1897 Human Embryology 26

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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Chapter XXVI. The Mouth Cavity And Face

The face may be said to be a characteristic of the higher vertebrates, and to acquire increased importance as we ascend the series. In the marsipobranchs, ganoids, and selachians, the face does not form a projecting apparatus, there being merely an area on the ventral side of the head, which is distinguished by including the mouth and the nasal pits. The primitive arrangement is somewhat masked in the marsipobranchs by the modification of the mouth into a large sucking apparatus, but in ganoids and elasmobranchs it is preserved throughout life with little alteration. That the vertebrate mouth belongs primitively on the under side of the head, and is at first a simple transversely expanded orifice, is clearly established by the embryology of every vertebrate class. Balfour ('* Comparative Embryology," II., 317) seems to have been the first to definitely formulate this generalization. The evolution of the face, so far as we could judge at present, depended, Jirst^ upon the enlargement and fusion of the oral and nasaJ cavities, which involved a change of site for the hypophysis; second^ upon the partial separation of the nasal and oral cavities, leaving the posterior nares open ; third, upon the growth and specialization of the facial region, of which the elongation of the jaws is the most conspicuous indication; fourth, upon the development of a prominent external nose. At the same time there occur modifications of position in the face in relation to the brain and its case or cranium, which it will be well to mention briefly in order to render the following sections of this chapter clearer.


The position of the face, or oral region, is originally determined by the head-bend, as is more fully explained in the following section, see also Fig. 319. If we imagine a median longitudinal section of the head to occupy a rectangular area divided into quarters, then we may say the lower posterior quarter corresix)nds to the mouth region, the other three quarters to the brain. As development progresses, the oral quarter enlarges out of proportion to the rest of the head so as to project forward in front of the fore-brain ; in this stage, which is represented by the adult amphibians, the bulk of the facial apparatus is very great proportionately to the cranium. In the reptiles, the oral region is elongated still further in front of the brain case, resulting in the long snout. In mammals a third stage is established by the great increase in size of the brain, especially of the cerebral hemispheres, in consequence of which the brain comes to extend over the snout, as it were; in man, whose brain has the maximum enlargement, the facial apparatus is ahnost entirely covered by the brain. The modifications involved in the increase of the brain in mammalia, so far as the skull is concerned, have been considered p. 467 ; they are well indicated by Wiedersheim in his '* Gnmdriss der vergleichenden Anatomie," 2te Aufl., Fig. 84. In brief, the facial apparatus, 1, underlies the hind brain, as in elasmobranchs ; 2, projects in front of the brain (amphibia, reptiles) ; 3, is covered by the cerebrum (manmials).


Formation of the Oral Cavity

When the medullary tube enlarges to form the brain — see Chapter XXVII. — the end of the head bends over to make room for that enlargement. The bending of the head carries the oral plate over on to the ventral side of the freely projecting head, compare p. 2G2. In Fig. 100, the head-bend is just developing; Ent^ indicates the anterior extremity of the entodermal canal, and the reference line crosses the oral plate, or membrane formed by the union of the entoderm and ectoderm ; the oral plate occupies the entire space between the fore-brain, /6, and heart, ftf, and there is as yet, properly speaking, no oral cavity, but it arises by the next changes which occur. The changes which develop the mouth cavity are the growth of the brain and of the pericardial canity, both of which expand ventrally, leaving a space — the mouth cavity — between them. Fig. 170. Laterally the cavity is bounded by a wall or sheet of tissue, which stretches from the pericardial somatopleure to the head and is the anlago of the cheek; it may be called the cheek plate (Wangenplatte) , The mouth cavity is now a shallow fossa between the head and the heart, and still without connection with the entodermal canal (Iiuman embryo of 2.15 mm. with two aortic arches). The fossa cannot, strictly speaking, be regarded as an invagination, such as is the invertebrate vorderdarm, p. 20 1, but is rather the result of the growth of the parts surrounding the oral plate. The oral pit is lined by ectoderm.


While the oral fossa is developing, the formation of the gill pouches begins. About the time the third branchjal arch is formed, the oral plate ruptures in the human embryo, and the oral fossa communicates widely with the pharynx, Fig. 320. Upon the lateral and ventral sides no boundary can be found later, but upon the dorsal or cranial side a projection persists, Fig. 319, in front of which apj)ears an evagination of the oral fossa, to constitute the anlage of the hypophysis cerebri or pituitary body (see below, p. 571), and behind which appears a second evagination from the pharynx to constitute the so-called Sei^sel's pocket, p. 208. The oral cavity proiwr and the pharjTix are now merged into a single wide cavity, Fig. 320, for which wo have in English no special term — in German it is sometimes called the Mundrachenraum (His, ** Anat. monschl. Embrvonen," Heft III., 20). The ectodermal mouth cavity, or oral fossa, does not correspond to the mouth cavity of the adult, for the adult cavity must include part of the pharynx, since it includes the tongue, which is developed from the flc^:)r of the pharynx, and in fact His has shown ("Anat. raenschl. Embryonen," Heft III., 31) that the arcus palato-glossi, which are taken as the boundarj' between mouth and pharynx in the adult, are derived from the se(*ond or hyoid arches of the pharjmx. Hence the adult mouth cavity includes the ectodermal oral fossa plus the region of the first gill-arches of the pharynx


ln human embryos of the third week the mouth is a five-sided orifice, and I observe that the same »hape appears in other mammalian embryos, and also in both amphibian and elasmobranch embryoB, Fig. 31G, hence it is probably characteristic of all vertebrates in the stage with five unmodified aortic arches. The mouth is bounded (His, "Anat. menschl. Embryonen," Heft III., 30) anteriorly by the wall (Stirvumlat) of the head covering the forebrain between the nasal pits, Fig. 310, A, laterally by the maxillary processes, Mx, and latero-posteriorly by the mandibular processes, Mfl; the latter are the first branchial arches, and unlike the following arches, br, they meet in the median ventral line.


Another important factor in the development of the oral region is the descent or migration of the heart. It will be remembere<l that the aortic end of the heart moves from the anterior or buccal end of the pharj-nsc, tailward. The change in the heart's position leaves the greater part of the pbarj'nx free to be <lifferentiated in intimate association with the oral region, and the change aJso separates the mouth and the heart, so that very early we find the caudal or lower boundary of the mouth to be no longer the pericardial somatopleure, but the mandibular processes or arches, the ventral ^. v"^'^;£. ^^ ends of which are developed behveen the month and mandi'huiar proceml heart. hi. p.M.ttion ot h^rt;


In certain teleostit, sorae time after the first pair of gill-pouches develop, the raouth breaks through in the ventral region of these pockets hh a bi-lateral involution of the ectoderm, fusing with the entoderm and opening on each side of a central partition; neither involution crosses the median lino. The double oral invagination was discovered by A. Dohrn, 82. 1, and the discovery has Ixjen confirmed by J, B. Piatt, 91.1, -iH-i. In other t«let)sts (Mcintosh and Prince. 90.1, TT3) the mouth is single and median in origin, an in the remaining vertebrates. The significance of the al)errant double origin is unknown, though Dohm interprets it as evidence of the evolution of the mouth by the fusion of two gill-clefts.


