Book - A Text-book of Embryology 16

From Embryology


Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Heisler 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

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In the organs of the .senses we have to do with peripheral nervous mechanisms of greater or less degrees of complexity, the essential elements of which are elalK>ratelv modified or specialized neiiro-epithelial cells. These neuro-epithelial structures are specialized cells of the ectoderm, derive<l from it either directly, hy the infolding of patches of ectodermic epithelium, as in the case of the olfactory cells, or indirectly, by growth outward from the central nervous system, as in the case of the retina. The organs of the sense of touch, the tactih' corpuscles of the skin and mucous membranes, are distributed somewhat irregularly, while such highly Sjweialized struc^tures as the organs of the special senses of vision, hearing, smell, and taste are provided with special protective and accessory apparatuses.

The Development of the Eye

It will perhaps larilitatc th<* eoinpreliension of the general principles involved in the (Ifvclopincnt of the eye if its function as the organ of vision is. krpt in mind, and if, therefore, the retina and the o|)tic nerve are recognized as the essential parts (»f the organ, and the other structures as accessories. The retina an<l the optic nerve are an outgrowth from the brain, the rod- ai]<I eone-visual <'ells of the former being epithelial cells so specialized as to serve as j)ercipient eh-ments, while the (»j)tic nerve-fibers are the (-onducting medium. To alh)Wof tlu' penetration and refra<'ti<m of the ravs of liirht, the (»verlving epich^rmis differentiates into a transparent and refra<*tive medium, the crystalline lens, and the necessary prote<»tion an<l means of nourishment are provided by the other constituent of the eyeball. Further protection is furnished by two folds of modified skin and subcutaneous tissue, the esrelida, and lastly for the lubrication and still further protection of the exposed part of the eyeball, there is formed still another set of accessory organs, the l&crimal appuratiu.

The first step in the development of the eye is the growth of a diverticulum from the side of the primary fore-brain vesicle (Fig, 160). These optic evaginations are qnite large

II of two-d«y- chlck-embryn ; B. bmln of huni»n embrro of Oiowii the development of tbe opllc leslclva and bnlD-resl< h. inter-brain; ob, optic veulcles.

as com{)arGd with the h rain- vesicle. They begin to be evident even before the neural tube is completely closed. As the attached part of the diverticulum expands less rapidly than the distal ]>ortioii, the evagination soon assumes the form of a sac or vesicle, the optic vesicle, connected by a hollow stalk with the primarj' fore-brain. When the secondary fore-brain vesicles gn)w out anteriorly from the primary vesicle, the region of the latter that Itecomes in consequence the inter-brain is the part to which the stalk of the optic vesiele in attachetl. Hence the optic vesicle is an appendage of the inter-brain or thalamoncephalon and its point of attachment to the latter Is at the lateral iwrt of the base, in front of the region of the infundihiihim (Fig. 147, A and C).

The optic vesicle expands laterally and dorsally lies immediately lieneath the epidermis, forming a prorai

nenee on the side of the head (Fig. 62). The ectoderm at the point of omtaet with the optic ver^iele becomes thick* ened and depressed, the dilTerentiation of this lens-area being the >tartin^ p«.»int of the erystaUine lem. The depressed |iatoli of ectixlemi, sinking more <k»eply, is converted into a sac, the lens-vesicle, the o >nnt*etion of which with the surfacecells is <mM\ l«»st. The ilistal wall of the optic vesicle, upon coming into contact M'ith the lens-vesicle, undergoes invagination, this wall sinking in until the cavity of the vesicle is ahno-t obliterated. Thus the vesicle is converted into the druilile- walled optic cnp, the o|K'ning of which looks laterally toward the surface of the head, and is occupied by the leusvesiele.

The invaginated wall of the vesicle — thatis, the layer nearer the c<*nter of the cuji — becomes the retina, except its pigment-layer, the latter resulting fn)m the outer layer of the cup. The stalk <»f the cup l)ecomes the optic nerre. The surroimding mesodermic tissue grows into the openings nrferred to above, and gives rise to the vitreous hnmor, while the mesodermic cells that closely envelop the optic cup produce the uveal tract and the sclera and cornea.

Having tra<*ed briefly the develripment of the organ, its sevcnil parts may now bo considered in detail.

The Retina and the Optic Nerve

These two structures, as stated abov<*, an; directly derive<l from the optic vesieh; and its stalk.

To rep4*at, for the sake of continuity, some points already mentioned, tin; optic vesicle grows forth as a diverticidum from the side; of tin? primary fore-brain vesicle, its appearance beting foreshadowed by a lateral bulging of this vesicle even before the neural canal is com[)letely eloswl. When the primary fore-bniin vesicle divides into the secondary fore-brain vesicles and the vesicle of the inter-brain, the regir>n of origin of the optic vesicle falls to the latter, the jM»int (»f attachment being at the outer edge of the base of the vesicle in front of the infundibular evagination. The optic nerve is to be regar<le<l th(Tefore as springing from the inter-brain or thahunen<*e|)halon.

Fig. 162.— Three BUMtmlvc sMgca of dsTelopnient of the eye, showinE fOmt.tlon or aeenndary nptic cup and crrilaltlne leiu in human embryos or 4 mm. < J), fl mm. (B), &nd H mm,(C]. (Tonnieui): a, a, primltlTe optic veilclei; b, extern *l Uyer of neciuidary opllc cup (fliture pfttmenl-Uyor of rellna) ; c. Inner layer of cnp (re linn proper) ; il, lens-pit llhlekened and depreued eatudena):

diattfly under the epidermis, separated from it by nnly a thin layer of embryonal connective tisBtie. This lateral position of the optic vesicles is characteristic of the early stages of development. After the end of the first month the eyes gradually move forward and downward toward their permanent position, which is approximately attained probably early in the third month.

Shortly after the fourth week the distal or lateral wall and the under surface of the optic vesicle l>ecome invaginated. The invagination begins when the vesicle comes into contact with the lens-vesicle (Fig. 162). When the infolding is complete, the vesicle has l>econie the secondsiy optic cap, which latter consists therefore of two layers, an inner and an outer. The month of the cup, which faces away ffom the niMliaii plane of the head, is occupied by the lens-vesicle. Since the under surface of the vesicle ]>artici))a(es in the invnginating pniee.'is (Fig. 163) there is also in this wall of the enp an ajwrture, which is known as the choroidal flsmre. The invagination affects the under siirfatre of the tubular .-^lalk of the vesicle sii that it is c<mvertc<l into an inverted (loultle-Iaycred trough. These invaginations bear an important relation not only to the further nK'taniorphosis of the optic vcsieli" lunl its stalk into the retina and thf.iptic- nerve, but also to the (levc4o|)nnTit iif the vitreous body ami of llic r<-iiti-al artery of tlic n.'lina. TliMs, the vitreous binly is PrmIiiwiI in yurX at !ca>t by the Kie^iilcrmic tissue that finds access to the cui) tlin.Li<rh llic ehoniidai fiTisure, and tlic arloria wntralis n-i'mw is di'v.IojK'd in the vascular ■ tissue iliat invagliiates -uriaee of the stalk of


the I the '


gnuhially 1-oiitraeis after ilic enti-!in<-e of the niescRlerni, and in the liist motitli of fetal life it entirely closes. Tiie mouth of ilie o]Hie cup einlinices tite lens, its rim being always on the ilislal side of, or su]>crlii-ial to, that .-itructure. This iijieiiiiig repiTM-ntf* the pupil of later stagi-s.

The further metamorphosis of the optic cup includes alterations peculiar to each of the two layers and also to the different regions of the cup. The mouth of the cup contracts somewhat by increased growth of the wall, and thus there is a zone bordering this orifice which is anterior to the lens, holding the same relation to the latter body that the future iris holds. A second zone corresponds with the periphery of the lens, while a third region, the fundus of the cup, includes all the remaining part of its wall.

