed at an extraordinarily early epoch in the Chick. For as soon as the structureless sheet previously mentioned (fig. 269 A) has attained a certain thickness, mesenchymatic cells proceeding from the margin spread themselves out on its inner surface as a single-layered thin cell-membrane. With this begins also the formation of the anterior chamber of the eye. For the thin fundament of the cornea, which at first lay in immediate contact with the front surface of the lens, now becomes somewhat elevated from the latter, and separated from it by a fissure-like space filled with fluid (humor aqueus). The fissure is first observable at the margin of the secondary optic cup, and spreads out from this region toward the anterior pole of the lens. The anterior chamber of the eye does not, however, acquire a greater size and its definite form until the development of the iris. Two opposing views exist concerning the origin of the structureless sheet which has been described as constituting the first fundament of the cornea in the Chick. According to KESSLER it is a product of the secretion of the epidermis, whereas the corneal corpuscles migrate in from the mesenchyma. In his opinion, therefore, the cornea is composed of two entirely different fundaments. According to KOLLIKER, on the contrary, all its parts are developed out of the mesenchyma, and the homogeneous matrix simply outstrips the cells in its growth and extension. In Mammals (fig. 266) the conditions differ somewhat from those of the Chick ; for as soon as the lens-vesicle in Mammals is fully constricted off, it is already enveloped by a thin sheet of mesenchyma (fi) with few cells, which separates it from the epidermis. The thin layer is rapidly thickened by the immigration of cells from the vicinity. Then it is separated into two layers (fig. 267), the pupillar membrane (tv) and the fundament of the cornea (A). The former is a thin, very vascular membrane lying on the anterior surface of the lens ; its network of blood-vessels communicates on the one hand posteriorly with the vessels of the vitreous body, together with which it constitutes the tunica vasculosa lentis, and on the other anastomoses at the margin of the optic cup with the vascular network of the latter. The fundament of the cornea is first sharply delimited from the pupillary membrane at the time when the anterior chamber of the eye and is formed as a narrow fissure, which gradually increases in extent with the appearance of the iris.

Fig. 270. Section through the margin of the optic cup of an embryo Turdus musicus, after KESSLER.

/, Retina ; pi, pigmented epithelium of the retina (outer lamella of the optic cup) ; It, connective-tissue envelope of the optic cup (choroidea and sclera) ; * ora serrata (boundary between the marginal zone and the fundus of the optic cup) ; ck, ciliary body ; 1, 2, 3, iris ; 1 and 2, inner and outer lamellae of the pars iridLs retinae ; 3, connective-tissue plate of the iris ; Ip, ligamentum pectinatum iridis ; sch, canal of SCHLEMM ; D, DESCEMET'S membrane ; h, cornea ; he, corneal epithelium.

During those processes the condition of the optic cup itself has also changed. Its outer and inner lamellae continually become more and more unlike. The former (figs. 2G6, 2G7 pi) remains thin and composed of a single layer of cubical epithelial cells. Black pigment granules are deposited in this in increasing abundance, until finally the whole lamella appears upon sections as a black streak. The inner layer (?), on the contrary, remains entirely free from pigment, with the exception of a part of the marginal zone ; the cells, as in the wall of the brain vesicles, become elongated and spindleshaped, and lie in many superposed layers.

Moreover the bottom of the cup and its rim assume different conditions, and hasten to fulfil different destinies; the former is converted into the retina, the latter is principally concerned in the production of the ciliary body and the iris.

The edge of the cup (fig. 267 rz, fig. 270*, and fig. 271) becomes very much reduced in thickness by the cells of its inner layer arranging themselves in a single sheet, remaining for a time cylindrical, and then assuming a cubical form. But with its reduction in thickness there goes hand in hand an increase in its superficial extent. Consequently the margin of the optic cup now grows into the anterior chamber of the eye between cornea and tbe anterior surface of the lens, until it has nearly reached the middle of the latter. Then it at last bounds only a small orifice which leads into the cavity of the optic cup the pupil. The pigment layer of the iris is derived from the marginal region of the cup, as KESSLER first showed (fig. 270 l and 2 ). Pigment granules are now deposited in the inner epithelial layer, just as in the outer lamella, so that at last the two are no longer distinguishable as separate layers.

The mesenchymatic layer which envelops the two epithelial lamella keeps pace with them in their superficial extension. It becomes thickened and furnishes the stroma of the iris with its abundant non-striated muscles and blood-vessels (fig. 270 s ). In Mammals (fig. 267 x) this is for a time continuous with the tunica vasculosa lentis (tv), in consequence of which the pupil in embryos is closed by a thin vascular connective-tissue membrane, as has already been stated.

The part of the optic cup which is adjacent to the pigment layer of the iris and surrounds the equator of the lens, and which likewise belongs to the attenuated marginal zone of the cup (fig. 270 c), undergoes an interesting alteration. In conjunction with the neighboring layer of connective substance, it is converted into the ciliary body of the eye. This process begins in the Chick on the ninth or tenth day of incubation (KESSLER), in Man at the end of the second or beginning of the third month (KOLLIKER). The attenuated epithelial double lamella of the cup, in consequence of an especially vigorous growth in area, is laid into numerous, [nearly] parallel short folds, which are arranged radially around the equator of the lens. As in the iris, so here, the adjacent mesenchymatic layer participates in the growth and penetrates between the folds in the form of fine processes. A cross section through the foldeel part of the optic cup of a Cat embryo 10 cm. long (fig. 271) affords information concerning the original form of these processes in Mammals. It shows that the individual folds are very thin and enclose within them only a very small amount of embryonic connective tissue (bi ') with fine capillaries, and that, unlike the pigment epithelium of the iris, only the outer of the two epithelial layers (ab) is pigmented, whereas the inner (ib) remains unpigmented even later and is composed of cylindrical cells.

Fig. 271. Cross section through the ciliary par of the eye of an embryo Cat 10 cm. long, after KESSLEB.

Three ciliary processes formed by the folding of the optic cup are shown, li, Connective-tissue part of the ciliary body ; ib, inner layer, a I. outer pigmented layer of the optic cup li', sheet of connective tissue that has penetrated into the epithelial fold.

Subsequently tho ciliary processes become greatly thickened through increase of the very vascular connective-tissue framework, and acquire a firm union with the capsule of the lens through the formation of the zonula Zinnii. In Man the latter is formed, according to KOLLIKER'S account, during the fourth month, in a manner that here, as well as in other Mammals, is still incompletely explained.

LIEBEEKi'JKN remarks that the zonula is distinctly recognisable in eyes which have attained half their definite size. If one takes out of an eye the vitreous body together with the lens, and then removes the latter by opening the capsule on the front side, the margin of the capsule appears surrounded by blood-vessels which pass from the posterior over on to the anterior surface.

" At the places where the processus ciliares are entirely removed, tufts of fine fibres are to be seen which correspond to, and fill up, the depressions between the ciliary processes ; but between these tufts is also to be seen a thin layer of the same kind of finely striate masses, which must have lain at the same level as the ciliary processes." Furthermore LIEBERKUKN states that " there lie within this striated tissue numerous cell-bodies of the same appearance as those that are found elsewhere in the embryonic vitreous body at a later period."

ANG-ELUCCI believes that the zonula arises from the anterior part of the vitreous body ; at the time when iris and ciliary processes are developed he finds the vitreous body traversed by fine fibres, which extend from the ora serrata to the margin of the lens. He describes as lying between the fibres sparse migratory cells, which are maintained, however, to have no share in the formation of the fibres.

The fundus of the optic cup (figs. 266, 267, 270) furnishes the most important part of the eye the retina. The inner lamella of the cup (r) becomes greatly thickened, and, in consequence of its cells being elongated into spindles and overlapping one another in several layers, acquires an appearance similar to that of the wall of the embryonic brain. Subsequently it becomes marked off by an indented line, the ora serrata (at the place indicated by a star in fig. 270), from the adjoining attenuated part of the optic vesicle, which furnishes the ciliary folds. It also early acquires at its two surfaces a sharp limitation through the secretion of two delicate membranes : on the side toward the fundament of the vitreous body it is bounded by the membrana limitans interim ; on that toward the outer lamella, which becomes pigmented epithelium, by the membrana limitans externa.

In the course of development its cells, all of which are at first alike, become specialised in very different ways, as a result of which there are produced the well-known layers distinguished by MAX SCHULTZE. I shall not go into the details of this histological differentiation, but shall mention some further points of general importance.

As WILHELM MULLER in his " Stammesentwicklung des Sehorgans der Wirbelthiere " has clearly shown, the development of the originally similar epithelial cells of the retina takes place in all Vertebrates in two chief directions : a part of them become sensory epithelium and the specific structures of the central nervous system ganglionic cells and nerve-fibres ; another part are metamorphosed into supporting and isolating elements into MULLER'S radial fibres and the granular [reticular or molecular] layers, which can be grouped together as epithelial sustentative tissue (fulcrum). Finally, with the descendants of the epithelium are associated connective-tissue elements, which grow from the surrounding connective tissue into the epithelial layer for its better nutrition, in the same manner as in the central nervous system. These ingrowths are branches of the arteria centralis retinae with their extremely thin connective-tissue sheaths. The Lampreys alone form an exception, their retina remaining free from blood-vessels. In all other Vertebrates bloodvessels are present, but they are limited to the inner layers of the retina, leaving the outer granular (Kb'rner) layer and that of the rods and cones free ; the latter have been distinguished as sensory epitheliuni from the remaining portions with their nerve-fibres and ganglionic cells the brain-part of the retina.

Of all the parts of the retina the layer of rods and cones is the last to be developed. According to the investigations of KOLLIKER, BABUCHIN, MAX SCHULTZE, and W. MULLER, it arises as a product of the outer granular (Korner) layer, which, composed of fine spindle-shaped elements, is held to be, as has been stated, the essential sensory epithelium of the eye. In the Chick the development of the rods and cones can be made out on the tenth day of incubation. MAX SCHULTZE states concerning young Cats and Rabbits, which are born blind, that the fundament of the rods and cones can be distinguished for the first time in the early days after birth ; in other Mammals and in Man, on the contrary, they are formed before birth.