The Evolution of the Vertebrate Mouth is still one of the most puzzling of the unsettled problems of morphology. The increase of extent of the mouth cavity in the higher as compared with the lower vertebrates is discussed in the next section on the hypophysis. The present section treats only of the origin of the verterate mouth. The first question is, necssarily, whether the mouth of vertebrates is homologous with the mouth of invertebrates or is a new structure. The fi>rniation of the embryo by concrescence enables us, I think, to decide between those alternatives. In Peripatus, the leeches, and the annelids with well-marked concrescence, the nnion of the octental lines is incomplete, the anterior and posterior ends not meeting, but leaving the two ends of the elongated gastrula mouth open, to fonii the mouth and anus respectively ; the mouth is carried inward by the invagination of the vorderdarni, and the primitive mouth is thereafter merely the opening of the vorderdarm or oesophagus into the archenteric canal.* In those invertebrates in which the process of concrescence is plainly marked, the mouth is seen to be the anterior extremity of the gastrula mouth, and to he bounded by the ectental line; the site of the invertebrate mouth is where concrescence begins, and it is, therefore surrounded by the ectodennal neural plate, + forming the brain {Scheitelplatte ocesophageal commissures, and ventral nerve chain {Bauch(fanglienkette). The corresponding point in the verte\ brate embryo is easily found, \ being between the optic \ evaginations at the place ■ marked m, in Fig. :n7, and ': which probably corresponds ' to the future infundibulum / in position. So far as I am / aware, the relations at this point during early stages in ;ii-^„^;^.' j^^"*^^ vertebrates have never been


Fig. 317.-Bla8t<xlenn of a Do^-Fi^h, Acimthlas, thoroughly Studied witll the with commeneiDf? (.'oncrescencv. A/, Point corre- intention of aSCertainin&T spondiofi: to invertebrate mouth; i?, blastodermic u xv j. r

rim. whether any traces of a communication with the archenteron could be found. Until this is done, there can be, in my judgment, little hope of our knowing what has become of the invertebrate mouth.


The above determination of the site where we have to search for the original mouth may be accepted with considerable confidence. If it is correct, it sets aside two hyp<^theses which have attracted attention : Jirsty the hypothesis that the vertebrate mouth is identical with that of the invertebrate, and, second, the hypothesis that the old mouth is represented by the hypophysis,! for neither of these structures are derived from any part of the gastrula mouth. That both these hypotheses are untenable is evidenced by the deductions involved in their adoption. The annelid brain lies in front of the mouth ; if, therefore, either the hypophysis or the mouth of vertebrates is identical with the annelid mouth, then the brain and spinal conl must correspond to the ventral nerve chain only, and the annelid brain must have entirely disapix^ared. The vertebrate brain and eyes thus be(»ome new structures — a conception which seems to me

  • The nieanini; of the (lou>)1e origin of the month described by C. Semper in biuldinj? annelids and by Kleinenberi; in I>iiHidorh.vnchus has not fMH^n exftlnined. That it has the Hicmiflcanee Attributed to it by KleinenU»rjc can hardly l>e a<lmitt<'d. for th«»re is no «»vi«lenee Uiat It represent* a primitive mode of development.

tit seemH to me justiflable to speak of this an a continuous neural plato. althouifh there is a certain independence of development betwe«*n the "Scheitelplatte " and ventral chain, and although the cr)mmiHsures develop later.


That the hypophysis represents the annelid (psophafnis was first susr^restetl by A. Dohm. 75f.a. but he has since withdrawn his opinion. Similar was Richard Owen's infelicitous homoloiiry of the hypophysis, infundibulum. and pineal jrland with the old «psophairus ( Proc. Linn. Soc. . London, xvi). Beanl has revlveil I kdim'** theory, init has not suci*ee<ie<l in rendering it more plausible, to my judgment. Com|>are A. l)ohm,'H3. 1.

indefensible. Another deduction involved in the views under discussion is that a line of concrescence runs from the hypophysis or mouth to the fore-brain, representing the closure of the gastrula along that distance — yet of such a line not a trace can be detected.


As the infundibulmn is an invagination of the ectoderm toward the archenteron developed at or near the point where the invertebrate mouth lay, it is quite possible that it corresponds to the oral invagination (vorderdarm) of annelids. This identification has been more or less in the minds of morphologists for twenty years past, but no one has yet brought decisive evidence to justify it; nevertheless, its plausibility must be admitted.


Since the vertebrate mouth is regarded as a new structure, the second question comes : How did it arise? As we have seen, the first trace of the mouth is the oral plate, p. 2G2, formed by the union of the ectoderm and entoderm over a small area without mesoderm in front of the brain ; by the development of the head-bend the plate is carried over on to the ventral side and the oriil cavity is developed. There is nothing in this history which we can recognize as a clew to the origin of the mouth, but, on the other hand, there is nothing in it strictly incompatible with Anton Dohm's hypothesis that the mouth of vertebrates represents two gill-slits united in the median ventral line. The chief facts in favor of Dohm's suggestion seem to me to be : firsts that the trigeminal nerve shows the same relation to the mouth as other cranial nerves (facial, glosso-pharyngeal, and vagus) to the gill-clefts: second, that the gill-clefts approach the median line anteriorly, the first pair being nearest, the last pair furthest from the middle plane; thirds that the oral plate is formed like the membrane across a gill-cleft (Verschliissplatte)^ p. 2G4, of ectoderm and entoderm united without mesoderm. Dohm has recmTed repeatedly to this h>T)othesis in his " Studien."


Two other theories have to be mentioned, namely, Semper's and Balfour's. The former, 76.3, 330, observed a small ectodermal pit on the dorsal side of the head of a leech, which suggested the possibility of such a pit deepening and becoming connected with the archenteron, and so creating a new mouth. Balfour ("Works," I., 31)2-394) has suggested that vertebrates and annelids arose from an ancestor with lateral nerve cords, and that in annelids the cords united to form a median ventral chain, in vertebrates to form a median dorsal chain, so that in the fonner there is, in the latter there is not, an oesophageal ring. The development of both types by concrescence proves that the neural sides are identical in annelids and vertebrates. Therefore Balfour's hypothesis falls — and with it. of course, Gegenbaur's — that the brain is the same in both tj^pes, but that the vertebrate spinal conl is an outgrowth of the annelidan Rupra-oesophageal ganglion, the annelidan ventral chain being lost in vertebrates.


Hypophysis

The hypophysis cerebri, Rathke's pocket or pituitary body {Hirnanhan<j)/\^ a structure of very problematical significance, which has been much studied and speculated upon by embryologists. It arises in all vertebrates as an evagination of the ectoderm near the dorsal lx)rder of the oral plate, but is separated from the plate by a fold of the ectoderm. In Petromyzon, Fig. 318, the fold, F, acquires great size, and ie shown by Dohrn, 83.1, to develop into the roof of the mouth and the upper lip; accordingly the hypopbys^ invagination, hy, is outside the oral cavity proper, and more intimately associated ; with the olfactory area and nasal pit, N. The liypophysis runs toward the end of the Dotochord, iich, and is nearly met by a small blind diverticulum of the archenteron, which is presumably homologous with Seesel's pocket, p. 26M, of amniota; in the lamprey the hj"pophysis early gives rise to glandular diverticula, : and itself becomes the adult nasal duct of authors. In , elasmobranchs, owing, probunuu no ..u™. ...». .no i...u-o™^n ^(A ^^ly, to the increased head^itachord"' ^1. pntraWm: 'mM,"'mBH™!iirm; jf! bend and sizeof the foro-lirain, SSSth:*AV'^^ph^ia; '!v,'^S^*'p^"A(w'r*c. the region between the nose ^"P"*'- and oral plate is turned in ho

as to be almost wholly included in the oral cavity, and accordingly liie fold, Fig. 319, F, and hypophysis, ky, now appear as appendages of the oral ca^nty, for I homologize the transverse fold, Fig. ;nO, F, which borders the hypophysis in shark embryos, with the foki. Fig. 318, F, which forms the upper lip in the lamprey. In amphibians, according to A. Goette's ooservations, 75.1, 288, 317, u[x>n Bombinator. the hypoj)hysis arises as a solid ingrowth from the nervous layer (f/. p. 541*) of the ectoderm, in front of the mouth, and, as development proceeds, there follows the inclusion of the hypophysal



'lion or an Arnnthla-i Eiiilirv<M>f I3.a mm f.h. Forv-hralu : m.b. ucBia: till, hyixniliyiils i-vaittnalloii : F. fiM xciitiratini! hypoiibyBia i: hi, tHran; Li. aa\aev vt llvtr; Vt,^ yi.lk-sliilk.


