Paper - The comparative anatomy of the lips and labial villi of vertebrates

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Anson BJ. The comparative anatomy of the lips and labial villi of vertebrates (1929) J. Morphol. 47(2): 336-406.

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This historic 1929 paper by Anson describes lip development.


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The Comparative Anatomy of the Lips and Labial Villi of Vertebrates

Barry J. Anson

Department Of Anatomy, Northwestern University Medical School, Chicago, Illinois

  • The preparation of manuscript and drawings for publication was continued at Northwestern University, where Professor Arey provided technical and other assistance which is gratefully acknowledged; Miss M. Walsh drew many of the figures included in plates 1, 4, and 6.

Five text figures and nine Plates


Author's Abstract

An attempt is made to define lips, and on Danforth's interpretation of homology, homologous lips are found at certain stages of development in some representatives of all classes of vertebrates. The primary lips characteristic of selachians, after the maxillary and premaxillary hones have developed within the territory of the upper lip (toadfish, may disappear (trout, Spelerpes), accompanied by a forward migration of the lower jaw. The secondary lips of higher forms are first indicated in certain teleosts and amphibians. Lips vary in structure to accord with their physiological functions, whether sensory, prehensile,, or adhesive. Lips of the cod are highly sensory: those of the tadpole and of grazing animals, in different ways, are notably prehensile: the lips of x>etromyxoii and the vampire, having abundant villi, are most effectively adhesive. Therefore, the smaller villi of the lips in suckling animals are presumably for tight adhesion to the nipple. The opossum and rat, however, nurse before their lips have developed. No free macroscopic villi are found on human lips, hut there is a zone of thick epithelium tending to form villi. Such a zone is shown to be a widespread feature of vertebrate lips.

Introduction

At the meeting of the American Association of Anatomists in 1924, Dr. F. T. Lewis described the nasomaxillary angles of human lips, both embryonic and adult, and suggested that the transient labial villi found in tiers between the teeth and the skin might represent abortive generations of primitive teeth. Interested in determining further the real significance of these peculiar villi, he proposed that I should make a comprehensive study of the problem, "which has been done, under his general supervision, during the four years that I have held a National Research Council Fellowship at the Harvard Medical School.^ All reasons for regarding the villi as denticles have been presented in an earlier paper, in "which the remarkable tooth-like processes within the upper lip of the cat, mimicking on a small scale the forms of selachian teeth, are described in detail apparently for the first time. My conclusion then was that a first step had been taken in establishing the ‘denticle’ interpretation, but that a further one was needed, and it wms stated that a study for that purpose was in progress. This has now been completed, and any relationship between villi and ancestral teeth might be definitely denied were it not for the iconoclastic interpretation of cranial homologies earnestly advocated by Kesteven in a long series of papers. Probably Professor Starks is in accord with most authorities when he comments on Kesteven’s work— “His view, ingenious as it is, I do not find myself able to accept.” Were it correct, a most interesting relationship between the villi and the premaxillary teeth of teleosts might exist, as will be shown incidentally in its proper place. Our primary interest, however, is in the comparative anatomy of the lips, with a view to interpreting their conditions in man. But who knows what a lip is, or which vertebrates have them?

Meckel has been the guide and leader of the G-erman anatomists in this discussion.® In 1811, he wrote :

Die Lippen fehleii den nieisten Saugthieren und alien tiefer als sie stehenden Wirbelthieren, erseheinen aber aueh bei dem menselilieben Embryo nieht vor Ablauf der ersten beiden Monate nach der Bmpfangniss.

Little by little the German writers have admitted a wider occurrence of lips. Wiedersheim, in early editions of his Lehrbuch,* emphasized the muscular element as essential to true lips, and saw in mammals “hier zum erstenmal auftretenden eigentlichen Lippenbildungen. ” But, somewhat more lenient than Meckel, he adds: “Die Cetaeeen und Monotremen sind die einzigen Saugethiere, welche der Lippenbildun gen ganzlieh. entbehren.” Kiikentbal was astounded by this statement, in so far as it refers to Cetacea.


  • a personal communication.
  • Meckel, J. F. Beytrage zur vergleiehenden Anatomie, Bd. 2, Heft 1, Leipzig, 1811, S. 45.
  • Wiedersheim, K. Lehrhuch der vergl. Anat. der Wirbelthiere, 2. Auflage. Jena, 1886, xS. 482.


He writes :

Diese Bemerkung Wiederslieim’.s ist um so unverstandlicher, als ja gerade bekanntermaassen bei den Bartenwalen die Erhebung der Unterlippe eine ganz ausserordentlich grosse ist und bei der Nabrungsaufnahme eine wiehtige Rolle zu spielen hat.

And be quotes Escliricbt, “sclion vor fast einem halben Jabrbundert,” for details. Kopscb, in Rauber’s Lehrbucli,. does not beed tbis correction, but Wiedersbeim bas done so, and tbe last word on tbe subject, from Schumacher, is as follows :

Eigentliehe, d. li. mit Muskulatur I'^ersehene, Lippen flnden sich erst bei den Sdugern. Die „Lippen“ bei Fischen diirfen nicM mit den muskulosen Lippen der Sangetiere homologisiert werden; sie stellen nur Hautfalten mit senkrecht sich uberkreuzenden Bindegewebsziigen dar (Oppel). Die fleischigen Lippen der Sangetiere, in Gemeinschaft mit den Backen sowie mit der beweglichen, muskulosen Zimge, ermoglichen das Saugen und stehen auch in wiehtiger Beziehung zur artikulierten Sprache des Mensehen. Den Monotremen fehlen Lippenbildungen.

Thus tbe German anatomists wbo see lips as primarily a muscular suckling apparatus, consistently restrict them to mammals ; but others wbo consider that lips devoid of muscle would still be lips, find them more widely distributed. We shall see that, in tbe nursing stage, tbe lips of mammals are very often but slightly developed.

  • Kukenthal, W. VergL-anat. und entw. Untersucliungeii an Walthieren. Tli. 2, Jena, 1893. Denksclir. d. med.-naturw. Ges. zu Jena, Ed. 3, S. 317.
  • der Tierreihe kommen Lippen nur den Saugetieren zu, dock fehlen sie den Monotremen und Walen.^^ (Eauber^s Lehrb. d. Anat. d. Mensck., 7. Aufl., Abt. 4, 1907, S. 15).
  • Hdb. d. mikr. Anat. d. Mensch., kerausgegeben von v. Mbllendorff, Bd. 5, Teil 1, Berlin, 1927, S. 13,
  • Most of this passage is an unacknowledged quotation from Wiedersheim. But Keibel independently prefers to restrict the term lips to the structures in man and mammals. selbst ziehe es vor, den Ausdruck Lippen fiir die fleischigen Hautmuskelwiilste zu reservieren, welehe den Mund der Sauger und des Mensehen umgeben^^ (Hertwig's Handb. d. vergL ii. exp. Entw. d. Wirbeltiere, Bd, 1, Teil 2, 1906, S. 156.)


A very different position lias been taken by French and English anatomists.® Cuvier, while admitting that many sorts of fish are lipless, found that more often their months are bordered with soft and extensible lips, sometimes ^tres grandes’ or 'tres proiioncees. ’ Among amphibians ^4a boncliG est bordee de levres dans la sirene et le proUe .... La pean forme autour de la bonclie (des ampliiiima) des levres tres marquees.” Although Cuvier found no mobile lips in reptiles, and only so-called lips in birds, he recognized their presence ^^daiis tons les mammiferes, sauf dans les echidnes. .... Dans Vornitlwrhynqiie elles n’ont aucune mobilite.”^®

Similarly, Owen, who knows that fishes ^Oiave not so distinct a sphincter oris as mammals,” ascribes to some of them ^ thick fleshy lips,’ In reptiles — Ophidia, Sauria, and Crocodilia —

a narrow tract of soft and vascular integument intervenes between the scale-clad border of the mouth and the jaws, sinking into a more or less shallow groove which defines the lips and receives the secretion of a row of mucous crypts : but such lips are hard and inflexible : in certain frogs and toads they are of softer texture: but in none are produced or prehensile.^ ^

With this Goppert agrees, differing from the prevalent opinion of German anatomists when he writes :

Lippen bcsitzen ferner die Mehrzahl der Eeptilien. Sie beherbergen hier die Glandulae labiales. Ihr Pehlen bei den Cheloniern erklart sich aus eiuer Exiekbildung. . , . Bei den Saugetieren .... durch den Besitz einer Muskulatur stehen sie auf besonders hoher Entwieklungsstufed ® Compare Ilondeletms, G., Libri de piscibus, Lugduni, 1554, Lib. Ill, Cap. 8, p. 57: Whoever credulously believes that there are no lips in fishes •wdll at

once change his mind when he sees and handles rock-fishes, which have fleshy, true and perfect, mobile lips. . . . Very many fishes then lack lips: some have them, like the rock-fishes which browse on moss, and take their food with lips instead of hands, as oxen, sheep and the like, which live by grazing; and they must have lips since their tongne is imperfect.^’ (Note by Dr. Ijewis.)

  • Cmder, O. Legoiis d^aiiatomie compar^e. 2. 4d. T. 4, jjart. 1, Paris, 1835, pp. 381-409.
  • Owen, E. On the anatomy of vertebrates, Vol. 1, London, 1866, pp. 410-411; 434.
  • In Hertwig’s Handbuch, Bd. 2, Teil 1, 1906, S, 79.


Rejecting facial musculature as the essential feature of lips, they may be defined as fleshy folds bordering’ the entrance to tbe mouth, placed anterior to the skeletal supports of the jaws, and separated from the dentigerous zone by a groove, the sulcus labialis or vestibulum oris ; lips may be partially supported by labial cartilages, and may be rendered mobile through the invasion of skeletal muscle. In man tbe lips possess two zones, described by Luschka as the pars villosa and pars glabra, and after a lapse of some years these terms and the parts which they stand for have become quite familiar. It is, however, a question whether in the human lip these villi are distinct and separate in life, or wdiether tall connectivetissue papillae extend into a very thick epithelium, which through easy maceration becomes cleft to form the villi. Though the pars villosa is a very distinct zone in the lips of newborn infants, separate villi are not ordinarily seen, even with a hand lens. Rounded or, at best, conical papillae were observed by Dr. Levds in the untreated lips of a dead infant kept a short time in cold storage, but the same specimen after a night in Ringer’s solution, or after hardening and shrinking in Zenker’s fluid, showed separate villi very clearly. He has called my attention to the statement by Ruysch (Thesaurus anatomieus vii, No. xl, Not. 3) that “in the lips the papillae do not come into view unless the epithelium has first been removed, whereas in the intestine elevations in the form of silky hairs (villi) are seen like a fleece tvithout ablation of the integument or epithelia.” “As to function,” Ruysch continues in Not. 5, “I think that the papillo-nervons body is an instrument of sensation (seiisus) and hence such fierce pain where, by humors, bitter medicines or other things, the epithelium has been removed, so that the papillae are affected.” There is a measure of truth in this statement, and a valid criticism of some more modern accounts, but a discussion of these details finds its place in a final section of this paper.

True lips, then, appear to be essentially protrusible folds in front of the teeth, over which the skin connects with mucous membrane, and on the surface of which two zones may be distinguished — an outer smooth zone and an inner papillary (‘villous’) zone, perhaps sensory in function. The animals studied in search for lips to be recognized by this new criterion will be presented in the conventional zoological order. For this investigation 302 sagittal series in the Harvard Embryological Collection were utilized, together with 162 partial series specially prepared for the purpose. Either in the gross or with the binocular microscope, 825 whole specimens were examined. The figures accompanying this paper were selected from 900 tracings and 228 photographs or sketches collected during the progress of the work.

CTCLOSTOMES

Unfortunately, we begin with an aberrant form, the hypophysis of which, instead of remaining in the roof of the mouth, nxigrates to the top of the head. Were it considered an essential landmark, lips, if present, would be in no relation with the mouth. This difficulty has been considered by His, who proposed to place the lips of Petromyzon in a class by themselves, naming them ‘Rachenlippen’; but Keibel, who has arranged an interesting series of embryonic sections of petromyzons and rabbits to illustrate this feature, would not call them lips at all.^^ It becomes a question as to whether the hypophysis is an all-important landmark in this orientation. After it has migrated beyond the oral territory, the mouth becomes overhung by a hood which in descriptive literature is often called a lip. Thus Dohrn ( ’82), when writing of larval lampreys, refers to the ‘extraordinary growth of the upper lip” which he figures together with the ‘Unterlippe."* For this he is criticised by His ( ’92) for the obvious reasons already noted, which were of course well knowm to Dohrn.

  • This is in Keibel's important chapter dealing in a general way with the entire subject of our thesis (Kap. 6 in Hertwig’s Handbuch, Bd. 1, Teil 2, 1906, S. 156 and 158). See also His, Arch. f. Anat. u. Entw., 1892, S. 410.
  • Mitth. d. zool. Stat. zu Neapel, Bd. 4, S. 177.


Prominent in larval stages, the hood or upper lip later retracts, and in continuity -with the fold •which represents a lower lip it bounds the funnel or round mouth of the cyelostomes. In relation with the fleshy circumference of this suctorial mouth, the much-discussed horny teeth are developed and, in front of them, a fringe of tentacles possibly comparable with villi. In greater detail the development of the mouth may be described as follows.

In an embryo of 1.8 mm. the pharyngeal cavity has formed, hut there is only a slight corresponding invagination of the ectoderm. In the 2.7-mm. embryo (fig. 6) the anterior extremity of the foregut has encountered the ectoderm at the bottom of a considerable depression, deeper, in fact, than is usually found in the stomodaeum of vertebrates. Just in front of the ectodermal cavity there is a hj’pophyseal pocket in association with the median olfactory pit they communicate with the exterior by a common opening. In the 4.75-mm. embryo (fig. 7) the naso-hypophyseal invagination has shifted toward the dorsal side of the head, and the hood which has undergone extraordinary growth now forms the roof of the stomodaeum ; at the bottom of the stomodaeal cavity the ectoderm has come in contact with the entoderm to form the pharyngeal membrane. In the 6.8-mm. stage the hood or upper lip has become straightened, and posterior thereto, on the dorsal surface, is located the opening of the naso-pituitary invagination. The dorsal surface of the hood is appreciably thickened, but the epithelium of its inner surface is relatively thin. Villi have appeared within the stomodaeum arranged in annular manner in front of the velum. The latter is a derivative of the pharyngeal membrane which remains into late stages of Petromyzon and marks the boundary between stomodaeal and pharyngeal cavities. These relations, as seen in an older specimen (27.6 mm.), are sho^wn in figure 8.

In the larva of 42 mm. (fig. 9) the villi, apparently involved in the general anterior migration, have moved forward, and.

Compare Bohrn, loc. cit. ; Ahlborn, Zeitsehr. f . wiss. Zool., 1883, Bd. 39, S. 191-294; Kaensche, ZooL Beitr., Breslau, 1890, Bd. 2, S. 219-250.



in the young adult (fig. 10), are marginal in position. The upper lip in its later stages, as shown by Eathke’s dissections, consists ‘ ‘ chiefly of muscle fibers .... by wdiich the lip after expansion becomes again contracted.” The lower lip, which is mostly connective tissue, contains a thin layer of muscle, and may be ‘ ‘ somewhat contracted. ’ Eathke regarded the villi as organs of taste.

