Talk:Book - A Laboratory Manual and Text-book of Embryology 6

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As the average student will not have time to study series of embryos sectioned in different planes, dissections may be used for showing the form and relations of the organs. Cleared embryos mounted whole are instructive, but show the structures superimposed and are apt to confuse the student. Pig embryos 10 mm. or more in length may be easily dissected, mounted as opaque objects, and used for several years. Success in dissecting such small embryos depends: (1) on the fixation and hardening of the material employed; (2) on starting the dissec tion with a clean cut in the right plane; (3) on a knowledge of the anatomy of the parts to be dissected.

Fixation and Hardening of Material.— Embryos fixed in Zenker's fluid have given the best results. They should then be so hardened in 95 per cent, alcohol that the more diffuse mesenchyma will readily separate from the surfaces of the various organs, yet the organs must not be so brittle that they will crumble and break. Embryos well hardened and then kept for two weeks in 80 j)er cent, alcohol usually dissect well. Old material is usually too brittle; that just fixed and hardened may prove too soft. As a test, determine whether the mesenchyma separates readily from the cervical ganglia and their roots.

Dissecting instruments include a binocular dissecting microscope, a sharp safety razor blade, large curved blunt-pointed dissecting needles, pairs of small sharp-pointed forceps, and straight dissecting needles small and large.

Methods of Dissection. — In general, it is best to begin the dissection with a clean, smooth cut made by a single stroke with the safety razor blade, which should be flooded with 80 per cent, alcohol. The section is made free hand, holding the embryo, protected by a fold of absorbent cotton, between the thumb and index finger. Having made preliminary cuts in this way, the embryo may be affixed with thin celloidin to a cover glass and immersed in a watch glass containing alcohol. We prefer not to affix the embryo, as the celloidin used for this purpose may interfere with the dissection. Instead, a cut is made parallel to the plane of the dissection so that the embryo, resting in the watch glass upon this flat surface, will be in a fairly stable position. It may thus be held in any convenient position by resting the convex surface of a curved blunt dissecting needle upon some part not easily injured. The dissection is then carried on under the binocular microscope, using the fine pointed forceps, dissecting needles, and a small pipette to wash away fragments of tissue.

Whole Embryos. — For the study of the exterior, whole embryos may be affixed with celloidin to the bottoms of watch glasses which may be stacked in wide-mouthed jars of 80 per cent, alcohol. The specimens may thus be used several years at a saving of both time and material. Preliminary treatment consists in immersion in 95 per cent, alcohol one hour, in ether and absolute alcohol at least thirty minutes, in thin celloidin one hour or more. Pour enough thin celloidin into a Syracuse watch glass to cover its bottom, and immerse in this a circle of black mat paper, first wet with ether and absolute alcohol. Pour off any surplus celloidin, mount embryo in desired position and immerse watch glass in 80 per cent, alcohol, in which (he specimen may be kept iadeSnitelv. Embryos may also be mounted in gelatinformalin solution m small sealed glass jars.

Lateral Dissections of the Viscera. — Dissections Eke those shown in Figs. 144 and 145 may easily be prepared in less than an hour, and make valuable demonstration and laboratory specimens. Skill is required to demonstrate most of the cerebral nerves, but the

Fig. 144.— Lateral dissection of an 18 mm. [Ug embryo, showing the n the right side. X 8.

central nervous system, cerebral and spinal ganglia, and viscera may easily be exposed. Starting dorsaUy, make a sagittal section of the embryo slightly to one ade of the median line and avoiding the umbilical cord ventrally. With the embryo resting on the flat sectioned surface, begin at the cervical flexure and with fine forceps grasp the ectoderm and dural anlage at its cut edge, separate it from the neural tube and pia mater, and strip it off ventralwards exposing the myelencephalon and cervical portion of the cord. As the mesenchyma is pulled away, the gangUa and roots of the cerebral nerves will be exposed. The mesenchyma between the ganglia and along the nerv^ may be removed with the end of a small blunt needle. Care must be exercised in working over the mesencephalon and telencephalon of the brain not

Fig. 145. — Lateral dissection of a 35 nun pig embryo to show the ni right side. X 4.

to injure the brain wall, which may be brittle. By Btartiog with a clean dissection dorsaily and gradually working ventrad, the more important organs may be laid bare without injury. The beginner should compare his specimen with the dissections figured and also previously atudy the reconstructions of Thyng (191 1) and Lewis (1903).

Lateral dissections of embryos 18 mm. and 35 mm. long show infinitely better than sections the form and relations of the organs, their relative growth, and their change of position (Figs. 144 and 145). Compare the organs of 6, 10, 18, and 35 mm. embryos and note the rapid growth of the viscera (see Figs. 95 and 120). Hand-in-hand with the increased size of the viscera goes the diminution of the dorsal and cervical flexures. In the brain, note the increased size of the cerebral hemispheres of the telencephalon and the presence of the olfactory lobe of the rhiftencephalon. The cerebellum also becomes prominent and a ventral flexure in the region of the pons, the pontine flexure, is more marked. The brain grows relatively faster than the spinal cord, and, by the elongation of their dorsal roots, the spinal ganglia are carried ventral to the cord. The body of the embryo also grows faster than the spinal cord, so that the spinal nerves, at first directed at right angles to the cord, course obliquely caudad in the lumbo-sacral region.