area in the general mouth cavity ; there is no distinct fold between the hypophysis and the oral plate. In amniota nearly the whole ectodermal area l)otwe«>n the oral plate and the nasal pits is turned in and incorjmratcd with tiic mouth cavity lM>fore tht' evagination to form the hypophysid appears; hence, the orgau developti as an out growth of the oral chamber. The comparative embryology of the pituitary body teachoB UB that the mouth cavity increases, as we asc^id ue vertebrate series, by the annexation of neighboring territory, and that the primitive upper lip of vertebrates disappears, with the further consequence that in cyclostomes the homologue of the maxillary proceea is to be sought, not in the lip, but between the hypophysis and nasal pits.


in mammals the hypophysis is first indicated (£oUiker, "EntwickelungBgesch.," 1879, p. SOU) by a slight groove a little in front of the oral plate, but it does not have the form of a distinct evagination until after the oral plate {Rachenhaut) is ruptured. The ectoderm of the mouth over the hypophysal area hes against, and is apparently intimately soldered to, the ectoderm of the brain, a point which has been generally overkxjketl, but which seems to me of great im[>ortance. It is commonly stated, e. g., by Kraushaar, 86.1, 87, that, when the oral plate ruptures, a portion of it persists upon the dorsal side, and is the Ijeginning <if the fold which separates the hypophysis from the pharj-iix. I think that this is probably not the case, but that all trace of the oral jilate <li8am>ear8 and that a new fold arises as a duplicature of the et-tixlorm fiUe<l with me»odemi. Fig. 320, F. This new fold I honiologize with the lip of Petromyzon, Fig. 318, F. The hypophysis is now. Fig. 3-2(). hif, a diverticulum of the oral cavity, with one wall attached to the brain, and the other formed by a fold dividing the liypiipliysis from the mojuth. The epithelium of the mouth is one-layered, and not thickened, as is that of the hypophysis ; the cells are multiplying rapidly in the stage figured, there being numerous karyokinetic figures, which, so far as I have seen, are always near the free surface of the epithelium. The relations of the notochord to the hypophysal wall have been discussed, p. 183; in the specimen figured above, there is a connection between the chorda and the lower posterior part of the hypophysis. The organ in the stage of open invagination was described by Rathke, hence the invagination is often called "Rathke's pocket;" Rathke supposed, erroneouslv, that it was developed from the archenteron (pWrnx).


Fig. 110.— Hedlan Section of the Head o( a Rabbit En bryo ot thtrteen and one-haU Dni fb. Fore-brain; mb. mid-bratn; rU. i:«vbe11um; Ab, hinJ-brala; ncft, notochord; hf. hypODbfhIb; F, fold cormq>ODdlnK to the lip of Pptrouiyioa; Ec, eclodermi P, sooiatApleuric wall of pericardium; Md, manilible; Aa. wall ot the aorta,


The hypophysal diverticulum now elongates and its upper end expands to a considerable vesicle, the lower end remaining narrow as the pedicle. At the same time the floor of the brain forms an outgrowth behind the hypophysis, which is the anlage of the infundibulum — compare Chapter XXVII. The two diverticula have their walls united. It is pn)bable that the cementing together over the hypophysal area of the buccal and cerebral ectoderm is the mechanical condition causing the formation of the two diverticula. The hypophysis now grows rapidly; the pedicle elongates and its lumen is obliterated ; the mesenchyma meanwhile condenses to form the base of the skull (sphenoid) ; the pedicle aborts entirely (in the rabbit by the sixteenth day) but the position for its passage through the sphenoid is marked a little longer, but is ultimately obliterate<l by the growth of the sphenoidal cartilage. According to MikluchoMaclay (70.1, 40, Anm.) the passage persists in sharks. Lanzert (see Henle's Jahresbericht, 18<38, p. 88) found traces of the passage, named by him canalis cranio-pharyngens, in children at birth in ten cases out of one hundred. There is then left merely the upper end of the hypophysis as a closed epithelial vesicle lying in the future sella turcica close to the infundibulum. The vesicle becomes flattened in the longitudinal direction, and the flattened vesicle soon acquires, at feast in the pig, a yoke shape in section by becoming first convex toward the fore-brain, then concave in its centre, toward the infundibulum, as may be observed in a pig embryo of 18 mm. (KoUiker, ^^Entwickelungsges.," 2te Aufl., Fig. :52I».) The vesicle completes its development by sending out hollow buds from its anterior wall (rabbits, 20-30 mm.); in birds, according to W. Miiller, 71.4, and Mihalkovics, 77.1, buds arise from both walls. The buds elongate and branch; numerous bhxxl-vessels are developed between them ; the buds separate from the parent Vesicle (rabbits of 40 mm.), but continue to grow; their lumen disappears, and they produce a highly vascularized complex of hypo[)hys{il cords. Kolliker thinks (*'Entwickelungsgeschichte," 187H, 531) that the main vesicle persists i-crogniz^ibly in man into adult life.


The infundibulum also contributes to the pnxhiction of the adult hypopliysis of mammals, although in lower vertebrates it persists as an integral portion of the brain, and is different iattnl into ganglionic tissue. As first shown by Vs . Miiller, 71.4, the j)oint^Ml end of the infundibulum undergoes in amniota an enlargement, In'ginning in sheep embryos of 35 mm., in pig embryos of Wl nun. (Kolliker, "Entwicjielungsges.," 1879, 531). The knob-like enlargement loses its cavity, and although the differentiation of nervous tissue begins in it, its cells early acquire an indifferent character, and it is penetrated by blood-vessels and connective tissue ; the connection with the brain is permanently retained. The knob is designated in the adult as the posterior lobe of the hypophysis, although it can in no sense be regarded as part of the true hypophysis.


Historical Note. — The following memoranda are taken from Mihalkovics, 77.1, and Kraushaar, 85.1. The older authors regarded the hypophysis as part of the brain ; this conception was held by Von Baer "Entw.-Ges.," I., 104, 103, and II., 293, and foimd as late as 1862 a defender in F. Schmidt, 62. 1, 51, although Rathkehad discovered the hypophysal evagination in 1838, 38.1, and Rathke's discovery had been confirmed by KoUiker (Entwickelungsges.," 1861, p. 242) . Rathke subsequently, 6 1 . 1 , 100, withdrew his opinion that the evagination formed the hypophysis, but W. Miiller, 71.1, demonstrated that it was unquestionably correct, but retained the erroneous opinion that the evagination was developed from the archenteron. That the evagination belongs to the oral cavity was finally


Sroven for amphibia by A. Goette, 75.1, and for manmials by [ihalkovics, whose researches, 77. 1, 83-04, are the most important yet made on the organ. Mihalkovics' results on mammalia have been confirmed by KoUiker, 70.2, Kraushaar, 85.1 (His, "Anat. menschl. Embryonen ") , and others. The development of the hypophysis in the lamprey has been especially studied by Dohm, 83. 1, whose results have been confirmed by subsequent investigators (Scott, 83.2, Shipley, 88.1, Kupffer, 90.1).


That the notochord had some connection with the hypophysis has been held by several authors. C. B. Reichert, 40.1, 179, regarded the pituitary body as the end of the notochord, but twice later, 1861 and 1878, changed his opinion. Dursy, 69. 1, maintained that the notochord was united with the pocket of Rathke, and formed part of the hypophysis ; see also J. B. !Platt, 91.1.

