Book - A Text-book of Embryology 9

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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The evolution of the £ace depends so largely upon the growth of the parts concerned especially in the production of the mouth and nose that any account of its development must deal for the most part with the development of those structures. In tracing the earliest stages of facial growtli, it will be well to consider the face as a whole before proceeding to a detailed description of its several parts. If we seek the principles underlying the conformation of the face, we shall find that its apertures and chief cavities are merely so many provisions for bringing the central nervous system and the aliraentarv tract into relation with the outside world. It will be seen, for example, that certain small depressions apiH'ur U|H>n the surface ; that one of these, which is destined to me the mouth and the respiratory part of the nasal dviticss a-ssumes relationship with the alimentary tract and with Its offshi^ot, the respiratory system ; thatother depres^MtSs. whioh s«l>scquently develop into the olfactory parts >^< th^ iwfcal ohanilH^rs, come into relation with (»utgrowths hmiiu the olfactory bulbs ; and that still another acttwltum In^comes the lens-vesicle, which likewise mvi>^ *»ttlk Att vHii^r\>wth from the brain to become a jmrt of ^ ^*-»f»4KMnt ^*ttp!*>H>npin* the eye.

avp itt tht* ditferentiation of the face is the fort«i|tH>.r >4 iiKi <ml flilt» the earliest indication of the future •t%Hu^. ^V' vHiJ i^iUit^ ;jip(H'ars on the twelfth day, and con* * >iH«ut ,^*vH of wtiHlerra and entoderm, the meso*^«4k|^.iU<ai« U icL ^tuatcd on the ventral surface of the head-end of the embryo, which already presents the enlargement of the cerebral vesicles. The oral plate becomes relatively depressed by the upgrowth of the surrounding tissues, the fossa thus produced constituting the oial pit or Btomodsnin (Fig. 57). The oral plate is now the pbatyngeal » (Fig. 66). Reference to the sajpttal section will

Fig, W.— Medl«n setllon tlirough the henil of an emhryo rabbtt 6 mm. long (arttr MlhslkovlcB): rA. laerobrene bulvtiin stomndsuni and 10 ru^t. pharyngeal membrane (Rachenhauli; lip, place rrom which the hypophysla Is developed; A, heart: kil, lumen or fare-gul: cH. ehorda; c, ventricle of (he cerebrum; i>). third ventricle, ihat of the belween-braln ithalamenccphalonl ; e', fourth ventricle, that or the hind'brain and after-brain tepeneephalan and tnctcncephHiun, or medulla obloni^ta) ; ft, central canal of (he spinal cord.

show that the oral pit corresponds in position to the headend of the gut-tract. The formation of the pit is, in effect, a piishin^-in of the surface ectoderm to meet the alimentary entoderm.

A second important factor in the development of the face is the appearance of the first and second visceral arches, which occurs in the third week. As pointc<l out in a preceiling section, the flnt viBceral arch divides into the mandibular areh and the maxillary process (Fig. 62), the latter being the smaller and appearing to spring from the mandibular areh. Both the maxillary processes and the mandibular arehes grow toward the mc<lian lino of the ventral surface of the body. Owing to the growth of these struct

itren and to the sharp flexion of the head and neck that MM!iirH between the twenty-first and the twenty-third day, the (inil pit becomes very much deeper and acquires more definite boundarie.s. During the third week it is a iossa of |H<nUif|^oul outline. Its upper boundary is ibrmed by the unpaired nasobontal or nasal process (Fig. 67, A), which is BAHentiully it thickening on the ventral wall of the forebrain vcHiele, brought int<> close relalion with the to^ea by the flexion almve reierredto. The lower boundary is formed by the tnundibular arches, while the lateral extent of the fosea 'i» limited liy the maxillary process of eaeh side.