In all Vertebrates, as long as rods and cones are not present, the inner layer of the optic cup is bounded on the side toward the outer layer by an entirely smooth contour, due to the membrana limitans externa. Then there appear upon the latter numerous, small, lustrous elevations, which have been secreted by the outer granules or visual cells. The elevations, which consist of a protoplasmic substance and are stained red in carmine, become elongated and acquire the form of the inner limb of the retinal element. Finally there is formed at their outer ends the outer limb, which MAX SCHULTZE and W. MULLER compare to a cuticular product, on account of its lamellate structure.

Inasmuch as the rods and cones of the retinal cells grow out in this way beyond the membrana limitans externa, they penetrate into the closely applied outer lamella of the optic cup, which becomes the pigmented epithelium of the retina (figs. 266, 267, 270 pi} ; their outer limbs come to lie in minute niches of the large, hexagonal pigment-cells, so that the individual elements are separated from one another by pigmented partitions.

A few additional words concerning the connective tissue enveloping the fundament of the optic cup. It acquires here, as on the ciliary body and the iris, a special, and for this region characteristic, stamp. It is differentiated into vascular [choroid] and fibrous [sclerotic] membranes, which in Man are distinguishable in the sixth week (KOLLIKER). The former is characterised by its vascularity at an early period, and develops on the side toward the optic cup a special layer, provided with a fine network of capillary vessels, the membrana choriocapillaris, for the nourishment of the pigment-layer and the layer of rods and cones, which have no blood-vessels of their own. It further differs from the ciliary body in the fact that at the fundament of the optic cup the choroid membrane is easily separable from the adjoining membranes of the eye, whereas in the ciliary body a firm union exists between all the membranes.

If we now glance back at the processes of development last described, one thing will appear clear to us from this short sketch : that the changes in the form of the secondary optic cup are of preeminent importance for the origin of the individual regions of the eye. Through different processes of growth, which have received a general discussion in Chapter IV., there have been formed in the cup three distinct portions. By means of an increase in thickness and various differentiations of the numerous cell-layers, there is formed the retina ; by an increase of surface, on the contrary, is produced an anterior, thinner part, which bounds the pupil and is subdivided into two regions by the formation of folds in the vicinity of the lens. From the folded part, which joins the retina at the ora serrata, is formed the epithelial lining of the ciliary body ; from the thin portion which surrounds the pupil and which remains smooth, the pigniented epithelium (uvea) of the iris. Consequently there are now to be distinguished on the secondary optic cup three regions, as retinal, ciliary, and iridal parts. To each of these territories the contiguous connective tissue, and especially the part which becomes the middle tunic of the eye, is adapted in a particular manner ; here it furnishes the connective- tissue plate of the iris with its non-striated musculature, there the connective-tissue framework of the ciliary body with the ciliary muscle, and in the third region the vascular choroidea with the choriocapillaris and lamina fusca.

In the development of the optic cup there arose on its lower wall a fissure (fig. 265 cms), which marks the place at which the fundament of the vitreous body grew into the interior of the cup. What is the ultimate fate of this fissure, which is usually referred to in the literature as choroid fissure It is for a time easily recognisable, after pigment has been deposited in the outer lamella of the optic cup. It then appears on the lower median side of the eyeball as a clear, unpigmented streak, which reaches forward from the entrance of the optic nerve to the margin of the pupil.

The name choroid fissure takes its origin from this phenomenon. It was given at a time when the formation of the optic cup was not adequately known, when the pigmented epithelium was still referred to the choroidea. Therefore in the absence of pigment along a clear streak on the under side of the eyeball it was supposed that a defect of the choroidea a choroid fissure had been observed.

The clear streak afterwards disappears. The fissure of the eye is closed by the fusion of its edges and the deposition of pigment in the raphe. In the Chick this takes place on the ninth day, in Man during the sixth or seventh week.

In still another respect is the choroid fissure noteworthy.

In many Vertebrates (Fishes, Reptiles, Birds) a highly vascular process of the choroidea grows through the fissure, before its closure, into the vitreous body and there forms a lamellar projection, which extends from the optic nerve to the lens. In Birds it has received the name " pecten," because it is folded into numerous parallel ridges. It consists almost entirely of the walls of blood-vessels, which are held together by a small amount of a black pigmented connective tissue.

In Mammals such a growth into the vitreous body is wanting.

The closure of the choroid fissure takes place at an early period and completely.

Occasionally in Man the normal course of development is interrupted, so that the margins of the choroid fissure remain apart. The usual consequence of this is a defective development of the vascular tunic of the eye at the corresponding place an indication of the extent to which the development of the connective-tissue envelope is dependent on the formative processes of the two epithelial layers, as has already been stated. Both retinal and choroidal pigment are therefore wanting along a streak which begins at the optic nerve, so that the white solera of the eye shows through to the inside and can be recognised in examinations with the ophthalmoscope. When the defect reaches forward to the margin of the pupil, a fissure is formed in the iris which is easily recognised upon external observation of the eye. The two structures resulting from this interrupted development are distinguished from each other as choroidal and iridal fissures (coloboma choroidere and coloboma iridis).

The Development of the Optic Nerve

The stalk of the optic vesicle (fig. 272), by which the vesicle is united with the between-brain, is in direct connection with both lamellae of the optic cup, the primary optic vesicle having been infolded from below by the fundament of the vitreous body to form the cup. Its dorsal wall is continuous with the outer lamella or pigment-epithelium of the retina ; its ventral wall is prolonged into the inner lamella, which becomes the retina. Thus, aside from the formation of the vitreous body, the development of a choroid fissure also has a significance in view of the persistence of the direct connection between retina and optic nerve. For if we conceive the optic vesicle invaginated merely at its anterior face by the lens, the wall of the optic nerve would be continued into the outer, uninvaginated lamella only; direct connection with the retina itself, or the invaginated part, would be wanting.

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Fig. 272. Plastic representation of the optic cup with lens and vitreous body.

ab, Outer wall of the cup; ib, its inner wall ; h, space between the two walls, which afterwards entirely disappears ; Sn, fundament of the optic nerve (stalk of the optic vesicle with groove-formation along its lower face) ; ims, choroid fissure ; yl, vitreous body ; I, lens.

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Originally the optic nerve is a tube with a small lumen, which unites the cavity of the optic vesicle Avith the third ventricle (tig. 264 A). It is gradually converted into a solid cord. In the case of most Vertebrates this is produced simply by a thickening of the walls of the stalk, due to cell-proliferation, until the cavity is obliterated. In Mammals only the larger portion, that which adjoins the brain, is metamorphosed in this manner ; the smaller part, that which is united with the optic vesicle, is, on the contrary, infolded by the prolongation of the choroid fissure backward for some distance, whereby the ventral wall is pressed in against the dorsal. Consequently the optic nerve here assumes the form of a groove, in which is imbedded a connective-tissue cord with a blood-vessel that becomes the arteria centralis retinae. By the growing together of the edges of the groove, the cord afterwards becomes completely enclosed.

For a time the optic nerve consists exclusively of spindle-shaped, radially arranged cells in layers, and resembles in its finer structure the wall of the brain and the optic vesicle. Different views are held concerning its further metamorphoses, and especially concerning the origin of nerve-fibres in it. Differences similar to those concerning the origin of the peripheral nerve-fibres are maintained. Upon this point three theories have been brought forward.

According to the older view, which LTEBERKUHN shares, the optic fibres are developed in loco by the elongation of the spindle-shaped cells. According to His, KOLLIKER, and W. MULLER, 011 the contrary, the wall of the optic vesicle furnishes the sustentative tissue only, whereas the nerve-fibres grow into it from outside, either from the brain toward the retina (His, KOLLIKER), or in the reverse direction (MULLER). The stalk of the optic vesicle would constitute, according to this view, only a guiding structure as it were would predetermine the way for its growth. When the ingrowth has taken place, the sustentative cells are, as KOLLIKER describes them, arranged radially and so united with one another that they constitute a delicate framework with longitudinally elongated spaces. In the latter are lodged the small bundles of very fine non-nuclear nervefibres and numerous cells, arranged in longitudinal rows, which likewise belong to the epithelial sustentative tissue and help to complete the trestle-work.

The embryonic optic nerve is enveloped in a connective-tissue sheath, which is separated, as in the case of the brain and secondary optic cup, into an inner, soft, vascular and an outer compact fibrous layer. The former, or the pial sheath, unites the pia mater of the brain and the choroid membrane of the eye ; the latter, or the dural sheath, is a continuation of the dura mater and at the eyeball becomes continuous with the sclerotica. Later the optic nerve acquires a still more complicated structure, owing to the fact that vascular processes of the pial sheath grow into it and provide the nerve-bundles and the epithelial sustentative cells belonging to them wit] i connective-tissue investments.

As has been previously stated, the direction in which optic fibres grow into the stalk of the optic vesicle is still a subject of controversy. His, with whom KOLLIKER is in agreement, maintains that they grow out from groups of ganglionic cells (thalamus opticus, corpora quadrigemina), and are only secondarily distributed in the retina. He supports his view on the one hand by the agreement in this particular which exists with the development of the remaining peripheral nerves, and on the other by the circumstance that the nerve-fibres are first distinctly recognisable in the vicinity of the brain.

W. MULLEE, on the contrary, believes that the outgrowth takes place in the opposite direction ; he maintains that the nerve-fibres arise as prolongations of the ganglionic cells located in the retina, and that they enter into union with the central nervous apparatus only secondarily. He is strengthened in his opinion by the conditions in Petromyzon, which he declares to be one of the most valuable objects for the solution of the controversy concerning the origin of the optic nerve. I refer, moreover, in connection with this controversy, to the section which treats of the development of the peripheral nervous system (p. 452).

The Development of the Accessory Apparatus of the Eye

There are associated with the eyeball auxiliary apparatus, which serve in different ways for the protection of the cornea : the eyelids with the Meibomian glands and the eyelashes, the lachrymal glands and the lachrymal ducts.

The eyelids, the upper and under, are developed at an early period by the formation, at some distance from the margin, of the cornea, of two folds of the skin, which protrude beyond the surface. The folds grow over the cornea from above and below until their edges meet and thus produce in front of the eyeball the conjunctival sac, which opens out through the fissure between the lids. The sac derives its name from the fact that the innermost layer of the lid-fold, which is reflected on to the anterior surface of the eyeball at the fornix conjuiictme, is of the nature of a mucous membrane, and is designated as the conjunctiva, or connecting membrane, of the eye.