Nasal Pits

In this section the development of the cavity of the nose is taken up — for the history of the olfactory organ proper, see Chapter XXVIII. The formation of the nasal pits begins with the differentiation of the olfactory plates, which are two areiis of thickened epidermis situated just in front of the mouth and in actual contact with the wall of the fore-brain. The plates give rise to the olfactory epithelium of the adult. In Petromyzon instead of two plates there is a single median one, which extends to the anlage of the hypophysis. Fig. 318. This fact renders it probable that primitively there was a single median plate in vertebrates, which has become divided ; in the lamprey such division is established later. H. Ayers, 90. 1, 240, however, states that the nasal area or olfactory plate of the lar\'al lamprey isdivide<l by a median non-olfactive raphe into two lateral ix>ckets, right and loft, to which t\n} right and left olfactory nerves are respectively distribnted. It is ]X)ssible that more exact observation will show that in all vortel)rates there is at first a single plate, which is early divided. Balfour, *'Comp. Embr>'ol.," II., 533, regards the condition in Petromyzon as secondary, but gives no evidence to support his opinion, which was, perhaps, really due to the tradition which says the vertebrate olfactory organ is paired.


The nasal pits proper are developed, as pointed out by A. Goette,

76. 1 , not by the inva^nation of the olfactory plate, which ia apposed

to the brain ab initio, but by the upgrowth of the ectudemi and mesoderm around the plate. The upgrowth takes place on the medial, upper, and lateral side of each plat<-, and hence forms two pits with a partition, the future septum narii, between them They are the nasal pits and communicate along their whole lower side directly with the mouth cavitj , Fig. 322. The mode of deielopment of the nasal pita or sacs renders it highly probable, it seems to me, tluit the essential mechanical condition is, as with the hypophysis, the union of the epidennalplate with the brain wall. The nasal pit IB at first very shallow. Fig. 321, and the olfactory plate is exposed laterally; and there can be seen at its lower part a small <le]}res8ion, the anlage of the organ of Jacobson. The growth of the uaaal pittt in man has been described by His

("Anat. meuschl. Erabryonen," Heft III., 45-56). There are two principal changes, 1, the gniwth of the tissues around the olfactory plate; 2, the migration of the pits away from the brain. Fig. 323 gives a view of an early stage in which the pits are small and sliallow and the tissue is forming a ridge around them, which, however, does not extend on to the oral side, so that the pits open freely into the month cavity. The nasal pits are widely separated by a projecting mass of tissue, which I propose to call the lutsal process, and which is the Sfirufortsaiz of German embryologists. Between the nasal pit on each side and the mouth the aiihige of tl e nasal process is thickened and rounde<1, making a protuberance — the procesHHH fjliybtilaris of His. The nasal process includes the juirtition lietween the two na^l chambers, the anlage (if the future nose r and of the future intennaxillarj- region of the upper lip. The maxillary pniei'ss extends between the mouth and eye, towani the nawd pit, and later by joining the processus globularis begins the sojuiriitionof the nasal and buccal chambers and completes tlio permanent iippor bonier of the mouth — compare Fig. 324, L, Mx. A a developmeut proceeds, the tuteral ridge, see Fig. 321, grows forward and covers in the uasal pit from the side, and may therefore be regarded its the anlage of the wing of the adult nose. We now have the two external nares. Turning to tho j^Towth .)f tin; iiariiil chambers, we find that they enlarge ari tbe wbole liife enlargea, and that they occupy an increasing space, Fig. 333, jVA, opening widely into the mouth cavity above the palate shelf. Tho figure shows that the palate develops from the walls of the mouth cavity, and the space above it is, therefore, oral, not naBid; hence the nasal cavity of the adult includes more than the nasal pit of the embryo. It is from the nasal pits proper that the so-called labyrinth of the nose is formed. , The development of the labyrinth begins with the appearance — in man during the third month — rtE three projecting folds on the lateral wall of each nasal cluimber. Fig. 32(i the folds are the upper, middle, and lower turbinal folds (Xasejiviu^ikeln) &tid consist at first each of a duplication of the ectoderm tilled with indi&'orent meseiichyina, which, however, verj' eiuly changes BS^t^^W^T^f^outSl^J^ryt' into cartilage; the turbinal eartihige

.V*. Dual tnvlty: (A. Tmtnfhonlj



BAthk<^'s poek«t; ig a conseuiience, not a cause, as


M^W'«Di»ge; VfTm™"" ofton Stated, of the development of '■ the turbinal fold. The formation

of the labyrinth advances by the formation of ontgrowtlis, which become the ethmoidal sinuses, by the appearance — in man during the sixtli month — of the antrum Higbmorii, or expansion of the nas^ cavity, into the region of the superior maxilla, and finally by the evaginations to fonn the sphenoidal and frontal sinus, which, however, do not arise in man until after birth. Finally wo consider the separation of the otfactorj' plate from the brain. This does nut take place until the olfactory ganglion develops from the epithelium (ectoderm) of the plate. The olfactory nerve fibres nre developed very early, in the chick during the third day — compare Chapter XXVII. The fibres lengthen, the olfactory and neunil epithelia separate, and ultimately the osseous cribriform i)Iate is developed between them.

For observations on the development of the ixjsterior narcs, see Fr. Hochstetter, 91.2.

Jacobson's Obgan, — The oi^iui of Jjicoltson arises verj' early as a small distinct invagination, on tho me<ijal w^all of the nasal pit, as first stated by Dursv. 69. 1. Our knowledge of its development is due chiefly to KoUiker, 77.3, 79.2. 7«fl, and Fleischer, 78.1. At four months it is a cj'lindrical blind canal, i-unuing from ita

original orilice backward in the septum narii. It is surrounded by a small cartilage (Jacobeon's cartilage) near its orifice; this separate cartilage is derived from a growth of the main cartilage of the septum. The canal ia innervated by the olfactory nerves, and in certain mammals it is much more developed than in man.

The external nose is developed toward the end of the second month by a growth of the nasal process (His," Anat. menschl, Embryonen," III. , 35) . It is at first short and broad, having at three months very nearly the shape which is permanent in certain negro racew. The external nares and wings of the nose are carried forward witii the general nasal upgrowth. At three months the external nares are usually completely closed by the growth of their epitheliiun, which forms a plug of gelatinous consistency. The plug disappears after the fifth month {KoUiker, "Entwickelungsges.," ISTfl, 707).

Maxillary Process. Reference has already been made to that thickening of the upper edge of the mouth, which appears almost a« a continuation of the mandibular arch, and which is known as the maxillary process, or sometimes as the superior maxillary process (Oberkie/erforisatz). It is termed a process, because from its small size and position it appears at first like a bud from the mandibular arch. Later it stretches farther forward, and when the mouth has changed from its original pentagonal shape to a transverse slit. Fig. 322, the maxillary process no longer appears specially connected with the mandibular arch, but is united with the edge of the nasal process as above described, p. 57C. A thorough study of the primitive relations and growth of the maxillary process is much needed. It is possible that, as several authorities hiwe maintained, it is morphologically the upper part of the mandibular arch, which, in consequence of the head-bend, makes an angle with the mandible proper. Althougli this hypothesis commends itself to the embryologist, it needs a firmer basis than it yet has to stand upon.

Mandibular Arch

The first branchial arch forms the lower boimdar^ of the mouth, and by its long-continueil growth develops into the projecting lower jaw. The history of the skeleton and muscles of the lower jaw are treateil, p. +44 and 47S, respectively. The chin is at first retreating and does not \re! come distinctly iin>minent tmtil the fifth month. The growth of the jaws increases the separation ot the mouth from the heart.

Lips and Gums. — Very soon after tho upper jaw has been formed bj the union of the maxillary and

otaHumBD Embrjo. nn, Narrs; Op fji- ^asal prOCeSSeS, itS Ordl SUrfaCC do L. portion or lip devfiopwi from 'ihe^nagai velops two parallel ridgos. Fig. 'Mi. oiwd from tbf muiiLoir prwres: /iTdeDui of wbich the outer ami more bulky. 5!S2?VHi8,'^°" "^^ xHdiaa» Y^ ^j^ jg jjjQ anlage of the upper lip, and the inner and smaller, /), the anlage of the gums (gingtvoi, dental ridge) . At about the same time, or a little later, similar ridges develop on the lower jaws.