The flmdas of the cup undergoes much greater specialization than the other regions. The outer layer of the cup remains thin, consisting of a single layer of cells which assume the cuboidal form and become infiltrated with pigment-granules. This forms the pigmenlrlayer of the retina. The inner lamina of the cup thickens, by the multiplication of its cells, and soon consists of numerous spindle-shaped cells. The thickened fundus is marked off from the zone that surrounds the periphery of the lens by a slight groove which corresponds in position with the future ora serrata. These early spindlc-cells give rise to two kinds of elements, the stroma of the retina, or Miiller's fibers, and the various nerve-cells, including the highly specialized rod- and cone-visnal cells.

The principal sustentacular elements, or Miiller^s fibers, like the spongioblasts of the neural tube, are radially arranged and extend throughout the entire tliickness of the retina. Their inner expanded extremities, in close contact with each other, form the inner limiting membrane, while their outer ends, in the same way, constitute the outer limiting membrane, which latter is in contact with the pigmentlaver. The stroma of the retina receives a small contribution from the mesodermic tissue, which grows into it through the choroidal fissure to furnish the vascular supply.

Of the nerve-cells, those near the pigment-layer undergo great alteration in form and become the sensory epithelium — that is, the rod- and cone-visual cells. At first these lie entirely internal to the external limiting membrane, which separates them from the pigment-layer. After a time, liowever, processes grow out — that is, away from the center of the eyeljall — and ]>erforate the external limiting membrane to i»eiietrale Iwtwecn the cells of the pigment-layer. These

jmxresscs are llu' rods and fonen, and colWtively constitute the layer of rods and conea of (lit- iidult The bodies of the

KoMlsni pi. pi BtQ* 11 led I'plthcUiiTii iif Ihc eju I'luttr luniella uf the opUc enp,fl^ aceondary optic veiklcl ; r.rolinii(iniiprlain*llB of the optic cup); it, mKi^iUln nf (he optic cup. which farma the pun clllarls el Iriills rellnie: g, Til with blood-TCBKla : fv, tunlm tucuIou leiilla; U. blood-uurpuwlei : tA.ehon tf, lens-flben: It, leiu-eplthcllum ; r.nnu of the leni-tlbcr nuclei; ft, ftindaB at the iK>niea; he, exlernAl corneal epithelium.

rod- and cone-visual cells, situated on (he inner side of the i membrana limitan.s externa, are elongated into narrow ele- j ments, the position of tho nnctei being indicated by slight I enlai^ments. They constitnte the outer nuclear layer of ' the matnre retina. The outer nuclear layer and the layer of rods and cones are to be regarded, therefore, as one layer of highly specialized neuro-epithelium, made up of the rodvisual cells and the cone-visual cells, the inner segments or bodies of the cells being only apparently isolated from the outer segments, the rods and cones respectively, by the fact that the latter proj(?ct through minute apertures in the external limiting membrane. The axis-cylinder pnxjcssesof these cells pass toward the center of the eyeball.

The neuro-epithelium of the retina is the last of its elements to develop. In man and in many mammals, it is present at birth. In the cat and the rabbit, the rod- and cone-visual cells develop after birth, and hence the new-born of these species are blind. The macula lutea is developed after birth.

The cells of the inner part of the retina differentiate into the remaining nervous elements, some becoming the bipolar and other cells of the inner nuclear layer — the ganglion retinae — while others form the large ganglion cells of the ganglion-cell layer. The axis-cylinder processes of the ganglion cells are directed inward to form the nerve-flber layer, the fibers of this layer converging from all parts of the inner surface of the retina toward the optic disk or papilla. Here they perforate the retina, as well as the choroid and sclera, to pass, as optic nerve-fibers, to the brain.

This part of the optic cup, the ftmdus, produces then, in the manner descrilKjd above, the functionating portion of the retina, or the pars optica retinae, the anterior termination of which is indicated by the orra serrata.

The lenticular zone of the optic cup, which is in relation with the peripherj' of the lens, undergoes comparatively slight specialization. Its outer lamella is pigmented, as in the fundus of the cup. Its inner layer remains very thin and consists of cells which at first are cuboidal, but which later become cylindrical. At the end of the second month, or the beginning of the third, the two layers of the lenticular zone become plicated, owing to excessive growth in superficial extent. The folds are nearly parallel and are arranged radially with reference to the lens, the margin of which they surround. These folds are the first indication of the ciliary processes. The mcstKlermic tissue immediately external to th(» <)j)ti(! cup (lifforentiates into the uveal tract, the part corresponding with the lenticular zone of the cup furnishing the ciliary hody. The young growing connective tissue penetnitos between the folds of the lenticular zone of the cup, acupiiring intimate union with the pigment-layer, and thus provides the connective-tissue basis of the ciliary processes. This lenticular zone of the two hiyers of the optic cup, therci'on*, <'onstitutes the lining, or internal covering, of the ciliary ImmIv, and must necessarily be reganled as the continuation of the retina, it is known as the pars ciliaris retiiUB of the fully developed (?ye.

The marginal zone of the optic cup, or the region bordering its orilice, is also related in its further growth with the uveal traet. Although in the earlier stages of development the lens lies in the mouth of the cuj), as time goes on the relation is so altered that the a[K»rture and the zone which borders it occupy a position in front of the lens. In this marginal zone both lamelhe of the cuj) become pigmented and aecjuire union with the layer oi' mes(Klermic tissue which is dillerentiatiiiir into the iris, and thev therefore contribute to the formation of that structure, ecmstituting its pigmentlayer. Th(^ pigment-layer of the ]>osterier surface of the iris is, therefore, an extended but rudimentary part of the retina. It is called the pars iridica retinae.

FnMu what has been said, it will be ap])arent that the retina f(>rms a (Mnnplete tunie with an anterior perforation, the pupil, and that it consists of the funetionally active part, or retina proper, th(» pars optica retinae; of the pars ciliaris retinae, marked olV iVom the latter by the ora s(»rrata ; and of the pars iridica retinae, which terminates at the margin of the pupil.

The evolution of the optic cuj) or secondary oj)tic vesicle mav b(» thus summarized :

I. Marginal or most anterior The thin atrof)liic pare iridica reregion of cup. tinir, or pi^rnicnt layer of the iris.

n. Ltnticular zone of cup. Pars ciliaris rt-tina*, covering inner

surface of ciliary body.

III. Fundus of cup. Functionating part of retina, or pars

optica retins, including : A. Outer layer. A. Pigment-layer of retina.

£. Inner layer. B. 1. Neuro-epithelial layer, made

up of layer of rods and cones (the processes of the rod- and cone-visual cells) ; membrana limitans externa; outer nuclear layer (the bodies of the rodand cone-cells).

2. Cerebral layer (representing an interpolated ganglion with connecting fibers), consisting of :

Outer reticular layer ; Inner nuclear layer ; Inner reticular layer; Ganglion-cell layer; Nerve-fiber laver.

The optic nerve is the metamorphosed stalk of the optic vesicle. When the distal and under surfaces of the vesicle suffer invagination, the stalk participates in the process, its under surface being marked by a groove which is a prolongation of the choroidal fissure of the optic cup (Fig. 163). By this infolding, the cavity of the stalk is obliterated and the stalk is converted into a double-walled tube enclosing mesodermic tissue which follows the invaginating ventral wall. In this mesodermic tissue is developed the arteria centralis retina. In mammals the invagination affects only the distal part of the stalk, the segment included between the eyeball and the point corresjionding in the adult to the place of entrance into the nerve of the central artery. It must be apparent that the outer layer of the tube thus formed is directly continuous with the outer layer of the optic cup, while the invaginated lamina is the prolongation of the inner wall of the cup or of the part that becomes the retina proper, since not only the distal wall of the optic, vesicle is invaginated, but its under or ventral wall as well.

the primitive optic nerve at this stage consists of layers of spindle-shaped cells, with a central core of vascular connective tissue.