In many Mammals and likewise in Man there is during embryoniclife a temporary closure of the conjunctival sac. The edges of the lids become united throughout their whole extent, their epithelial investments fusing with each other. In Man the concrescence begins in the third month, and usually undergoes retrogression a short time before birth. But in many Reptiles (Snakes) the closure is permanent. Thus a thin transparent membrane is formed in front of the cornea.

In Man during the concrescence of the eyelids there are developed at their margins the Meibomian glands. The cells of the rete Malpighii begin to proliferate and to send into the middle connectivetissue plate of the eyelid solid rods, which afterwards become covered with lateral buds. The glands, at first entirely solid, acquire a lumen by the fatty degeneration and dissolution of the axial cells.

At about the time of the development of the Meibomian glands, the formation of the eyelashes takes place ; this corresponds with the development of the ordinary hair, and therefore will be considered along with the latter in a subsequent section of this chapter.

In most of the Vertebrates there is associated with the upper and under lids still a third, the nictitating membrane or membrana nictitaiis, which is formed at the inner [median] side of the eye as a vertical fold of the conjunctiva. In Man it is present only in a rudimentary condition as plica semilunaris. A number of small glands which are developed in it produce a small reddish nodule, the caruncula lacrymalis.

The lachrymal gland is an additional auxiliary organ of the eye, which is destined to keep the sac of the conjunctiva moist and the anterior surface of the cornea clean. In Man it is developed in the third month through the formation of buds from the epithelium of the conjunctival sac on the outer side of the eye, at the place where the conjunctiva of the upper lid is continuous with that of the eyeball. The buds form numerous branches, and are at first solid, like the Meibomian glands, but gradually become hollow, the cavity beginning with the chief outlet and extending toward the finer branches.

A special efferent lachrymal apparatus, which leads from the inner angle of the eye into the nasal cavity, has been developed for the removal of the secretions of the various glands collected in the conjunctival sac, but particularly the lachrymal fluid. Such an apparatus is present in all classes of Vertebrates from the Amphibia upward ; its development has been especially investigated by BORN in a series of researches.

In the Amphibia it begins to be formed at the time the process of chondrification becomes observa.ble in tho membranous nasal capsule. At that time the mucous layer of the epidermis, along a line that extends from the median side of the eye directly to the nasal cavity, undergoes proliferation and sinks into the underlying connectiveI issue layer as a solid ridge. Then from the nose to the eye the ridge becomes constricted off, subsequently acquires a lumen, whereby it is converted into a canal lined with epithelium, and opens out into the nasal cavity. Toward the eye-end the canal is divided into two tubules, which at the time of detachment from the epidermis remain in connection with the conjunctival sac and suck up out of it the lachrymal fluid.

In Birds and Mammals, including Man (fig. 273), the place where the lachrymal duct is located is early marked externally by a furrow which runs from the inner angle of the eye to the nasal chamber. By means of this furrow two ridges, which play an important part in the formation of the face, the maxillary process and the outer nasal process, are sharply marked off from each other ; these will engage our attention later. According to COSTE and KOLLIKER the lachrymal duct arises by the simple approximation and concrescence of the edges of the lachrymal groove. These older conclusions have been contradicted by BORN and LEGAL, one of whom has investigated Reptiles and Birds, the other Mammals. According to them there arises, in nearly the same manner as in Amphibia, through proliferation of the mucous epithelium, at the bottom of the lachrymal groove an epithelial ridge, which becomes detached but is not converted into a canal until a rather late period. When we raise the question, how phylogenetically the lachrymal duct may have first originated, we shall doubtless find that it has been derived from a groove, by means of which the sac of the conjunctiva and the nasal chamber are first put into connection. When, therefore, we see the lachrymal duct established from the very beginning simply as a solid ridge, as for example in the Amphibia, we must call to mind how in other cases also originally groove-like fundaments, such as the medullary furrow, make their appearance, under special circumstances, as solid ridges,

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Fig. 273. Head of a human embryo, from which the mandibular processes have been removed to allow a survey of the roof of the primitive oral cavity.

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Finally, as far as regards the development of the lachrymal tubules in Birds and Mammals, BORN and LEGAL refer the upper tubule to the proximal part of the epithelial ridge, and maintain that the lower one buds out from the upper. EWETSKY, on the contrary, declares that the proximal end of the epithelial ridge expands at the inner angle of the eye, and becomes divided by the ingrowth of underlying connective tissue, and metamorphosed into the two tubules, so that both arise from a common fundament.

Summary

1. The lateral walls of the primary fore- brain vesicle are evaginated to form the optic vesicles.
2. The optic vesicles remain united by means of a stalk, the future optic nerve, with that part of the primary fore-brain vesicle which becomes the bet ween -brain.
3. The optic vesicle is converted into the optic cup through the imagination of its lateral and lower walls by the fundaments of the lens and vitreous body.
4. At the place where the lateral wall of the primary optic vesicle encounters the outer germ-layer, the latter becomes thickened, then depressed into a pit, and finally constricted off as a lens-vesicle.
5. The cells of the posterior wall of the lens-vesicle grow out into lens-fibres, those of the anterior wall become the lens-epithelium.
6. The fundament of the lens is enveloped at the time of its principal growth by a vascular capsule (tunica vasculosa lentis), which afterwards entirely disappears.
7. The membrana capsulo-pupillaris is the anterior part of the tunica vasculosa lentis and lies behind the pupil.
8. The development of the vitreous body causes the choroid fissure.
9. The optic cup has double walls ; it consists of an inner and an outer epithelium, which are continuous with each other at the opening of the cup, which embraces the lens, and at the choroid fissure.
10. Mesenchymatic cells from the vicinity grow in between the lens and the somewhat closely applied epidermis to form the cornea and DESCEMET'S membrane, the latter being separated from the tunica vasculosa lentis by a fissure, the anterior chamber of the eye.
11. The optic cup is differentiated into a posterior portion, within the territory of which its inner layer becomes thickened and constitutes the retina, and an anterior portion, which begins at the ora serrata, becomes very much reduced in thickness, and extends over the front surface of the lens, growing into the anterior chamber of the eye until the originally wide opening of the cup is reduced to the size of the pupil.
12. The anterior attenuated portion of the cup is, in turn, divided into two zones, of which the posterior becomes folded at the periphery of the equator of the lens to form the ciliary processes, whereas in front it remains smooth ; so that in the whole cup three parts may now be distinguished, as retina, pars ciliaris, and pars iridis retinae.
13. Corresponding to the three portions of the epithelial optic cup, the adjoining connective -tissue envelope takes on somewhat different conditions as the choroid proper, and as the connective-tissue framework of the ciliary body and that of the iris.
14. The skin surrounding the cornea becomes infolded to form the upper and lower eyelids and the nictitating membrane, of which the last is rudimentary in Man, persisting only as the plica semilunaris.
15. The epithelial layers of the edges of the two eyelids grow together in the last months of development, but become separated again before birth.
16. The lachrymal groove in Mammals passes from the inner angle of the eye, between the maxillary and outer nasal processes, to the nasal chamber.
17. The lachrymal duct for carrying away the lachrymal fluid is formed by the downgrowth and constricting off of an epithelial ridge from the bottom of the lachrymal groove, the ridge becoming hollow.
18. The two lachrymal tubules are developed by the division of the epithelial ridge at the angle of the eye.

The Development of the Organ of Hearing

In the case of the ear numerous parts of quite different origin unite, in much the same manner as in the case of the eye, to form a single very complicated apparatus ; of these, too, it is the portion to which the auditory nerve is distributed the membranous labyrinth with its auditory epithelium that is by far the most important, outstripping as it does all the remaining parts in its development : it must consequently be considered first.

The Development of the Otocyst into the Labyrinth

The membranous labyrinth is preeminently a product of the outer germ-layer. However great its complication in the adult is, a complication that has given it the name labyrinth, its earliest fundament is exceedingly simple. It arises on the dorsal surface of the embryo in the region of the medulla oblongata (fig. 263 gb), above the first visceral cleft and the attachment of the second visceral arch (fig. 274 above the numeral 3). Here over a circular territory the outer germ-layer becomes thickened and soon sinks down into an auditory pit. This process can be traced very easily in the embryo Chick on and after the end of the second day of incubation, and in the embryo Rabbit fifteen days old. The auditory nerve makes its way from the brain, near at hand, to the fnndus of the pit, where it terminates in a ganglionic enlargement.

The Bony Fishes alone exhibit a deviation from these conditions. Just as the central nervous system was in their case formed not as a tube, but as a solid body, and the eye not as a vesicle, but as an epithelial ball, so we see here that instead of an auditory pit there is formed by means of the proliferation of the outer germ-layer a solid epithelial plug. This, like the brain-tube and the eye-vesicle, acquires an internal chamber at a later period only namely, after being constricted off.

The next stage shows the pit converted into an auditory vesicle. In the Chick this takes place in the course of the third day. The invagination of the outer germ-layer grows deeper and deeper, and by the approximation of its margins becomes pear-shaped ; soon the connection with the outer germ-layer becomes entirely lost, as is shown by a section through the head of an embryo Sheep (fig. 275 Ib).

In nearly all Vertebrates the auditory vesicle is constricted off from the ectoderm in the same manner. The Selachians are an exception : here the auditory vesicle which is metamorphosed into the labyrinth retains permanently its connection with the surface of the body in the form of a long narrow tube, which traverses the cartilaginous primordial cranium and is in union dorsally with the epidermis at the sin-face of the body, where it possesses an external opening.

Fig. 274. Head of a human embryo 7.5 mm. long, neck measurement. From His, "Menschliehe Embryonen." The auditory vesicle lies atove the first visceral cleft. In the circumference of the visceral cleft there are to be seen six elevations, designated by numerals, from which the external ear is developed.