The histogenesis of the lips and giims has not been investigated. From the Btudy of sections of the lower lip of a fcetus of six months, which I have prepared, I cooeider it probable that the peculiar epithelium of the lips arises, 1, by the disappearance of both the epihichium and stratum lucidum, and, 2, the distention of the remaining cells — a basal growing layer being retained. In a rabbit of thirteen days, the epitrichium runs over the region of the future lip. In a pig embryo of about 3.5 cm., the epitrichium is still present, and the cells below are enlarging and beginning to comify.


The glands of the lips, according to Kolliker, "Entwickelungsges.," 187(), 828, arise during the fourth month as solid ingrowths of the epithelium, and later send out each eight to ten branches, which, while still solid, form a pretty rosette.

Formation of the Palate

As soon as the external nares are separated from the mouth, there is a partition between the nasal pits and the mouth This partition, lu which the intermaxillary bone is differentiated later, is supplemented by another partition, the true palate, Fig. .324, Pal, which shuts off the upper part of the oral cavity from the lower, thus adding the upper fwrt to the uasal chambers. The paliite is a secondary structure, which divides the mouth into an upper respiratory passage and a lower lingual or digestive pastmge. The palate arises as two shelf-like growths of the iuuer side of each maxulary process, Fig. SH, Fat, and is completed by the union of the two slielves in the median line. As seen in a side view the shelves are represented in Fig. ZZS, Gl, they arch so as to descend a certain distance into the pharynx, but in the phaniix their growth is arrt»jted, though they may be still rQC<^nized in the adult. In the region of the tongue, which includes more than the primitive oral cavity, the palate shelves continue growing. At first they proje^-t obliquely downward toward the floor of the mouth, Fig. '.yio, pat, and the tongue, T, rises high between them, and appears in sections which, like the one represented in Fig. 325, pass through the internal nares, to be about to join the intemasal septum. As the Kiwer jaw grows, tho floor of the mouth is lowered and the tongue is thereby brought further away from the intemasal septum. At the same time the palate shelves take a more horizoutiil position and pass towanl one another alK)vo the tongue and below the nasal septum, and meet in the middle line where they unite. From their original position, see Fig. 325, pat, the shelves nece.-*sarily meet in front (toward the lips) first, and unite Itohind (toward tho pharj'nx) later. In the human embiyo the union begins at eight weeks, and at nine weeks is completed for the regi<in of the future hiird palate, and by eleven weeks is usually completeil for the soft pidato also. The palate shelves extend back across the second and third branchial archee: by the migration of the first gill pouch, or, in other words, of the Eustachian tube, the Eustachian opening comes to lie above the palate (uvula) while the second cleft remains lower down and lies below the pialate, as the anlage of the tonsil. His, "Anat. menschl. Embryonen," Heft III., 82. The tivula appears during the latter half of the third month as a projection of the border of the soft palate. Soon after the two palatal shelves have united with one another the nasal septima unites with the palate also, Fig. 326, and thereby the permanent or adult relations of the cavities are established.

Lachrymal Duct

The canal which leads from the comer of the eye to the nose {Thrdnennasengang of G. Bom) is not found in fishes, but only in amphibia and amniota. The site of this duct is very early marked out by the lachrymal groove. Fig. 322, running down from the eye to the invagination, or to the nasal pit as soon as the latter appears. This groove is bordered above by what is known as the lateral nasal process or prominent surface between the nasal pit and the eye — compare Fig. 322 — and is bordered below by the maxillary prodess. This groove soon disappears and leaves, so far as known, no trace. It was supposed by Kolliker (" Entwickelungsgesch.," 1879, 409) to be the anlage of the duct — an opinion which Bom's observations on amphibians, 76. 1, and on Sauropsida, 78. 1. 83.3, followed by those of Legal, 81. 1, 83. 1, on mammals, showed to be erroneous.


The duct arises along the line of the lachrymal groove as a thickening of the under side of the epidermis, which appears about the time that the cartilage develops around the nasal cavities — in man, according to Ewetzky, 88. 1, at the end of the fifth or beginning of the sixth week. The thickening increases until it forms a ridge, which finally separates as a solid cord from the epidennis, except at each end ; the cord then acquires a lumen, thereby becoming an epithelial canal. In man the upper end of the solid cord broadens out at the inner canthus and then divides into two forks, each of which acquires a lumen, with the result of producing a bifurcation of the duct (Ewetzky) . In the pig, the bifurcation is developed, but one fork aborts, according to Legal, 83.1.

Teeth

The development of the teeth in man and other mammals has been much studied, and has been repeatedly described by competent authorities in comprehensive summaries. I have, therefore, deemed it unnecessary to go over many of the original articles carefully, and instead base the following synopsis chiefly upon Waldeyer, 72.1, Kolliker, 79.2, 815, Tomes ("Dental Anatomy"), Von Ebner, 90.1, and O. Hertwig. The list of authorities is given in my " Bibliography" under " TV^fA," but it is far from complete ; for further lists see Waldeyer and Von Ebner. For a very admirable critical synopsis of the various notions that have been advanced concerning the histogenesis of the teeth, see Von Ebner, 90.1, 249-252. It must be remembered that most of the articles upon the human teeth are by more or less incompetent writers.


Dermal Teeth of Sharks

The teeth were primitively organs of the skin and widely developed over the surface of the body, and as stated before, p. 401, they have played an important role in the genesis of the skeleton. It is, therefore, to the fishes that we must turn to ascertain the primitive mode of tooth formation, choosing the sharks, since they have been the most thoroughly studied in this regard, thanks chiefly to O. Hertwig, 74.1,2. The teeth of shar^ are generally known as placoid scales. The tooth b^ins as a mesenchymal papilla, Fig. 327, composed of crowded cells and projecting into the epidermis. The layer of epidermal cells overlying the papilla changes in character, its cells gradually lengthening into very long cylinders, and be comes the enamel oi^n. By further development the epidermis thickens, the papilla projects into it, and becoming narrow and longer, and taking an oblique posirton, gradually assumes the shape of the tooth. Ossification now begins over the surface of the papilla; there arises a layer of epithelioid osteoblasts, and between these and the enamel organ the development of bone, or, as it is called in teeth, of ivory, begins ; the osteoblasts persist, and the bony structure is developed only between them and the epidermis, forming a stratum which grows in thickness. At the same time the enamel oi^n begins to deposit the calcified layer, known as enamel, over the papilla. Later the tooth acquires a support by the direct ossification of the connective tissue at its base, and is then a completed "placoid scale."

The teeth of the mouth depart from this primitive mode of development, for they do not arise on the surface, but deep down, Fig. the Acanttiias Embryo o( 10 cm. En. Em p, papilla; £}i. epid«rmlB; Cu, dermis.


dentiferoiis epithelium grows down into the dermis forming an obh jue shelf, which ia&\ be regarded as a spe ciS tooth form ng organ. On the un der side of the shelf the teeth are devel oped in the same way as over the miiK; =,». skin, although they *;jj«™l colta; J*.<lent«Jpapilli.; D S. denlAl slielf. After O, Hen are much larger.

The teeth are, however, in various stages of development, and only one is fully exposed ; when, aa happens in time, it is lost, the next tooth behind replaces it, and since the production of new tooth germs goes on in adult life, the replacement of teeth in the shark's jaw continues indefinitely; hence sharks are termed poli/phyodont. Mammals have two sets of t«etb, and hence are called diphyodont.



"We learn from the sharks that a tooth is a papilla which projects into the epidermis, aod, ossifying in a peculiar way, changes into ivory around the soft core or pulp: to the papilla the epidermis adds a layer of enamel. The tooth proper unites with a small plate of dermal bone at its base. By a modification in the jaws, the epidermis hrst grows into the dermis, and then the dermal tooth papilla is developetl. In the higher vertebrates the teeth of the jaws only are developed, and they arise in the modified way we have noted in the selachian jaw.