In the young adult (fig. 58) the villi are macroscopic, very vascular, and covered with an epithelium containing an abundance of mucous cells. These cells have a very thick (4 p) and markedly striate cuticula, which, resisting compression in empty cells, may form a hemispherical cap over a downwardly tapering body. Thin dark cells wdth flat tops are frequently seen. But the well-preserved formalin specimen studied does not show that the cuticula is interrupted in places by sensory cells, each with five to ten stiff hair-like processes, in the manner described by Langerhans.” In alcohol-fixed material Ivaensche also was unable to find them, but he notes that Langerhans used fresh specimens. Thus the existence of sensory cells, so definitely described by Langerhans, awaits confirmation.

After the development of the villi, and altogether independently of them (Ivaensche), the horny ‘teeth’ first appear, like the lips on the wrong side of the hypophysis. Their position within the funnel-shaped mouth, posterior to the villous cirri, is shown in figure 10. They are supported by a bar of cartilage (the annular cartilage) which, in sections, is a large object, rising into the pulp of the tooth (fig. 57). This pulp is a papilla of corium, capped by a thick and modified epithelium, the broad germinative layer of which is followed by a clear cornified zone. In the latter nuclei are absent, but intercellular spaces are retained. The cornified layer represents the corium of a developing tooth.

  • Eathke, H. Anatoniie ties Querders. Beitrage, Abtli. 4, Halle, 1827, S. 75 ^ 77 .
  • Langerhans, P. ITntersucliungen iiber Petromyzon planeri, Preiburg, 1873, S. 19-20.


External to it (fig. 57) there is a sheet of non-eoriiifiecl cells, aceumiilatiiig in a thicker mass toward the apex of the functioning tooth. These cells Kaensche named ‘stellate,’ since he observed their spiny intercellular processes as a conspicuous feature. It is an inappropriate term for the flat cells in my specimen. An outer zone of cornified cells, quite like the inner, completes, the picture. Since the process of corniflcation is an interrupted one, provision is made for shedding the teeth, which are merely these cornified caps, one within another. In other vertebrates, the teeth are calcified cores of dentine, with a cover of enamel, which breaks through the epithelium to appear on the surface. Such a tooth of a dogfish embryo is shown for comparison in figure 59.

Henii (1923) finds that the teeth of cyclostomes are without genetical relations to the teeth of other groups.^® The small size of the dermal papilla, the absence of dentine, and the epidermal character of the tooth as a whole in Petromyzon support such a view. Yet, as Beard has pointed out, in Myxine and Bdellostoma the dermal papilla produces an imperfectly calcified tooth beneath the epidermal cornification.'** Warren (’02) and Bridge (’10)^” are of the opinion that epidermal teeth represent a primitive stage in the evolution of teeth and dermal spines, to be followed by a later stage in which calcification supersedes cornification as a method of hardening. Until odoiitologists can agree whether Petromyzon has teeth, a decision as to the presence of lips may well be reserved. The mouth is certainly bounded by a muscular fold provided with vascular tentacles; and in describing the villi on the lips of human infants. West, in an important paper, has recently stated, ‘ ‘ One is struck by the resemblance which they bear to the condition found in the mouth of Petromyzon. ’

  • In Dean’s Bibliography of Pishes, voL 3.
  • Anat Anz., 1888, Bd. 3, S. 169-172 ; ZooL Jahrb., 1889, S. 727-752.
  • Warren, E., Quart. Journ. Micr. Sci., N. S., voh 15, pp, 631-636; Bridge, T. W., Cambridge Nat. Hist., vol. 7, p. 248.
  • Carnegie Pub. no. 361 (Contr. to Embryology, vol. 16), 1925, pp. 23-45.


ELASATOBRANCHS

The mouth of elasmohraaichs, commonly ventral, is large and crescentic. In some sharks, for example, in Prionace gla-nca, Ehinohatus lentiginosus, and Sqnalus acanthias (fig. 61) it has no connection with the nasal pits; but in the torpedoes and rays generally there is a pair of oronasal or nasobuccal grooves (fig. 63). The month opens into a spacious oral cavity. On its floor the mucous membrane is raised by the basihyal cartilage into a fold, more or less pronounced, forming the imperfect tongue, — “a protrusible tongue is never developed in fishes. Small folds, generally corresponding in direction with the curve of the mouth, are present in front of the tongue. Cells secreting mucus are found, but there is absence “of all glands which are characteristically present in higher forms. Owen refers to the lips of most sharks and rays as “partially supported by labial cartilages”; Daniel recognizes the “membranous folds or lips” which bound the mouth ; Gregory sees ' ‘ all the elements of the face of man” and specifically includes lips.^^ A more adequate description follows.

Dogfish

Over the maxilla of Squalus acanthias (fig. 61) a prominent fleshy fold projects do-wnward, almost concealing the teeth of the upper jaw. It is limited within by a pronounced groove, which in large specimens may attain a depth of 4 or 5 mm. The fold and groove (fig. 62) resemble to a striking degree the lip and labial groove of the mammalian mouth, and the cavity of the groove is properly a vestibulnm oris. But toward the angle of the mouth the lip bifurcates, its two arms enclosing a depression into which fits an arm of the lower lip containing a labial cartilage. The upper arm of the upper lip likewise contains a cartilage, and as the cartilages of the two lips come together at the angle of the mouth they resemble, on a small scale, the cartilages of the jaws themselves. Accepting Gegenbaur’s interpretation of the labial cartilages as the upper and lower segments, respectively, of a premandihnlar branchial arch, Sewertzoff interprets the pocket behind this arch (seen between the diverging arms of the upper lip in fig. 62) as a rudimentary gill cleft which does not break through to the outer surface.®^ He goes even further and finds in the embryo of Acanthias a more anterior branchial outpocketing, which should indent the upper arm of the upper lip in figure 62. The chief labial cartilage of the upper lip lies behind this slight pocket; a smaller labial cartilage is in front of it and, as a whole, nearer the median line. Thus each half of the upper lip contains two cartilages, instead of one, as in the lower lip. For us it is a recondite problem whether these skeletal elements of the lips of sharks are regressive structures, as viewed by Gegenbaur and Sewertzotf, or are new acquisitions in process of further development as indicated by conditions in the higher vertebrates.

  • Bridge, Cambridge Nat. Hist., vol. 7, 1904, p. 252.
  • Daniel, Elasmobraneh fishes, Berkeley, 1922.
  • Gregory, Amer. Mns. Journ., 1917, vol. 17, p. S79.


The relations of the upper lip of the dogfish, as seen in section, are shown in figure 14. Toward the median line it contains no cartilage. Its interior is filled with loose connective tissue, having many elastic fibers, sharply differentiated with Weigert’s resorcin-fuchsin. It is not very vascular, but contains several nerve trunks, presumably sensory. Beneath the thick epidermis of the outer surface there is a dense tendon-like corium consisting chiefly of very coarse fibers parallel with the surface, crossed by perpendiculars at fairly regular intervals. The corium is bounded internally by a layer of fine elastic elements. Externally, an elastic basement membrane stretches under the epithelium, and on encountering the dermal spines above their expanded basal plates, it ensheathes them closely, and can be followed as far as their enamel. These spines, however, as seen in figure 14, are beyond the free portion of the lip. The epithelium is specially thickened toward the free edge of the lip and contains a few scattered slender papillae, sometimes distinctly vascular. On the inner surface of the lip the epithelium is thinner and the corium looser. This tough and elastic lip contains no muscle. The ‘levator labii superioris’ arises from the cranium, passes along the upper labial cartilage -without being attached to it, and is inserted into the mandibular fascia; its function is not to raise the upper lip, but to protract the ja-R’^s.^® None of the other muscles attached to the ja-vvs have been found to enter the lips.


  • SewertzofP, A. N. Die Morphologie des Tiseeralappavates der Elasmobraneliier. Anat. Anz., 1923, Bd. 56, pp. 389-410.


Internal to the upper lip there is a ridge of tissue covering the free margin of the palatoquadrate cartilage (which is the skeletal element of the upper jaw in selachians). This ridge bears the teeth, which develop along its inner surface, pass over its crest, and are then shed along its outer surface, their sites being marked by papillae of infiltrated or scar tissue. The dentigerous ridge is the gingiva of figure 14.

Internal to the gingival ridge there is a transverse fold of soft tissue -which is described in Cuvier’s Auatomie comparee of 1835 as an internal lip (levre interieure).^'^ We had named this structure the palatine fold or valve (fig. 14, v.pal.) before reading the opinion of Allis that, under certain conditions, the corresponding structure in Chlamydoselaehus “would strikingly suggest if it does not actually foretell, the secondary palate of mammals. It had been evident that if the nasal pits should acquire internal openings above it, separated by a median septum which should grow down to meet it, mammalian conditions would result. But we had abandoned this idea as too speculative, before finding it expressed on the high authority of Allis. A corresponding infrapalatine fold or mandibular valve, though much less developed, rises toward it from the floor of the mouth.

The lower lip of the dogfish is similar to the upper, though much less developed (fig. 14). It is limited to the lateral two-thirds of each half of the jaw. Medially, its absence suggests a greatly broadened frenulum.^® The labial groove attains its maximum depth (4 mm.) near the angles of the mouth, and there the mouth ends in a pair of oblique clefts which, if prolonged, might reach the nasal pits, as in the skate.


  • Haller. jEntwieklung, Bau uiid Mechanik des Kieferapparates cles Doriiliais. Zeitschr. f. mikr-anat. Forsehung, 1£)26, Bd. 5, S. 783. Tome 4, Partie 1, p. 399.
  • Allis, E. P. Homologies of the pahTto-quadrate of selaeliiaus. Anat. Anz., 1913, Bd. 45, S. 355.



The embryological development of the lips in Squalus may he studied in the sagittal series of the Harvard Collection, which are among the specimens used by Scammon for his Normentafeln in 1911.®" At 5 mm. ectoderm and entoderm have fused to form the oral plate. At 22 m m. (fig. 11) the plate has gone, except for a remnant, just anterior to which the hjqpophyseal diverticulum is still attached to the roof of the mouth. The oral roof and the floor of the forebraiu are no longer in complete apposition. Mesenchyma has begun to separate these two epithelial surfaces, and the roof of the mouth shows certain obscure transverse folds. In the 50-mm. embryo (fig. 12) the significance of these folds is evident. There are then three of them, of which the most anterior, limited behind by a groove, will form the lip of the adult ; the middle one is the gingival ridge ; the most posterior, situated just behind the dental lamina, is the palatine valve. A later stage is shown in figure 13, from an embryo of 159 mm. The balanced relation between the palatoquadrate cartilage and Meckel’s cartilage, each with a labial lamina or groove in front of it and a dental lamina or groove behind it, is strikingly apparent. Usually the elasmobranch lips are set off by a groove. This is true of the upper lip, and of the lower lip laterally ; but more medially the latter presents a solid lamina (fig. 13), which is the common condition in embryonic mammals.

Raja and Torpedo

In Eaja, but not in Squalus, the oblique clefts at the angles of the mouth extend forward to the nasal pits, thus forming the oronasal grooves. They extend also posteriorly; and curving toward each other beneath the mandibular rami, they cross the midline as a series of inframandibular sulci (fig. 63). Human embryos of the 8 to 10-mm. stage, and larger, show a corresponding furrow beneath the lower jaw, which accounts for their characteristic ‘double chin.’^*^ But the dogfish (fig. 61) has nothing of the sort. In the skate, between the upper lip and the teeth, there are variable loose folds, quite conspicuous on the right of figure 63. They seem to be merely the slack when the jaw is retracted. The upper lip is much better developed than the lower, and in the section of a 48-mm. embryo (fig. 18) with a distinct labium superius, no trace of a lower lip is indicated.


  • According to Jordan and Everraann, Chiniaera has 'Uips tliickish, the lower with a frenum.
  • Scammon: Squalus aeanthias, Heft 112 in KeibePs Normentafeln zur Entw. d. Wirbeltiere, 1911.


The embryological origin of the lip in torpedoes is shown in figures 15 to 17. These sections are quite like those of Squalus, except that the dental laminae open into sulci toward the surface, and the inframandibular furrows have appeared in figure 17. The cartilage shown in the upper lip is a median structure not found in the dogfish. Such examples of lips as have been presented presumably represent the typical selachian condition.

Selachian lips in relation to denticles and villi

'Williamson’s paper in ’49, “On the microscopic structure of scales and dermal teeth, ’ ’ and the brilliant studies of Owen and Hertwig which followed, have established as a most familiar fact that selachian teeth and placoid scales are ‘ ‘ identical in essential structure as well as in their manner of development.”®^ But we cannot agree with Bridge that in the embryo dermal spines and teeth form a continuous series and “it is only later, when lips become apparent, that the continuity of the teeth and dermal spines is interrupted and the two structures assume their distinctive characters.”^® That conclusion he supports with Gegenbaur’s figure of the lower jaw of a young Scyllium, showing the teeth with their spines pointing inward, and scales of the adjacent skin with spines pointing outward, between which there are scales with spines pointing both ways, making a perfect transition. We have found Gegenbaur’s figure decidedly misleading, as also his statement that “the presence of these structures [the teeth] in the primitive buccal cavity is explicable from the fact that it was formed by an invagination from the exterior.


Compare His as to the ^Doppelkinn^ — ^Die Entwiekelung der menschliehen unci thierischer Physiognomien. — Eine Skizze. Arch. f. Anat, u. Entw., 1892, S. 394.

Williamson, W. C. Phil. Trans. Boy. Soc. London, 1849, pp. 435^75: Owen, E- Odontography, London, 1840-1845: Hertwig, O. Ban u. Entw. d. Placoidschnppen u. d. Zahne, Jena. Zeitschr., 1874, Bd. 8, S. 331-404; Hautskelet cl. Pische, Morph. Jahrb., 1876, Bd. 2, S. 32S-395.



There is apparently no selachian mouth which has a general lining of dermal denticles, certain of which become hypertrophied to form the teeth. On the contrary, the teeth appear as primary structures, decidedly segregated in early embryonic stages. They arise as specialized papillae of the corium along the labial wall of the dental lamina; the lingual wall of the lamina, lacking the power to form teeth, serves as a covering — ^the ‘thecal lamina’ of Bolk. Subsequently, teeth form in other situations. The dermal denticles of the lower jaw of the dogfish are directed distaUy, so as not to oppose the forward movement in swimming (fig. 14, with an enlarged one at &) and the nearest teeth point into the mouth, to retain food (fig. 14, a). Very likely in other forms, as figured by Gegenbaur, the transition between teeth and scale is bridged by T-shaped structures of intermediate size, which point both ways. Thus the teeth on either half of the lower jaw of Heptanchus, as figured by McDonald and Barron,®® are serrated predominantly to the right and left, respectively, with a median tooth serrated both ways. But this is no evidence that the right teeth were derived from the left or vice versa. The fact which Gegenbaur well knew, that teeth may develop in entodermal territory, is in itself sufficient to show that they are not merely scales impocketed into tke montli and there hypertrophied.

Cambridge Nat. Hist., vol. 7, pp. 248-249.

Gegenbaur, C. Elements of compcarative anatomy. London, 1878, p. ooO. The figure is in Yergl. Anat. d. Wirbelthiere, Bd. 2, Leipzig, 1901, S. 40.

®“See Daniel, p. 124.