In its first fundament the organ of hearing in Vertebrates resembles in the highest degree those structures wliicli, in the Invertebrates are interpreted as organs of hearing. These are lymph-filled vesicles lying under the skin, which are likewise developed out of the epidermis. Either they are wholly constricted off from the epidermis, or they remain connected with it by means of a long, ciliate, epithelial canal, as in the Cephalopods, even after they have become surrounded by connective tissue. In both cases the vesicles are lined with epithelium which consists of two kinds of cells : first of low, flat elements, which ordinarily exhibit ciliary movements and thereby put in motion the fluid within the vesicle, and secondly of longer cylindrical, or thread-like, auditory cells with stiff hairs, which project into the endolymph. The auditory cells are either distributed individually over the inner surface of the auditory vesicle or arranged in groups, or they are united at a particular place into an auditory epithelium, the auditory patch (macula acustica) or the auditory ridge (crista acustica), which may be either single or double. To all the auditory vesicles of the Invertebrates there is sent, moreover, a nerve which ends at the sensory cells in fine fibrillre. Finally, there is present as a characteristic structure a firm, crystalline body, the otolith, which is suspended in the midst of the endolymph and is ordinarily set in vibration by the motion of the cilia. It consists of crystals of phosphate or carbonate of lime.

Sometimes there is only a single large, in most cases concentrically laminated, spherical body, sometimes a number of small calcareous crystals, which are held together by means of a soft pulpy substance.

Fig. 275. Vertical cross-section through the vesicle of the labyrinth of an embryo Sheep 1-3 cm. long, after BOETTCHER. Magnified 30 diameters. wh, Wall of the after-brain ; rl, recessus labyrinthi ; Ib, vesicle of the labyrinth ; gc, ganglion cochleare, which is in contact with a part of the labyrinth-vesicle (dc) that grows out into the ductus cochlearis.

It is difficult to follow the formation of the otoliths within the otocyst. In one case, which FOL was able to follow, they were developed by an epithelial cell in the wall of the vesicle. The cell secretes small calcareous concretions in its protoplasm, becomes enlarged in consequence, and protrudes as an elevation into the endolymph. When it has become more heavily loaded with calcic salts, it is connected with the wall by means of a stalk only, and finally it becomes entirely detached from the wall and falls into the cavity of the vesicle, in which it is kept floating and rotating by the ciliate cells.

In Vertebrates the otocyst, which, as we have seen, agrees in its first fundament with the organ of hearing in Invertebrates, is converted into a very complicated structure, the membranous labyrinth, -the evolution of which in Mammals I shall describe in some detail. It undergoes metamorphoses, in which the formation of folds and constrictions plays the principal part (fig. 276). The auditorv sac detached from the epidermis, and lying at the side of the after-brain, soon exhibits a small, dorsally directed projection, the recessus labyrinthi or ductus endoly niphaticus (fig. 275 rl). Probably we have to do in this with the remnant of the original stalk by means of which the auditory vesicle was connected with the epidermis. According to some investigators, on the contrary, the .stalk disappears entirely and this evagination is a new structure. The first assumption is favored especially by the previously mentioned condition in the Selachians the presence of a long tube, which maintains a permanent connection between labyrinth and epidermis.

Fig. 276. Membranous labyrinth of the left side of a [human] embryo, after a wax model by KRAUSE.

//, Recessus labyrinth! ; dc, ductus cochlearis ; hb, pocket from which the horizontal semicircular canal is formed ; a in', enlargement of the pocket which becomes the ampulla of the horizontal canal ; am (rb), vb', * common pocket from which the two vertical semicircular canals are developed ; am (vb), enlargement of the common pocket from which the ampulla of the anterior vertical canal arises. An opening (w) has been formed in the pocket, through which one sees the recessus labyrinthi. * Region of the pocket which becomes the common arm of the two vertical canals (sinus superior) ; rb', part of the common pocket which furnishes the posterior vertical canal.

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Later this appendage of the labyrinth (figs. 276-9 rl) grows out dorsally to a great length, during which its walls come into dose contact with each other, excepting at the blind end, which is enlarged into a small sac (fig. 279 rl*).

Meanwhile the auditory sac itself (figs. 275-7) begins to be elongated and to be formed into a ventrally directed conical process (dc), the first fundament of the ductus cochlearis, which is curved inward a little toward the brain (fig. 277 nh), and the concave side of which lies in close contact with the previously mentioned ganglionic enlargement (yc) of the auditory nerve (hn).

Fig, 277. Cross section through the head of a Sheep embryo 1'6 cm. long, in the region of the labyrinth-sac. On the right side is represented a section which passes through the middle of the sac ; on the left, one that is situated somewhat farther forward. After BOETTCHER.

hn, Auditory nerve ; vb, vertical semicircular canal ; gc, ganglion cochleare (spirale) ; dc, ductus cochlearis ; /, inward-projecting fold, whereby the sac of the labyrinth is divided into utriculus and sacculus ; rl, recessns labyrinth! ; tth, after-brain.

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It will be serviceable in the following description if we now distinguish an upper and a lower division of the labyrinth. They are not yet, it is true, distinctly delimited from each other, but in later stages they become more sharply separated by an inward-projecting fold (figs. 277-9/).

The upper part (pars superior} furnishes the utriculus and the semicircidar canals. Of the latter the two vertical canals arise first, the horizontal canal being formed later. The method of their origin was early ascertained by the zoologist RATHKE in the case of Coluber. Recently KRAUSE has still further elucidated the interesting processes by the construction of wax models of the conditions in mammalian embryos.

As is to be seen from the various sections (figs. 277, 278), but still better from the model (fig. 276) produced by reconstruction, the semicircular canals are developed by the protrusion of several evaginations of the wall of the sac, which have the form of thin pockets or discs (hb, vb) with a semicircular outline. The marginal part of each such e vagination now becomes con side i 1 ably enlarged, whereas the remaining portions of the two epithelial layers come into close contact and begin to fuse. As the result of this simple process -the enlargement at the margin and the fusion of the walls which takes place in the middle there is formed a semicircular canal, which communicates at two places with the original cavity of the vesicle. At one of its openings the canal is early enlarged into an ampulla (fig. 276 am and am'}. The middle part, in which the fusion has taken place, soon disappears, the epithelial membrane being broken through by a growth of the connective tissue (fig. 276 6).

There exists an interesting difference between the development of the horizontal and the two vertical canals, which was discovered by KRAUSE. Whereas the horizontal canal is established as a small pocket by itself (fig. 276 hb), the two vertical canals arise together from a single large pocket-like fundament (fig. 276 am (vb), *, vb').

Fig. 278. Cross section through half of the head of a foetal Sheep 2 cm. long, in the region of the labyrinth, after BOETTCHEK. Magnified 30 diameters.

rl, Recessus labyrinth! ; vb, hb, vertical and horizontal semicircular canals; U, utriculus ; f, inward-projecting fold, by which the labyrinth-sac is divided into utriculus and sacculus ; dc, ductus cochlearis ; yc, ganglion cochleare.

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The walls of this large pocket come into contact with each other and fuse at two different places. At one of them there has already been formed, in the preparation from which this model (fig. 276) was constructed, an opening (o) by the resorption of the fused epithelial areas, whereas at the second place (vb 1 ) the epithelial membrane is still preserved. Between the fused parts of the pocket there remains open a middle region, which is indicated in the model by an asterisk, and this becomes the common arm (sinus superior) of the two vertical canals. Thus embryology furnishes for this peculiarity, too, a simple satisfactory explanation.

Fig. 279. View produced by combination from two cross sections through the labyrinth of a Sheep embryo 2-8 cm. long, after BOETTCHKR. rl, Recessus labyrinth! ; rl*, its flask-like enlargement ; vb, hb, vertical and horizontal canals ; U, utriculus ; S, sacculus ; /, fold by means of which the labyrinth is divided into saccuhis and utriculus ; cr, canalis reunions ; dc, ductus cochlearis ; kk, cartilaginous capsule of the cochlea ; sp, sinus petrosus inferior.

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That which remains of the upper portion of the auditory vesicle, after the semicircular canals have grown forth from its wall, is called the utriculus (figs. 278-80 U).

Meanwhile no less significant and fundamental alterations take place in the lower part of the auditory sac and lead to the formation of sacculus and ductus coc/ilearis.

By a continually deepening constriction (fig. 279 /) the lower portion (S) is delimited from the utriculus (U), and finally remains connected with it by a very narrow tubule only (canalis utriculo-saccularis figs. 280 R and 282 2 ). Since the constriction affects exactly that place of the labyrinth-sac from which the recessus labyrinth! arises, the opening of the latter subsequently comes to lie within the territory of the canalis utriculo-saccularis, at about its middle (figs. 280 R and 282 2 ). In this manner there is produced an appearance as though the recessus labyrinthi were split at its beginning into two narrow tubules, one of which leads into the sacculus, the other into the utriculus.

By a second deep constriction (figs. 279, 280, 282) the sacculus (S) is separated from the developing ductus cochlearis (dc). Here also a connection is maintained by means of an extraordinarily fine connecting tubule only (cr), which HENSEN discovered and has described as canalis reuniens. The ductus cochlearis itself increases greatly in length, and at the same up in spiral turns in the soft, envt loping, embryonic connective tissue, until in Man it describes two and a half turns (figs. 280 C and 282 Con). Since the first whorl is the largest, and the others are successively narrower, it acquires a great resemblance to a snail-shell.

Fig. 280. Diagram to illustrate the ultimate condition of the membranous labyrinth, after WALDEYER.

U, Utriculus ; S, sacculus; Cr, canalis reunions ; R, recessus labyrinthi ; C, cochlea ; K, blind sac of the cupola ; V, vestibular blin 1 sac of the ductus cochlearis.

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The alterations in the external form of the vesicle are accompanied by changes in the nature of its epithelium also. This is separated into the indifferent epithelial cells, which simply serve as a lining, and the real auditory cells. The former are flattened, becoming cubical or scale-like, and cover the greater part of the inner surface of the semicircular canals, the sacculus, the utriculus, the recessus labyrinthi, and the ductus cochlearis. The auditory cells, on the contrary, are elongated, become cylindrical or spindle-shaped, and acquire at the free surface hairs, which project into the endolymph. By the separation of the vesicle into its various divisions the auditory epithelium is distributed into an equal number of separate patches, to which then the auditory nerve is distributed. Accordingly the auditory epithelium is resolved into a macula acustica in the sacculus and another in the utriculus, into a crista acustica, in each of the ampulla} of the semicircular canals, and into an especially complicated termination in the ductus cochlearis. Hero the auditory epithelium grows out into a long spiral band, which is known under the name of CORTI'S organ.