The manner in which the nerve-fibers are developed is still a matter of controverjsy. According to His and K5lliker, the fibers gi*o\v out from the ganglion-cells of the optic thahimi and the anterior corpora quadrigemina^ while Muller and Froriep believe that they are the prolonged axis-cylinder processes of the ganglion-cells of the retina. According to Ramon y Cajal, growth occurs in both directions. In either ease, the cells of the optic stalk would furnish only the sustentative tissue of the nerve. There is also a contribution of sustentative tissue or stroma fnmi the mesoderm, as in the case of the central nervous svstem.

The Crystalline Lens

The lens, exclusive of its capsule, is, like the retina, of ectodermic origin. The first step in its <leveIopm(»nt is the formation of a thickened and deI)ressed patch of the ectoderm on the lateral surface of the hea<l, this area being situated at the place where the optic vesicle is nearest the surface (Fig. U)2, By d). The depression is the lens-pit. It soon becomes converted into a closed sac, the lens-vesicle, by the gradual a])proximation and union of its edges. The pit receding from the surface as its lips come together, the completed vesicle lies under the surface ectoderm, witli wliieh it is for a time connected by the slender stalk of tho invagination. Upon the disapi)earanc(» of the strand of cells constituting the stalk, the lensvesicle is completely isolated from the outer germ-layer (Kig. 1(12, (\i'l

The lens-voside in birds is a hollow epithelial sac several lavers thick, but in nuunmals the central cavitv contains a mass of (jells, which latter disappear in the later stages of development.

I^pon the invagination of th<» optic vesicle to form the se(M>ndary optic cu|>, the lens-vesich? is embracwl by the lips of the cup and still later Cannes to lie within the cup, near its orifice (Fig. 1^)4).

The further alterations in the vesicle are de|KMident primarily upon changes in its deep and sujH^rficial walls resjH^ctively, each of wliich consists of several layers of cylindrical cells. the cells of the sui>erficial wall alter their form, bcH?oming cul)oi<lal, while the posterior or decjwr cells lengthen so as to become fibers. Thus the deeper wall of the vesicle thickens at the expense of the central cavity — the central mass of cells at the same time disappearing — while the superficial layer remains thin. The two strata are continuous with each other at the equator of the lens, one form gradually merging into the other at this region, which is a zone of transition (Fig. 164).

The lens at this stage is composed, therefore, of a thin superficial or anterior stratum of cuboidal epithelial cells and a much thicker posterior or deep layer of so-called fibers, the latter being simply the greatly elongated cells of the posterior wall of the vesicle. Between the two laminse is a small remnant of the cavity of the vesicle. The epithelial layer persists throughout life as the epithelium of the lens, while the fibrous layer is the basis of the lens-fibers of the mature condition. The cavity sometimes persists as a small space containing a few drops of fluid, the liquor of Morgagni.

The next important stage in the development of the lens is the formation of additional lens-fibers. These result from the proliferation of the cells of the epithelial or anterior layer. The lens-fibers are formed in successive layers, as may be made evident by the maceration of a lens. Each fiber extends from the anterior to the posterior surface of the lens. The ends of the fibers meet each other along regular lines, producing thus the characteristic three-rayed figures or stars of the lens, one of which belongs to each surface. Hence, while the lens-fibers first formed are the elongated cells of the posterior layer of the lens-vesicle, the fibers of later gro^vth originate from the cells of the anterior wall. The epithelial character of the lens-fibers is evinced by the presence of a nucleus in each fiber of a young lens.

The lens-capsule results from the differentiation of the mesodermic tissue which surrounds the lens. It is from this enveloping vascular lamina, the tunica vasculosa lentis, that the growing lens derives its nutrition. The capsule is well marked in the second month. Its blood-vessels are derived from those of the vitreous body. At the end of the seventh month this well-developed, highly vascular membrane begins to undergo retrograde alterations, the final result of which is its transformation into the thin, non-vascular^ transparent capsii le of tlio mature lens.* The most active growth of the lens itself occurs prior to the degeneration of the tunica vasculosa lentis, so that even before the end of fetal life the lens has nearly attained its full size. Thus the weight of the lens of the new-born child is 123 milligrammes, while that of the adult lens is but 11)0 milligrammes (Huschke).

Hence the crystalline lens has a double origin, the lens-substance or lens proper being derived from the ectoderm, while the capsule oriirj nates from the mesoderm.

The Vitreous Body, — The vitreous body has been regarded usually as a roniparatively slightly differentiated form of connective tissue, and as being derived from the middle germ-layer. Recent investigtitions show, however, that it originates in ])art at least from ectodermal tissue. According to these observations, processes grow forth from those stromal elements of the optic cup which afterwanl Ix^come Midler's libers, and these processes, advancing toward the lens-vesiele, interlace to form a network, the primitive vitreous (Kolliker, Froriep). This ])roeess continues for a longer time at the marginal zone or month of the cup than elsewhere, the j>roto|)lasniie fil>ers which grow from this future ciliary and iridal j)ortion of the cup contributing to the lorniaiion of the zonule of Zinn. In mammals the cells of the lens-vesicle, another ectodermal structure, also send torth processes which, according to Lenhossek, bear a prominent part in the d<velo|)ment of the vitreons body. The mesodermic tissue, already in the sta<x<* <>f cnd)rvonal connective tissue, now gains access to the optic (Mip through the choroidal fissure (Fig. 1 <>•>), its ingrowth, in fact, accomjKinying the invagination of the un<l(M' >urta<M' of the opti(^ vesicle, and constitutes what K(")lliker designates the mesodermal vitreous. The intermingling of these two constituent ele ' It sonu'titiu's li:i|>|»«.Mis that parts of the fetal lons-capsulc persist. The most ootnmon exani]>le of 8\i<h persistence is the so-called meiiihrana pupillaris soinetinK*s present at hirth, pnuhicinK rmnjnn'tal otrtAia of the pupil. This results from the persistence (»f that part of the fetal capsule which is situated on the anterior surface of the lens, hehind the pupil.

ments produces finally the definitive vitreous. Since the inferior surface of the stalk of the vesicle — the future optic nerve — participates in the invagination of the optic cup, the mass of mesodermic tissue which helps to form the vitreous is continuous with that which invaginates the primitive optic nerve to produce the central artery of the retina. As a consequence, the blood-vessels which soon develop so plentifully in the vitreous b(xly are extensions from the central artery of the retina, the latter itself being continued forward as the hyaloid artery. The terminal branches of the hyaloid artery pass on through the vitreous body to terminate in the vascular capsule of the growing lens, constituting the blood-supply of that structure.

The intercellular substance of the young tissue undergoes but little differentiation, while the cells become gradually reduced to a few stellate elements which ultimately entirely disappear. The peripheral part of the tissue develops into the hyaloid membrane, which anteriorly acquires union with the capsule of the lens.

The blood-vessels of the vitreous disappear during the last two or three months of fetal life. The hyaloid artery persists, although in reduced form, for a longer time than the smaller vessels. Upon its final degeneration it is replaced by a canal, the hyaloid canal, or canal of Stilling, which is present in adult life.

The Middle or Vascular and the Outer or Fibrous Tunics of the Eye

The outer fibrous coat of the eye, including the sclera and the cornea, and the middle tunic or uveal tract, comprising the choroid, the ciliary body, and the iris, are structures of mesodermic origin, being directly produced by the mesodermic tissue surrounding the optic cup. The richly cellular mesoderm applies itself to the exterior of the cup and differentiates into the two layers in question, the changes involving on the one hand the metamorphosis of the mesodermic cells chiefly into muscular and vascular elements, and on the other hand the evolution of a tissue essentially fibrous in structure. These two tunics are distinguishable in the sixth week.

the cornea is formed from the thin laver of mesoderm that penetrates l)et\veen the lens-vesicle and the surface ectoderm. The lens- vesicle lies very near the surface, and the thin stRitum of mesoderm that is interposed between the two is the anterior layer of the lens-capsule (Fig. 164). This anterior layer thick(»ns by the immigration of other cells and subsequently splits into two lamime, a superficial one which produces the cornea (Fig. 104, A), and a deeiK?r, which is now the proper anterior wall of the lens-c^apsule. Thus a space filled with fluid a pjwars between the primitive cornea and the lens, which (H)rresponds with the future anterior and posterior chambers of the eye, the <livisicm of the sj>ace into these two chambers being eflectcd subsequently by the development of the iris. The further (leveloi)meiit of the cornea consists simply in the differentiation of the mesodermic cells and the int(M'eellular substance into the several characteristic elements of the adult structure.