This rather fine distinction is of special importance in our particular problem. No denticles are found on the inner surface of the selachian lip, i.e., in the. region of transition between scales and teeth, where in the cat the tooth-shaped villi are located. The latter, therefore, cannot be interpreted as selachian denticles. Furthermore, the labial villi of mammals are non-calcified structures rising above the general level of the mucous membrane. Although Bose insists (against Hertwig) that “die erste Anlage der Zahne zeigt sieh bei Crocodilns Inporcatus in Form von freien, fiber die Oberflaehe der klundschleimhaut hervorragenden Papillen,”®'* it is certainly characteristic of selachian teeth and spines that they do not project until calcified.

Toward the upper lip of the young dogfish (fig. 14) the dermal spines end quite abruptly. An occasional broad and characteristic papilla on the outer surface of the lip, causing no elevation, is evidently a denticle in process of development, or perhaps in arrested development. But the epithelium toward the labial margin becomes very thick. With the scattered slender vascular papillae which enter it, though causing no elevations, it is quite suggestive of the pars villosa of the human lip. The relation, if any, between the slender papillae of the dogfish lip and the broad ones which are abortive denticles, cannot be easily determined: they appear to be altogether independent formations.

TELEOSTS

The extraordinary development of the lips of teleosts makes them, of all classes, the most interesting for the present study. That was recognized by Allis, whose account of “the lips and the nasal apertures in gnathostome fishes” constitutes the essential literature of this subject.^^ The primary, secondary, and tertiary lips wMcli lie described in tlie teleosts, and found reprevsented in selachians and dipnoans, will here be presented in a simpler manner, and quite differently, since it is the result of a wholly independent investigation; yet the main conclusions of Allis are strikingly verified. We have selected three common and widely distributed fishes — the cod, carp, and trout — as presumably typical of the vast array. The toadfish has been added, since embryological sections of that species were already at hand.


^*R-ose, C. Ueber die Zahneiitwieklimg der Eoptilien. Deutsehe Monatsselirift f. Zahnlieilkiinde, 1892, Jahrg, 10, p. 129.

Allis, B, P. Jour. Morph., 1919, vol. 32, pp. 145-205. (Paper dated 1916. Through an oversight, the plates, though aceompaiiied by a list of abbreviations, are wdthont lettering and eonsequently sometimes difficult to interpret.)




Fig. 1 ToacMsh (Opsanus tau). Sagittal section of tlie head. X 3- d.'gal.f d.p-mx.j d.mn.j palatine, premaxillary, and mandibular teeth, respectively j Z.L, lower lip; Z.5., upper lip; mx., maxilla; premaxilla; s.i-lab.j infralabial sulcus; v.myi.f mandibular valve; v.mx., maxillary valve; x and y, morphologically equivalent sulci.


Toad fish {Opsanus tau)

In a parasagittal section of an adult, passing along the medial surface of the orbit (fig. 1), the single row of strong, blunt palatine teeth {d.pal,) in the upper jaw and the corresponding row of mandibular teeth in the lower jaw {d,mn.) serve as the essential landmarks for comparison with the selachians. The palatine valve of the shai’k, behind the palatine teeth, has disappeared, but the corresponding structure of the lower jaw — the mandibular valve (v.mn .) — ^is well developed. Just behind the palatine teeth, and also in front of them, there is a row of soft nodular elevations of the mucous membrane. Several rows of similar nodules are seen on either side of the mandibular teeth.

Eeplacing the simple curtain-like upper lip of the dogfish, there is here a protractile upturned structure, containing two bones instead of two cartilages, and provided with a row of premaxillary teeth {d.p-mx.) which does not extend very far laterally. These teeth, like the others, are bordered on either side by a line of vascular nodular elevations of the mucosa. Between the teeth and the anterior beaded fringe there is a groove of some depth, forecasting a new labial sulcus; and in front of the beaded fringe a shallow depression may be considered as separating an internal pars villosa from an external pars glabra. However, this new secondary upper lip is still very ill-defined. Behind the anterior row of teeth there is a new palate-like fold, called the maxillary valve (v.mx.), which functionally replaces the palatine valve of selachians.

Cuvier recognized that back of the anterior teeth, and almost always in both jaws, “there is a sort of membranous velum or valve .... the effect of which is to prevent food, and especially the water taken in for respiration, from escaping through the mouth.” He found this valve well marked in the genus Zeus, and noted that it exists ‘dans une infinite d’autres poissons’ (Hist. nat. des poissons, Paris, 1828, T. 1, p. 497). Dahlgren, watching living fishes in aquaria, saw the valves in action in over fifty species — ‘no teleost has been found without them’ — and unaware of Cuvier’s reference, he wrote his excellent account of ‘the maxillary and mandibular breathing valves of teleost fishes’ (Zool. Bull., 1898, vol. 2, pp. 117124). Cuvier or perhaps his editor, Duvernoy, made no distinction between the palatine valve of selachians and the maxillary valve of teleosts, both being included under ‘levres interieures’; but their radical topographical difference has been noted by Allis and others.

The lower lip of the toadfish is by no means a replica of the upper, though embryologically it arises in similar fashion (fig. 20, from an 8-mm. specimen). Medially, on either side, a premandibiilar cartilage develops within it (shoA\m in section in fig. 1), hut no bone, and it has not teeth. No breathingvalve is associated with it. Its crest shows medially some obscure nodules, apparently comparable with those of the upper lip which are in relation with the premaxillary teeth. The entire structure is rather loosely attached to the lower jaw, since a deep infralabial sulcus {s.i-lah.) rises from below toward a groove, x, which separates the premandibular formation from the. nodulous gingival zone. Morphologically corresponding to x is the groove y in the upper jaw. This arrangement suggests that the premandibular lip will be suppressed in higher vertebrates or will merge in a secondary low-er lip, w^hereas the primary upper lip will become an integral part of the upper jaw with or without the formation of a secondary upper lip.

A curious condition, in the toadfish, results from having one set of teeth in the lower jaw and twm sets in the upper. In general, in the animal series, the primitive occlusion of the mandibular teeth with the palatine gives place to occlusion of the mandibular teeth with the maxillary and premaxillary, accomplished by a forwmrd growdh of the lower jawL Subsequently the palatine teeth disappear. The toadfish is in a transition stage. Laterally, its mandibular teeth bite against the palatine. Medially, they bite against the central premaxillaries, but the more lateral premaxillaries bite against the lower lip. This extraordinary maladjustment of upper and lower teeth is shown in figure 21, a section of a 42-mm. specimen. Comparison with figure 20 shows how different is the course of development taken by the two primary lips and summarizes what we have said of this fish. In the cod, what seems a far more satisfactory occlusion has been effected.®*

I am indebted to Br. Lewis for formulating tlie comparison between the fish and amphibian lips as presented throughout this and the following sections. He has made the finished drawings of figures Ifl to 21, 25 and 26, 28 to 32, and some others; figures 1 to 3 and 23 were redrawn hy Mr. Aitken under his direction, during my stay in Cliicago.


Cod {Gadiis morrhua)

Tlie several folds about the moxitli have been sketched hi figure 23. The entire margin of the upper jaw is supported by the premaxillary bone. Internal to the premaxillary at the angle of the mouth, and extending forward above the premaxillary under the eye, is a fold formed about the maxillary bone. It passes out of the region of the lip as seen in figure 2 (mx., compare fig. 1).

As stated by Kesteveu,^° the maxillae and premaxillae of the majority of teleostean fish constitute an adventitious jaw. That they are “homologous with the labial cartilages of the elasmobranch fish is based on the relation of both structures to the forepart of the skull and to the lips.” With that portion of his contention we can agree, but not with his ‘somewhat startling ideas’ regarding their homologies in the higher vertebrat&s. The older view seems unassailable, namely that the premaxillary bone loses its lateral part and becomes intermaxillary, allowing the maxilla to form all the lateral portion of the jaw in higher vertebrates.

Above tbe maxillary fold in the cod there is a supramaxillary fold, which in part of its course contains a bone variously called lacrimal, peri-orbital, infra-orbital, etc., "with the homologies of which happily we are not concerned. This fold represents the tertiary lip of Allis, which, as a lip, is ‘found only in the Dipneusti’ (l.c., p. 184). In mammalian embryos its homologue is a portion of the maxillary process bounding the lacrimal groove laterally (Allis, p. 193). Thus it extends downward from the region of the inner canthus of the eye to the middle of the cheek, and in none of the animals which we have studied is there valid reason to consider it a lip.

The loTver jaw is formed by a broad mandibular element, which is seen both above and beloxv the premandibular fold lying in front of it. This premandibular fold tapers laterally, contains a cartilage, and constitutes the primary low-er lip. That it is comparable with the primary upper lip has been shown in the toadfish. But whereas the secondary upper lip is only a small part of the primary upper lip, the secondary lower lip includes all of the premandibular fold together with a portion of the mandibular fold. In fishes in which the premandibular fold is not developed, the lower lip is altogether a derivative of the mandibular fold.


^ Kesteven, H. L. A new interpretation of the bones in the palate and upper jaw of hshes. Jourii. Anat., 1922, rol. p, 315,



Fig. 2 Codfish (Gadus morrliua). Sagittal section of the head. Natural size. Lettering as in figure 1, with the following additions : p-mn., premandibular fold ; s.i-7nn.f inframandibular sulcus; supramaxillary fold; vih, labial villi; vom., vomer.


The paramedian section of the cod’s head (fig. 2) is readily comparable with that of the toadfish. In the cod the row of palatine teeth is lacking, but the median extension of that rowon the vomer is retained; a few vomerine teeth appear in the section, "with wrinkled and somewhat nodular folds of mucous membrane on either side, suggestive of conditions in the toadfish. There are several ranks of premaxillary teeth, occupying a crescentic dentigerous area widening to 11 mm. medially, but ending abruptly before reaching the midline. The mucous membrane of the roof of the mouth can thus pass uninterruptedly to the freuular portion of the upper lip. Internal to the upper teeth there is a row of nodules along the attached border of the somewhat rudimentary maxillary breathing-valve. Externally there is a w’ell-defined labial sulcus, in places 3 mm. deep. The lip, without magnification, shows a fringed inner part or \’illosa, and smooth outer glabra. It is overhung by the supramaxillary fold.

The lower jaw has several rows of mandibular teeth, which now occlude effectively with the premaxillaries. Within the mouth these teeth are bordered by nodular excrescences of mucous membrane which, as in the upper jaw, are along the attached border of the rudimentary breathing-valve. The extensive lower lip contains the premandibular cartilage in its medial part, and show's a fringed crest and smooth outer declivity. Since the villous fringe follow's the line of the teeth, it is only medially that it can be regarded as crowning the premandibular fold (fig. 2). There is a deep infralabial sulcus as in the toadfish, and a still deeper inframandibular sulcus. A tongue, much more like that of higher animals than is found in most fishes, rises from the floor of the mouth.

A sagittal section of the lips of a full-grown cod, not far from the median line, is shown in figure 22. The skin of the pars glabra is thinner, softer, and less deeply pigmented than ordinary skin. It is crossed by shallow furrows extending out from the spaces betw'een the villi. A better idea of the villi is obtained from the photograph, figure 64. The largest of them, 3 mm. tall, are found on the upper lip near the median line. These are compound structures, consisting of a central mound with ten to tw’^enty rounded marginal lobulations. Laterally, they become smaller, lose their secondary outgrowths, and are reduced to simple nodules 0.5 to 1.5 mm. high. Even these disappear toward the corners of the mouth, where the crest of the lip loses all its serrations and zones, and becomes a simple smooth ridge or fold. The villi in a single row along the lower lip are not so broad as those of the npper lip, and in many instances are not compound. They, too, become smaller and disappear towmi-d the corners of the mouth. These eserescenees or villi occur not only pressed against the teeth, but in the form of small buttonlike nodules, similar to the secondary outgrowths of the large labial villi, they may be found throughout the length and breadth of the dentigerous zones. Among the teeth they are irregularly distributed. They occur also on the frenula of upper and lower lips. Internal to the plates of teeth, they form another continuous fringe. This distribution may be compared with that of the human villi, found both on the lips and on the gums.

Each villus consists of a very vascular connective-tissue core, covered with thick stratified epithelium. The connective tissue forms a single primary papilla in the simple villi, which, in compound forms, sends a digitation into each of the peripheral nodules. Within the larger villi secondary papillae may be found at the top and adjacent portion of the sides of the primary papillae. The epithelium is thinnest over the tops of the secondary papillae. It heaps up over the lateral surfaces, where it is commonly from fifteen to twenty cells deep. The outer cells are not greatly flattened. Scattered about, but most abundant at the base of the depressions between the mounds, there are cells distended with mucus. Of greater interest is the profusion of taste buds, which, though found elsewhere, are particularly abundant in the labial villi. These buds occur on the free surface of the villi, at the top (fig. 64), and not on the sides as, for example, in the foliate lingual papillae of the rabbit. Conspicuous nerve trunks entering the villi are further evidence of their highly sensory function.

Jonathan Couch (Trans. Linn. Soc., London, 1825, vol. 14, p. 72) reported that “filaments between the teeth and lips of the cod seem designed to enable it to discover and select its prey. And how well they are able to fulfill their object appears from the instance of a codfish” .... [etc., referring to a large one in good condition, which seemed congenitally blind .


Leydig (Zeitsehr. f. wiss. ZooL, 1851, Bd. 3, and again in his Lehrbueh, 1857, p. 197) figured a very round taste bud on the top of a vascular papilla from the lip of Leuciscus dobula, a eyprinoid roach. P. E. Schulze (Ueber die becherformigen Organe der Pische, Zeitsehr. f. -wiss. Zool., 1863, Bd. 12, S. 218-222) assigned to them a gustatory function, which Herrick has conclusively established (Jour. Comp. Neur., 1901, vol. 12, pp. 329-334; Bull. U. S. Pish Com., 1903, vol. 22, pp. 237—272). Bateson (Journ. Marine Biol. Assoc., London, 1890, N. S., vol. 1, p. 225-256) records their presence on the lips of four species of Gadus, but does not associate them with labial villi. I find them present in the epithelium of the dentigerous zone, especially on the low villi found there, and also on the nodular formations internal to the dental areh, but nowhere in the codfish are they more abundant than on the labial villi. None were observed on the villi of the toadfish.


Carp ( Cyprinus carpio)

This fish has thick pouting lips, the limits of which are illdefined, since no teeth develop in either jaw (fig. 3). “The margin of the. upper jaw is formed by the premaxillaries alone,” and beyond this bone the lip projects forward. Suspended above it there is a fold suggestive of the supramaxillary fold in the cod, but since it contains the maxillary bone, w^e consider it the maxillary fold. If that is correct, then the supramaxillary fold of the carp is limited to the sides of the head. Comparison of figure 3 with the section of the toadfish (fig. 1) corroborates this opinion.

The position where the premaxillary teeth should erupt is indicated in the carp by the tract of nodules beneath the premaxillary bone. In front of them is a distinct labial sulcus, and then the lip, feebly furrowed anteroposteriorly. In other eyprinoids the lip may be more deeply plicate (as in Catastomus duquesnii), or tubereulate, i.e., with low nodules or ‘villi,’ as in C. oceidentalis. In the latter its margin is upturned, as it is in the toadfish. In all cases the furrowed or nodular portion of the lip bears taste buds, which are abundant in the section of 0. duquesnii photographed as figure 65.

The carp has a large maxillary breathing-valve (fig. 3), behind which is a palatine prominence — “thick, soft, minutely granular, and very sensitive .... popiilarlv named ‘carp tongue.’