KiHin atWr the appearance of the oml pit, the future nnre.s are fiifeHhadowed by tile development of the two oUactorr plates, flititiited one ou each side of the nasofrontal process, widely iH'iHinited from each other. These epithelial areas, which (HHin beeome dejiressions, the nasal pits, are closely united witli llie wall of the fore-brain vesicle from the first; they develop mibitefpiently into that [«irt of the nasal mucous inemlii'Utie which Is concerned esjiecially wilh the sense of Bmell. Thi.H fact Iwcomes very significant when it is rememberttd that the olfaotory bulbs, with which the olfaetorj- epithelium aNsumes intimate relationship, are outgrowths from the bniiii.

The nasofrontal process, during the fifth week, becomes mui-h ihii'ki'tu'd idoug ils lateral margins, forming thus the Clobular pTOCosses (Fig. 07.^1), which constitute the inner l)ouiKlnrii!.H of the nasal pits. At the same time, there grow downward and forwanl from the nasofrontal process two ridges, one on each t<tde, the lateral frontal processes, which form the out«r Imundaries of the nasal pits (Fig. (i7, A). In this manner the pitd become mmh increased in depth. The lateral frontal process projects between the nasal pit and the maxillary process, its line of contact wilh the latter strui'tiirc being marked by a groove, the uaso-optic fttirow or lacrimal groove. This groove later completely disappears; it i;t of imixirtiuiee, however, as indicating the position of the now developing nasal duct, which will l)e referred to hereafter. The nasal pit-* are widely in commnniciition wilh the cavity of the primitive mouth. About the fortieth day, however, the extremities of the maxillary processes have grown so far towani the median line that they have met and united with the lateral frontal processes and with the nasofrontal process (Fig. 67, B and C). In this manner the nasal pits become separated from the oral fossa, each of these openings aeqiiirinfi more definite lionndaries. It is apparent from this description that the upper boundary of the primitive oral cavity is not identical with that of the adult mouth. The nasofrontal process is the forerunner of the intermaxillary portion of the upper jaw, including the corresponding part of the upper lip and of the nasal septum and bridge of the nose.^ The lateral frontal process becomes the wing of the nose. By the completion of the changes here noted the face aajuires more distinctive form. It will be seen that the upper jaw proper results from the metamorphosis of the maxillary processes. The manner in which its sinus, the antrum of Highmore, is added, as well as the ossification of the jaw, will be considered hereafter.

Fig. ei — DeTeloproent of the Cuw ot the bummn cmbirn lllie) : A, emtiDo of ■bout twenly-nine dsyi, Tbe iiuaarrontai plalu difltrvuliBllnH Into pi-cicoiBUii BlobDlana, tow md which tlio maxillary proeenBea of Ural viscenil mreh are cxlendiDg-. B. embiyo of atioul thiny-(>iur ilayB : the glubular. lal«nil fKinUl, and mailllai; processuK are In apposition : thu primitive npcnln; a now belter detlned. C, embryo of about thu elvblh week. Immediate bnundarlei of moutb are more definite and the now.) iirincesnre partly (brmed. external ear appearing. D. embryo at end of second month.

The development of the eye will be described in connection with that of the sense-organs. In so far as the eyes have relation to the external form of the face, it will be sufficient to say that the surface ectoderm is iuvaginated in the fourth week to form the lens-vesicle, this sac, which gives rise to the crystalline lens, being covered by two little folds of iVtcKlerm, the primitive eyelids; that the organ is situated on the siilo of the head, in marked contrast to its position in tho matun* state ; and that the naso-optic ftirrow, previously ivK^rnnl to» jmssrs from the inner angle of the eye toward iW winvr f the nosi*. The development of Ihe face having Uv« ivMutinl t>ut in a giMieral way, the individual parts may tv vXHwivlonxl j^»|Kirately.