Upon the separation of the auditory epithelium into maculae, cristae, and organ of CORTI, the originally single auditory nerve distributed to the auditory vesicle is likewise resolved into separate branches. We distinguish in the case of the auditory nerve the nervus vestibuli, which is in turn divided into numerous branches distributed to the maculae and cristse, and the nervus cochlece.

The originally single ganglion acusticum belonging to the auditory nerve also becomes differentiated into two separate portions. The portion belonging to the nervus vestibuli is in the adult located in the internal auditory meatus far from the terminal distribution, forming here the well-known intumescentia gangliformis Scarpae ; the portion belonging to the nervus cochleae, on the contrary, adjoins the terminal distribution of the nerve. In the embryo it (figs. 277, 278 gc) is closely united with the fundament of the ductus cochlearis, and as the latter increases in size grows out to the same extent in the form of a thin band, which reaches to the blind end of the ductus and is known under the name of ganglion spirale (fig. 283 gsp).

(b] Development of the Membranous Ear-Capsule into the Bony Labyrinth and the Perilymphatic Sj)aces.

All of the changes which have been mentioned hitherto have proceeded from the epithelial vesicle which was constricted off from the outer germ-layer. It is now my purpose to direct attention to a series of processes which take place around the epithelial cavities, in the mesenchyme in which they are imbedded. The processes lead to the formation of the bony labyrinth, the perilymphatic spaces and soft connective-tissue layers, which are intimately joined to the purely epithelial structures hitherto treated of, and with the latter are embraced in descriptive anatomy under the name of membranous labyrinth. Changes take place here similar to those in the development of the neural tube and of the eye, in which cases also the connective-tissue surroundings are modified in a special manner and with reference to the epithelial parts. In the present instance there are produced structures which are comparable with those existing in the former cases, as has already been pointed out by Ko" LLIKER, SCHWALBE, and others.

The comparison may be carried into details. The parts arising from the primitive auditory vesicle are at first surrounded by a soft, vascular connective-tissue layer, as the neural tube and the epithelial optic cup are. To the pia mater of the brain corresponds the vascular membrane of the eye and the soft ear-capsule, or the connective-tissue wall of the membranous labyrinth. Around all three organs a firm envelope has been developed for the purpose of protection ; around the brain the dura mater with the cranial capsule, around the eye the sclerotica, and around the organ of hearing the bony labyrinth with its periosteum. To these is to be added still a third noteworthy agreement. In all three cases the soft and firm envelopes are separated by more or less considerable fissure-like spaces, which belong to the lymphatic system. Around the neural tube the subdural and the subarachnoid spaces are found, around the eye the perichoroid fissure, around the organ of hearing the perilymphatic spaces, which have received in the cochlea the special names of scalse (fig. 283 ST and SV}.

The details of the formation of the enveloping structures around the epithelial auditory vesicle are as follows : Soon after the auditory sac is constricted off from the epidermis it is enveloped on all sides by a richly cellular mesenchyme, the individual cells of which lie in an extremely scanty, soft, and homogeneous intercellular substance, and possess each a large nucleus with a thin protoplasmic covering having short processes. Gradually the envelope is differentiated into two layers (figs. 279, 281). In the vicinity of the epithelial canals the soft intercellular substance increases in amount ; the cells become either stellate or spindle-shaped, in the former case sending out long processes in various directions. There is formed here that modification of connective substance known as mucous or gelatinous tissue (figs. 281 and 283 </), in which there are also blood-vessels. Outside of this the cells remain smaller and more closely crowded together, and are separated from one another by thin partitions of a firm intermediate substance. With an increase of the latter the tissue soon acquires the character of embryonic cartilage (&&).

The further changes must be followed separately in the semicircular canals, the utriculus and sacculus and the ductus cochlearis,

The throe semicircular canals do not lie exactly in the middle of the cavities of the embryonic cartilage containing the gelatinous tissue, but are so situated that their convex borders are in almost immediate contact with the cartilage, whereas their concave sides are separated from it by a thick layer of gelatinous tissue. The latter is differentiated into three layers : into a middle portion, in which the gelatinous intercellular substance is greatly increased in volume, and becomes at the same time more fluid, and into two limiting layers, which are converted into fibrous connective tissue. One of the two [the inner] is intimately united to the epithelial tube, for the nutrition of which it provides by means of a close network of blood-vessels distributed through it ; the other [the outer] lies on the inner surface of the cartilaginous envelope and becomes its perichondrium.

Fig. 281. Section through the cochlea of a Sheep embryo 7 cm. long, after BOKTTCHER. Magnified 20 diameters.

/,/, Cartilaginous capsule of the cochlea ; S, sacculus with the nerve (ns) distributed to it ; U, utricle ; gs, ganglion connected with the cochlear nerve (nc) and sending nerve-fibres (ns) to the sacculus ; gsp, ganglion spirale ; dc, ductus cochlearis ; C, CORTI'S organ ; g, gelatinous tissue in the periphery of the ductus cochlearis ; x, more compact connective-tissue layers.

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The gelatinous tissue of the middle layer is of only short duration. It soon shows signs of degeneration. The stellate cells become filled with fat granules in the vicinity of their nuclei and in their long processes ; later they disintegrate. In the gelatinous matrix there are formed, by a continually advancing process of softening, cavities filled with fluid. These increase in size and then become confluent, until finally there has arisen between the connective-tissue membrane of the semicircular canals and the perichondrium, in place of the gelatinous tissue, a large space filled with perilymph, which is indicated in the diagram, fig. 282, in black. Here and there, however, connective-tissue cords remain running from one layer of connective tissue to the other, and serving as bridges for the nerves and bloodvessels which are distributed to the semicircular canals.

Finally, a last alteration takes place in the cartilaginous envelope by its becoming converted into bone-substance by endochondral ossification. Thus the membranous semicircular canals are enclosed in the bony semicircular canals (fig. 282 a and 1> KL], which are enlarged reproductions of the former.

Fig. 282. Diagrammatic representation of the whole organ of hearing in Man, from WIEDKKSHEIM.

Outer ear: M, M, auricle; Mae, meatus auditorius externus ; 0, its wall; Mt, membrana tympani. Middle car: Ct, Ct, cavum tympani ; O' , its wall; SAp, sound-conducting apparatus, which i.s drawn as a simple rod-like body in place of the auditory ossicles ; the place f corresponds to the stapedial plate, which closes the fenestra ovalis ; Tb, tuba Eustachii ; Tb 1 , its opening- into the pharynx; 0", its wall. Inner car: the bony labyrinth (KL, KL') for the most part cut away ; S, sacculus ; a, b, the two vertical membranous and osseous semicircular canals ; S.e, D.e, saccus and ductus endolymphaticus, of which the latter is divided at 2 into two arms ; Cp, cavum perilyinphaticum ; Cr, canalis reunions ; Con, membranous cochlea, which produces at + the vestibular cceciun ; Con 1 , bony cochlea ; Sv and St, scala vestibuli and scala tympini, which at * communicate with each other at the cupula tenninalis (Ct) ; D.p, ductus perilymphaticus, which arises from the scala tympani at (I and opens out at D.p 1 . The horizontal semicircular canal is not specially designated, but is easily recognisable.

Corresponding changes (fig. 282) are also accomplished in the periphery of the utriculus and sacculus (#), and lead to the formation of (1) a perilymphatic space (Cp), which is in communication with the perilymphatic spaces of the semicircular canals, and (2) a bony envelope (A"7/') of the atrium or vestibulum, which constitutes the middle region of the bony labyrinth.

The envelope of the epithelial cochlear duct, which becomes the bony cochlea with its scalse, undergoes a more complicated alteration. It is already differentiated, at the time when the duct (fig. 279 dc) makes only half of a spiral turn, into an inner, soft and an outer, firm layer, the latter becoming cartilage (kk). The cartilaginous capsule (fig. 281 kk), which is continuous with the cartilaginous mass of the remaining parts of the labyrinth and together with them constitutes a part of the ospetrosum, afterwards encloses a lenticular cavity and possesses below a broad opening, through which the cochlear nerve (no) enters. The resemblance to a snail-shell is not yet observable ; it takes place gradually and is produced by two changes : by the outgrowth of the epithelial duct and by the differentiation of the soft tissue surrounding it into parts which are fluid and such as become more firm.

In its outgrowth the epithelial ductus cochlearis describes within its capsule the previously mentioned spiral turns (dc), shown in cross section in fig. 283 ; at the same time it remains quite closely approximated to the inner surface of the capsule (kk). The cochlear nerve (nc) ascends from its place of entrance straight up through the centre of the turns, consequently in the axis of the capsule, and gives off numerous lateral branches to the concave side of the cochlear duct (dc), where they are enlarged into the ganglion (gsp), which has now also grown out into a spiral band. The nutritive blood-vessels have taken the same course as the nerves.

When the development has advanced as far as this, there still remains to be accomplished only an histological differentiation in the soft mesenchyme which fills the cartilaginous capsule in order to produce the parts of the finished cochlea that are still wanting the modiolus, the lamina spiralis ossea, the bony cochlea, and the vestibular and tympanic scalse (fig. 283). Here, as in the vicinity of the semicircular canals the utriculus and the sacculus, the mesenchyme is differentiated into a firmer connective substance, which becomes fibrous, and into a gelatinous tissue (g), which is continually becoming softer. Fibrous connective substance is developed first around the trunks of the nerves (nc) and blood-vessels that enter the cartilaginous capsule ; furnishing the foundation of the future bony axis of the snail-shell (M), secondly it furnishes an envelope for nerve-fibres (N) that run from the axis to the epithelial cochlear duct, for the ganglionic cells (ysp), and for the blood-vessels, and constitutes a connectivetissue plate which is subsequently ossified to form the lamina spiralis ossea. Thirdly, it clothes with a thin layer the epithelial ductus, serving for the distribution of the blood-vessels on the latter, and together with it is designated as the membranous ductus cochlearis. Fourthly, it lines the inner surface of the cartilaginous capsule as perichondrium (P). Finally, fifthly, there is formed a connective tissue plate (Y) extending between the cartilaginous ridge which, as previously described, projects inward from the capsule and the connective -tissue axis of the cochlea (M). It is stretched out between and separates the successive turns of the membranous cochlear duct, so that the latter now comes to lie in a large canal, the wall of which is in part cartilaginous, in part membranous. This canal is the foundation of the bony cochlea.