The floor of the mouth displays no striking features; a tongue, such as is found in the cod, is wanting. There is no premandibular cartilage. The infralabial sulcus, which does not cross the midline, is well marked laterally. The lip thus becomes a thick, rounded, and somewhat everted margin of the jaw. In Catastomus duquesnii the lower lip is further everted, growing back under the ramus of the jaw on either side as a rounded lobe; such bilobate lips are described as ‘deeply incised.’ Like the upper lip, the lower, in certain species, may be plicate, and tbe crests of the anteroposterior plicae may be crenate, so as to appear fringed.



Fig. 3 Carp (Cyprinus earpio). Sagittal section of the head. X 3. Lettering as in figures 1 and 2.


^®Yarrell, British Fishes, 3rd ed., London, 1859, vol. 1, i>. 356.



Brook trout (Salvelinus fontinalis)

As compared with the section of the toadfish (fig. 21), that of the front is simple and compact (fig. 19), in many points resembling the amphibians. The premaxillaries are no longer protractile. The masillaries haA^e descended to form the lateral margins. of the upper jaw, and bear teeth which are lateral extensions of the premaxillary series. The primary upper lip, in which the premaxillary bone developed, has become thoroughly incorporated in the front of the head and there is no secondary upper lip. Maxillary valve and palatine teeth make clear the orientation in relation Avith figure 21. The lower jaAV of the trout dispenses with the premandibular lip and cartilage — ^large structures in toadfish and cod, which are in process of elimination in the carp — and in this it is strikingly amphibian. The mandibular valve is shown in the section, and lingual teeth make the tongue conspicuous. Since the skin of the front of the head turns in smoothly at the mouth and arrh’es at the dental area in both jaws with no indication of a labial groove, the trout is a fish with no trace of lips.

AMPHIBIANS

Urodeles

In the embryo of Amblystoma the mouth is at first A^'entral, and at 8 m m . (fig. 24) is closed by a plug of cells extending to the hypophyseal outgrowth (some of the cells in this plug contain yolk granules, as noted by Eyeleshymer in Necturus). Subsequently, the mouth becomes terminal, and sagittal sections of the head at 26 mm. are very much like those of the trout at 25 mm. (compare figs. 19 and 25). In neither animal has the mandible adAmnced far enough to bring the mandibular teeth into effective occlusion with the premaxillary; in both, the vomeropalatine teeth are well back in the roof of the mouth. But Amblystoma lacks the breathing-valves ; and laterally the lower lip, feebly marked out in figure 25, becomes an overhanging structure with a deep iufralabial sulcus. This is shown in figure B2A, from a 12-mm. specimen. Still more laterally (fig. 32B) the labial margin of the upper jaw descends below the lower lip. It then enters the infralabial sulcus to join the lower lip and form the corner of the mouth. The resulting condition is readily comparable with that in the dogfish (fig. 62) if we consider that in Amblystoma the inner ramus of the upper lip has been suppressed.

Although such corners of the mouth can be observed in adult Amblystoma, they are more strikingly present in adult Necturus, in which Cope described the lips as follows:

The tipper lip is rather full and has a thin edge. It overhangs the lower lip, concealing the posterior [i.e., posterolateral] part of it, and embracing it at the eanthus, since it is attached within the groove which bounds it below. The lower lip is deeurved, and the anterior part is deeper, or more convex downwards, than the posterior half, and is separated from the corresponding part of the opposite side by a considerable interspace which is without groove.*^

A section of Necturus (31 mm.), for comparison with Amblystoma, is shown! in figure 26. It presents a primitive feature in that the mandibular teeth occlude wdth the vomeropalatine. There is a thick lower lip wdth a clumsy labial sulcus, and tissue in front of the premaxillary teeth is ready for invasion by a corresponding sulcus, wdien it wdll form an upper lip.

In a large Cryptobranchus (35 cm.), though there is a shallow labial groove which follows the curve of the broad upper jaw’ to the ends of the dental area, shown! at its deepest in figure 27, there is no corresponding groove on the low^er jaw. There is no groove for either lip in the 60-mm. Spelerpes (fig. 28). This section shows the mandibular and premaxillary teeth in occlusion, with the vomeropalatine teeth far back in the mouth. Transverse maxillary and mandibular folds {v.mx. and v.mn.), suggestive of the breathing- valves of fishes, appear in this section; hut the mandibular fold rises from the floor of the mouth instead of extending backward from the mass of tissue investing the teeth; and the maxillary fold, though correctly placed, is hardly thin enough to be valvular. An interesting feature of Spelerpes bilineatus, as described by Cope,'*^ is the “slight obtuseness of the lip on each side of the muzzle to represent the cirrous appendage of the larva, which is sometimes persistent, thus presenting the cliaracters of the supposed species of S. cirrigera.” In the type specimen of cirrigera Cope describes the cirri of the upper lip as “cylindrical and a little knobbed at the ends, extending downward past the lower jaw : thej' are about 0.05 of an inch in length : the appearance presented is not unlike that of the muzzle of a walrus. ’ ’ We have had no opportunity to observe the larval cirri of Spelerpes.

Cope, E. D. Batraehia of North America. Bull. U. S. Nat. Mus., 1889, DO. 34, p. 24.



Fig. 4 Mouth of a IT-mm. toad-tadpole (Bufo lentiginosus amerieanus). X 20.


Anurans

The tadpoles of frogs and toads, unlike those of urodeles, pass through a stage in which the mouth is at the bottom of a pocket (vestibulum oris, Mundbucht, vestibule buccal) formed by specialized projecting lips, each of which bears several rows of hooked cornified cells or 'teeth’ (fig. 4). At the angles of the mouth there are conspicuous cirri or papillae. These familiar structures have been described many times, perhaps first by Swammerdam. In Floyd’s translation, with reference presumably to the tadpole of Eana fusca, we read :


^^Loe. cU,, p. 164.


The aperture of the mouth consists of an under jaw and an upper one, both moveable, of an extreme blackness, and armed with very small teeth lilie a saw, with which, considering its strength and size, the little animal is able to bite exceeding hard. These parts seem to be made of a slender, horny, and pretty flexible bone. There are, moreover, both above and below the opening, a great many little horny bones of the same kind, furnished with a multitude of little black teeth. All these little bones are placed on some muscular and very white plaits which serve the animal like so many lips. The skin lying on each side beneath the mouth, consists of a great number of white papillae (Swammerdam. Book of Nature, or History of Insects. London, 1758, p. 116).

Excellent modern descriptions are those of Heron-Eoyer and van Bambeke, with the supplementary study by Gutzeit, and a brief but valuable report by Miss Hinckley.*® At a stage when this apparatus is well developed (fig. 29 A), a rostral cartilage (Duges; cartilago labialis inferior, Gaupp**) supports a continuous cornified beak, composed of V-shaped cellular laminae stacked one within another. Suspended below it is the lip, deeply undercut by an infralabial groove such as was seen laterally in Ambly stoma (compare fig. 32). But in the tadpole the lip extends across the median line,* and its everted inner surface — ^the outer surface as it hangs pendent — ^bears three transverse ridges of thick, stratified epithelium, within which vertical columns of cells become cornified, forming incurved horny hooks or ‘teeth.’ Gutzeit estimated that there were 385 of these teeth on the lower lip of Eana fusca. The base of each functioning tooth is invaded by the cm’ved top of one destined to replace it, and this in turn by another, so that they have been likened to ‘piles of liberty caps.’ In section they are seen in the photograph, figure 60, and in van Bambeke’s lithographs, but it remains for some one to reconstruct the individual cells before this most remarkable transformation of stratified epithelium can be adequately visualized, xin epithelial mound with its crest of horny teeth constitutes a ‘pectinate ridge’ (van Bambeke) or ‘fringed fold’ (Miss Hinckley).


Heron-Rover et Ch. van Bambeke, Arck. de Biol., 1899, T. 9, pp. 185-309 ; Gutzeit, Zeitschr. f. Zool., 1890, Bd. 49, S. 43-70; Hinckley, M. H., Proc. Boston Soc. Nat. Hist., 1882, vol. 21, pp. 307-315.

Earlier literature is comprehensively treated by Gaupp, Primordial-Cranium von Rana fusca, 1893, Morph. Arbeit, 1892, Bd. 2, pp. 275-481. We cite also Duges, Sur Post^ologie et la myologie des batraeiens, Paris, 1834, pp. 1-216.



The inferior rostral cartilages appear to arise in continuity with each other and with the anterior ends of Meckel’s cartilage. Stohr states explicitly. Die ersten Skeletanlagen des Anurenkopfes sind: 1) Untere Lippenknorpel, Meckel ’scher Ivnorpel und Quadx'ata, die zusammen ein Continuum bilden; diese xlnlage ist unpaar, jedoch verrath die Cruppirung der sie konstituirenden Zellen eine Zusammensetzung aus zwei Stiicken.^® A median, more cellular disc separates the lateral halves of the rostral cartilage, so that it is usually described as paired. A more difficult question is its relation to Meckel’s cartilage. Though there is cartilaginous fusion and continuity between the two, the rostral cartilage appears as an independent element, spliced to Meckel’s cartilage, and overlapping it so that laterally, toward the corners of the mouth, a single sagittal section may show detached sections of both elements (fig. 30). It can hardly correspond with the premandibular cartilage of fishes, since the mandibular bone develops in front of it (fig. 31). Thus it is either a new element, or, as Stohr believes, an adaptive modification of the terminal part of Meckel’s cartilage.

Hear the median line, on either side, and extending straight back from its attaclnnent to the under surface of the rostral cartilage, there is a long slender mixscle — the genio-hyoid of Duges (figs. 60 and 30, m.g-h.). A much shorter and smaller muscle arises from the posterior side of Meckel’s cartilage near its tip, passes toward the depth of the infralabial sulcus, and enters the lip close to its inferior epithelial surface (figs. 29 A and 30, m.L). It is inserted into the adjacent subepithelial tissue of the lip, with fibrous strands radiating toward the pectinate ridges. These muscles are very definite, but other strands described by Duges — a ‘rostro-labial’ muscle — we have not been able to identify. The lips are predominantly fibrous and not muscular, and yet, as Duges states, they “jouissent d’une mobilite notable, et meme d’une eertaine force.” Swammerdam observed that they can be “opened, shut, and moved in various ways, seizing food and helping to convey it to the mouth.” Miss Hinckley reported that the fringed folds “are capable of being laid back when the tadpole wishes to reject any substance held by the fringe.”


"Stohr, Ph. Zur Entw. des Anurenschiidels. Zeitschr. f. wiss. Zool., 1881, Bd. 36, S. 79.



The upper lip is external to a cornified beak which descends in front of the lower beak when the mouth is closed (fig. 29A). The upper beak is supported by a rostral cartilage or pair of cartilages (clearly modeled by Gaupp in E. fusca). According to Stohr, Die oberen Lippenknorpel entstehen durch Abschniirung von den Balkenanlagen und dokumentiren sich hierdurch als vorderste Abschnitte der seitlichen Schadelbalken.^® Gaupp suggests that their interpretation calls for further investigation of the upper labial cartilages ‘bei den Fischen.’ They seem to be a maxillary or. intermaxillary cartilage — a new formation, destined to disappear. Anterior to the upper beak in the tadpole is the upper lip, less developed than the lower, and having only two pectinate ridges, the inner being deficient toward the median line. There is nothing to correspond with the infralabial sulcus. This lip in our sections is notably non-museular.

At the corners of the mouth there are papillae (fig. 29B) which, judged by uninjected specimens, are quite vascular. Strieker, Schulze, and others, as reviewed by van Bambeke, have found within them transversely placed cells suggestive of tactile corpuscles. Nerves enter them (Gutzeit), but it is admitted that they require investigation with special methods to confirm their presumably tactile function. No sensory buds were observed in tbeir epitbelium. Witbin tbe oral cavity there are occasional eonspicuons plicae, or tall wide villi, of similar structure, Avhieli disappear during metamorphosis.

^®Loe. cit., p. 86.


In the older tadpoles the horny teeth, and ultimately the liorny beaks, are shed (fig. 30) and the lips atrophy. They have wholly gone in the 10-mm. toad (fig. 31), which shows the oral contour of the adult. Spicules of bone are developing in front of the rostral cartilage of the lower jaw, and the remnant of the cartilage in the upper jaw is completely surrounded by the intermaxillary bone. Lipless, toothless jaws are the attained result.

Reptiles

Crocodilia

In the unhatched crocodile, what appear to be hard and inflexible lips are readily found (fig. 5A, p. 375), but the teeth emerge through their crests, and the labial tissue, if present, is adherent to the jaws: “Maxillae sinuosae, labiis liberis destitutae.”^^ Barge, in a special study of the lips of reptiles, goes further and finds in crocodiles “not a trace, not an indication of a lip. ”•*8 (A further comment on the labial region of crocodiles vdll be made in the section on birds, with which they have features in common.)

Lacertilia

Young embryos of a common geckonid lizard of Jamaica, Aristelliger praesignis, are without lips at the stage when the dental laminae invade the mesenchyma of both jaws (fig. 33) . The mandibular dental lamina grows down behind the bone developing around Meckel’s cartilage. At 8.8 mm. (fig. 34) another lamina has begun to descend in front of the bone of the mandible. This in its upper part is the labial lamina, but below it gives rise to a series of ramified glands and constitutes the ^glandular baiid^ of Bolk,*^'^ or lamina glandularis.

Hoffmann, in Bronn >s Tliierreieh, Bd. 6, Aljth. 3, S. lOoS, referring to all CroeofXilina.

  • “Barge, J. A. J. Zur Morpliologie der Lippe. I. fiber Lippenbildung bei den Eeptilien. Zeitschr. f. Anat. n. Entw., 1927, Bd. 82, S. 694-719. (I am indebted to Dr. H. L. Weatherford for calling attention to this important paper, and for other valuable suggestions.)



Originally the excretory ducts to which the glandular lamina gives rise ^^open on the surface in all reptilian embryos/’ but later they may closely approach the alveoli of the teeth, with which they correspond in number. Bolk therefore criticises those who call them dabial’ glands, though he does not wish to stress the fact that ‘dips in the accepted sense ai'e wanting in reptiles,” and proposes to name them ‘tooth-glands.’ However, he recognizes that they may remain independent of the teeth, “in wdiich case a groove is usually formed, bounded on the medial side by the os dentale, and laterally by a fold of soft tissue, on the bottom of which the ducts open: by the development of this iirotecting wall of soft tissue there comes about an anatomical condition not unlike that in mammals with real lips.” It is also not unlike that in Amblystoma (fig. 25) or in Sqnalns (fig. 14), except that in them no glandular structures are found at the bottom of the groove. Bolk’s pnpil, Barge, who continued the investigation, still uses the term labial glands, though he, too, is of the opinion that any comparison between reptilian and mammalian lips must be “zwar rein formlich .... die innere Bedeutung dieser Gebilde doch tatsachlich eine ganz aiidere ist.”^^

In the adult Aristelliger the line of orifices of these glands in the lower jaw is freely exfjosed, so shallow is the labial groove as it runs between the row of teeth internally and the hard inflexible margin of the mandible externally. In the upper jaw a corresponding groove is scarcely perceptible. Barge places the allied Hemidactylus with lizards having a vestibnlum oris, and hence lips, on both jaws, though in Hemidactylus very poorly developed. In Gecko he finds none at all.