The Mouth

tV vx'x ik^w hrlotly, for the sake of convenience and clear*»%^s I'V s\<vli\^r history of the development of the mouth, Hv 'UkI »{io tiiM stop to Ih» the appearance, at the twelfth ^b\;k »4 t^\^ ^^ y4»W« By the enlargement of the anterior . .ui '4 ilv iKinnt tuU* to torm the cerebral vesicles, and bv

the development of the visceral arches^ this area becomes a depression, the oral pit. The pit is at first bounded caudad by the cardiac prominence and cephalad by the fore-brain vesicle (Fig. 57). In the third week the oral pit becomes a five-sided fossa, owing to the growth of several new structures. These are the unpaired nasofrontal process, which bounds the fossa above, the mandibnlar arches, which bound it below, and the maxillary processes, which form the lateral boundaries (Fig. 67). The mandibular arches do not actually unite with each other until the thirtv-fifth dav. A transverse groove appears on the outer surface of the united mandibular process, the elevation in front of which is the lip ridge, while behind the groove is the chin ridge; these ridges respectively produce the lower lip and the chin. The angle between the maxillary process and the mandibular arch corresponds to the angle of tli(» future mouth. In the sixth week — about the fortieth day — the oral fossa acquires a new upper boundary, whi(!h separates it from the nasal pits, by the growth of the maxillary and lateral nasal processes The primitive oral cavity, as before mentioned, is at first separated from the gut-tract by the pharyngeal membrane (Fig. 6G). This structure ruptures at some time during the fourth week, thus bringing the mouth into communication with the upper end of the gut-tract. The exact location of the pharyngeal membrane with reference to the adult pharynx is somewhat difficult to define; it is certain, however, that the primitive mouth includes more than the limits of the adult oral cavity, comprising, in addition to the latter, the anterior part of the adult pharynx. Reference to a sagittal section, as in Fig. 66, shows the relation of the oropharyngeal cavity to the brain-case ; in the tissue separating the two the floor of the cranium is subsequently formed. A little evagination from a point (Ap, Fig. 66) in the back part of the primitive oral cavity becomes the anterior portion of the pituitary body or hsrpophysis, the posterior lobe of which develops as an evagination from the floor of the primary fore-brain vesicle. With the development of the floor of the cranium, the hypophysis becomes entirely isolated from the oral cavity. A little pouch or recess usually demonstrable in the median line of the roof of the pharynx of the child, though not always present in the adult, is the persistent pharyngeal end of the diverticulum that forms the hypophysis; it is known as the pharyngeal bursa or Bathke's pocket.

Very soon after the formation of the upper jaw in the nuinnor above described, the oral surface of the jaw presents two parallel ridges. Of these, the outer, which is the larger, doV('h)pH into the upper lip, while the inner smaller ridge betH>iU(»H the gum. The lip and gum of the lower jaw are produiHHl Hiniihirly, at the same time or a little later. So far, tho only d^Mnarcation between the mouth and the nasal levity in furniHhed by the tissue representing the united linmthHMital, hitenil nasal, and maxillary processes, the nares ii|H'hihft widoly into the cavity of the mouth posterior to this


'V\\\> rorniation of the palate, however, effects a separation Ih^Iw^h^U thr two that gives to each space its permanent limitau ihr inner or oral surface of the upper jaw two «ih\'iritko phJiH^tiouH appear, one on each side, which are n of the Aiture palate. These gradually grow V\»n\«U'aI iHioh othrr, the tongue, which has meanwhile been slv\\^l\»|»iu)^, p»H»hvting upward between them. In the eighth VUH Ki Mhiou v»l* \\w^K' two lateral halves of the palate begins ^l' \\w^\ wwWvwA' <»\li*«»inities. By the ninth week union has Uks^u I'lmn* a** lar back as the extent of the future hard ^H^iU^s mid. I\N llio t^h'venth week, the constituent halves of ihiv joll W have uuiteil also. As these two halves ap^»u^^v^K \H\K'\\ \»tlirr the tougu<» recedes from between them, v(iH»wth t»r the lower jaw, so that, when union imui ', thai oi^au iHHMipies its normal position under the ^^^I»U^. ( Vv.Hum t\irmation within the soft tissue first formed puulurr-' \\w }mlute piHuH^sses of the superior maxillae and of tl*o juiluio Imiiert, wliioh prtH*esses collectively constitute the hard palutr of the adult. The intermaxillary bones are toiiiu'd wiihiu the primitive |mrtiti(m between the mouth and the uau*.-.. The rompletiou of the palate definitely marks otl tho uasil t'tiambers fixuu the mouth, thus dividing the early oral luivity into a lower sjmkh*, the true mouth, and an upper region, which is essentially a part of the respiratory system.