That portion of the mesenchyme which is not converted into fibrous connective tissue becomes gelatinous tissue (g and </). It forms bet\veen the parts just mentioned two spiral tracts, one of \vhich is located above and the other below the membranous ductus cochlearis and the membranous lamina spiralis. The tracts therefore occupy the place of the scala vestibuli (SV) and the scala tympani (>ST). The latter arise, even before the process of ossification begins, in exactly the same w r ay as the perilymphatic spaces in the case of the semicircular canals and the vestibule. In the gelatinous tissue the matrix becomes softer and more fluid, and the cells begin to undergo fatty degeneration. Small fluid-filled cavities make their appearance ; these become joined to one another, and finally the \vhole space occupied by gelatinous tissue is filled with perilymph. The process of softening begins at the base of the cochlea in the region of the first spiral (ST and SV), and advances slowly toward the cupola. Here vestibular and tympanic scalse finally unite, after the last remnant of the gelatinous tissue has been dissolved. Figure 283 exhibits a stage in which, at the base of the cochlea, the perilymphatic spaces (SV and >ST) have been formed, and only small remnants of the gelatinous tissue ((/') are present, whereas at the apex of the cochlea the process of liquefaction of the gelatinous tissue (y) has not yet taken place.

With the development of the scala? the membranous ductus cochlearis changes form. Whereas its cross section was formerly oval, it now assumes the form of a triangle (dc). For those portions of the wall which are adjacent to the vestibular and tympanic scalse, and which have been named from them, gradually become flattened, and are stretched out smoothly between the free margin of the lamina spiralis and the inner wall of the cartilaginous capsule. In this process the tympanic wall (C) comes to lie in the same plane as the lamina spiralis, the vestibular wall (Iv) forms with the tympanic an acute angle, and the third wall (x) is everywhere in close contact with the perichondrium of the cartilaginous capsule.

Fig. 283. Part of a section through the cochlea of an embryo Cat 9 cm. long, after BOETTCHER.

kk, Cartilaginous capsule, in which the cochlear duct describes ascending spiral turns; dc, ductus cochleares ; C, organ of Coim ; ie, lamina vestibularis ; x, outer wall of the membranous ductus cochlearis with ligamentum spirale ; SV, scala vestibuli ; ST, ST', scala tympani ; g, gelatinous tissue, which still fills the scala vestibuli (sv') in its last turns ; g', remnant of the gelatinous tissue, which is not yet liquefied ; M, firm connective tissue surrounding the cochlear nerve (nc) ; gsp, ganglion spirale ; N, nerve which runs to CORTI'S organ in the future lamina spiralis ossea ; Y, compact connective-tissue layer, which becomes ossified and shares in bounding the bony cochlear duct ; P, perichondrium.

The epithelial lining of the membranous duct us cochlearis assumes very different conditions in the three corresponding regions of its wall. Whereas the epithelial cells of the vestibular and the outer walls become in part cubical, in part quite flat, those of the tympanic wall become elongated, and are in connection with the terminal filaments of the cochlear nerve ; they produce the complicated organ of CORTI (C), which, like the auditory ridges and auditory patches of the ampullre, the sacculus and utriculus, contains the terminal ends of the auditory nerve.

The construction of the intricate cochlea approaches completion with the beginning of the process of ossification. The latter is accomplished by two methods. First, the cartilaginous capsule ossifies in the endochoiidral manner, as does the whole cartilaginous os petrosum, of which it constitutes a small part. The osseous tissue thus formed is for a long time spongy and provided with large medullary spaces. Secondly, the previously mentioned fibrous connective-tissue layersthe partitions between the cochlear canals, the connective-tissue axis or the niodiolus and the lamina spiralis undergo direct ossification. At the same time compact bone-lamell?e are laid down from within on the spongy bone-tissue formed from the cartilaginous capsule; these lamelke are formed, as BOETTCHER has shown, by the original perichondrium, which becomes the periosteum. Consequently the bony cochlear capsule, since it is produced by periosteal secretion, may be easily detached from the loose osseous tissue of endochoiidral origin during early post-natal years.

Development of the Accessory Apparatus of the Organ of Hearing

(Middle and External Ear.)

With the membranous and bony labyrinth, which are together called the inner ear, there is associated a subsidiary apparatus, in the same way that the eye-muscles, the lids, and the lachrymal glands and ducts are added to the eyeball. It is made up of structures which are wanting in the lower Vertebrates (Fishes), but, beginning to be developed in the Amphibia, become more and more complete in the higher forms. Their function is to transmit vibrations to the labyrinth, and consequently they are together called the conducting apparatus. From their position they are also known as middle and outer ear. The former consists in Mammals, where it attains its highest development (diagram, fig. 284), of the tympanic cavity (67), the Eustachian tube (Tb), and the three auditory ossicles ($Ap] ; the latter, of the tympanic membrane (M), the external meatus (Mae), and the external ear or auricle (M). The statement just made, that these parts are wanting in Fishes, is to be taken cum grano salis : it is as a sound-conducting apparatus only that they are wanting, for they are present even in the case of Fishes, but only as structures of a different function and in a more simple condition. For the various accessory apparatus of the, organ of hearing are developed out of the first visceral cleft and certain parts which are located in its periphery.

Here also it will be well to acquaint ourselves with the originalthe initial condition, for which the Selachians may serve as an example.

In them the greater part of the first visceral cleft, which is situated between the mandibular and hyoid arches and between the nervus trigeminus and n. acustico-facialis, disappears ; at the side of the throat it becomes closed, remaining open only at the origin, or base, of the two visceral arches. It then has the form of a short canal, which possesses a small round opening at its inner and another at its outer end, and which passes in very close proximity to the labyrinth-region of the skull, in which the organ of hearing is located. The canal, here called the spiracle, has no longer anything to do with respiration, since the branchial leaflets on its wall have undergone degeneration. Owing to its position in the immediate vicinity of the labyrinth, it presents, even in the Selachians, the best course for the propagation of the sound-waves to the inner ear, and this is the chief ground for its entering wholly into the service of the organ of hearing in the remaining Vertebrates, and for its being developed in a more serviceable manner for this particular function.

The structures in the higher Vertebrates corresponding to the spiracle of the Selachians are (fig. 284) the tympanic cavity (Ct), the Eustachian tube (77>), and the external meatus (Mae). They likewise are developed out of the upper part of the first visceral cleft. Although it lias recently been asserted by certain investigators (URBANTSCHITSCH) that they have nothing to do with the first visceral cleft, but are established independently by the evagination of the pharynx, this view is opposed not only to comparativeanatomical considerations, but also to statements of KOLLIKER, MOLDENHAUER, and HOFFMANN, which relate to the development in Reptiles, Birds, and Mammals.

In the classes of Vertebrates just mentioned the first visceral cleft is closed in its upper part also, contrary to the condition in Selachians.* The closure becomes more firm and complete owing to the ingrowth of a connective-tissue layer between the inner and outer epithelial plates. Remnants of the first visceral cleft are preserved on both sides of the closing membrane as depressions of greater or less depth ; an inner one on the side toward the pharyngeal cavity, and an outer one which is surrounded by ridges of the first and second visceral arches.

• See the statements discussed in a previous chapter (p. 2X7), concerning the mooted question whether the visceral clefts remain closed by means oi' an epithelial membrane or are temporarily open.

Fig. 284. Diagrammatic representation of the whole organ of hearing in Man, from WIKDEKSHEIM.

Outer car: M, M, auricle; Mac, meat/us axiditorius externus ; O, its wall; Mt, membrana tympani. Middle car: Ct, Ct, cavum tympani ; O 1 , its wall; SAj>, sound-conducting apparatus, which is drawn as a simple rod-like body in place of the auditory ossicles; the place t corresponds to the stapedial plate, which closes the fenestra ovalis ; Tb, tuba Eustachii ; Tb 1 , its opening into the pharynx; 0", its wall, fiitu .rmr: the bony labyrinth (KL, A'Z 1 ) for the most part cut away ; S, sacculus ; a, b, the two vertical membranous and osseous semicircular canals ; S.e, D.e, saccus and ductus endolymphaticus, of which the latter is divided at 2 into two arms ; Cp, cavum perilymphaticum ; Or, canalis reuiiiens ; Con, membranous cochlea, which produces at -t- the vestibular ccecuru ; Con 1 , bony cochlea ; Sv aad St, scala vestibuli and scala tympani, which at * communicate with each other at the cupula terminalis (Ct) ', D.z>, ductus perilymphaticus, which arises from the scala tympani a b d and opens out at D.p\ The horizontal semicircular canal is not specially designated, but is easily recognisable.

The inner depression, which is called canalis or sulcus tubo-tym panicus (pharyngo-tympanicus), is located, like the spiracle, between trigeminus and acustico-faeialis, It becomes the middle ear, and is enlarged by an evagination that is directed upward, outward, and backward. The evagination inserts itself between the labyrinth and the place of closure of the first visceral cleft, and takes the form of a laterally compressed space, which is now to be distinguished as tympanic cavity from the tubular remnant of the sulcus tympanicus, or Eustachian tube. Its lumen is very small, especially in the case of advanced embryos of Man and Mammals, its lateral and median walls being almost in immediate contact. This results chiefly from the fact that there is present beneath the epithelial lining of the middle ear a richly developed gelatinous tissue. The latter still encloses at this time structures, the auditory ossicles and the chorda tympani, which later come to lie, as it were, free in the tympanic cavity.