Amniote Tooth-Germs

The first indication of the de\ elopment of tooth-germs in mammals is the appearance of a thickening of the epithelium covering the jaw ; tlio thickening forms a curving ridge on the under side of the epithelium According to C. Rose, 91.2, 4j1 ^p' '^7^ the ridge appears in the human embryo during the sixth week. The ' ^p

go expands. Fig. S29, and subdivides into an outer portion, L.gi the anlage of the groove between the lip and gum, and an inner por tion, a.sh, the dental shelf, which grows obliquely inward ; on the under side (in the upper jaw on the corresponding upper side) of the Fig. sffl.-sectfonoiPiirtoriheLowOTJav shelf arise the dental papillse, Pp. "/ a iimnan Embiro ot 40 mm. Ep.i. Epithplium of Up: o.jp, oral pplthflium: Lor, the dental shelf (Zaknleiste) aniacpof itjiBrr-m-: rf^.ifenmiaiiHf:^. is homologous with the similar p"*""* ^f'"^"* structure in the shark. Its historj- in the human embryo has been investigated by C. Rose, 91,3. The ^pillae for the milk-teeth are formed on the under side of the shelf, Fig. 32i), and it is thus possible for the shelf to continue growiug t<jward the lingual side, so that a second set of germs is developed for the i>ermanent teeth. The end of the shelf toward the articulation of the jaws is prolonged without retaining the direct connection with the epithelium, and from this ])rolongation arise the enamel organs for the three permanent molars. Wherever a tooth-germ arises, tho dentjil shelf is locally enlai^ed, and the local enlargement constitutes an euiunel organ which projects from the under side of the shelf. The jiortions of the shelf between the enamel organs gradually break up, forming firet an irregular network, and later separate fragments, which may persist throughout life and lead to various patholi^ical structures ; while the permanent germs are forming the shelf is solid between them, although it has assumed the reticulate stmoture lietween the germs of the milkteeth. In consequence of the reticular formation, the fully developed enamel organs have several bands or threads, by which they are connected with the dental shelf proper.

Fig. 330 represents the under side of a mo<lol of tho epithelium of the gum of the upper jaw of a human embryo of i{> mm. reconstructed by 0. Rose from the sections. Fig. li'i'.t. L.gr is the ridge corresponding to the groove between the lip and gum; pal is the surface of the palate; d..sfi is the denfeil shelf, the ten cups or depressions on which correspond to the papilla; for the ten milk-teeth.



Fig S9) — EiplsDStii


After the shelf has developed somewhat, its line of connection with the epithelium of the gum becomes marked by a superficial groove, as may be seen in the human embryo of eight to ten weeks, Fig. 32i, D. This groove was formerly supposed to be the first trace of the dental shelf, but Rose's obser\ ations correct the supposition.

The second step in mammals IS the formation of outgrowths (in mull ten in each jaw) from the under side of the dental shelf; each outgrowth is the anlage of an enamel organ for a milktooth The derivation of the enamel organ from the epidermis was discovered by Kolliker. The outgrowth is covered toward the mesoderm by a layer of cylindrical epithelial cells, the continuation of tho basal layer of the epidermis, while the core is filled with iKilygonal cells, which resemble those of the middle part of the Malpighian layer of the skin. Tho outgrowths, after penetrating a short (listance, expand at their lower ends, but remain each connected by a narrow neck with the overlying epidermis. The expanded end is tho enamel germ proper; it very soon assumes a triangular outline as seen in sections, owing to the flattening of its under aide, an<l at tlio same time it moves somewhat toward the lips. Meanwhile the pbelf continues growing on the lingual side of each ingrowth, to produce the enamel organs destined for tho second or jiennanent teetli. At this stage we notice tiiat the mesenchyma under the flattened end of the enamel organ has become more dense, to form the aulage of tho dental papilla, and is beginning to develoii fibrillie around both tho enamel germ and the papillary aniage. The fibrillar envelope is the future dental follicle (Zahnsack) .


The third step is the final differentiation of the enamel organ and the accompanying shaping of the papilla. The enamel organ. Fig. 331, continues growing and becomes concave on its under side, so that the mesoderm imdemeath acquires the shape of a papilla. It is now that the form of the tooth is determined by the form assumed bj" the papilla, which in its turn is probably determined by the growth of the enamel organ. Vim Bruan, 87.1, has sliown that the enamel organ exteuils over the papilla of various mammals not only as far as the enamel is formed, but also as a thin layer to the base of the papilla, or over the future root. Over the root, after the tooth is shaped, the enamel oi^an aborts. The apex of the root is never covered. C. Rose, 91.2, has shown that in man also the enamel organ extends at first over the root, but subsequently aborts. A fully developed tooth germ consists of. 1, the follicle, 2, the enamel genu with its neck running to the dental shelf, the edge of which grows on, Fig. 331, B, to form the secondary teeth, and, 3, the papilla.


The follicle is merely an envelope of connective tisdue. Fig. 331, in which we can distinguish, according to Kolliker, an outer denser and inner looser layer ; in the latter the cells are mere distinct aud the fibrillee are less numerous than in the former. A rich network of capillary vessels is developed in the follicle, Fig. 331, v, and appears in part aa a series of villous-like growths into the enamel organ. The follicle develops first over the lower part of the papilla, then over the enamel oi^an, the neck of which aborts and the follicle closes over, completely separating the enamel organ from its parent epidermis.


Fig, 381— Vertical Section or a MolarTooUiOerm of a Human Embryo of 1«0 mm. Ep. Epithelium of thedentalfumm'; B. bud for secondary Kerm: Bi, central cellnot the enamel or([an; c. enunel cells; p, meaeuch^mal papilla: v, tolliculitr envelope wllb blouti- vessels.


The enamel organ changes greatly in appearance. The layer of cj'linder cells is well preservtS only over the concave side. Fig. 331, c, where the epithehum in in contact with the dental papilla. In the neck the cells become appressed and irregular in form. Over the convex surface of the enamel organ the cells become lower and cuboidal, and ultimately atrophy and datten out, but, so far as I know, no exact study has yet been made of the changes they pass through. The convex surface becomes very irregular by upgrowths of cells, crowded together; it ia betweeu these upgrowths that the vascular villi of the follicle are formed. The layer of cylinder cells over the papilla become much elongatecl and aa their nuclei, after the enamel has begun to form, are nearly all placed at about the same level, they constitute one of the most l)eautifuUy regular epithelial layers known. These c«lls covering the papilla are known as the enamel cells (Schmelzzellen, aineloblastf, membrana adaiiHiiitina of Raschkow) because they produce the enamel, aa described below. The enamel cells average alx>ut 40,'i in length, and at birth about O-7/i in width ; their outer ends, i. e. away from the papilhi, are furnished with prickles or thread-bridges by which the cells are connected, Fig. 'i'-i'i, with one another and the neighboring cells of the enamel organ; the bodies of the cdls are finely granular, and not infrequently have larger glistening granules at their lower or papillary ends; their nuclei are elliptical and l(*-l-in long; before the enamel api^ears they Ho at various levels ; after it appears they are found, with rare exceptions, Fig, 332, b, near the upper ends of the cells, all at one level. The lower or papillary ends have the processes of Tomes, so named from their discoverer; these appear when the enamel begins to fonn; they are short, thick, and tapering, one on each cell; they often seem fibrillated, and are always separate*! from the cell proper by a small cuticular Ijorder; while in situ Tomes' processes are fitteil into sockets on the surface of theenamel. The enamel colls have, probably, no membrane on their sides. After the formation of the enamel is completed the enamel cells degenerate and are lost, except, 1, that ; their border persists as a honiy membrane, ; cuticula eboris, covering the enamel, and. 2, ,. that a few groups of cells may remain for a wdTams'. long time as isolated epithelial bodies in the dental follicle (Malassez), The celts in the centre of the enamel organ undergo a verj' peculiar metamorphosis. They remain united together by a few thread-like prcx-esses, and, therefore, have a certain degree of resemblance to the (^mlirj-onic connective tissue cells, but the intercellular spaces do not contjiin in the enamel organ any homogeneous matrix, but merely fluid. The Bte|)s by which this metamorphosis of the central cells is .'iccomplished are still imperfectly known. A few layers of the central cells of the enamel organ retain more of their primitive character. Fig. 332, c, These cells constitute the intermediate layer of Ktillikcr; they are polygonal, granular, and connected with one another by intercellular threads (prickles).