The uveal tract closely corresponds in extent with the two layers of the oi)tic cup. The choroid is differentiated from that portion of this primitive uveal tract which envelops the pars optica of the retina. In this region the enveloping layer of nicso<lcrniic cells develops into the several elements of the choroid, the most C()ns])icuous of which are an inner layer of capillary vessels, tlu^ choriocapillaris, and an outer lay(»r of larg(?r vessels, th(» stroma-layer of the choroid. the development of the clioroi<l bears a certain relation to the choroidal fissure of the optic cup. This tissure has been referred to as a gap in the iMi<lcr surface of the eup corresponding with the line of invagination through which the mesodermic tissue, of which the developing choroid is a part, grows into the cup to pnxluee the vitreous. Although normally this fissure in the retina entirely disjippears, its site be<*onies pigmented later than other regions of the pigment«laycr of the retina, and h<*nce there is, for a time, a clear streak in this part of the retina which has the appeanince of a fissure in that membrane. As the pigment-layer of the r(»tina was formerly assigncnl to the choroid, this streak appeared to be a breach of continuity of the ch<»roid ; hence the term choroidal fissure. In some cases, however, the choroidal fissure fails to close, and as the development of the choroid is largely dependent upon or is governed by that of the retina there remains a corresponding gap in the choroid. This defect enables the sclera to be seen from the interior in a line extending forward from the optic nerve entrance. It is known as coloboma of the choroid.

The ciliary body is developed immediately in advance of the choroid and from the same layer of mesodermic tissue. The deeper parts of the tissue in this region correspond with the plications of the ciliary part of the retina, sending processes into and between the radial folds of this part of the two layers of the optic cup, with which latter the highly vascular mesodermic tissue acquires firm union. This results in the formation of the ciliary processes. Some of the cells of the more peripheral part of this zone are converted into unstriated muscular tissue, thus producing the ciliary muscle. All the characteristic or important elements of the ciliary body are, therefore, derived from the mesoderm, while the thin layer of tissue on its inner surface, representing an undeveloped part of the optic cup, the pars ciliaris, is of ectodermic origin.

The iris, the most anterior zone of the uveal tunic, is produced from the same mesodermic tract that gives rise to the choroid and to the ciliary body. As stated above, soon after the lens-vesicle becomes constricted off from the surface ectoderm, it is enveloped by a mass of mesodermic cells which constitute its primitive capsule, and the layer of these cells lying between the lens-vesicle and the surface ectoderm splits into an anterior layer, which becomes the cornea, and a posterior stratum which is the anterior wall of the lens-capsule. This produces a space between the lens and the cornea. The lens now recedes farther from the surface, and the margins of the optic cup advance, so that the lens now lies within the cup, the marginal zone of the cup being in front of the lens, between it and the cornea, while its equator is in close relation with the ciliary regions of the cup and of the uveal tract. Thus the space between the lens and the cornea is

H divided into an iiiitcrior cdnipartnicnl, tlic anterior chamber,

H ntid a jmsterior fl])ai-p, tin- posterior chamber, the orifice of

■ tlic cup being a mi-nns of comiTHinication l>plnceii the twd

H and representing ttie pupil of a iaier sla^, the niar^ual

H zone of the cuj) furnishes the gniding line for the develo]* ^1 ment of the iris, The niesf«lcniiie tissue in rclutioD with

tm. Iffi.— BtgltEal iiectlon IhroUBh ttif pyp nf an embryo i

lUyi X 30 (Kailkerl: o,o|illc nttrc; j.. licmgimBl Hmni-nl-U

CllUry pun nf the rellnt ; p'. fi>rppiirl "f llie i>l.ll« PUp (rudiment of thB il

the srterli twnlralls retlnit unler II: f. trls; mj,, nienibr«n» pupllUriii «,«  with Piilib*tluro (,- iMiliwbne; I. lem; V. Icnt-cpllhcHum ; /. MlwottBjJ

the outer surface of the marginal stone of the euji difTerc tiatca into the vascular, muscular, and couneclive-tiasuc elM tnenta of the iris pn)iwr, while ils pfisterior pignient-laycr fa constituted by the slightly specialized layers of the mot anterior part of the optic cup, the part that in known a* th< para iridica retinre. Recent investigations (Xushhauni, Her^ zog, etc.) indicate that the circular and the radial muscular'J

fibers of the iris develop from the outer epithelial layer of the optic cup or, iu other words, from the part of the optic cup that becomes the pars iridica retina?. The circular fibers, sphincter pupillee, are distinguishable in the fourth month, the radial or dilator fibers, in the seventh mouth.

Since the anterior and posterior chambers of the eye are spaces hollowed out of the mesoderm, they represent a lymph-space and are, as such, lined with endothelial cells.

The cleft in the inferior wall of the optic cup referred to above as the choroidal fissure necessarily affects the marginal zone of the cup as well as the region posterior to it. If this part of the fissure persists, as it sometimes does, it may be accompanied by a corresponding deficiency in the tissues of the iris projjer. Such a congenital defect, appearing as a radial cleft in the lower half of the iris, is known as coloboma of the iris.

The Eyelids and the Lacrimal Apparatus

The eyelids are developed from folds of the primitive epidermis that form over the superficial part of the developing eyeball (Fig. 165, pp and pa). After the separation of the lens- vesicle from the surface ectoderm, the latter pouches out into two little transverse folds for the upper and lower lids respectively. Each fold includes a certain quantity of mesodermic tissue, from which are produced the connectivetissue elem.ent^ of the lids, as the tarsal plates, etc. After the folds attain to a certain degree of development their eilges approach each other and become adherent, thus enclosing a space between the primitive lids and the front of the eyeball. The infolded ectodermic layers lining this space acquire the characteristic features of mucous membrane and constitute the epithelium of the coAJnnctiva, the part of this membrane that covers the cornea adhering closely to that structure as its anterior epithelial layer. The union of the edges of the lids begins in the third month and lasts until near the close of fetal life. A short time before birth the permanent palpebral fissure begins to form by the breaking down of the adhesions.

A part of the mesodermic tissue of the lids undergoes conversion into fibmiis connective tissue, thus producing the tarsal plates of the upper and lower lids^ with the iMJpebral DasciflB and tarsal ligaments by which the plates are attached to the margins of the orbit.

During the period when the edges of the lids are adherent, the Meibomian glands and the eye-lashes are formed. The ghmds develop from solid cords of epithelial cells that grow from the deepest or Malpighian layer of the primitive epidermis into the tarsal plates. The conls become hollow tubes by degeneration of their eentnd cells.

In addition to the two principal folds that produce the lids, a third, vertical fold app(\ars at the inner, nasal side of the conjunctival space, beneath the lids. This fold remains quit(i small in man and forms the plica semilunaris, but in most other vertebrates it attains much greater size as the third eyelid or nictitating membrane. A small part of this third fold develops sebacw)us glands and a few hair-follicles and becomes the lacrimal caruncle.

The lacrimal gland is devel(>]KHl in the same manner as tl)(i Meibomian glands, by the growth of solid epithelial cords from the conjunctiva. The cords grow into the underlying inesodcnn at th(» outer part of the line of reflection of the conjunctiva from the inner surface of the upj)er lid to the front of the eyeball. The conls acquire lateral branches and then become liollowcd out to form the secreting tubules and efferent ducts of the gland, the connective-tissue stroma of which is contribut<'d by the surrounding mesodermic tissue. The orifices of the adult efferent ducts in the upj>er outer j)art of the conjunctival sacj corr(»sjK)nd with the points from which the primitive cell-cords first grow forth.