The tympanic membrane also is now in a condition very unlike that which it afterwards acquires. The history of its formation is by no means so simple as was formerly supposed. For it is not derived exclusively from, the narrow closing membrane of the first visceral cleft ; the neighboring parts of the first and second membranous visceral arches also participate in its production. The embryonic tympanic membrane is therefore at first a thick connective-tissue plate, and encloses at its margins the auditory ossicles, the tensor tympani, and the chorda tympani. The reduction in the thickness of the tympanic membrane takes place at a late period, simultaneously with an increasing enlargement of the tympanic cavity. Both are brought about by shrinkage of the gelatinous tissue, and by an accompanying growth of the mucous membrane lining the tympanic cavity. Wherever the gelatinous tissue disappears the mucous membrane takes its place, inserting itself between the individual ossicles and the chorda tympani, which thus come to lie apparently free in the tympanic cavity. In reality, however, they lie outside of it, for they continue to be clothed on all sides by the growing mucous membrane, and are connected with the wall of the tympanic cavity by means of folds of that membrane (malleusfold, incus-fold, etc.), in much the same manner as the abdominal organs which grow into the body-cavity are invested by the peritoneum and supported from its walls by the mesenteries.

With a reduction in the thickness of the tympanic membrane there occurs a condensation of its connective-tissue substance, whereby it is enabled to fulfil its ultimate function as a vibrating membrane.

A more extended discussion of the development of the auditory ossicles will be deferred to a subsequent section, which deals with the origin of the skeleton. At present, only a few words further concerning the formation of the external ear, which, as has already been stated, is derived from a depression on the outer side of the place of closure of the first visceral cleft. Its development has been minutely investigated in the Chick by MOLDENHAUER and in the human embryo by His. As the lateral view of a very young human embryo (fig. 274) shows, the first visceral cleft is surrounded by ridge-like margins, which belong to the first and second visceral arches, and are early divided into six elevations designated by Arabic numerals. From these is derived the auricle, which therefore involves a rather extensive tract of the embryonic head (the pars auricularis). The pocket between the ridges, at the bottom of which the tympanic membrane is met with, becomes the external meatus, This is continually growing deeper owing to the surrounding wall of the side of the face becoming greatly thickened ; finally it is developed into a long canal, the wall of which is in part bony, in part cartilaginous. The six elevations mentioned, which surround the orifice of the external ineatus, together constitute a bulky ring. The accompanying representation (fig. 285) shows clearly its metamorphosis into the external ear. It shows that out of the elevations 1 and 5 the tragus and antitrasrus are developed, out of 2 and 3 the helix, and out of 4 the antihelix. The lobule of the ear remains for a long time small ; it is not until the fifth month that it becomes more distinct. It is derived from the hillock marked with the numeral 6. At the close of the second month all the essential parts of the external ear are easily recognisable ; from the third month onward tho upper and posterior part of the auricle grows out more from the surface of the head ; and it acquires greater firmness upon the differentiation of the auricular cartilage, which had already begun at the end of the second month,

Fig. 285. Fundament of the outer ear of a human embryo, after His. The elevation marked 1 produces the tragus ; 5, the antitragus. The elevations 2 and 3 produce the helix ; 4, the antihelix. From the tract 6 is formed the lobule. K, Lower jaw.

Summary

1. The most essential part of the organ of hearing, the membranous labyrinth, is developed at the side of the after -brain above the first visceral cleft from, a pit-like depression of the outer germlayer.
2. By closure the auditory pit becomes the auditory vesicle ; it sinks down and becomes imbedded in embryonic connective tissue, from which the cranial capsule is subsequently developed.
3. The auditory vesicle acquires the complicated form of the membranous labyrinth by various evaginations of its wall, and becomes differentiated into the utriculus, with the three semicircular canals, into the sacculus with the canalis reunions and the cochlea, as well as into the recessus vestibuli, by means of which sacculus and utriculus remain permanently connected with each other.
4. The auditory nerve and the auditory epithelium, which are at first single, are likewise divided as soon as the vesicle is differentiated into a number of regions into several nerve-branches (nervus vestibuli, n. cochlea?) and nerve-terminations (the cristse acusticse of the three ampullae, a macula acustica for the utriculus and another for the sacculus, and the organ of CORTI).
5. The embryonic connective tissue, in which are enclosed the auditory vesicle and the products of its metamorphosis, is differentiated into three parts :
   1. Into a thin connective-tissue layer, which is closely applied to the epithelial wall and together with it constitutes the membranous labyrinth
   2. Into a gelatinous tissue, which becomes liquefied during embryonic life and furnishes the perilymphatic spaces (in the cochlea the scala vestibuli and the scala tympani)
   3. Into a cartilaginous capsule, from which there arises by a process of ossification the bony labyrinth.
6. The middle and outer ear are derived from the upper part of the first visceral cleft (the spiracle of Selachians) and its periphery.
7. The tympanic membrane, which at first is rather thick and only gradually becomes reduced to a thin, tense membrane, is developed out of the closing plate of the first visceral cleft and the adjacent parts of the visceral arches.
8. The tympanic cavity and the Eustachian tube are developed out of a depression on the median side of the tympanic membrane, the sulcus tubo-tympanicus, and out of an evagination. from it extending upward, outward, and backward.
9. The tympanic cavity is at first extremely small, the connective tissue of the mucous membrane that surrounds it being gelatinous [and voluminous].
10. . The auditory ossicles and the chorda tympani lie at first outside the tympanic cavity in the gelatinous tissue of its wall ; it is only after shrivelling of the gelatinous tissue that they come to lie in folds of the mucous membrane, which project into the now more capacious tympanic cavity (incus-fold, malleus-fold).
11. The external meatus is developed from the periphery of the depression that lies on the lateral side of the tympanic membrane ; the auricle arises from six elevations, which are converted into tragus, antitragus, helix, antihelix, and the lobule of the ear.

The Development of the Organ of Smell

The organ of smell is, like the eye and ear, a product of the outer germ-layer, from which it is developed somewhat later than the two higher sensory organs. It first becomes noticeable, at either side of the broad frontal process (fig. 274) previously described, as a thickening of the outer germ-layer which His has designated in human embryos as nasal area. Both fundaments soon become more distinct owing to the fact that each nasal area becomes depressed into a kind of trough, the edges of which rise up as folds (fig. 286). An olfactory lobe, which has been formed meantime by an evagination of the cerebral vesicle, grows out on either side to the thickened epithelium of this area, where its nerve-fibrillce terminate.

The two olfactory pits, which are formed in a similar manner in all Vertebrates with the exception of the Cyclostomes, in which only an unpaired pit arises, are separated from each other by a considerable distance. They therefore appear at first as distinctly paired structures, whereas in their ultimate condition in the higher Vertebrates they have approached each other toward the median plane and become an apparently unpaired organ, the nose,

The study of tho development of tin 1 origin of smell acquires additional interest, when one lakes into account the comparative - anatomical conditions. It is then found that the various stages through which the organ of smell passes during embryonic life, in Mammals for example, have been preserved as permanent conditions in lower classes of Vertebrates. Thus in the case of many groups of Fishes the organ of smell is preserved, as it were, in its initial stage in the form of a pair of pits. Upon closer histological investigation, however, this condition acquires a special interest, because it presents points of comparison with simpler sensory organs which are distributed over the integument. As BLAUE especially has shown in a meritorious work, the olfactory nerve does not terminate in this case in a continuous olfactory epithelium, but in individual, sharply differentiated organs (fig. 287 rk), which, although closely crowded in an indifferent ciliate epithelium (fe), are nevertheless separated from each other.

Fig. 286. Frontal reconstruction of the oro-pharyngeal cavity of a human embryo (Rg of His) 11.5 mm. long, neck measurement. From His, " Menschliche Bmbryonen." Magnified 12 diameters.

The upper jaw is seen in perspective, the lower jaw in section. The posterior visceral arches are not visible from the outside, since they have moved into the depths of the cervical sinus.

Fig. 287. Longitudinal section through three olfactory buds from the regio olfactoria of Belone, after BLAUE. Highly magnified.

rk, Olfactory bud ; fe, indifferent ciliate epithelium in several layers ; n, branch of the olfactory nerve.

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The organs (rk) consist of numerous fine, rod-like cells, which at their free ends bear fine bristles and are united into bundles that are distinctly delimited from the ordinary cells of the epidermis. They closely resemble the sensory nerve-terminations which are abimd;intly and widely distributed in the epidermis of Fishes and other lower Vertebrates the beaker-like oryans or the nervous end-buds. BLAUE has therefore named them olfactory buds. He proceeds from the conception that, like the similarly constructed gustatory buds of the oral cavity, they are descended from the sensory organs distributed over the whole integument. The organ of smell is simply a depressed patch of the skin richly provided with terminal nerve-buds, which, undergoing a change of function, has come to subserve a specific sense. The continuous olfactory epithelium of the higher Vertebrates has arisen from the originally scattered, isolated olfactory buds (fig. 287 rk) by a process of fusion, the indifferent epithelium (fe) having gradually disappeared. In certain species of Fishes and Amphibia such a transition can be demonstrated.

The further development of the organ of smell is especially characterised by the olfactory pits coming into relation with the oral cavity. Each of them (fig. 286) develops a furrow which runs downward to the upper margin of the mouth and receives on its outer side the previously described lachrymal groove, coming in an oblique direction from the eye. Nasal pit and nasal furrow become deeper in older embryos (fig. 288), owing to their margins protruding outward as ridges and forming the so-called inner and outer nasal processes. The two inner nasal processes are separated from each other by a shallow furrow running from above downward ; they together produce a thick partition between the two olfactory pits that in the higher Vertebrates subsequently becomes more and more reduced in thickness. They also furnish the middle of the roof <i)f the mouth. The outer nasal processes (also called the lateral frontal processes by His) form on either side a ridge protruding between the eye and the organ of smell, and furnish the material for the formation of the lateral walls of the nose and the alee. Their lower margins meet the front end of the transversely located maxillary processes, from which they are delimited externally by the lachrymal grooves.

Fig. 288. Fundament of the nose and the roof of the primitive mouthcavity of a human embryo (C. II. of His), seen from below after removal of the lower jaw. From His, "Menschliche Embryonen." Magnified 12 diameters.

On the median wall of the nasal pit there exists a special small depression, which was first found by Dims Y in mammalian embryos, and which is also observable in human embryos at a very early stage (His). It is the fundament of JACOBSON'S oryan, which afterwards makes its way into the septum of the nose. It receives from the olfactory nerve a special branch, which is indeed of remarkable size in embryos.