Bolk, L. Oclontologieal essays. III. On the tooth-glands in reptiles and their rudiments in mammals. Journ. of Anat., 1921, vol. 55, pp. 219-234 (pp. 230-234 here cited).

Loc. cit., p. 719.


In addition to certain lizards witli no lips, witli lips on both jaws, and with lips on the lower jaw only. Barge describes several with psendovestibnla, which are clefts separating what appears to be gingival tissue from the teeth, — “eine Spalte zwischen der nackten Kieferoberflache mid der Sclileimliaut.” Although true lips rest against the ‘naked teeth’ and by suppression of gingival tissue would rest against the ‘naked jaw,’ Barge considers that a fold of tissue so situated cannot be a true lip — it is a ‘labiale Schleimhautfalte’ of gingival nature. In Calotes jubatus a cleft posterior to the teeth produces in each jaw a similar ‘linguale Schleimhautfalte’ (apparently inadvertently labeled ‘labiale’ in his Abb. 13, 14, and 15). These folds in the figures of Calotes, Draco, and especially of Hatteria suggest the internal lips or breathing-valves of teleosts, which, as in the toadfisli and cod, bear a nodulose border pressed against the teeth anteriorly.

Barge’s subdivision of the tissue in front of the teeth, in Calotes and Draco, into a pars gingivalis between the pseudovestibulum and true vestibulum, and a pars labialis anterior to the true vestibulum,. seems to correspond with our pars villosa and pars glabra in the cod, the gingivalis or villosa being molded against the teeth in both cases, and his terms may prove the better ones. In Hatteria he shows both parts as having indented crests; and toward the corners of the mouth, where the vestibulum is deepest, “es macht den Eindruck, als ware die Lippe mit Papillen besetzt.” Altogether there is a most striking analogy between the conditions in Hatteria and the cod, as may be seen by comparing Barge’s figure 1 with our figure 2.

Ophidia

In Typhlops, a Cuban ‘blind snake,’ there is no depression comparable to a labial groove and consequently no lip-like fold on either jaw (fig. 40 Instead, a cornified epithelium extends well into the month, forming a sort of beak. This epithelium consists of t’wo very distinct strata — an inner layer, with distinct cell walls, and an outer layer, less than half as thick, composed of homogeneous keratinized substance. The horny stratum, in sections, is often stripped away from the productive stratum beneath, x^borally it is continuous with the similar layer belonging to the cycloid scales which cover the body uniformly. Within the mouth, before reaching the transverse line of teeth in the ujiper jaw, it is interrupted liy large glands, as shown in the figure; there are no teeth in the lower jaw.


=^The adult speeimens examined were recently collected in Cuba by Professor Bremer, who obtained also the Aristelliger embryos in Jamaica. I am indebted to him for the opportunity to report upon them.


Bothrops, as described by Barge, has a lower, but no upper lip; Tropidonotus and Python have both. In Tropidonotus he finds that the outer surface of the lip is scaly; the passage from skin to mucous membrane occurs along a sharp border ; between the gingiva and this border there is a rather wide depression “which is to be considered a vestibulum oris”; in the floor of the vestibulum gland-outlets are iiresent in a regular succession. In Python, lips are present anteriorly in both jaws, — “nach vorn .... eine gut aiisgebildete Lippe.” The vestibulum, which is Barge’s criterion of a lip, does not extend far posteriorly ; and in the median line of the lower jaw it is interrupted by what might be called a frenulum.

Chelonia

Turtles, though Barge dismisses them with the comment, “Pur eine Studie der Lippenbildung konneii diese Eeptilien nicht in Betracht kommen,” present in the embryo a labial groove (Lippenfurehe) identified and figured by Eose.®® His description is very brief, and his interpretation of the groove is justified only because it resembles closely the labial groove in birds. A single inverted drawing of the upper jaw in an advanced embryo of the green turtle (Chelonia mydas) accompanies Edse’s description.

“ Rose, C. fiber die Zahnleiste und die Bischwiele der Sauropsiden. Anat. Anz., 1892, Bd. 7, S. 754.


In older embryos of Clirysemys marginata (as in the 27mm. specimen, fig. 35) this groove, running along the side of the upper jaw, may readily be identified. There is no corresponding groove on the lower jaw. It is placed somewhat above the epithelial thickening which is to form the horny beak, and is not merely a boundary of the eornified region. The beak crowns the maxillary bone, and medially ends at an epithelial thickening (fig. 35, x, corresponding with y on the lower jaw) which is apparently Rose’s dental lamina (Zahnleiste). Fortunately, its interpretation is not within the field of the present inquiry. The labial groove becomes shallow and disappears posterioiiy, and also anteriorly, for it does not cross the median line. It seems unquestionably comparable Avith the labial groove in birds, to be described in the following section.

The best evidence that the labial groove in turtles has been correctly interpreted is afforded by the ‘Lippenschildkroten’ — the group for which Geoffroy St.-Hilaire, in 1809, proposed the generic name Trionyx, remarking, “As to the true lips found in these turtles, it is a feature all the more surprising. ...” A section of a newiy hatched Trionyx ferox (fig. 36) show’s these lips {l.s. and l.i.) to be extensive everted folds of A’aseular connective tissue, free from muscle. Their histological structure has been desci’ibed in some detail by Hoffmann,®® who could find no epithelial sense organs. Considering the relation of the upper lip to the tooth-like epithelial cornification (fig. 36, a-h), it seems clear that the lip arises above the labial sulcus, which is represented either by the ‘pseudovestibulum’ (&) or by the shallower, more lateral groove on the under side of the lip. We have not the intermediate stages needed to decide behveen these alternatives. On either side of the hard dental formations the soft tissue rises in a fold, as is so generally the case in the low'er vertebrates.

Toward the corners of the mouth, in the adult, the everted condition of the upper lip comes to an end. The lip is then a thin fold -whicli descends below the lower liii and conceals its posterolateral portion, mncli as in Necturus. It does not, however, turn upward into the infralabial sulcus of the lower jaw. Both the upper and lower lips end abruptly before reaching the median line; and this, for the lower lip, is the condition in Necturus. But whereas the amphibian has a blunt snout, that of Trionyx is much prolonged, and the lips do not extend toward its tip.

®In Broun Thierreich, Ed. 6, Abth. 3, Erste Halfte, S. 233.


Birds

“In birds the purposes of nutrition and defence are fulfilled by a bone-like beak, which forms a compound substitute for teeth and lips.” Such was Aristotle’s oinnioii.“‘ In chicks about to hatch, Aldrovandi could add that on the tip of the upper bill, there is “something whitish, cartilaginous, and rather hard, round and like a millet-seed.” He thought it guarded the tip like the button on a fencing foil, lest the beak puncture some veins, membranes, or other delicate structure, “and women say that chicks cannot pick up food until it has been removed.” But his artist overlooked this rostral callus when he drew the chick ‘in utero,’ as did Harvey also. In arguing against Fabricius, who thought that the hen broke open the egg on hearing the chick peep within it ( — ^move within it, according to the Hippocratic writings),®* Harvey

paxtibus animalium. Oxford ed., transl. by W. Ogle, 659^ 20, This reference and the discussion of the literature which follows have been borrowed from notes made by Dr. T*. T. Lewis.

““ Ornithologiae toinus alter, 1600, p. 21S. Compare Bloch, Abd. d. k. Leop.Carol. Akad., 1904, Bd. 82, S. 307.

®®An opinion which will not die. Beaumur (Art de faire eclorre, etc., Paris, 1749, T. 1, pp. 311-342) accounts for it in three ways: the hen may have been seen removing fragments of shell from the nest; the eggshell may at first be broken as if from the outside, since the shell membrane can remain intact; the hen in turning the eggs may break them, a catastrophe. Saee (^47) finds that the mother greatly aids the chick in getting out, by breaking carefully (cassant avec precaution) the shell around the aperture first made by the chick. Voeltzkow reported to the Berlin Academy (Sitz.-ber., 1891, Erster Halbband, S. 115-120) that Madagascar crocodiles in the egg, buried 2 feet in the sand, make known by a sound audible ‘the length of a room’ their wish to be freed; whereupon the mother animal, which sleeps upon the nest, scrapes away the sand so that was content to comment, ‘^neither is tlie bill of the chick so vsoft, nor yet so far from the shell, that it can not pierce through the prison walls.” The first and, until Gardiner’s, the best figures of this structure are those Avhich John Hunter was arrang‘ing on the evening before his sudden death (in 1793) His notes refer to the 'Gittle horny knob at the end of the beak with which the gosling breaks the shell” as if it were already known; but it was new to Yarrell in 1826, when he wrote On the small horny appendage to the upper mandible, describing it in pigeons, chicks, ducks, and geese. ‘^Having performed the important office of dividing the shell, it is easily separated by the edge of the thumb nail of the attendant .... or by the chicken itself in its early attempts to pick up food.”’^ In Hunter’s figures, and especially in those of Gardiner, this horny appendage is remarkably mammiform, aird is surmounted, in loco papillae, by a hard spine which feels like a pin-point.®^ The callus rostralis, or from its function raptor putaminis,^^ culminates in a spina rostralis (sen spina ruptoris). It is shown in longitudinal and transverse section in figures 38 and 39, from chick embryos of 43 and 31 mm., respectively.

In 1841, Mayer reported that turtles and crocodiles have a callosity like that in bii*ds.^'^ It appears in the 26-mm.

the young can escape. This does not involve any "breaking of the shell on the imrt of the mother, yet to Sluiter it seems 'almost fabulous.^ Beese notes that 'like the alligator, the young crocodile makes a squeaking noise shortly before hatching and the mother doubtless opens the nest’ (Alligator and its allies, 1915, p. 42).

Works of Wm. Harvey. Transl, by Willis. Syd. Soc,, p. 267.

John Hunter’s observations on animal development. Ed. by R. Owen, 1841, pi. 76, figs. 17, 18, et al., of the goose; also p. 31.

^®Zool. Journ., 1826, vol. 2, p. 433-437.

Gardiner, E. G. Beitrage zur Kenntniss des Epitnchiums und der Bildung des Vogelsehnabels. Inaug. Diss., Leipzig, 1884, figs. 22 and 23.

Welches sieh wie eine Steeknadelspitze anfiihlt” — Rosenstadt’s apt description, Duval (Atlas, 1889, p. 47) notes that it is called 'le diamant,’ referring presumably to the glass-cutter’s implement.

“Literally 'shell-breaker,’ which in English is Hermann’s term for it. He found it in the heron, eider duck, avoeet, coot (Pulica), and snowy owl (The origin of birds, London, 1926, pp. 94-95). It is also called the 'egg-pip.’ Apparently there is no species of bird in which its absence has been established.

“Froriep’s Notizen, Oct, 1841, Bd. 20, S. 70.


Chrysemys (fig. 37). Unfortunately, jMayer referred to this rostral callus as the ‘egg-tooth’ (Eizahn) in which he has been followed by uncritical writers, Rose and Sluiter protesting vigorously. Mayer asserted that the spina ruptoris of the chick is sometimes double, and described it (when magnified four times) as consisting clearly of two pointed, conical, bright yellow crystals or teeth, placed quite near each other in pockets of the skin of the beak, from which they project obliquely out^vard toward either side. “They are not al-ways equally developed, and sometimes only one is present.” He does not describe this spina duplex as calcareous, but refers to dust from the shell found around it when an egg is opened after the chick has peeped; and lie mentions that it ‘rubs through’ the shell.

Sacc, in 1847, was sure that “it is not through wearing away the shell, by rubbing it with its beak, that the chick breaks it open, but by striking it violently.” He associated this act with respiration and raising the head. “The beak of chicks,” he continues, “at the time of hatching is so weak, that it would be absolutely impossible to break the shell, if nature had not placed there this little calcareous tubercle, which becomes detached soon after birth ; all the chicks which lack this outgrowth perish in the egg. ’ ’ Similarly, Dr. Horner reported to the British Association in 1853,®® “that the opinion that the shell is broken by a cutting or scraping motion of tlie bill, through the agency of the pointed horny scale at its end, is fallacious, as the membrane which lines the shell is at first left entire, while the shell itself outside has been chipped or broken off. ’ ’ He observed that the shell is broken generally “by a single smart bloTV only, though in some instances the blow is immediately repeated, or doubled ; and each strike is made with considerable power, as is not only seen, but felt and heard.” Keibel finds that the hypertrophied musculus complexus is adapted to thrust the rostral spine powerfully against the shell and to crack it, and is thus the chief active agent in the process.®® Other muscles may play a part.®^ The spine is serviceable in breaking the shell if it merely serves to concentrate the eruptive force at a point. As Professor P. W. Bridgman expresses it, “If the total amount of force which can be exerted upon the shell is fixed, the intensity of the counterforee in the shell must be greater if the action of the breaking force is confined to a small area.”

^ Sacc. Ann. des sei, iiat., 3* Ser., ZooL, T, 8, p. 168. Paris. 1847.

Brit. Ass. Adv. Sci.j 1853, pp. 68-69.



Turtles use the rostral spine in making “a small rip at one end of the egg, which they enlarge by moving the head from side to side, sometimes aided by the claws of their forefeet.” ®® They do not, like the chick, turn around within the egg, detaching a cap of shell, but come out through a terminal cleft. The difference which Weinland alleged, that in turtles the spine ‘is gradually worn off,’ whereas in birds it drops off, does not exist. Dr. H. L. Babcock informs us that in spotted turtles the spine may be spontaneously shed on the tenth day, at which time they have all become readilj’’ detachable. In crocodiles, according to Voeltzkow, the spine comes in contact with one end of the egg and by its mechanical operation, which ‘genau wie ein Bohrer wirkt,’ it pierces the shell.®®

Mayer’s report that there are two diverging rostral spines in the chick is explicitly rejected by G-ardiner and by Heilmanii, though the latter found in the avocet a V-shaped base leading to the single apex. In three chick embryos of about fourteen days in the Harvard Collection, the structure is precisely and symmetrically median, with no suggestion of a paired origin, and the same appears to be true of turtles. But in the crocodiles it is double, as Mayer stated. Voeltzkow and Sluiter agree that in them it develops from two separate epithelial mounds. According to Voeltzkow, a little papilla then arises between them, forming a bridge from one to the other, the end result being a sjunmetrically bifid median structured® In the two specimens drawn in figure oB and C, the entire spine is toward the left, and in one instance its right and left portions are quite unequally developed. The pit shown in figure 5B may indicate that the spine on the right has been suppressed, in which case the bifid structure would be wholly a left spine, but this single specimen by no means justifies so strange a conclusion.


  • Keibel, F. Wie zerbricht der ausselilupfende Yogel die Eischale? Anat. Anz., 1912, Bd. 41, S. 381-382.

Poblman. Aiiat. Rec., 1919, vol. 17, pp. 89-104.

Cochran, B. M. The box turtle. Nature Magazine, Sexit., 1927, p, 152.

Voeltzkow, A. KSitz.-ber. Akad. Wiss., Berlin, 1891, Erster Halbband, S,. 115120: Ann. Nat. Hist., 1892, vol. 9, pp. 66-72.



Fig. 5 A, head of an nnhatehed Crocodilus americanus (total length 133 inni.). X 2. Bf dorsal aspect of the ruptor of the same embryo. X 12. C, raptor of a 137-mm. C. johnstoiii. X 12. B, beak of a parrakeet embryo (Melopsittacns undulatus), after Braun, Arb. zool. Inst., Wiirzburg, 1882, Bd. 5, Taf. 8, Fig, 15. X 4.