The DENTAL PAPILLA consists at first, as stated above, of crowded mesenchymal cells. Blood-vessels appear in it very soon after the enamel organ has become concave on the lower side. The papilla acquires very nearly its permanent shape before any further differentiation of its tissue begins. The shape of the papilla is probably determined entirely by the enamel organ, by which it is completely embraced, see above. During the fourth month the cells nearest the surface enlarge — principally by the growth of their protoplasm. They appear as a continuous layer next the enamel organ ; their function is to produce the dentine between themselves and the enamel organ, hence they are called odontoblasts {membrana eborisy KoUiker) ; they are to be regarded as modified osteoblasts. The deposit of dentine begins in the milk-teeth toward the end of the fourth month. In a vertical section of a developing papilla, one can see several stages, because the development advances more rapidly toward the apex and more slowly toward the base of the papilla. The tissues underneath the odontoblast layer constitute the so-called pulp of the tooth. The connective-tissue cells of the embryonic pulp are small and have numerous very fine and branching processes which impart a fibrillated appearance to the tissue, but so far as known there are no true intercellular fibrillae in the pulp. The cells are somewhat more crowded directly imder the odontoblasts than in the interior of the papilla.

Enamel

The deposit of enamel begins on the milk-teeth toward the end of the fourth month. According to our present knowledge, the formation of enamel must be conceived about as follows : Each enamel cell forms an enamel prism by the metamorphosis of the lower end of the cell into a calcified column ; a cement, which is also calcified, holds the prisms together; the cement is presumably a derivative of the inter-cellular substance between the enamel cells. Enamel is, therefore, essentially different from bone and dentine, in neither of which do the cells calcify, yet the enamel cells resemble odontoblasts in many respects. The first step toward the production of an enamel prism is the change of the protoplasm at the lower or papillar}^ end of the enamel cell into a homogeneous mass, resembling a cuticular cell border ; by the union of the borders of adjacent cells, a continuous membrane or cuticula is generated. We must assume that this membrane grows upon its upper side by apposition from the enamel cells, and becomes modified on its lower or papillar}- side at nearly the same rate. The modification consists in the production of the fibrous tuft, Fig. 332, a, described above, at the end of each enamel cell. The lower end of this tuft (Tomes' process) calcifies and becomes the beginning of the enamel prism. The enamel prisms begin small in diameter with considerable cementing substance between, but, as they lengthen, their diameter increases so much that there is little or no space for cementing substance between them. The enamel prisms lengthen by apposition on their ends adjoining the enamel cells, yet for a long time the cells maintain their size, perhaps nourishing themselves at the expense of the central cells of the enamel organ, which gradually atrophies as the enamel thickens. From their mode of growth, it follows that the prisms stretch through the whole thickness of the layer of enamel.


Since the enamel prisms widen out toward the surface of tho tooth, it is probable that the enamel cells iiiorea»e in diameter as the enamel is deposited. The cells cease multiplying by the time the enamel b^ns to form. The enamel prisms undergo further changes after birtli. They become harder and thicker at the expense of the cementing substance between them. At birth it is still relatively easy to break up the etiamel into its prisms, and to a certain extent to break the prisms so as to obtain indications of fibrillated Btnicture.

Dentine

The odontoblasts, as stated above, are modilied mesenchymal cells, which form an epithelioid layer over tho surface of the papilla. The odoutoblitsts are, at first, short cylinder cells, each with an oval nucleus toward tlio end of the cell fai-thest from the enamel oi^.ui. They keep their mesenchymal character in that they are connected by processes with one another and with the underlying cells of the papilla. The first change in the odontobhmts preimratory to the deposit of dentine is the appearance of the so-called meiiihrana privfttrnHitivfi, a clear homogeneous membrane ciaisisting apparently of anisotropic intercellular substance. The membrana always lies next the odontoblasts and is best interpreted as the layer of uncalcified dentine, see C. Hose, 91.2, 470. There now arise the dental processes, which are prolonga^ t' of the odontoblasts toward the enamel organ aa f as the niembrana ptEefornmtiva. The process? vary much in size, but are generally alK>ut xth to one-fourth the diameter of tlio colls; u. ell usually has one dentinal process only, b 8 metimes there are two, and even as many as X have been seen by Boll. Between the denn processes a clear anisotropic subst^mce is rm d, which gradually increases in thickness, ocesses lengthening correspondingly, iiiitil a I n derable layer, which imiy be described as

j' u ified dentine, intervenes between the odon F has and tho enamel organ. Calcification sets

, n n xt the enamel and progresses towanl the

' p a; at the same time the deposit of uncalcified " / e is continued by the odontoblasts. The

] // fication is incomplete: tho uncalcifie<l sjxits a e known in the adult tooth as the interglobular

^ sp . Tho niembrana pneformativa cannot, as

t . 8 gt,ested by Von Ebner, 90.1, 244, be resorbed

'Vt j. e enamel orgjtn, since it is not in contact

^ ^ t, but it is to 1)6 observed in well-developed and is perhaps jiresont throughout life. It tha given rise to many misconceptions. The I ma \ of the dentine was sup|>ose<l by Waldeyer

^ asa od to- t^, i^ mxluced by a met«i«ori»liosis of the proto« as of the odontoblasts, but this point is open

■ran. d cussion. The question is [Mirt of the more

ge al one — What is the origin of intercellular ub tan C mp i'J'J. As the deiitini' increases in thickness

od toblas bet m longer and narrower. Fig. '.V.iS, B, and the dentinal processes finer, more numerous and branching, the branches anastomosing with one another. The processes persist and never calcify, the spaces they occupy being the dental canaliculi of the adult. The ends of the odontoblasts toward the dentine become, for the most part, as it were, squared off, while the lower ends become more or less pointed. Fig. 333. The odontoblasts lose much of their regularity of arrangement, as the dentine nears completioi), but they are still found in the adult. In old age they become comparatively inconspicuous and assume a rounded or ovoid shape (Tomes' *' Dental Anat.," 187(5, p. 97).

The cement is merely a layer of bone developed by ossification of the dental follicle over the root of the tooth. It differs from onlinary bone by the greater abundance of Sharpey's fibres in it. Its development begins on the milk-teeth during the fifth month, and takes place after the type of periosteal ossification.

Age of Development. — The following table indicates approximately the ages at wliicli the various stages of development are passed by the different teeth. To complete the table it must be added, 1, that the fii*at permanent molar arises the fifteenth or sixteenth week like a milk-tooth as a bud from the epithelium of the dental groove; 2, that the second molar begins as a bud from the neck of the first about the third month after birth, and, 3, that, according to Magi tot, the germ of the tliiixl molar, or wisdom-tooth, begins as an enamel bud from the neck of the second molar, about the third yeiir (C. S. Tomes, "Dentiil Anat.," 187G, p. 128.)


l*«*mianent teeth (except ' fi

Wwk8. ! Milktmh. molars). i First molars.

Ttli Dental >fnK>ve and ri«lp*.

Hth I'lnaniel organs bud. !

9rh KDAiiiel orfcaii concaves. '

li)tli Follicular wall.

Enamel organ fully differentiated iEnamel bud

I Follicle clones al>ove jcerm. 'i appears.

16th I - Neck of enamel orgau re- I

I sorlied Enamel buds appear

j-j^j t Dentine appear8 on incisors

/ and canines j Papilla.