The uvula appears during the latter half of the third month as a small protuberance on the posterior edge of the soft palate.^

The Teeth

The teeth, morphologically considered, are calcified papillae of the skin, capped by a layer of peculiarly modified and calcified cells of the epidermis. Although in man and the higher mammals the teeth are found only upon the gums, in certain lower types they have a much wider distribution, occurring upon the roof and floor of the mouth and in the pharynx, and also, in selachians, upon the general skin-surface, in which latter case they are so modified as to constitute scales.

The dentine and cementnm of the tooth, as well as its pnlp» are derived from the mesoderm ; the enamel is a direct derivative of the overlying ectodermic epithelium. Mammals are said to be diphyodonty since they develop two sets of teeth ; while such groups as sharks, which continue to produce and lose new teeth throughout life, are denominated polyphyodont

The development of the teeth is inaugurated in the sixth week of embryonic life by the multiplication of the epidermal cells covering the surface of the gums to form a linear ridge. The growth of the ridge is away from the surface, so that the new structure projects into the underlying mesoderm. This horseshoe-shaped ridge, which corresponds in direction and extent to the line of the gum, subdivides into two parallel ridges, of which the outer marks the position of the future groove between the gum and the lip ; the inner is the dental shelf or dental ridge, which must be regarded as the earliest indication of the future teeth. The dental shelf extends into the underlying mesodermic tissue, not directly

  • Deficiency of union of the halves of the palate, resulting in a median

fiuBure, constitutes the deformity, cleft palate. This deficiency may be limited to the hard or to the soil palate, or it may affect both, or it may be seen in the uvnla, either alone — deft or bifid uvula— or in conjunction with cleft palate.

iliiwiiwiinl l>iit in an oblique direction toward tbe inner op llitKiitil wirfnoi- of the gum. While the dental slielf is growliiKt it" liiK' of L^ounectiuD with the surface ectoderm is iiiui'ktKl liy tiw superficial dental groove, wliich at one time wtifl hHiktKl iiiKiii as being thu first evidence of tooth-formation.

lI|Min lh« itidd of the dental shelf opposite the free or oitil Niirfiici), individual protuberances develop, corresponding ill niuiilHtr to that of the teeth of the temporary set — ten for riioh Jiiw. ICach little projection consists of a mass of ectoili)i'inli^ I'clU. which soon becomes expanded at its deep exIroinity, iMTomiri)^ thus club-shaped and later Ha-sk-tihajied, anil which in <ni]led llie enamel-sac or primitive enamel-Kenn, nUlcv tJio eiitiiucl of the tooth is developed from it (fig. <JS),

Mcntiwhile the continnity of the original dental shelf is broken by the di»ip|K-uranoc of the t*Il3 in the intervals between tlio in<)iviilual ennmel-germs, each germ iiccoming thereby isiilnti-d from itn neighbors. The neck of the flask shaped enamel-germ becomes reduced to a slender strand of cells and finally disappears, so that there is no longer any connection between the enamel-sac and the ectodermic cells of the free surface of the gum. While the enamel-sacs for the temporary teeth are growing in this manner, the corresponding structures for the teeth of the permanent dentition bud from the inner side of the dental shelf — that is, the side looking toward the tongue — except those for the three permanent molars, w hich grow backward toward the articulation of the jaw from the position of the second temporary molar.