In 1857, Weinland reported that he had caught a young sandpiper (Tringa piisilla, Wils.) which “still wore the hard horny tubercle on the upper biir’ and to his surprise he found “a similar armature on the lower bill, though less prominent.’’ “I suppose,” he continues, “that the horn on the lower bill serves only as a support for the upper biU while knocking.”'^ Unfortunately, Weinland did not fix the location of the structure on the lower bill. In a heron (Ardea nyetieorax) about to hatch, which by chance was brought to the laboratory, the mandible is tipped with a well-defined pointed projection which lies within the trough of the upper bill wdien the beak is closed. It does not correspond in position with the rnptor of the upper bill, which accordingly has no counterpart below; a slight diffuse swelling involving the symphysis seems irrelevant. But Eosenstadt, with no reference to Weinland, has recorded in cytological detail the production of a rudimentary rostral callus (Eizahnanlage) in chick embryos of fourteen to fifteen days, situated der mittleren Partie des Unterschnabels ' ’ and extending ‘'in einen dunnen Streife auf beide Seiten. Its presence there confirms his opinion that even on the upper bill it is of no use in breaking the shell, being a formation without the slightest physiological significance. We have seen no indication of a rostral callus on the lower bill of chicks, which moreover lacks the pointed tip observed in the heron.


Voeltzkow, A. Abb. d. Senekenberg. ISTaturf, Ges., 1899, Bd. 26, S. 74-77.

'^Proe. Essex Inst., vol. 2, 1856-1860, pp. 115-116.


Functionally similar to the rostral callus of crocodiles, turtles, and birds, but structurally totally different, is the well-known egg tooth of snakes and lizards. In 1839, Johannes Muller reported to the Berlin Academy his discovery of a peculiar, flat or upturned, intermaxillary tooth, found at the time of hatching in snakes and lizards generally. He described it as growing forward beyond the line of the upper jaw, and apparently serving as a chisel to cut through the egg shell. But he found it equally developed in those species which bring forth their young alive.

In 1853, Dr. Weinland, of Cambridge, Massachusetts, a student of Muller ^s, observed the snake’s egg tooth in action. It is a true incisor putaminis, or, from its rostral position, a dens rostralis, and produces "a long sharp slit through the thick leathery shell . . . . cut as if by a sharp knife.” Weinland made a microscopical preparation, which showed that this tooth consists of dentine with characteristic canaliculi, bounding a pulp cavity, and capped by a transparent, structureless layer which has since been described as enamel, but in which Weinland failed to find enamel prisms. It is a true tooth developed solely “to cut open the eggshell, which it may perform in the time of a second, and soon after it drops.” Weinland discourses on Nature’s elaborate preparations for so brief but important an event. It remained for Sluiter to show^ that in its embryological development the shell incisor of lizards is a perfectly characteristic tooth. In Gecko, he found a pair of such teeth, close together, and so beveled that they combine to make a Y-shaped apex.

” Bosenstadt, B. Arch, f . mikr. Anat., 1912, Bd. 79, S. 632.

”Arch. f. Anat. Physiol, n. wiss. Med., Jahrg. 1841, S. 329-331, with a plate of excellent figures.

Weinland, Proc. Essex Inst., Salem, vol. 2, pp. 28-32. But after ^cracking the shelP it takes the ringneck snake, Diadophis, nearly a day to emerge. It may withdraw its head and later project it through the same or a new opening, these openings being somewhat on the side of the egg and not quite apical. ^ ^ Cracking the shell seems to be produced by the thrusting movements of the head or the turnings of the body of the snake. ... A small eggtooth may be partly responsible for the first break in the shell. (Blanchard, F. N., Michigan Acad, of Sei,, 1927, vol. 7, pp, 284-285.)


In Mabnja and Lacerta, he saw that the left member of the pair is small and iinernpted, when the right is functional and so displaced as to appear median; and in Calotes and several snakes the median shell incisor is presumably a right one, with the left entirely suppressed/^

Mayer correlated the spine of birds, turtles, and crocodiles with the calcareous egg shells in those groups, and the tooth of snakes and lizards with leathery or membranous shells. This opinion Rose adopted, but Sluiter ruled out, noting that many turtles produce eggs with leathery envelopes, and the eggs of some lizards have very hard, limy shells. It is important that no species has been found having both spine and tooth.

Recent writers agree that the rostral callus is non-caleified. It is a horny emergence of the subepitrichial epithelium, with no connective-tissue core. Figures and detailed histological descriptions have been supplied by Gardiner, and especially by Eosenstadt. Since it occurs within the area of greatly thickened epithelium bordering the oral aperture, where proliferative activity takes many forms, it might possibly have some relation to labial villi. It seems, however, to represent wiiat in mammals w^onld be the tip of the nose. That w^ould mark the external boundary of the labial territory in birds. The internal bonndary depends upon the position of the missing teeth.

In locating the dental area of birds, parrots have been particularly serviceable, ^Gin heureux hasard ayant mis a la disposition de M. Geoff roy un foetus de per roquet pres d’eclore.’’ Geoffroy reported to the Academie, in 1821, that he saw’', along its beak, a row^ of tubercles ^'presentant toiites les apparences exterieures des dents’’ (compare fig. 5D). Although these tubercles were not implanted in the bone of the jaw% there were canals in the bone, corresponding in number with the tubercles, w^hich transmitted vessels and nerves to their gelatinous central pulps. In lieu of enamel, the crenulate margin of the beak W'as covered with a tenacious white transparent layer, passing continuously from one elevation to another, and suggesting to Geoffroy the compound tooth of the elephant! Soon after hatching, progressive cornifieation obliterated the tubercles, but the permanent serrations in the bills of certain ducks were considered comparabled.

Sluiter. tlber clen Eizahn und die Eischwiele einiger Reptilien, Morph. Jahrb., 1893, Bd. 20, S. 75-89.


Since that time, parrot embryos have been studied frequently. E. Blanchard, in cockatoos, thought he saw dentine and bony sockets for the tubercles Fraisse, in parrakeets, found no trace of dentine, and no alveoli. The papillae are very vascular, and may become elongated, so that after removal of the horny layer they are soft and float about in the fluid in which they are examined'^® — an observation suggesting' what happens Avith Imman labial Adlli. Fraisse reasserts that the horny teeth of the mergansers are comparable formations. Braun’s studies provided the excellent figure here copied in part as figure Gardiner thought it ‘possible

but far from probable’ that the elevations were due mechanically to the beak becoming hooked but Hide shows that this is a futile suggestion.®® Gardiner more reasonably stated (I.C., p. 44) that the papillae in the parrot may functionally be correlated Avith the production of the horny beak, since they resemble the papillae which he found in hoofs. In the beaks of embryo chicks, ducks, pigeons, and haAvks he found papillae similar to those of the parrakeet, except that they failed to produce nodular eleA'ations. In the duck he recorded only a single row in the upper jaiv, but three or four roAvs in the lower. In the parrot additional small nodules occur on the roof of the mouth. Fiirbinger, who reviewed the literature and sought for tubercles in the Laridae and Limicolae, concludes that in recent birds “elevations like dental papillae arise, largest apparently in the parrots,” but they are destitute both of enamel epithelium and of dentinal cells.

Geoffrey St.-Hilaire. Mem. et Hist, de PAead. des Sci, Annee 1821, Paris, 1826. Analyse des travaux (par Cuvier), pp. ISO—lDl.

” Blanchard, Emile. Comptes rendus de PAead. des Sei., Paris, 1860, T. 50, pp. 540-542.

Fraisse, P. Verh. d, phys.-med. Ges., Wurzburg, 1881, Bd. 15 (Sitz.ber. fiir 1879-80, S. iii-vi).

^ Braun, M. Die Entwicklung des Wellenpapagei's (Melopsittaeus undulatus). Arb., zool.-zoot. Inst., Wurzburg, 1882, Bd. 5, S. 178-179, Taf. 8, Pig. 15.

Ihde. Arch. f. mikr. Anat., 1912, Bd. 79, S, 255.


As seen by comparing figure 5J. -with 5Z>, Geoffroy’s dental tubercles of the parrot closely resemble the alveolar mounds along the jaws of the crocodile, through each of which a tooth is soon to emerge.®^ It seems probable, therefore, that the tubercles in the parrot indicate the position of the missing teeth. The dental lamina of Rose is, however, immediately internal to their bases, and therefore quite separate from them (as may be inferred from fig. 39). Ihde has marshaled the evidence of those who question or deny the dental nature of this epithelial proliferation or ‘lamina.’

Above the row of rostral tubercles shown in figure 5D, there is a groove, which Gardiner, of Boston, in his Leipzig dissertation, interpreted as a labial groove (Lippenfurche). Rose comments: “Nach meinen Untersuehungen ist es zweifellos, dass Gardiner sehr im Rechte ist, wenn er in dieser Furche ein Aualagon der Lippenfurche vermutet. Ihde, as critic, finds that ‘Homologon’ — ^not ‘Analagon’ — should have been Rose’s term, and continues :

Ob die Fiirehe tatsaclilicli eiiie rudimeiitare Lippenfurche ist, weiss ieh nicht. Sie liegt im ObeLschnabel beim Hiilmehen und Papagei auswarts vom Sehnabelrande und ist noeh im vorgeriickteu Alter des Tieres nachweisbar. Die Deutung als Lippenfurche ist eine rein willktirliehe und solange haltbar, als keine bessere da ist, wobei ieh allerdings gestehen muss, da.ss mir eine bessere und wahrscheinlichere nicht bekannt ist.®'*

Gardiner’s labial groove and Rose’s dental lamina (or sulcus) are readily found in sections of the chick (fig. 39). Although the labial groove should be on the under surface of the upper jaw and directed upward (as in lizards and snakes), its lateral position in crocodiles and birds is not as fatal to homology as might he supposed. For in the upper jaw of a chick of 4.5 mm. the labial groove is ventral in its hinder part, shifting to a lateral position anteriorly. It increases in depth toward the point where it crosses the median line. In Trionyx the upper lip unquestionably is in relation with a corresponding groove. The relations in the lower jaw of the chick are similar to those in the upper, though the dental lamina is hardly appreciable. The lower labial groove also is deep anteriorly, and the epithelium between it and the oral cavity is invaded by a row of large papillae, quite round in transverse section, which clearly represent the tubercles of the parrakeet. As seen without magnification on the beaks of chicks toward the time of hatching, it certainly marks off a lip-like area of the horny beak.

Fiirbingerj M. Unters. z. Morph, u. Svst. d. Vogel, Amsterdam, 1888, Bd. 2, S. 1074-1075.

®^Rose has figured the toothed embryo of Croeodilus biporeatus in the Aiiat. Anz., 1892, Bd. 7, S. 757.

Rose, he., S. 753.

Ihde, I.C., S. 270.




Accepting the labial groove as defining the inner margin of the lip, the labial territory of birds becomes the exterior of the cornified beak. Histologically, it suggests the mammalian lip only in the remarkable thickness of its epithelium.

Mammals

Monotremes

In the absence of suitable specimens of either duck-bill or echidna, Avhich lack lips according to Wiedersheim, Kopsch, and Schumacher (as noted in our ‘Introduction’), the following citations may be of interest. Burrell finds that the oral integument of the duck-bill becomes horny and like a bird’s beak only in dried specimens. He describes it in life as soft and moist, and supplied with innumerable tactile corpuscles — “the most sensitive portion is undoubtedly the anterior border of the upper lip.” He defines the lips of the duck-bill as that “pliable tissue extending beyond the jaw bones,” and states that the lips may be puckered to the extent of forming a small central suction tube.®"’

“ Burrell, H. The Platypus, Sydney, 1927, pp. 9-10 and 68.


In Echidna, Ruge, who makes no mention of lips, describes the anterior fibers of the buccinator muscle as assuming the character of a sphincter oris, though divided very definitely into lateral halves.®® But Owen srav “lips transitorily manifested at the suckling period”; in the adult they become reduced to the scarcely movable margin of the small terminal oral orifice.®'^

On the tip of the .snout of Echidna, Semon found “an epidermal thickening — a small projecting mound — which certainly, as in Sauro'psida, plays a role in breaking the egg.shell.” “After hatching it gradually vanishes.”®® But here it is in nasal, and not in labial, territory. In the duck-bill Owen had described, “on the middle line of the upper mandible and a little anterior to the nostrils, a minute fle.shy eminence .... of which the adult presents no trace . . . . obviously analogous to the horny knob of birds.”® Burrell thinks that a ruptor is not needed by the restless muscular embryo within so thin a shell, and finding that the caruncle is at first soft, becoming hard and sharp a couple of weeks after hatching, he conjectures that it may serve as a ‘miUv-spur’ to incite lactation (l.c., p. 185).


Marsupials

Apparently all marsupials have lips, the muzzle generally being red from the vascularity of the integument (Owen). The lips of thirty-seven species have been studied macroscopically for elevations by F. E. Schulze,®® whose propensity for Greek nomenclature is suggestive of Heusinger’s. In the kangaroos the vestibnlum (‘chilocoel’) extends back laterally to a transverse fold — the ‘crista transversa superior’ in the upper jaw and ‘crista transversa inferior’ in the lower. These folds cross the vestibnlum in the interval between the cutting teeth and the molars, and tend to divide the mouth into anterior and posterior chambers. The posterior portion of the vestibnlum, passing into the cavity of the cheeks, is the

®^Ruge, G. Die Hautmuskulatur der Monotremen. Jenaische Denksehrifteii, Y (Senion, ZooL Forscliungsreisen, ii), 1895, S. 135.

Comp. Anat., vol. 3, p. 383.

Semon, R. Jenaisehe Denkseln*., Y, 1894, S. 73.

Trans. ZooL Soc., London, 1835, vol. 1, p. 224.

““ Schulze, F. E. Die Erhebungen auf der Lippen- und Wangenschleimhaut der Saugetiere. II. Die Beuteltiergattung Macropus. III. Marsupialia. Sitz.-ber. d. Akad. d. Wiss., Berlin, 1913, S. 384-395; 1916, S, 43-65.


‘pariocoeL’ The lips are hairy externally, with the upper divided into right and left halves hy an interval of bare skin.^’i Each lip may have an “evenly rounded, smooth outer margin” (Avith shallow indentations laterally, or sometimes conical or hooked papillae) which is perhaps a pars glabra— the ‘ektochiP of Schulze. Internally, in place of a villosa, Schulze finds two partly overlapping ridges— ‘entochiP and ‘paraehil,’ respectively — the former nodiilar, and the latter eomb-like with a row of pointed horny spines. At the angle of the mouth the nodular ridges of upper and lower lip meet, and are continued along the inside of the cheek as a single row of nodules for a short distance, when the row divides into an upper and a loAver branch. Between them there is sometimes a velvety area of small papillae. This is the general plan in marsupials, the many deviations from which, in various genera, are clearly shown in Schulze’s drawings and photographs.

The development of the marsupial lip, which Schulze did not consider, is shoAvn in figure 55 — a sagittal section of a 23-mm. opossum from the pouch, including the maternal nipple in its mouth. The dental lamina grows down behind the bone of the lower jaw, and the labial lamina in front of it, so like the condition in lizards (fig. 34), Avhich in turn is like that of sharks, that we conclude that all of these animals have at least the beginning of true lips. In the upper jaw of the small opossum there is a mai’ked epithelial thickening, which extends beyond the limits of the ill-defined labial lamina, and forms a projecting shelf of cornified cells pressed against the nipple. With this apparatus, at a stage before characteristic mammalian lips have developed, nursing’ is very successfully accomplished.