Th(> efferent lacrimal apparatus, consisting of the nasal or lacrimal du<'t and the canaliculi, is related genetically to the growth of the nose and the upjxT jaw. S(M)n after the appearance of the nasofrontal ]>rocess, a lateral projection, the lateral nasal process, grows from its side near the base and advances <lownward so as to form the outer boundary of the nasal j)it and consequently of the future nostril (Fig. ()7, A, i>). This lateral nasal process is separated from the maxillary process of the first visceral arch by an oblique furrow, the naso-optic groove, which extends from the inner angle of the orbit to the outer side of the nostril, or, before the separation of the nasal pit from the primitive mouth, to the upper boundary of the latter orifice. The naso-optic groove indicates the situation of the lacrimal duct. By some authorities — Coste and Kolliker — it is believed that the duct results from the union of the edges of the groove. Later investigations seem to indicate, however, that the duct is formed by the hollowing out of a solid cord of epithelial cells that appears at the bottom of the furrow. In either case the epithelial lining of the duct is an ectodermic involution. When the nostrils are separated from the oral aperture by the union of the nasofrontal, the lateral nasal, and the maxillary processes (p. 133), the lower end of the furrow is obliterated, and the partially formed duct is made to terminate in the nasal cavity.

The canaliculi, representing the bifurcated upper extremity of the duct, result, according to one view, from the division of the upper end of the epithelial cord into two limbs, one for each lid, and their subsequent hollowing-out ; according to another, from the continuation of the cell-cord into the upper lid and the later addition of a limb for the canaliculus of the lower lid. The lacrimal sac is merely an expanded part of the duct.

The Development of the Organ of Hearing

As in the case of the other sense-organs, the auditory apparatus consists of highly specialized nenro-epiiheliiim, connected by nerve-fibers and interpolated ganglia with the central nervous system, and of protective and auxiliary structures. The neuro-epithelial structures, including the organ of Corti and the cells of the cristsB and maculae acusticae, result from the specialization of certain of the epithelial cells which line the membranous labyrinth. The perilymphatic space, which is a lymph-space, together with its bony walls, the osseous labyrinth, serve for the protection of the delicate neural elements, while the middle ear

ami tlio nxleriial oar act ; siinuroiis vibratiims.

Till' iiitcrniil ciir Iwiiig the css^scntial [lart of the organ of hciiriiif^ ami hriiij; alsd the part first formed may i»ro|wrIy n-ccive tirst eoiisiih-nit^m.

The Internal Ear. — The membriuioiis labyrintli uf the

Uc vesicle of > ilors pit: B. Ihtf ol[c Malcle; rftirc Lrloderm,

iiilerMal ear is tlic <il<h'.-l part df the "i-jran i>f licanng. Its DiiM-iii is from a lliiekeiie.l .-in-iilar |»ateh i.|' eetiHlerm on the liorsulateral Mirl'ace i.l" the hea.l-rrjrimi of the eiiihryo near the liiirsil lerriiiitalioii i.f the fir.-l oilier viseerjil furrow. The tiiieketi.'.! arra M1ll^^ l>i-lo\v the Mirfar*', forniiiig thus the auditory pit, whic-li is iin>eiit in itie ihinl week (Kig, lOd, .1 ).

the 1 nrs .h-e|.rr, it^ e.lut- a|.|.rua.-h eaeh other and

tiiially iiieel ami niiitc lo form the otic vesicle or otocyst. Tills "lilile e]>ithelial sae fira.lnally nve<les from the nirface eeli«l.'i-m. At llii,- Ma.L'i- "f d-veloimieiit then- is no eraiiial eapsule oiher than tlir imlilti'r<'nt itu-oileriaie tissue \v I lieli surrounds ihe hi-ain-ve.-ieles ; heuee, the otic vcsiele, cmbedded ill tlii,- ti»m-, lies in elo^,- proxhnily to tlw aftor!>n»iti, and e.mies into r<>Iation with the neiisli«)(aeial ^Mtiglion (jt. ;t"21). Till' vcsiele, at lirst s|ilierii-al, soon heeoiiies

pear-shaped owing to the protrusion of its tlorsal wall. Tliis dursal projection, the recesaos vestibtiU or la.b3rrmthi (Pig. 16ii, C), lengthens out into a slender tube, the ductus endolymphaticns (Fig. ^^^), llie slightly dilated end of wlneli, the aaccua endolymphaticuB, is found in the adult occupying the aqucdiictus veslilndi of the tempoml bone. ,

Fig. 187.— Development or the membranniu labjrrlnth of tbo human ear (W. Hta, Jr.l : A. left labyrinlli of embryo of about four U'ecke, outer ildc; i>E, velUbutar and cochlear poitlons : rl. recessua Jabyrliithl. B, left labrrlnlh with part* nftoclalanaaudllfiry nerre* of embryo of about four ani a half weeka: rl. reoealus labyrintlii ; hc. ptc, ok, saperlar. poaterior. and external nemlelrciilar canalg ; I. Haccule: <. cochlea: vn./R. vdlbular and facial aervea; rg- rfi SB- veatlbular. cochlear, and genlciilatcganElin. C, left lahyrlnlh of embryo of about (Ivcwoclu, bom wllhnlit and below: labelllni; as In preredlngflKure.

The opiHJsite, anterior or ventral extremity of the otic vesicle tilsti bulges out into a small cvagiuatioD, wliieh gradually elongates until it is a tapering tube, slightly curved inward toward the median piano. This lengthens still more and becomes spirally coiled, forming the cochlear duct or scala media of the future cochlea (Fig. 168). The venicle it'^e If becomes constricted in such manner by an inward projection of its wall as to indicate its <livision into an upper larger and a lower smaller sac, the terms upper and lower referring respectively to the head-eud and the tail-end of the embryonic body. Before the con.strietion occurs, the wall of that part of the vesicle which is to become the future iipjKT or utricular division presents two {)ouched-out areas (Fig. I(j7, B). One of these gives rise to the extenud semicircular canal, while from the other are formed the saperior and posterior canals. The pouch that produces the external

Fig. 168.— niafrram to illustrate the ultimate condition of the membimnotu lAbjMnth (after Wnldeyer): i/, utrieulus: if, sacculus; cr, oanaliH rcuniens; r, ductus endolymphaticus: r. cochlea; k, blind sac of the cupola; r, vestibular blind cae of the du(?tu8 coehleuris.

canal is scmieirciilar in form and flat, lying in the horizontal plane, its upper and lower walls bring in contact with each other. The oppo.sed walls fuse, except at the periphery of the pocket, and hence all that remains of its cavity is a small marginal tube or channel, corresponding with its border and opening at each end into tlu* cuvity of the vesicle. Throughout the region of fusion of the walls, the latter become thin and finally disaj)i)oar, being replaced by connective tissue. Thus a semicircular epithelial tube is formed^ which is the horizontal or external semicircular canal. One end of the tube being dilated, the ampulla of the canal is produced.

the superior and posterior semicircular canals are formed in a somewhat similar inanucr by the other evaginatcd ponch or ])ockct, which is irregularly globular. To pro<bi<*e this result, the walls of the pocket contract adhesions throughout two regions, which (U)rresp<md with the rcs])cctive sj)accs enclosed by <'acli of the* two future canals in (jiiestion. The fusion of the walls takes place in such manner as to leave two narrow channels or tubes, one of which ahnost encircles the inner or mesial asjKHJt of the pocket, while the other bears the same relation to its jH)stcrior wall, the inner limb of the latter semicircle coinciding with the posterior limb of the former. The result of this arrangement is that two vertical semicircular canals are formed with their planes at right angles to each other, the two communicating with the otic vesicle by three openings, one of which is common to both canals. The other two apertures, being dilated, are the ampullated individiial orifices of the posterior and superior canals.

The constriction in the otic vesicle referred to above increases until this sac is divided into two parts, a larger, which includes the region from which the semicircular canals have developed and which is now the utricle, and a smaller vesicle, the sacculOi comprising the part from which the cochlear duct was evaginated (Fig. 168). The line of division coincides with the middle of the orifice of the ductus endolymphaticus, the proximal end of which participates in the division. Thus the ductus endolymphaticus becomes a Y-shaped tube, and affords the only bond of connection between the saccule and the utricle (Fig. 168).