As the enamel-germs grow downward into the mesodermic tissue, the latter sends up a number of conical projections, the dental papills, one for each enamel-organ. This dental papilla, of mesodermic origin, is the parent of the dentine and of the pulp of the tooth. When the dental papilla and the enamel-sac meet, the sac becomes invaginated, its under surface assuming a concave form. The enamel-sac at this stage therefore is a double-walled cup which caps the dental papilla. It is at about this time thatthe connection of the enamel-organ with the surface ectoderm is lost.

The further evolution of the enamel-organ consists essentially in the arrangement of its constituent cells into three layers and the formation, by the deepest of these three layers, of the special elements of the fnlly-developed enamel — the enamel-prisms. The most superficial stratum of the enamelorgan is composed of low columnar or polyhedral cells; the deepest layer, that nearest the papilla, the so-called membrana adamantina, consists of beautifully regular columnar cells, the ameloblasts or adamantoblasts ; between the two is a group of less characteristic epithelial elements. The cells of the deep layer, the enamel-cells, are alone con(?erned in the production of the enamel. The enamel-organ for a time covers the entire dental papilla. During the course of development, however, the growth of that part of it covering the future root of the tooth aborts, leaving the crown alone covered with the enamel.

The first step in the formation of the enamel-prisms by the enamel-cells is that the protoplasm of the deep extremity of each cell becnmex homogeneoii!>, and a tuft develops on the end of the pell, projecting loward the papilla. By the calcifieatiuD uf this tuft the formation of an enamel-prism is l)Cgun (Fig. 69). The process of caU'J Heat ion continues to advance from the deep or papillary aspect of the enamel-org-aii toward the surface. From this it comes about thatthe newest enamel is next to the enaniel-celis, or, in other words, nearest the surface, and also that the cnamel-prisma are arranged in a directiun generally vertical to the free surface cf the tooih. The formation of the enamel of the milk-teeth begins in rii. M -Himii-rtiiiKniniiuaik (Is- the latter part of tUe/uurih monlh. iMieiiylng B.iiiiii«i.«rgnii (Tuur- The middle layer of the enamelii i,..i.iit™i™«moi- Q^^p becomes greatly altered in

mil; J mill K.oi'Umf Inner larer

conatttution, owing to the accumulation of fluid and to the reduction of its colls to the form of ihin plates, the appearance being rather that of eonii«;tive IImiic than of an rpithelial structure. The sujierlicial layer of w\U uiidi'rf{iii-M atrophy, their exact fate not being known. The ritii'ithic ii>uinaut of the onaniel-organ is found upon the IVee nuifac-c of (he tooth for a variable time after its (iniplion, I'oimtilultMg (he membrane of Naamyth.

'I'lie dintaJ papilla hax beeu relVrrcd to as the structure lliat K<veH rise to the dentine. It origitintes from active milltiplicatiiiii of llie mesiMlermii- cells. The number of |MpillH> cnrrcrtpondH to the number of enamel-organ^;. As the [itipillu grows toward the enamel -organ it early acquires vawnlurlty. The shu]ie of the papilla, whether that of an ineinor, of a L^nine, or of a molar tooth, is determined by the fthiipc which the eniiniel-orgiHi assnnics. the conneetive tissue cells upon the surface of the papilla assume distinctive character, becoming large and branched, and constitute the so-called odontoblasts (Fig. 69). They are virtually modified osteoblasts. Forming a continuous layer, they have been styled the membrana eboris. Between this layer of odontoblasts and the enamel-organ a layer of intercellular substance appears, the membrana prsformatiya. The odontoblasts now send out processes toward the enamel-organ, which are known as the dental processes. Calcification begins upon the surface of the papilla and progresses toward its center, but is not complete. Small uncalcified areas, corresponding to the globular spaces of the completed tooth, remain next the enamel. The dental processes likewise fail to become calcified, and these are the adult dentinal fibers occupying the dentinal tubules of the finished dentine. The odontoblasts continue the formation of dentine until the dental papilla is entirely surrounded by it. What remains of the papilla, upon the completion of the tooth, constitutes the pnlp, a highly vascular connective-tissue substance supporting upon its surface the odontoblasts. The deposition of dentine begins in the latter part of the fourth month.