At a length of 170 mm., when the opossum is about ready to shift for itself, there is stiU no ripper lip adjacent to the median line. There the thickened epithelium extends coiitinnonsly from the anterior portion of the palate to the exposed part of the snout. But laterally there is a deep labial groove or vestibulum, and toward the corners of the mouth all the features of typical mammalian lips are present (fig. 56), The hairy skin (pars cutanea) becomes a pars glabra at the outer edge of the lips, and this forms a very thick pars villosa, invaded by slender connective-tissue papillae on approaching the vestibule. The free surface of the villosa of the upper lip is merely corrugated, but on either half of the lower lip, it bears a row of some forty conical projections, about 0,25 mm. tall. There are a few low nodules within the cheeks. Each lip is plentifully supplied with striated muscle.


Schulze states that this hare median groove, present and reaching the nares in some, hut by no means in all, the herbivorous marsupials, can have no other significance than to convey the nasal secretion to the mouth to mix with the saliva (l.e., 1916, p. 65). This is precisely the unlovely method of the camel to conserve moisture, as described by Owen (Comp. Anat., vol. 3, p, 393).


Edentates

The anteaters have small mouths in which ^'introduced termites may be crushed by the action of the tongue against two callous ridges, which seem to occupy the place of teeth.’’ The tongue is their prehensile organ, but their lips are muscular. In Myrmecophaga jubata, Owen has dissected the orbicularis oris and the retractors of both lips.^^ In Manis i3entadactyla, Carus and Otto have figured the upper lip as extending well below the lower lip at the corners of the mouth®^ — a feature of the lips in Trionyx and Necturus. Many hairs directed backward are said to occur within the cheeks.

Ungulates

The lips of the pig have no villous or papillary outgrowths. The pars villosa is a band of thickened epithelium invaded by tall vascular dermal papillae (fig. 48, from a 182~mm. embryo ) In ruminants, free villi and perhaps the entire lips attain their maximum development. The moose, wi'ites Gilbert White, of Selborne, has "such a redundancy of upper lip as

Trans. Zool. Soc., London, 1862, voL 4, p. 133.

Carus, G. G., and Otto, A. W. TaBulae anatomiain eoniparativam illustrantes. Pars IV. Lipsiae, 1835, S. 14 and Taf. 7, Fig. 6.


I never saw before. ’ ’ The macroscopic and well-known villi extend in a broad band from the angles of the mouth along the inside of the cheek, and serve ‘as mechanical obstacles’ to the escape of regurgitated food, confining the soft slimy mass to the molar region during the second mastication. “Neither the hog nor the horse have such buccal papillae” (Owen). Schulze has studied their function more recently.*® In the adult sheep they are shoAvn in figure 68, and a section of those in the calf in figure 69. If the villi are followed from the cheeks to the lips in either animal, and the attempt made to identify a pars villosa and pars glabra, the following complication will be observed. The villi, replaced by low nodules, extend to the hairy skin and eliminate any pars glabra (compare fig. 49). The area thus occupied is quite extensive, since it spreads over the labial margin into what, in other animals, is a pars cutanea. In the Bovidae and Cervidae the labial texture extends broadly to the nose, forming the characteristic muzzle or ‘Flotzmaul,’ for which there appears to be no English name.

Sirenians

The lips of the manatee have been described and photographed by Murie,*® and their action in a living specimen has been recorded by Garrod,*'^ but without making clear the limits of villosa and glabra, if such subdivisions exist. The ‘comparatively insignificant lower lip’ has a relatively smooth oiiter portion “with its sinuous, bristle-clad, thick epidermis, ’ ’ succeeded within the mouth by a papillose area which some have termed an ‘inner lip.’ “In front, the inner lip is separated from the outer by a deepish furrow, and behind it stops short at the tip of the tongue, though it is continuous with the gums” (Murie). Since the incisors never erupt in the manatee, and since the epithelium of the inner lip is thinner than that of the outer (Murie), the papillose mandibular pad or ‘inner lip’ is presumably gingival tissue.


The elephant may here be a competitor, since the upper Up, blended with the nose, forms the trunk. Owen found that the muscles of the trunk are a development of the orbicularis oris. ^‘The under lip of the elephant alone is free and is produced into a pointed form.'^

Schulze, F. E. Die Erhebungen u. s. w. I. Euminantia. Sitz.-ber. d. Akad. d. Wiss., Berlin, 1912, S. 510-521.

®'*Murie, J. On the form and structure of the manatee (Manatus americanus). Trans. Zool. Soc., London, 1874, vol. 8, pp. 133-134 and 164-166.

®'Garrod, A. H. Notes on the manatee. Trans. Zool. Soe., London, 1879, vol. 10, pp. 138-139.



The great upper lip is bilobed, with a median notch crossed by ‘innumerable transverse muscle fibres’ (Grarrod). Through their agency, the contiguous bristly surfaces of the pendent lateral lobes may be brought together, seizing between them some portion of vegetable food, which is then drawn upward by a backward movement of the lower margin of the lip as a whole. The lobes are so vascular as to suggest erectile tissue. A somewhat horseshoe-shaped elastic pad is found on the roof of the mouth internal to the labial notch, which is studded wfith short erect papillae of two sorts — ^larger conical ones about half an inch high, with smaller setose forms in the interstices (Murie). This ‘inner lip’ is presumably gingival, and the ‘deep furrow’ between it and the ‘outer lip’ is then a vestibulum oris.

Not only are the lips bristly, but hairs are found mthin the mouth, where, as Murie suggests, they may he the homologiies of the whalebone of some Cetacea.

Cetaceans

According to Kiikenthal,®® the form of cetacean lips is wholly dependent upon their physiological function. “In the first place they must effect a firm, tight closure of the mouth, preventing the entrance of water: and further, they must serve in obtaining food, in two ways, first, — ^in securing the mother’s milk without admixture of water; and secondly, in the capture of edible organisms.” He shows, in diagrams, how the concave border of the upper lip in the toothed whales is mortised, as it were, against the convex upper edge of the lower lip when the mouth is closed. In the whalebone whales this arrangement is reversed: the upper lip has a convexity which fits into a concavity of the lower lip. “It falls down like a thick curtain some feet in depth” (Owen). Kukenthal describes the labial sulcus as present in some si^ecies laterally, but not medially; and in the lower jaw of a Balaenoptera musculus he finds something suggestive of the ‘false vestibule’ of reptiles — “Eitie eigentlich Lippenfurche, welche die Unterlippe von dem Kiefer trennt, fehlt, sie fallt zusammen mit der Kieferfurehe, an deren Grunde die Zahnreihe liegt. . . . Im Oberkiefer ist eine Lippenfurche dagegen vorhanden” (l.c., p. 321). In Hyperoodon rostratus, which has horny formations in place of teeth, the gingival mounds meet tightly when the mouth is closed and the rudimentary lips cannot be brought together. Nothing is said of villi in any species.


” Kiikentlial, W. Jciiaische Denksehriften, 1893, Bd. 3, S. 317-322.

Carnivores

The bilateral clusters of tooth-like villi within the upper lip of the eat have been the subject of a previous paper.®® These villi are covered with thickened epithelium which extends laterally, but Yfithout villi, toward the corners of the mouth, assuming a position on the free margin of the lips. There, in advanced embryos (of 42.5 mm., fig. 46), it may form tag-like masses, sometimes pedunculated, which contained epithelial pearls. In the newborn (fig. 47) the thickened band is still present ; and it may be recognized, though it is not well marked, in older cats. The pars villosa of the upper lip is thus represented laterally by a non-villous zone of greatly thickened epithelium. The lower lip has a similar zone which is altogether without viUi. From this ‘pars villosa,’ the pars glabra with thinner epithelium, is separated by a deep sulcus, characteristic of the carnivorous lower lip (fig. 44, s.e.l.)- Ill figure 45, farther toward the corner of the mouth, there is an accessory sulcus, so that the entire section resembles Schulze’s schematic figure of the lip in the kangaroo, with grooves separating ektochil, entochil, and parachil (Schulze, ’13, p. 387).

“Alison, E. .T. ‘Denticles’ .... on the inner surface of the lip of the eat. Anat. Eee., 1925, vol. 31, pp. 93-121.


In the newborn dog a band of thickened epithelium along the lower lip, in the same position as that in the cat, actually bears a row of villi, confirming its interpretation as a pars villosa. The villi, fifteen to tiventy on each half of the lip, extend from the corner of the mouth halfway to the midline. Most of them are shaped like human incisors. Some broad ones near the medial end of the series are bifid ; a few, laterally placed, are conical. Structurally, they resemble those of the upper lip of the cat. As seen from the outside, the entire villous crest, with its thick epithelium, is set off from the pars glabra below by a sulcus, as in the cat. The sporadic occurrence of nodular or low spinous excrescences over any part of the villous zone, as far as this sulcus, emphasizes the distinction between the two areas. In adult dogs the villi are smaller and apparently fewer than at birth. On the upper lip there are no villi at any stage, though in places the zone of thickened epithelium is crossed by furrows, producing a pebbled surface. In newborn dogs there are a few gingival villi, frequently constricted at the base, in relation with the cheek teeth.

Rodents

Flower and Lydekker^®® describe the mouth of rodents as “divided into two cavities communicating by a restricted orifice, an anterior one containing the large incisors, and a posterior one in which the molars are placed.” Across the diastema between the incisors and molars the hairy integument of the face is continued into the mouth and meets the sides of the tongue. “This peculiar arrangement evidently prevents substances not intended for food from entering the molar chamber, as when the animal is engaged in gnawing through an obstacle.” “In the hares and pacas the inside of the cheeks is hairy, and in the pouched rats and hamsters there are large internal cheek pouches lined with hairy integument. ’ ’ But Schulze, wdio names the hairy area inflexum pellitum, as if it W'^ere infolded skin of the cheek, suggests that the baekwardly directed hairs may be useful, not in keeping things out of the molar chamber, but in guiding food into itd”^ The lips are in relation with the anterior chamber. If the cheeks have developed by the fusion of the posterior parts of the lips, it may explain Schulze’s observation that rodent lips lack the subdivision into anterior and posterior parts wMch he found in marsupials.


^‘^Introduction to the study of mammals. London, 1891, p. 416.

A vertical section through the lower lip of a newborn guinea-pig is shown in figure 51. The vestibule is deep, corresponding with the length of the incisors. The ill-defined pars glabra is hairy, as is the villosa. The epithelium becomes gradually thickened, and develops tall dermal papillae, but is without projecting villi. These features are more pronounced in a horizontal section through the cheek at the angle of the mouth (fig. 52). The transitional zone seems to have the character of hairy mucous membrane rather than of infolded skin, for its epithelium is three times thicker than the epidei’mis and becomes, at its posterior limit, twelve times as thick.

Embryologically, the lips of the guinea-pig develop from massive epithelial proliferations which bifurcate into dental and labial laminae (fig. 53, 18.6-mm. specimen). The direction of the lower labial lamina already indicates an incurved lower lip such as is found at birth (fig. 51). The lamina for the upper lip also splits very early, so that at birth both lips are well developed. In the rat and mouse, however, although the adult mouth resembles that of the guinea-pig, the labial laminae of the newborn are still unsplit. The young animals have no lips (fig. 54) ; thick epithelium fills what would otherwise be a labial sulcus and extends inward over the teeth, forming a considerable mound over the lower incisors.

The absence of lips in the newborn opossum and rat is doubtless correlated with the immature condition of their young at birth. It cannot depend on the length of the maternal nipple— very long in the opossum and short in the rat — though Neustatter suggests that the special development of the human labial margin is because “die Warze beim Weib relativ viel kiirzer ist als bei den Tieren, selbst den Affen. ’ ’ The guinea-pig, with long nipples, has young with welldefined lips at birth, being in all respects far advanced at that stage. The rabbit is intermediate between guinea-pig and rat. Incomplete labial grooves are present. In the lower jaw, tow'ard the corners of the mouth, there is no vestibulum oris and the lip is very ill-defined (fig. 50).

Schulze, F. E. Die Erliebungen ii. s. w. IV. Bodentia cluplicidentata. Sitz.her. d. Akad. d. Wiss., Berlin, 1916, S. 781 j and V. Bodentia siinplieidentata, ibid., S. 12.83.

Bats

A mouth of unusual interest is that of the Mexican vampire — a Phyllostoma — figured by Carus and Otto.^"'^ The upper lip, along the external part of its margin, is beset with a great many soft, round, fleshy papillae, wiiicli become gradually larger towmrd the corners of the mouth. These are followed, along the inner margin on either side, by many long, soft spines or laminae, with apices directed toward the teeth; and in the region of the first molar there are several lesser fleshy papillae, likewise directed inward. The low^er lip also has macroscopic soft round tubercles along its outer margin, and sharp, inwardly directed laminae internal to them; in the median line there is a larger tubercle, and below it a crescentic series of soft fleshy papiUae. A large undulate fold of mucous membrane betw'een the lip and mandible on either side completes the list of villous structures enumerated by the authors cited. Functionally, they state that these lips effect a firm and air-tight application of the mouth to the bitten skin of some animal, and serve as a cupping-glass in sucking blood.

Neustatter, 0. TTeber den Lippensaum beim Meiiseheii. Jejiaisehe Zeitselir., 1895, Bd. 29, S. 385.

^“^Loc. cit. (in footnote 93), Tab. YII, fig. 1, and S. 12.


Man

First we review the embryological featiires and then those of the adult. The mouth of the human embryo of 10.2 mm. in sagittal section (fig. 41) is readily comparable with that of the 22-mm. Squalus (fig. 11) or 12.8-mm. Torpedo (fig. 15). All are without lips and in all the mouth is ventrally placed —in man far from its ultimate position. This was well recognized by Balfour, who concluded that the vertebrate mouth was of suctorial derivation (Comp. Embr., vol. 2, pp. 263264). We find this position correlated with functional palatine teeth and primary lips; and since such teeth do not develop in man, a forward migration of the mandible may be expected, causing the ectoderm to extend much farther along the roof of the oral cavity than along its floor. This migration is occurring in the 22-mm. embryo (fig. 42) in which the dental laminae are shown,' and, in the lower jaw, the labial lamina. The approximation of labial and dental laminae at this stage in mammalian embryos produces what has been called the labio-dental plate. The union of the two is massive in the guinea-pig (fig. 53). In the lizard Aristelliger (fig. 34), the labial and dental laminae of the lower jaw are close together, but do not unite; in the elasmobranchs there is a considerable interval between them. In the human embryo of 70 mm. (fig. 43) the labial lamina grows dovm toward the front of Meckel's cartilage, and the dental lamina behind it, precisely as in the lizard, amphibians, and elasmobranchs; and on this simple but convincing evidence we base the conclusion that these lower vertebrates possess true lips. The varying relation of the lips to the median nasal and maxillai’y processes we regard as secondary differences due to variations in the location and development of the nasal pits. Consequently, it is not justifiable, from our point of view, to make the homologies of lips depend upon their relationship to those processes (His, Keibel, et ah).

The most recent description of human lips at birth is that of Schumacher,^*'^ in which we find confirmation of our observations that the lips of living infants lack villi. His guarded expression is as follows :

Hdb. d* mikr. Aiiat. d. Menseh., herausgegeben von v. Mollendorff, Bd. 5, Teil 1, Berlin, 1927, S. 9.