The beginning of the cochlear duct, failing to keep pace in growth with the other parts, api)ears as a smaller tube relatively, and is known as the canalis reuniens (Fig. 168, cr).

The structures so far considered — the utricle, the saccule, the semicircular canals, and the cochlear duct — being the product of the ectodermic otic vesicle, represent simply the adult epithelial linings of those cavities. The fibrous layer of the membranous labyrinth, in common with the walls of the bony labyrinth, is a product of the enveloping mesodermic tissue. While the cells of the otic vesicle thus for the most part constitute the walls of the several sacs and canals of the primitive internal ear, some of the cells specialize into neuro-epithelium. The most marked specialization of this sort occurs in the cochlear duct, where most of the cells on that wall of the duct which may be called its floor — the part corresponding to the future membrana basilaris — undergo such profound modification in form as to produce the most highly specialized neuro-epitheliul cells anywhere to be found, the elements that constitute the organ of Corti.

In the utricle and the saccule, as well as in the ampulIsB of tlio semicircular C4inals, there is a similar but less marked sp(»cializati(m of epithelial cells to produce in the former case the maculae acusticse, and in the latter, the crista acusticfle of the ampnlhT. While, therefore, the cells of the otic vesicle which are to s(Tve as the lining mucous membrane of the membranous labyrinth become flattene<l polyhe<lral cells arranpMl as a sintrle lay(T, those cells which are to functionat(» as the periphenil part of th(» acoustic mechanism l>ecoine the specially modified C(>lumnar cells, many of them with cilium-like appendages, of the maculie, the cristae, and of the organ of Corti.

From the first the otic vesicle lies in close relation with the aeustieofacial ganglion (Fig. 167, />). As pointed out in a preceding chapter (p. 321), this ganglion subsequently divides into two parts, corresponding with the two divisions of the auditory nerve. This division (jf the ganglion and of th(* nerve is correlated with the separation of the otic vesicle into a coelilear part, the cochlear duct, and the two vestibular vesicles, the saccule and the utricle. While the cochlear duct IS still a short, slightly curved tube, the cochlear ])art of the ganglion lies in close proximity to the tube, in the concavity on its inner side. As the duct lengthens and becomes more coiled, tiie ganglion likewise lengthens into a band which follows the spiral course of the duct, lying parallel with the latter and on the side toward the axis about which it is coiled. Ai'ier the formation of the bony parts of the cochlea, this ganglion octMipies the sjnral canal of the modiolus and is known as the gangUon spirale. It helongs to the cochlear division of the auditorv nerve, which is distributed to the cochleii.

The remaining part of the acoustic ganglion becomes rather widely separated from the spiral ganglion, coining to occupy a position in tluMuternal auditorv meatus, and the part of the auditory nerve with which it is conne<*ted acquires relation with the macular regions of the utricle and saccule as well as with the crista' of the ampulhe of the semicircular canals. These nerve-fibers constitute the vestibular division of the auditory nerve, M'hile the ganglion is the vestibular ganglion or intumescentia ganglioformis of Scar])a.

The development of the bony labsrrinth of the internal ear^ as well as of the connective-tissue parts of the membranous labyrinth, is effected solely by the differentiation of the mesodermic tissue which surrounds the epithelial structures above considered. As previously stated, at the time when the otic vesicle is first formed there is no indication of a cranial capsule, the brain-vesicles being surrounded and separated from the ectoderm by indifferent mesodermic cells. During the progress of the alterations in the otic vesicle, this tissue undergoes condensation and alteration to form the membranous primordial cranium, and shortly thereafter the petrous portion of the temporal bone is outlined in cartilage by the further specialization of a portion of this primitive connective tissue. The formation of cartilage does not affect all of the tissue which is afterward represented by the petrosa, the region that borders the semicircular canals, the cochlear duct, the saccule, and the utricle remaining soft embryonal connective tissue. There is thus a cartilaginous ear-capsule produced which is more than large enough to contain the primitive epithelial labyrinth, and the walls of which are separated from the latter by embryonal connective tissue.

The bony semicircular canals are almost exact reproductions, on a larger scale, of the epithelial canals^ and they are formed by the ossification of the cartilaginous petrosa. Even before this ossification occurs the soft connective tissue between the cartilage and the epithelial semicircular canals differentiates into three layers. The inner layer, becoming more condensed, is converted into fibrous tissue, and, adhering to the epithelial walls of the canals, furnishes the connective-tissue component of the completed membranous canals. Its blood-vessels serve for the nutrition of the canals. The outer layer also undergoes condensation and forms a fibrovascular membrane, the perichondrium, which later becomes the internal periosteum of the bony canals. The middle layer, on the contrary, becomes softer — by the liquefaction nf tin* intercellular siul>stance ami the degeneration of the cells — so that gradually increasing, fluid-filled cavities make their ap|H*a ranee, and these latter becoming lar^(T and many of them coalescing, a ^pace is formed around the niemhranous canals which is filled with fluid, the perilymph. This perilymphatic space is bridged across at intervals by con nectivt»-t issue processes that serve for the convevanct* of l)hMMl-vess<*ls to the membranous canals.

The vestibule of the internal ear is formed in practically the same manner as the Imny semicircular canals, the epithelial saccule and utricle at'quirinj;^ their cimnective-t issue constituents in the same way. TheR* is the difference, however, that the bony v<»stibule dm^s not conform to the shape of the vestibular |mrts of the membranous labyrinth, since it is a sinjrle unilividinl cavitv enclosinjr the two little vesiclcs, the siiccule and the utricle.

The bony cochlea, while develo|K»d u|M)n the same general plan as the other parts of the bony labyrinth, presents certain cons|)iouous uKNlitications. The epithelial cochlear duct, as stati'd above, in its early sta^ is a short, tapering, and sli«rhtly curved tube. While it is still in this condition, chondritication of tlir petrous bone (K»curs, whereby the duct ac(|uircs its cartilaginous capsule (Ki^. 109, hk). This capsule is i)\\v\\ at the prnximal end of the duct and thronjrh this o|K*iiinLr lh<' cochlear bninches of the audittuy ncrvi' gain access U) the capsule, beinj; connected with the c(K'hlcar <livi>inn of the auditory «ran«rlion, which, owing to its prcviou>ly having a>>iuncd a jK>sitiou beside the duct, Ciuucs to be enclosed by th<* <'apsulc as the latter is formed (Kig. 1^>9, m\ <j^it). It is only after the chondrification that the ci>chlear duct lengthens out and becomes t>pirally coiled. The coiling is in such n)ann(>r that the cochlear nerve is surrounded bv the duct — thatis, it lies in the axis about which the duct is spindly wound. Within the cartilaginous capsidc, filling all the space not occupied by the spirally coiled duct and the ccH^hlear nerve with its lengthened-out ganglion, is the end)ryoni<* conn<»ctive tissue of which f(»nnerly the entin* cartilaginous {x^trosa ccmsisted.

Thu cochlea consists now of a spirally coiled epithelial tu!>e Ijiug within an eloiigati'd cavity in the cartilaginous petrosa, a cavity, the walls of which arc, therefore, cartilaginous. The peripheral wuU of the coiled tube is in contact with the inner surface of the wall of the iTnrtilaginous cnpfule (Fig. 169, x), a fact which has an imiHtrtaiit bearing upon ihc further stages of growth.