During the metamorphosis of the dental papilla the mesodermic tissue immediately surrounding it undergoes slight condensation to form the follicle of the develoj)ing tooth. As the enamel-organ recedes from the surface, the follicle increases in extent to such a degree as to envelop the entire rudimentary tooth. Only that part of the follicle which covers the future root of the tooth is of subsequent importance, however ; undergoing partial transformation into true bony tissue, it gives rise to the cementom or crosta petrosa, while the unossified external fibrous layer constitutes the lining periosteum of the alveolus (Fig. 68).

The development of the permanent teeth is precisely analogous to that of the milk-teeth. The enamel-germs for the permanent teeth, with the exception of the molars, bud from the lingual side of the dental shelf in the seventeenth week (Fig. 70), the germ for the first permanent molar appearing about a week earlier at the posterior extremity of the dental shelf after llie manner of a milk-tooth, The germ for the second molar XhvXa fr^ini the tieck of the first molar in the tliir<l month after Itirth, while that of the third molar, the wisdom tooth, springs from the neck of the second about the third year. At birth, therefore, the gums contain the

enua at o inl1k-t>i'>Ili and ul

, llilckenetl oral eplLLeUum: «i, i •rown out at u rrom the dicIe (>) of II. Uecki-raciLrtlligt^.

two sets of teetli except the second and third permanent molars.

The eruption of the temporarr teeth begins usually at about five and a half nioiilhs after birth wilh the appearance of the central incisors, and is coniidete at from eighteen to thirtysix months, when thu sccniul molars are cut. The first t«eth of the penuftnent dentition are the tirst mohii's, which are eru)ited at ahoiil the sixth year. The accompanying table shown the time and the onlir of eruption of the teeth :


Temporary Dentition.

Central incisors 5} to 7 months.

Lateral incisors 7 to 10 months.

First molars 12 to 14 months.

Canines 14 to 20 months.

Second molars 18 to 36 montha

Permanent Dentition.

First molars 6th year.

Central incisors 7th year.

Lateral incisors 8th year.

First premolars 9th year.

Second premolars 10th year.

Canines 11th to 12th year.

Second molars 12th to 13th year.

Third molars (wisdom teeth) 17th to 21st year.

The Salivary Glands

The salivary glands, which in mammals consist of three pairs, the parotid, the submaxillary, and the sublingual, develop as outgrowths of epithelium from the lining mucous membrane of the mouth. The epithelial elements of the glands are therefore of ectodermic origin. The growth of the submaxillary gland begins in the sixth week, that of the parotid in the eighth week. Each epithelial outgrowth is at first a solid cylinder, which undergoes repeated branching and acquires a connective-tissue framework and capsule from the surrounding mesoderm. It is not until the middle of the fifth month that the lumen of the gland appears. This is brought about by the moving apart of the epithelial cells composing the cylinders and their branches. The main duct of the gland first becomes hollow, then its branches, and finally the lumina of the alveoli make their appearance. The respective sites from which the several glands grow correspond in a general way to the |X)sitions at which the ducts of the adult glands open into the mouth-cavity.