Jedenfalls geht aus den Beschreibungen und aueh aas- meinen eigenen Beobaehtungen hervor, dass die Ausbildung der „Lippenzotten“ des Neugeborenen sehr grosseii individiiellen Sehwaiikungen unterworfen ist. Mitunter ist von Vorragungen an der Oberflache nberhaupt nichts zu sehen. . . .

At the suggestion of Dr. Lewis, who made similar observations some years ago, I examined the lips of newborn and somewhat older infants in the Boston Lying-in Hospital, and have been unable to see the villi with the unaided eye. By careful examination with a hand lens (magnifying 8 diam.), w'hen the lips were rolled outward, the mucous membrane wms seen to be studded with minute red points of low conical shape. The red dots, or striae, doubtless represent the tips of capillary loops in the connective tissue of the slender dermal papillae. In infants of about one week the vascular dots seem to be replaced by minute wiiitish eminences, w’^hich result perhaps from a drying, and an increasing though slight opacity, of the overlying epithelium.^**®

The epithelium over the labial papillae macerates readily in a peculiar way. In an infant at full term which died at birth, Dr. Lewis found the dark red inner part of the lip sharply marked otf from the pars glabra, but no villi could be seen in the fresh preparation. There w'^ere low rounded mounds, or at best low conical elevations, in the pars villosa.

These papillae are similar to those of the gums which were described by Henry Goadby in 1858, in a citation which Hr. Lewis has provided, from what he regards as the first original histology published in the United States text-book of vegetable and animal physiology/’ New York, 1858). Goadby has a curious colored figure of the injected mucous membrane of the human gums. Its papillae in the adult, he says, ‘^are somewhat long, and consist of a single looped capillary, which, through the tension of the injection, becomes more or less twisted. Their number is very considerable, presenting literally a forest of them. The papillae of the lips present a gorgeous sight, when well injected; the loops, as in the former case, twisting with the pressure of the injection.” These loops, however, are not all as simple as Goadby showed them. They often include one or more anastomosing vessels, though the network is never as elaborate as in intestinal villi.


After hardening thirty-six hours in Zenker’s fluid, sections of the left upper lip, near the corner of the mouth, showed distinct villi, as seen in the photograph, figure 67. Maceration of the lips from the opposite side of the mouth for twentyfour hours in Kinger’s solution showed much taller and more slender villi, unlike anything seen in life, photographed in figure 66. After continuing the maceration for forty-eight hours, the epithelium came away from the villi in flakes, leaving the cores very prominent. These connective-tissue cores, even when slender, are quite resistant. Toward the pars glabra the epithelium was more tenacious, and over the glabra and skin it would not brush off, nor could it be detached with forceps as a layer in such a way as to display the papillae beneath.

Although the possibility of individual variation remains, so that some human lips may show distinct macroscopic villi in life, they have not yet been clearly recorded by any one, so far as we know’. The figure of the human lips by Miss Eamm,^”® reproduced by Schumacher and in Keibel and Mall’s Embryologjq presumably represents a stage in the shrinkage or desquamation of the epithelium. It greatly resembles Ruysch’s historic figure, the manner of the production of which he 'well understood. Consequently, it had been far better to name the zone in question the pars papillosa rather than the pars villosa, or from the proliferative abundance of its epithelium, the pars epitheliosa. However, the localized peaks of epithelial cells above the dermal papillae, such as are showm in sections of human lips, may readily pass into macroscopic elevations like those of many mammals. A Semnopithecus entellus at term has a very thick epitheliosa without villi over most of the lips, but there are a few free villi near the corner of the mouth. The lips of an adult Macacus rhesus have no free villi; there is a thick epitheliosa, as in the human adult.

AI. TjpBer die Zotten der Mundlippeii .... beiin NeugeRorenen. AMt. Hefte, Aiith. 1, 1305, Bd. 29, Fig. 2, S. 74.

Conclusions

1. We agree with Daufortli^"^ that “homology may he regarded as a purely relative matter. ... It is in the causal factors and not in the structures themselves that the real basis of homology is to be sought. . . . Homology is consequently usually partial and not absolute.” On such a basis we find homologous lips at certain stages of development in some representatives of all classes of vertebrates.

  • Danfortli, 0. H. Hair in its relation to questions of homology and phylogeny. Am. Jour, Anat., 1925, vol. 36, pp, 66-67.

2. KukenthaPs statement concerning the lips of whales — Die Form der Lippen ist vollkommen abhangig von ihrer physiologischen Function — applies, with but slight restriction, to the lips of all vertebrates.

3. The lips of elasmobranchs are ‘primary lips.’ We qualify Allis’s statement that “in the Amniota the functional lips are the secondary ones ’ ’ by pointing out that the secondary lips in the upper and lower jaws are morphologically very different structures. In the upper jaw the secondary lip is a small subdivision of the primary lip. In the lower jaw the primary lip is suppressed altogether, or wholly merged in the secondary lip, correlated with the forward movement of the mandible and tlie persistence of the mandibular teeth. But we agree with Allis that “maxillaiy and premaxillary teeth or bones may be developed, as in the Teleostei, Holostei, and Crossopterygii, in relation to this secondary upper lip.” “In most of the Sauropsida both of these latter bones are actually developed in relation to this lip, and there are, accordingly, in the upper jaw of these vertebrates, two arcades, with or without teeth, an inner and primary arcade formed by the bones developed in relation to the palatoquadrate and an outer and secondary arcade formed by the maxillary and premaxillary bones.”

4. Since the upper teeth of the higher vertebrates are not represented in the elasmobranchs, but are a new formation within the territory of the primary lip, their derivation from placoid scales is secondary and remote. There is merely the slightest morphological resemblance between labial villi and denticles. Yet if the tooth-like labial villi of the cat, for example, should become teeth, and bone should develop in the lip in relation with therh, the process of evolution of the secondary upper lip would only be carried a step further, leading to the production of a tertiary jaw and lip.


5. The functions of the vertebrate lip are three — sensory, prehensile, and adhesive— which are developed in varying degrees. Of the lips studied, those of the cod have the most conspicuous sense organs; those of the tadpole with their epithelial teeth, and of grazing animals through their intrinsic muscles, are the most prehensile ; the lips of petromyzon and of the vampire, with abundant villi, are the most adhesive, and indicate that the smaller villi of the lips of suckling animals, together with their very vascular papillae, are for tight adhesion to the nipple. However, the fact that nursing may be accomplished without this apparatus and even before lips have developed, as in the opossum and rat, suggests an excess of ingenuity — an over refinement in nature — like that which produced the rostral callus, often considered unnecessary for breaking the eggshell.

6. Morphologically, the entrance to the mouth of vertebrates tends to be guarded by a pair of folds over which the skin makes a transition to the mucous membrane. This transition is effected typically through two zones, an outer pars glabra and an inner pars villosa (or epitheliosa). The latter is a region of very thick epithelium and of tall papillae of the corium, and is the seat of great proliferative activity, leading to the production of various papillae and villi, both in the lower vertebrates and in mammals.


Plates

Abbreviations

call,, callus

ch., chorda dorsalis

cir., cirrus

c.M.j cartilage Meekelii c.p,, cartilage palatina c,p’7nn,, cartilage premandibularis c,p-qu., cartilage palatoquadrata

c. r.i., cartilage rostralis inferior C.T.S., cartilage rostralis superior dxonu, dens eorneus

d. lin,, dens lingualis d,mn., dens mandibularis d.mx., dens ni axillaris d.pal., dens palatinus d.p-mcc., dens premaxillaris d.V’p., dens vomeropalatinus f.hyp,, fovea hypophyseos f,o., fissura oris

f,olf,, fovea elf aetoria f.pil., folliculus pili (jin.j gingiva gl.citt., glandula cutis gl.or,, glandula oris

iiivaginatio iiasoliypopliysealis l.dent., lamina dentalis J.gl.f lamina glandularis hi., labium inferius ling,, lingua l.lah., lamina labialis


l, s,, labium superius mand., mandibula

m. gdi., museulus genioliyoideus m.L, museulus labialis

m.pJi., membrana pharyngea os.mand,, os mandibulae p.cut., pars cutanea p.gJah,, pars glabra p.liyp*, pedunculus liypophyseos papilla mammae p-mx., premaxilla p.vih, pars viliosa 7‘ost., rostrum

r. put., ruptoT putaminis .s%den.f., sulcus dentalis

s. e.h, sulcus externus labii s.i-L, sulcus infralabialis

sulcus iuframandibularis s,h or sdal}., sulcus labialis squ,, squama

s,r, or s.rnp., spina ruptoris x.cer,, ventriculus cerebri 'k)el., velum vih, villus

v.mn., valvula mandibularis v.mx,, valvula maxillaris v.or.j vestibulum oris v.pah, valvula palatina

Plate 1

Explanation of Figures

Sagittal sections of the oral region of Petromyzon and Sqnalus

6 to 10 Petromyzon. Fig* Q, P. planer:, 2,7 mm. Harvard Embryological Collection, series 792, section 23. X 55. Fig. 7, P, planeri, 4.75 mm. H. E. C., 777, sect. 22. X 55. Fig. 8, P. fluviatilis, 27.6 mm. H. E. C,, 247, sect. 50. X 12. Fig, 9, P. fluviatilis, 42 mm. H. E. C., 250, sect. 66. X 12, Fig. 10, Petromyzon, 164 nun. X 10.

11 to 13 Squalus aeantbias. Fig, 11, 22 mm. H. E. C., 231, sect. 78. X 15. Pig. 12, 50 mm. H. E. C., 444, sect, 143. X 30. Fig. 13, 159 mm. X 17.


Plate 2

Explanation of Figures

Sagittal sections of the oral region of fishes

14: Squalus acaiithias, 520 mm. X 7.5. a, lower tooth from crest of the gingival ridge; and I, one of the nearest scales from the outer skin of the lower jaw, to show relative size, and direction of spines. X 20.

15 to 17 Torpedo ocellata. Pig. 15, 12.8 mm. H. E. C., 689, sect. 68. X 15. Pig. 16, 28.2 mm. H. E, C., 672, sect. 262. X 15. Pig. 17, 51.5 mm. H. E. C., 711, sect- 607. X 15 18 Eaja eriiiaeea. 47.7 mm. X 15.

19 Salvelinus fontinalis. 25 mm. H. E, C,, 581, sect. 85. X 30.

20 and 21 Opsanus tau. Pig. 20, 8 nim. H. E. C., 116, sect. 48. X 25. Pig. 21, 41.8 mni. H.E. C., 1177, sect. 423. X 25.


Plate 3

Explanation of Figures

Mouths of fishes and amphibians

22 and 23 Gadiis inorrlma. Pig. 22, sagittal sections of the lips of an adult codfish, near the median line. X 5. Pig. 23, sketch of the circumoral folds of the adult.

24 and 25 Amblystoma punctatum. Pig. 24, 8 mm. H. E. C., 187, sect. 50. X 25. Pig. 25, 26 mm. H. E. G., 655, sect. 145 (mth voineropalatine teeth as in sect. 141). X 25.

26 Iviecturus inaculatus. 31.4 mm. H. E. C., 537, sect. 121 (combined, as to teeth, -syith sections 120 and 122). X 25.

27 Cryptobranclius allegheiiiensis. 350 mm. (adult). X 10.

28 Spelerpes bilineatus. 60 mm. (adult). X 25.


Plate 4

Explanation of Figures

Moutlis of amphibians and reptiles

29 to 31 Sagittal sections of tadpoles of Bufo lentiginosus americanus. Pig. 29, 12 mm. H. E, 0., 1190. A, near the median line, sect. 126; B, through the corner of the mouth, sect, 81. X 54. Pig. 30, 22,4 mm. H, E. 0., 1153, sect. 144. X 35. Pig. 31, toad of 10 mm. H. E. C., 1135, sect. 167. X 32.

32 Amblystoma punetatum. 12 mm. H. E. C., 665. Sagittal sections appnmehiiig the corner of the mouth; A, sect. 70; B, sect. SO. X 37.

33 and 34 Aristelliger praesignis. Pig. 33, 7.2 mm. H. E. C., 1629, sect. 91. X 40. Pig. 34, 8.8 mm. H. E. C., 1716, sect. 115. X 40.

35 ChrYseniYs marginata. 27 mm. Transverse section of the mouth. IT. E, C., 1096, sect. 219. X 21.

36 Trionvx ferox. Lateral half of a transverse section through the mouth of a newly hatched specimen, X 15.

37 Chrysemys marginata. 26.4 mm. H. E. C., 1099, sagittal sect. 557, X 15.

Plate 5

Explanation of Figures

38 and 39 The beak of chick embryos. Pig. 38, 43 mm. H. E. C,, 509, sagittal sect. 356. X lo. Pig. 39, 31 mm. H, E, C., 1967, transverse sect. 1036. X 15.

40 Typhlops lumbriealis. Sagittal section of the mouth of an adult ^ blind snake. ^ X 40.

41 to 43 Human embryos, sagittal sections. Pig. 41, 10.3 mm. H. E. C., 736, sect. 147. X 20. Pig. 42, 22 mm. H, E. C., 851, sect. 277. X 20, pig. 43, 70 mm. X 15.

44 to 47 Pelis doniestica. Pigs. 44 and 45, vertical sections of the lower lip of a newborn kitten, 45 being lateral to 44. X 12.5. Figures 46 and 47, vertical sections through the npx)er lipj 46 from an embryo of 42.5 lum,, near the corner of the mouth, X 20 ; 47 from a newborn kitten, X 12.5.


Plate 6

Explanation of Figures


Mouths and lips of mammals

48 Pig. Vertical section of the lip of a 182-mm. embryo. X 7.5.

49 Calf, at birth. Vertical section through both lips. X 7.5.

50 Rabbit. Vertical section of the lower lip of a newborn rabbit, halfway between the median line and the corner of the mouth.

51 to 53 Guinea-pig. Pig. 51, vertical section of lower lip of a newborn guineapig, in front of the right incisor tooth. X 15, Pig. 52, horizontal section through the cheek of an adult, at the angle of the mouth. X 5. Pig. 53, embryo of 18.6 mm, H. E. G., 1787, sagittal sect. 200. X 20.

54 Mouse. Sagittal section of the mouth of a newborn Mus musculus. X 20,

55 and 56 Opossum (Didelpliys virginiana). Pig. 55, sagittal section of the mouth of a 23-mm. specimen, attached to the maternal nipple. H. E. G., 2078, sect. 269. X 10* Pig* 56, vertical section of the lips of a young animal, 170 mm. in length, toward the corner of the mouth. X 21.


Plate 7

Explanation of Figures

57 PetroiiiYzoR fluviatilis. Section of a lioriiy tooth, with deep layers representing a replacement tooth, situated above a section of the annular cartilage. Prom a young specimen (164 mm.). X 72.

58 Petromyzon fluviatilis. Section of oral villi from the dorsal part of the oral funnel. From the same individual as figure 57. X 85.

50 Squalus acanthias. Section of a tooth, from an immature specimen of 520 mm. X 40.

60 Bufo lentiginosus americanus. The mouth of a 12-mm. tadpole; the lower jaw shows the horny Ijeak and the three tiers of cornified teeth. H. E. C., 1190, sect. 131. X 125.



Cite this page: Hill, M.A. (2020, September 19) Embryology Paper - The comparative anatomy of the lips and labial villi of vertebrates. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_comparative_anatomy_of_the_lips_and_labial_villi_of_vertebrates

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