IPj^i^ I \ Dentine ap)>ears ou first and I

I second molars Follicle.

aoth Dentine caps, 0.(i4-().06 in. high. , Papilla formed i Follicle closes.

2^th ' - 0.()5-O,()7 ** ' Dentine appears

Enamel organ fuU.v diflferentiated; follicle well formed F<»llicle clos«»s above germ


28th " 0.08-0.00

32d ■ " 0.0J)A11

»)th " (Ml-O.l-i

JJWth I " 0.12-0.14

After birth


Cusps coalesce.

Enamel and d(*ntino apijear.

Double Dentition of Mammals

The manner in which the teeth are renewed in the shark's jaw has l)een described, p. 582, Fig. 328 ; the new t^x>th-germs arise lus outgrowths on the lingual side of the old. In mammals there is the same relation l)etween the earlier milk-teeth and the later permanent teeth. It is, therefore, justifiable to assume that the diphyodont mammal preser\'es in a reduced degree the piscian power of renewing the teeth, and that the milk-teeth represent the primary dentition. Such, however, is not the view of Flower, 67. 1, who considers that the present mammals aide derived from monophyodont ancestors, and have acquired the milk-teeth secondarily by interpolation. This conception has been more recently adopted and defended by Oldfield Thomas (Phil. Trans., 1887, 451). For criticisms of these authors see Lataste, 89. 1, who also advances a more complicated hypothesis. Flower's hypothesis was based on the belief that marsupials, which have only one set of teeth, possess the permanent set, but W. Kiikenthal, 91.1, has found that the teeth of Didelphys (opossum) correspond to the milk-teeth, and that the germs of the permanent teeth are present in the embryo and abort without forming any tooth except the third prsemolar (so-called first molar) of the upper jaw, which belongs to the second dentition.

As to the evolution of the complicated forms assumed by the teeth of mammalia, see E. D. Cope, 74.1, and H. F. Osbom, 88.2.

Salivary Glands

The mouth cavity of amniotes is furnished with numerous glands, which in Sauropsida are found in part variously gathered into groups, in part scattered singly. In mammals scattered single glands are found, but instead of groups of glands there are three pairs of large glands, each with a long single duct. The three pairs are the salivary glands and are kno\vn only in mammals. It has been suggested that each salivary gland corresjjonds to a group of oral glands in reptiles, but the attempts to detennine the homologies involved in this assumption have failed, compare Reichel, 83.1, and Ercole Giaccomini, 90.1. On the other hand the development, I believe, indicates clearly that each salivary- gland is a single oral gland greatly enlarged, for it arises from a single invagination and in an early stage has a marked resemblance to an ordinary branching gland of the mouth.

Concerning the development of the small oral glands in man, a few observations are recorded by Kolliker (** Mikrosk. Anat., II., 2, and " Entwickelungsges.," 1879, 828) who also gives a few data concerning the salivaries. The development of the latter glands is known to us chiefly through the reseiirches of J. H. Chiewitz, 85. 1. The glands appear in the following order : submaxillary, sublingual, parotid. The submaxillary anlage can be seem in a pig embryo of 21 mm. and in a human embryo of about six weeks; the parotid appears in man by the end of the eighth week. As to the position of the anlages: the mouth at the time they appear has a characteristic shape in section. Fig. 325, being — if we imagine the tongue removed — like an inverted j., and there is at each side an angle, a: it is from the epitheliimi along this angle that the solid outgrowth to form the parotis takes place. The base of the tongue forms an angle on each side with the floor of the mouth, Fig. '.Vlb ; it is from this angle that the solid outgro\vths of the buccal epithelium take place to form the sublingual and submaxillar}' glands, the former near the front, the latter near the back of the tongue. The anlages of the parotid and submaxillary are at first at alxnit the same distance back from the frenulum of the tongue, but as development proceeds the submaxillary orifice migrates forwanl, the parotid backward. The following measurements are from Chiewitz, 85.1, 422.

Af?e of embryo in weeks 6 8 10 12

Submaxillary gland, distance from frenulum. . 0.52 32 0.8<5 0.12 mm. Parotid gland 0.34 1.08 1.10 mm.


The outgrowth of the sahvary anlage is at first a cyhnder, which, however. Boon begins to lengthen and branch ; the ends of the branches enlarge, and ultimately develop into the alveoli. The gland is now further characterized by the condensation of the connective tissue about its branches into a globular mass, which is sharply defined, Fig. 334, a, against the neighboring looser connective tissue. The lumen of the gland appears first in the main duct, then in its branches, and, last of all, in the alvetili ; it develops, not by the abortion of the cells in the centre, but by the cells moving asunder so as to leave a central navitj', while they themselves assume an epithelial arrangement. The alveoli are still solid at the l»eginning of the fifth month, but in an embryo of twenty-two weeks were found by Chiewitz, I.e., 497, to be all hollow. At this time the epithelium consists of a single layer of cylinder cells; in the ducts the nuclei are so placed that they form, as in earlier stages also, Fig, ■V-M, D, two rows; the nuclei of the outer row are somewhat smaller and stain more readily than those of the inner row ; in the alveoli the cells are at first all alike, but after the alveoli become hollow some of the cells become enlarged to form muciparous beaker-cella, while others Temain smaller and protoplasmatic ; these smaller ceDs become partly covered in by the neighboring beaker-cells, and thus develop into the semilunar cells of the adult.



iiEllaT>- Gland of a Human Embryo of BEilj-Ihre* to Blxly-«tjdlt Ah; Alitxilus: a, connective- 1 Issue shestli of tcland; D. duct.


Between the anl^^s of the sublingual and submaxillary glands, there appear later — twelfth week in man — ^some eleven to thirteen gland anlages, which in their mode of development resemble small salivary glands, Chiewitz, 86.1, 423. These are termed by Chiewitz alveolingual glands, and have been often confounded with the true sublingual gland.

Tongue

Although the tongue is developed from the floor of the pharynx, yet it becomes so entirely an appendage of the mouth that it may be appropriately treated here. Our knowledge of the development of the tongue is derived chiefly from Dursy, 69. 1, and His, ("Anat. menschl. Embrj-onen," III., Ci-8I).

The first distinct trace of the tongue is a small tubercle which appears in the middle line on the floor of the pharynx between the ends of the nrst and second (i.e., mandibular and hyoid) arches. It was supposed by Dursy to be formed by the fusion of me lower ends of the mandibular arches, but His has shown that it is single and median, and accordingly has ') Q termed it tuherculum impar. Fig, J 1 1T7. Immediately behind the tubercle appears the evagination to fonn the thyroid gland, see Chapter XXIX. Very 8<x)n after the tuliercle has appeared the lower ends of the second and third arches fuse — human embryos of 7 mm. — and their fused ends constitute the [■ anlages of the back of the tongue. The tubercle now rapidly enlai^es. Fig. 33.'t, Tf7, and becomes easily recognizable! n« the front i>art of the tongue. The site of the thyroid evagination remains as a fixed point, which is often marked by a Fmall depression, the /orrtmcjt CfKfM?» of Morgagni; the duct of the thyroid sometimes persists and is then found starting from the foramen ctecnm. The front and hack of the tongue are marked off. Fig. :t3.'>, by two oblique lines, which start from th(> foramen, and together form a widely open V, This V can be traced — as pointed out by His, 7.C., 7t'— in the adult tongue; the part Iwhind the V lias its surface thrown into ridges, and over it there are glands, which appear during the tliinl month; the part in front has jKtpillie developed under its epithelium, aiid the papillie circunivallat.-e are situatol a little (S-fi mm.) in front of the V, but in lines parallel with it; thecircumvallate papilhe do not, therefore, represent the division line between the front and back of the tongue. The largest part of the tongtie is de\'oloi)ed from the tuherculum impar, the less part from the r^on of the second and third branchial arches — hence the tongue is a derivative of the pharj-nx and not of the oral ciivity.



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Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History



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