The Tongue

Although the tongue originates from tissues belonging really to the walls of the pharynx, its development may be conveniently considered in connection with that of the mouth because of its relations in the mature organism. This organ, composed chiefly of muscular substance, is formed from three originally separate parts, an anterior unpaired fundament, and two posterior bilaterally eymmetrical segments. The line of union of these three parts is indicated approximately in the adult organ by the V-shaped row of circumvallate [Htpilla; on the dorsum of the tungtie. The anterior part of the tongue develops from a small unpaired tubercle, tlio tu1)erculiim impar, which grows from j the median line of the floor or anterior wall of the pharynx between the first, or mandibular, and the second, or hyoid, areh {Fig. 71, 6). the posterior segment of the tongue results from thegrowing together of two lateral halves, which develop from the anterolatend walls of the pharynx at the position of the second and third visceral arches, but not from the i arches themselvos. These ridges arc sometimes described aa ' the fused anterior (ventral) extremities of the arches just mentioned. The unpaired tubercle increases in size to such an extent as to constitute the major part of the organ. In the median line of the anterior wall of the pharynx, immediately behind the tuberculum impar, the epithelial lining of this cavity pouches forward and downward to develop later into the middle lobe of the thyr()id body. As the ridges which are to form the posterior [wrt of the tongue lie laterally and posteriorly to this median evagination, they completely enclose it in the process of fusing with each other and with the anterior tubercle. In this manner a canal or duct is formed leading from the surface of the tongue at the angle of junction of its three segments down to the middle lobe of the thyroid body, the latter meanwhile having descended from its original position. This canal is the thyroglossal duct or canal of His. During the further progress of development, the canal suffers obliteration, its only vestige beiug the orifice, which is known as the foramen csecum of adult anatomy.

., mm. (nbout 25 lo 30 linja). /, II, III, 1 , , uumt ,»oeral lurrow*; v, «inu» pr»pervi«all«, comprtaliig third and Fourth outer furrows: l,S,3, 1., vlieeral archei, s>ch with lU v<8(vml-Arch vc«el; e, tubercalum Imparl 7, orlflce of larynx : S, pulmunary evaKiDalian.

The papillae of the tongue are found exclusively on the part derived from the tuberculum impar ; the line of union between the anterior and posterior parts lies therefore behind the row of circumvallatc papilla?. The papillae begin to make their appearance as early as the beginning of the third month.

Prior to the union of the two lateral halves of the hard palate, by which the primitive oral cavity is divided into the mouth proper and the nasal chambers, the tongue projects upward between the palate-shelves, almost completely filling the primitive mouth. As the palate-shelves approach each other, however, the tongue gradually recedes to its subsequent normal position.

The Development of the Nose

The nose being an organ of special sense, its development is described in connection with that of the other special-sense organs in Chapter XVI. Owing, however, to its important relation to the other parts of the face, it is desirable to refer to its evolution in this connection. For a more detailed account, the reader is referred to Chapter XVI.

The first indication of the organ of smell is in the form of the two patches of thickened ectoderm, the nasal areas or olfactory plates, which appear on the head ward side of the oral fossa in the third week of development. At the end of the fourtli week the areas are depressed and constitute the nasal pits (Fig. 67, A). The nasofrontal process, a mass of thickened mesodermic tissue, lies between them. During the fifth week the lateral edges of this process become thick and rounded, forming the two globular processes, while growing outward and downward from the sides of its base are the two lateral nasal or lateral frontal processes. Thus the nasal pits, which corresiK)nd with the position of the future anterior narcs, become bordered on the mesial side by the globular processes and on the outer side by the lateral nasal processes. Below, the pits arc continuous with the oral fossa. Owing to the continued growth of these masses the pits gradually become deeper. The lateral nasal process is separated externally from the maxillary process of the first visceral arch by a groove, the naso-optic fturow. The lower extremities of the maxillary and lateral nasal processes soon unite with each other and advance toward the median line below the nasal pit. In the latter part of the sixth week they unite with the nasofrontal ])rocess and thus separate the nasal pits from the oral fossa and furnish the basis of the up{>er lip. The nasal pits are now the anterior nares, and the nose is represented by the irregular masses of tissue surrounding them. While the orifices of the nares are separated from the orifice of the primitive oral equity, their deeper parts are continuous with the latter, there being as yet no hard or soft palate.

In the eighth week the nose first acquires definite form, owing to the continued growth of the masses of tissue referred to above. The nasofrontal process forms the bridge of the nose with the nasal septum, and also the intermaxillary part of the superior maxiihe and the oonnoetive-tissue parts of the upper lip. The lateral frontal process becomes the ala of the nose. The nose is still verv broad and flat in the third month, after which time it grailually assumes its characteristic form.