Fig. Ue,— Pan of ■ Bectloii Itiniugh the cocblea or an embryo Ml. II cm. (3.l> in.) long litter Boellchcrl: bt. cartl]*gln(nis capadte. In vhLch llie cocblmr duel describe* aseending »|"lral turns i ilf. duetUB t'orhletrin ; c. iirgun of Cortl; It, lamina vialibul«rt«: i, ouler wall of the membranous duclu«cochl«arl» nilh Ugamenlum aplrale; SV. acila vfitninll : ST*. S7'. acila tympanl; g. Kelallnoun tiaae, which itni Hlls the tcala Tvatlbulf inO In Ita I'M tniat: a\ remnant of the gelatinous tliiue, which I> not yet llquvllcd : M. Arm connective tluua summnillne the cochlear nerve lac); gip. ganglion splcnle; .v. nerrc which rum to Cortl'a or>n>n in (he nilure lamina aplrallanuea: r, c<im|<Hct cnnnL-dlre-tiune UfCT.wblch liepomo onined and shares In boundlBK ""o bone cochlear duct : P, pcrichon

The embryonal connective tissue within the capsule now undergoes important modifications, which vary greatly in different regions. That portion of this tissue which immediately envclo])ri the cochlear nerve becomes first dense connective tissue, which is afterward <lire<*tly coiiverte^l into bone, constituting the modiolus, or axis, of the cochlea. The processes of eondensation and subsequent ossification extend outward from the nuKliolus in a spiral line, which corresponds with the intervals bi^tween the successive turns of the cochlear duet, until they meet the wall of the original capsule, thus produein^ the bony cochlea. That is, by the development of this spiral plate and its connection internally with tlu» uKHliolus and externally with the wall of the capsule, a tub(* at first partly membranous and jwrtly cartilaginous, and at a later sta^e osseous, is produced, which encloses the mueli smaller cochlear duct, and like it is wound spirally arc>und the modiohis. To repcnit, the original sample eavity of the cartilaginous capsule is subdivided by the growth of the modiolus and of th(» spiral shelf in such manner as to become a t<jjinil/t/ coiUd tube.

The cochlear nerve, enclosed within the coil of the cochlear duct, semis branches (Fiji:. 1^^, -V) i»^ a continuous spiral line to th<* duct, and the soft tissue surrounding and supporting these branches condenses to form a connectiyetissue plate whicli extends outward from the modiolus tc the cochlear duct and which, therefore, has a spiral course about the modiobis, its entire inniM* edge being attached to that central axis, while; its outer border is, throughout its entire extent, in continuity with the inner wall of the duct. At a later sta^e tliis s])iral plate undergoes direct ossification to form tlic two lainclhe of the bony lamina spiralis. Thus it is that the ganglion s])ir:de and the successive terminal branches of the cochlear nerve come to be enclosed within th(; s[)iral lamina. Recalling the condition of the cochh»a before the growth of the spiral lamina, it will be seen that the latter, in connection with the epithelial cochlear duct, divides the tube into two parts (Fig. IfJO, aST, .ST). It will be evident, to(), that the epithelial cochlear duct now holds a relation to the* larger tube of the future bony cochlea which is similar in principh* to the relation of the membranous semicircular canals to th(» bony canals, but with the dilVcrcnce that the outer wall of the epithelial duct is in close contact with the outer wall of the future bonv canal at Xf and that the inner walls of the two are connected by a spiral plate, the lamina spiralis.

The cochlear duct, then, is surrounded by undifferentiated mesodermic tissue, except on the side farthest from the modiolus, where its wall is in contact with and finally adheres to the wall of the cartilaginous capsule. The lamina spiralis divides this tissue into two parts which respectively occupy the positions of the future scala vestibuli and scala tympani. Tliis soft embryonal tissue, as in the case of the corresponding tissue of the semicircular canals, develops differently in different regions. The innermost stratum, which is in relation with the epithelial cochlear duct, becomes fibrous connective tissue and constitutes the flbrons layer of the adult cochlear duct ; that is, on the side of the duct toward the scala tympani, it becomes the connective- tissue layer of the membrana basilaris, while on the side toward the scala vestibuli it forms the fibrous stratum of the membrane of Beissner (Fig. 169). The peripheral zone of indifferent tissue, that in contact with the now cartilaginous wall of the future bony cochlea, as well as that which lies against the lamina spiralis, also undergoes condensation and forms a fibrous, or fibrovascular, membrane, the internal perichondrinm or future periostenm. The tissue intervening between these two layers retrogrades, the cells degenerating and the intercellular substance liquefying, until finally the spaces known as the scala vestibuli and the scala tympani are hollowed out. These channels are lymph-spaces and the fluid they contain is the perilymph. This perilymphatic space is in communication with that of the vestibule. Therefore, while the cochlear duct or scala media encloses an epithelinm-lined space, as do the saccule, the utricle, and the membranous semicircular canals, and in common with those structures contains the so-called endolymph, the scala vestibuli and the scala tympani are in the same category with the perilymphatic spaces of the other parts of the internal ear.

The Middle and the External Ear

The middle ear, consisting of the tympanic cavity and the Eustachian tube, is devclojx}il from the back part or dorsal ond of the first inner visceral fUrrow. The external ear, comprising the external auditory meatus and the auricle, comes from the dorsal extremity of the first outer furrow and the tissue about its margins, the tympanic membrane representing in part the closing membrane which sepanites the inner furrow from the outer.

The first inner viscend furrow, in common with the other inner furrows, is an evagination of the lateral wall of the primitive pharvngcal ciivity, or head-end of the guttract. The ventral end of this groove suffers obliteration, but the dorsal s(»gment, designate<l the tnbotympanic sulcus, becomes converted into a tube by the growing together of its edges. The tube is composed therefore of entodermic epithelial cells. It elongjites in the dorsal and outward dire(?tion, and its dorsal extremity becomes enlarged to produce the cavity of the tympanum, the remaining part of the canal becoming the epithelial lining of the Eustachian tabe. The canal being formed before the development of the cranium, and approximat(?ly its posterior half being surrounded bv the mesodermic embryonal connective tissue that al'terward becomes the petrosa of the temporal bone, the tympanic, cavity and a part of th(» Eustachian tube come to be enclosed within that bone, while the connective tissue enchasing the anterior part of the tube differentiates into the curved plate of ciirtilage that forms the cartilaginous part of the Eustachian tube.

Since the posterior end of the primitive epithelial tube insinuates itself between the otic vesicle and the surface, the tympanum comes to o(*cupy its normal position on the outer side of the internal ear. The tympanum, being derived from the back part of the first vis(»eral cleft, is in close relation with the first and second visceral arches, and the ossicles of the mid<ll(» ear ar(» derived from the dorsal extremities of the cartilaginous bars (jf these arches in the manner described in C-hapter XVIII. Necessarily the primitive ossicles are exterior to the primitive epithelial tympanic sac, as is also the chorda tympani nerve, which jKisses ulcDg its outer nide. After the ossification of the temporal hone, these structures are emhedded within the abundant soft connective tissue which is between the epithelial sac, now the mucous membrane, and the bony walls of the tympanum. This mass of soft tissue undergoes verj' considerable diminution, owing to which the mucous membrane comes into contact with the bony walls, and as a result the ossicles and the chorda tympani are enclosed in folds of the mucous membrane and seem to lie within the tympanic cavity,' They are excluded, however, from the true cavity of the tympjinum, since they are exterior to the epithelial or mucous- membrane layer.

Fig. 170. -Showing the gracjua] cluvi;lii|iiiii.-iil uf tlio fiartiinf the external car ftom promlncncss upon the mandibular and hyoldean Tistcral archus lHI»),varlnmly magnlHeil : 1. 2. prunilnenceB on mandibular arcb : 3. prominence between the two archea, prolooged postertorly in leconil fixture tu Sr; t.b. and 0, pniminencea on hyoidcau nr lecond riaceral arph; ^.lowerjaw. Prominence I forms the li^rua; 2, 3. ^, the helix; 4. Ihu anllbelli ; G, Ihc antitragug: S, the lubule.

The external auditory meatus is simply the persistent posterior part of the fir«t outer visceral furrow or hyomandibular cleft (sec pp. \\2, 1 Hil, this cleft doffing completely everywhere but in this region. The closing plate of the firrt cleft becomes the tympanic membrane. Hiiice the outer layer of this membrane is of wt.xlennie orijjin, while the inner layer is entodermic, being continuous with the epithelial tympanic lining, and the middle fibrous layer is derived from the mesoderm. The relation of the malleus to the membrane and of the latter to the bony tyinjKinie plate which forms part of the wall of the meatus is dealt with in the chapter on the development of the skeleton.