The stage with the nasal groove exists as the permanent condition in many Selachians. In these cases the deep nasal pits, which are enclosed )in a cartilaginous capsule, and the mucous membrane of which is raised up into numerous parallel folds, lie on the under surface of the elongated snout or rostrum. Deep grooves, which are bounded by folds of the skin containing muscles, and which can be closed as if by valves, lead to the front margin of the mouth at some distance from its angle.

The next stage, which in human embryos is reached in the second half of the second month, exhibits the organ of smell converted into two canals, which have been produced by the fusion of the margins of the two grooves, especially that of the inner nasal process with the maxillary process, which advances toward the median plane. The canals now possess two openings, the external and the internal nasal orifice (fig. 289) or the nares. The two external nares lie only a little above the border of the mouth-opening ; the internal, in the roof of the primitive oral cavity, on account of which they have been named by DURSY the primitive palatal clefts. They are located far forward, only a little removed from the edge of the mouth, a position which they retain permanently in the case of the Dipnoi and Amphibia. At first round, they afterwards become elongated and assume the form of a fissure running from in front backward.

Fig. 289. Roof of the oral cavity of a human embryo with the fundaments of the palatal processes, after His. Magnified 10 diameters.

With the metamorphosis of the organ of smell into a canal leading into the oral cavity, which has been effected in all Vertebrates that breathe by means of lungs, a second function has been assumed. It is now not exclusively a sensory organ for the perception of odors, but serves at the same time to conduct currents of air both to and from the oral and pharyngeal cavities and the lungs. It has become a kind of respiratory atrium for the apparatus of respiration. The assumption of this accessory function gives a special stamp to the later stages of the development of the organ, and is to be taken into account in a proper estimate of it. For the course of the further development is most of all determined by the tendency to an extensive enlargement of the surface of the olfactory chamber. The increase of surface, however, does not affect the real olfactory mucous membrane or sensory epithelium, to which the olfactory nerve is distributed, but rather the ordinary ciliate mucous membrane. It is therefore less connected with an improvement of the sense of smell than with an accessory function in the process of respiration. By an increase of the surface of the soft, vascular mucous membrane the air that is swept over it becomes warmed and freed from particles of dust, which are caught by the moist surface. From this time forward therefore one must distinguish a regio olfactoria and a regio respiratoria. The former, which is derived from the sensory epithelium of the original olfactory pit, remains relatively small, receives the terminations of the olfactory nerve, and is limited in the case of Man to the region of the upper turbinal process and a part of the septum nasi. It is the respiratory function that causes the vast dimensions which the organ of smell attains in the higher Vertebrates.

The increase in the surface of the nasal cavity is produced by three different events : (1) by the formation of the hard and soft palate, (2) by the development of the turbinal bones, (3) by the appearance of the accessory cavities of the nose.

The first event begins in Man toward the end of the second month. There is then formed on the inner surface of the maxillary process (fig. 289) a ridge, which projects into the wide primitive oral cavity and grows out horizontally into a plate. The right and left palatal plates at first embrace between them a broad fissure, through which may be seen the original roof of the oral cavity and on this the inner nasal orifices, which become more and more slit-like and are separated by a bridge of substance which has arisen from the median frontal process and can now be designated as the nasal septum. In the third month fae embryonic palatal fissu/re becomes gradually narrower.

Fig. 290. Cross section through the head of an embryo Pig 3 cm. long, crown-rump measurement.

The nasal cavities are seen to be in communication with the oral cavity at the places designated by a * ; A', cartilage of the nasal septum ; m, turbinal cartilage ; J, organ of JACOBSON ; J', the place where it opens into the nasal cavity ; gf, palatal process ; of, maxillary process ; zl, dental ridge.

The horizontal palatal processes of the upper jaw increase in size, and finally their free edges encounter in the median plane the still broad nasal septum, which has grown down yet farther into the oral cavity. Then the parts mentioned begin to fuse with one another from before backward.

Two stages of this process are illustrated by the accompanyin g figures (figs. 290, 291), in which cross sections through the anterior end of two embryo Pigs are represented. Figure 290 shows the stage at which the palatal plate (gf ) of the maxillary process (of) has advanced close to the lower margin of the nasal septum. Oral and nasal cavities are still in communication by means of the very narrow palatal fissure indicated by an asterisk.

In figure 291 the fusion has taken place. In this manner the primitive oral cavity is divided into two storeys, one above the other. One, the upper part, becomes associated with the organ of smell, to the enlargement of which it contributes ; it is distinguished from the space that arose from the original olfactory pit, or the olfactory labyrinth, as naso-pliaryngeal passage,. This opens behind into the pharynx by means of the posterior nares. The lower part becomes the secondary oral cavity. The partition that has been formed from the maxillary process is the palate, which later, when the development of the bones of the head can be traced, is differentiated into the hard and the soft palate.

Fig. 291. Cross section through the head of an embryo Pig 5 cm. long, crown-rump measurement. k, Cartilaginous nasal septum ; m, nasal turbinal process ; /, JACOBSON'S organ with jk, JACOBSON'S cartilage ; zl, dental ridge ; bl, covering bone.

A small portion of the palatal fissure, which in young embryos traverses the palate from in front backward and unites oral and nasal cavities (fig. 290 *), is preserved in most Vertebrates and constitutes the ductus nasopalatinus or STEXSON'S duct. A probe may be passed through it from the nasal to the oval cavity. In Man the duct of STENSON is closed during embryonic life ; there is preserved, however, in the palatal process of the bony maxilla at the corresponding place a vacuity, the canalis indsivus, occupied by connective tissue, blood-vessels, and nerves.

Where the ducts of STENSON are present, there are found in their vicinity the organs of JACOBSON, concerning which the statement has already been made that they are established very early as special depressions of the two olfactory pits. In Man this organ is converted into a narrow tube, which lies a little above the canalis incisivus and " pursues a straight course backward and slightly upward close to the cartilaginous partition, ending blindly " (SCHWALBE). In Mammals the organ is more highly developed (figs. 290, 291 J) ; it is enveloped in a special cartilaginous capsule (JACOBSON'S cartilage, jlc] and receives a special branch of the olfactory nerve, which terminates in a sensory epithelium, which agrees with that of the regio olfactoria. Frequently (e.g., in Ruminantia) it opens into the beginning of STENSON'S canal, which in this case remains open as a communication between nasal and oral cavities.

I cited the formation of folds as the second means of increasing the internal surface of the organ of smell. These are developed in Mammals (figs. 290, 291) and in Man on the lateral walls of the nasal chambers; they run parallel to one another from in front backward ; their free margins grow downward, and in consequence of the forms which they assume are called the three nasal turbinated processes, while the spaces between them are designated as upper, middle, and lower nasal 2 )assa 9 es - From the cartilaginous cranial capsule they receive in Man as early as the second month a support, which subsequently ossifies. In many Mammals the turbinated processes acquire a complicated form owing to the production upon the first fold of numerous smaller secondary and tertiary folds, which become peculiarly bent and rolled up. On account of the complicated form resulting from, the production of the turbinated processes the olfactory sac has received the name of olfactory labyrinth.

Thirdly and lastly, the mucous membrane of the nose is increased in extent by the formation of evaginations which grow out partly into the ethmoid region of the cranial capsule, which consists of cartilage during early stages of development, and partly into a number of the covering bones (Belegknocheii).

In this manner are formed the numerous small cribriform pits in the cartilaginous cribriform plate. Somewhat later (in Man during the sixth month) an evagination into the upper jaw is developed into the antrum of HIGHMORE. Finally, after birth evaginations penetrate into the body of the sphenoid bone and into the frontal bone, producing the sinus sphenoidales and sinus frontales, which, however, attain their full development only at the time of sexual maturity. In many Mammals the enlargement of the nasal cavity takes place even farther backward into the body of the occipital bone (sinus occipitales). Inasmuch as the accessory cavities of the nose take the place of bone-substance, they naturally contribute to the diminution of the weight of the cranial skeleton.

In connection with the account of the organ of smell the formation of the external nose ought now to be briefly considered. It is developed out of the frontal process and the parts designated as nasal processes (figs. 286, 288, and 289), these becoming elevated more and more above the level of the surrounding parts. At first broad and bulky, the nose later becomes thinner and longer arid acquires characteristic forms. The nostrils, which at their formation are far apart, come together in the median plane. Whereas the distance in an embryo five weeks old is, as His has shown by measurements, 1-7 mm., it has become reduced in an embryo seven weeks old to 1-2 mm., and in one somewhat older to O'S mm. The median frontal process is correspondingly reduced in thickness and furnishes the nasal septum,

Summary

1. The organ of smell is developed out of two pit-like depressions of the outer germ-layer, which are formed on the frontal process at a considerable distance from each other.
2. At a later stage the pits are united with the angle of the oral cavity by means of the nasal grooves.
3. The inner and outer margins of the olfactory pits and the nasal grooves project out as ridges arid constitute the inner and outer nasal processes.
4. By fusion of the margins of the nasal grooves the organ of smell is converted into two nasal passages, which open out on the frontal process by means of the external nares and on the roof of the primitive oral cavity a little back of the upper lip by means of the internal nares.
5. The internal nares afterwards become fissure-like and move nearer together, owing to the nasal septum becoming thinner and growing downward somewhat into the primitive oral cavity.
6. The upper part of the primitive oral cavity shares in the formation of the organ of smell and serves for the increase of its respiratory region, since horizontal ridges (the palatal processes) grow inward from, the maxillary processes toward the lower margin of the nasal septum, with which they fuse, and produce the hard and soft palate.
7. In the organ of smell a further enlargement of the spaces serving for respiratory purposes is produced by
   1. The formation of folds of its mucous membrane, by which the turbinated processes arise
   2. Evaginations of its mucous membrane into the adjacent parts of the cartilaginous and bony cephalic skeleton (formation of the " cells " in the cribriform plate, the frontal and sphenoidal sinuses, and the antrum of HIGHMORE).
8. In human embryos there is early formed in the olfactory pit a special depression of the outer germ -layer as fundament of the organ of JACOBSON, which receives a special branch of the olfactory nerve.
9. JACOBSON'S organ comes to lie at the base of the nasal septum remote from the olfactory region.
10. The ducts of STENSON in many Mammals and the canales incisivi in Man are preserved as remnants of the so-called palatal fissures the original fissure-like communications between nasal cavities and secondary oral cavity.

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Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton

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