The auricle is ilerived from the tissue around the margin of the uucIoscmI hack part of the first outer cleft (Fig. 171, C). Six little elevations make their appearance here, the projections being mesodermic tissue covered with ectoderm. The mesodermic component of the elevations diiferentiates into the <':irtilaginous and other connective-tissue jKirts of the auricle. The nodules marked 2 and 3 in Fig. 170 bi^coming a continuous ridge, produce* the helix, while nodule 4 becomes the antihelix. The tragus and antitragus develop resijcctively from the projections 1 and 5. At the end of the second month, these parts are so far a<lvan(?ed as to be easily distinguishable, and the connective-tissue basis of the ridges and projections and the continuous plate-like mass to which they all are attache<l be^in to undergo chcmdrification. From the third month onward, this primitive auricle, by continued growth and greater sepanition from the si<le of the head, assumes more? and more the charactei>> of the fullv formed member. The lobule, however, which results from the growth of the little elevation marked G, lags behind the other parts in development and is rather indistinct until the fifth month, after which time it increases in size and gradually acquires its normal proportioiis.

The Development of the Nose

The nose is primarily a special sense-organ, although a part of its cavity serves, in air-br(»athing vertebrates, as an adjunct to the respiratory system. The evolution of the mature organ of smell may be epitomized by the statement that the olfactory epithelium, the ess(>ntial part of this senseorgan, is a patch of dei)ressed or infolded ecjtoderm, the cells of which are highly specialized and ar(» brought into relation with the central nervous system by means of the outgrowth from the latter of a part of its mass, the olfactory lobe.

Verv wirly in intra-uterine life — before the end of the thinl week — the olfactory plates apjn^ar as loealizt^l thicken


iiigs of the ectoderm situated just in front of or above the on»l fossH. Tbese nasal areas are the forerunners of the future olfactory epithelium. It is worthy of note that the olfiictory [ilatf.* un.- \n very close relation with thf iirimiiry

Devetupment or (he flu» nr ihc bum&neiDbryn (TUB) - A, embryo of it twenty-nluv dayi. The nuuCroalal plate dlOeren Hating into proccuui gioDularea, toward whlph the mailllnry proccsies of llret vlucral arcb are eitendiag. B. embryo of about Ihlrty-fourdajra: Ibe global ar, lateral frontal, and nuiillnry prufetwea are In apposition ; the prlmlllTe openlnt; la now better deflned, C, embryo of about the eighth week - Immediate Imiinilsrle* of moulh are more deflntte and the nasal oriHces are partly (brmed, external ear appearing. D. embryo at end of oecond munth.

fore-brain vesicle, being, in reality, on the outer surfiiee of the ectodermic covering of its ventral wall.

Owing to the rapid outgrowth of the surrounding tissue, the olfactory plates l)ecome relatively depressed, constituting now the nasal pits, which arc distinguishable at about the twenty-eighth day. The pits are separated from each other by a broad mass of tissue, the nasal or iia8ofh>ntal process (Fig. 171), which is, as it were, a localized thickening of the inesoderniic tissue on the ventral wall of the primary fore-brain vesicle ; and this process makes its appearance in the third week. During the fifth week the nasofrontal process thickens greatly along its lateral margins, the thick edges being known as the globular processes (Fig. 171, A, B). At the same time the lateral nasal processes bud out from the nasofrontal process, one on each side, above the nasal pits, and, growing downward, form the external boundaries of the pits, each of which depressions is bounded on its inner side by the corresponding globular process. The nasal pits, therefore, have well-marked walls on every side except below, where they are directly contimious with the oral fossa.

In the latter end of the sixth week the nasofrontal process, which, it will be remembered, constitutes the upper limit of the oral fossa, is joined on each side bv the united maxillary, and lateral nasal, processes. This effects a division between the oral fossa and the nasal pits, and forms, though as yet crudely, th(* external nose, and the upper lip as well. The detinite formation of the external nose may be said to be indicated about the cH/htli rack. The orifices of the na>al pit.-^ are now the anterior nares, while the pits themselves have bectnne short canals, opening by their deep orifices, the posterior nares, into the primitive mouth-cavity ahove the palatal shelves. The nanvs are separated from i'ach other by the still broad nasofrontal j)roeess. That portion of the nasofrontal process that separates the nares gradually becomes thinner and produces the septum of the nos(\ while its external or superticial })art gives rise to the bridge and tip of the organ.

The growth of the palate- shelves (Fig. 172) toward the median line, resulting in their union with each other and with the recently-fornn^d septum, definitely divides the nasal chambers from the cavity of the mouth, th(» posterior nares now opening into the pharynx. This separation is completed toward the end of the third month.

The complexity of the ndiilt nasal cavities js proclticetl W the formation of ridges ami jKHielies on (he lateral walls of the original nasal pits. Three inwardly projecting horizontal folds of the eeio<lernii(i lining of the cavity, tho superior, middle, and inferior tnrbi&al folds, appear njion the outer wall nf each nasal fossa (Fig. 173). Each fold contains a stratnni of mesodermie tissue which develops into cartilage and nubseqnenily into bone, forming respectively the three turbinated bones. The cartilaginons character of these folds becomes apparent at the end of the second, or the early part of the third, month. An cvagination on the lateral wall of each

Fig. .— Roof or Ihe oral rnvlly ofa human emhirj-o with the niiidiunenta of (he poUlal prouenes (alter HIa), < 10.

nasal fossa, between the middle and the inferior tnrbinnl processes, becomes the antnun of Higbmore ; this is fnrmcil in the sixth month. Other cvafrinalions jirtxiiiec the ethmoidal, the frontal, and the sphenoidal sinuses, the last two of which are not completed, however, until after birth. Very early in the development of the nose a small invagination appears on the mesial wall of the nasal pit. In the fourth month of gestation this invagination has become n canal in the iipptum (Fig. 173, /), running from before backward and ending in a blind extremity. It is the so-called organ of Jacobson, which, in man, is merely a rudimentary strnctnre, but which, in most other mammals, is more highly developed, Iwing surrounded by a cartilaginons capsule and receiving a special nerve-supply from the olfactory nerve.

The olfactory plates lictKime sejiaratod from the fore-brain vesicle and ajii.-cijiiontly from the later brain and its oulgrowtli, the olfactwry bulb, by the development of au intervening bony plate, the cribriform lamina of the etlimoid bone. The ectoderraic cells of the olfactory plates differentiate into the highly specialized nenro-epithelial elements of the olfactory mncoiis monibranc, the olfactory epithelliun, and I their asiioeiuted supporting cells. The axon*; of the neuro- I epithelial cells piws upwuiil (liroiifrli die tribrlform ]»late of the ethmoid bone as the olfactory nerve-fibers, and, entering the ventral surface of the olfac'lory Ijnlb, arliorize with the proces-ses of the mitnd cells of ilie bull), whereby they acquire J functional relationship with the olfactory centers in the brain, 1

Fig. 173.— Cr SB a i i tl t tl tl hmd f ai en I rj i orown-rump meuureuient The iiuml cavlllw Bre (leen lot wlih thcoralisTilyal IhepUieii dealsniileiJ by a* K cBrUlHseaf tbeoual Mp-fl tuni;iH,turblnal»rlIl&ge J argan i f Jucnbson J the place when ItopaulBttfl Ihe naiial dbtUs' ; gf, paUul proceis; of. mftxlUary proceM; il. dBntal rld|».| |Hert»[g).

The esternal nose, as previously stated, first acquires defi- 1 nite form about the eighth week by the union of the distal J ends of the lateral nasal processes with the nasofrontal proo* 1 C8.S, the former proilucing the ala and the latter thebiid|«| and the tip of the nose. In the third month the organ is j tmduly flat and broad, but from this time on it gradually j a-ssume-s the familiar characteristic form. From the third i month to the fifth each external naris is closed by a gelat- J inous plug of epithelial cells.