Median Sagittal Dissections (Figs. 146 and 147). — Preliminary to the dissection, a cut is made dorsally as near as possible to the median sagittal plane. Beginning caudally at the mid-dorsal line, an incision is started which extends in depth through the neural tube and the anlages of the vertebrae. This incision is carried to the cervical flexure, cranial to which point the head and brain are halved as accurately as possible. The blade is then carried ventrally and caudally, cutting through the heart and liver to the right of the midline and of the umbilical cord until the starting point is reached. A parasagittal section is next made well to the left of the median sagittal plane and the sectioned portion is removed, leaving on the left side of the embryo a plane surface. With the embryo resting upon this flat surface, the dissection is begun by removing with forceps the right half of the head. In pulling this away caudalwards, half of the dorsal body wall, the whole of the lateral body wall, and the parts of the heart and liver lying to the right of the midline will be removed, leaving the other structures intact. If the plane of section was accurate, the brain and spinal cord will be halved in the median sagittal plane. Wash out the cavities of the brain with a pipette and its internal structure may be seen. Dissect away the mesenchyma between the esophagus and trachea and expose the lung. Remove the right mesonephros, leaving the proximal part of its duct attached to the urogenital sinus. The right dorsal lobe of the liver will overlie the stomach and pancreas. Pick it away with forceps and expose these organs. Dissect away the caudal portion of the liver until the hepatic diverticulum is laid bare. It is whitish in color and may thus be distinguished from the brownish liver. Beginning at the base of the umbilical cord, carefully pull away its right wall with forceps, thus exposing the intestinal loop and its attachment to the yolk stalk. If in the caudal portion of the umbilical cord the umbilical artery is removed, the allantoic stalk may be dissected out. To see the anlage of the genital gland, break through and remove a part of the mesentery, exposing the mesial surface of the left mesonephros and the genital fold. The dissection of the metanephros and ureter is difficult in small embryos. In 10 to 12 mm. embryos, the umbilical artery, just after it leaves the aorta, passes lateral to the metanephros and thus locates it. By working carefully with fine needles the surface of the metanephros may be laid bare and the delicate ureter may be traced to the base of the mesonephric duct. The extent of the dorsal aorta may also be seen by removing the surrounding mesenchyma. With a few trials, such dissections may be made in a short time, and are invaluable in giving one an idea of the form, positions, and relations of the different organs. By comparing the early (Figs. 96 and 122) with the later stages {Figs. t*i and 147) a number of interesting points may be noted.

Chorioid plexus uffourlh Hypophysis Tda chorioidM foiirlh vrnlricle M yclcmephalen




Sltsonepkric duct

I. pig embryo, showinfc central nervous system in a in position. X 8.

In the brain, the corpus striatum develops in the floor of the cei^bral hemispheres. The interventricular foramen is narrowed to a stit. In the roof of the diencephalon appears the aniage of the epiphysis, or pineal gland, and the chorioid ptewtf of the third ventricle. This extends into the lateral ventricles as the lateral chorioid plexus. The dorso-Iateral wall of the diencephalon thickens to! form the thalamus and the third ventricle is narrowed to a vertical slit. The ' increased size of the cerebellum has been noted. Into the thin dorsal wall of the myelencephalon grows the network of vessels which form the chorioid plexus of the fourth ventricle, which is now spread out laterally and flattened dorso-ventrally. About the notochord mesenchymal anlages which form the centra of the vertebra are prominent.

—Median sagittal dissection of embryo.

Turning to the alimentary tract, observe that the primitive mouth cavity is now divided by the palatine folds into the upper nasal passages and lower oral cavity. In the lateral walls of the nasal passages develop the anlages of the turbinate banes. On the floor of the mouth and pharynx, the tongue and epiglottis become more prominent. The trachea and esophagus elongate and the lungs lie more and more caudad. The dorsal portion of the septum transversum, the anlage of a portion of the diaphragm, is thus carried caudad, and although originally, when traced from the dorsal body wall, it was directed caudad and ventrad, now it curves cephalad and ventrad, bulging cephalad into the thorax. The proximal limb of the intestinal loop elongates rapidly, and, beginning with the duodenum, becomes flexed and coiled in a characteristic manner. The distal limb of the intestinal loop is not coiled, but its diverticulum, the ccecunt, is more marked. Caudally, the rectum^ or straight gut, has completely separated from the urogenital sinus and opens to the exterior through the anus.

Of the urogenital organs y the genital folds have become the prominent genital glands attached to the median surfaces of the mesonephroi. The metanephroi have increased rapidly in size and have shifted cephalad. Proximal to the allantoic stalk the adjacent portion of the urogenital sinus has dilated to form the bladder. As the urogenital sinus grows it takes up into its wall the proximal ends of the mesonephric ducts, so that these and the ureters have separate openings into the sinus. Owing to the unequal growth of the sinus wall, the ureters open near the base of the bladder, the mesonephric ducts more caudally into the urethra. The phallus now forms the penis of the male or the clitoris of the female. Cranial to the metanephros a new organ, the suprarenal gland, has developed. This is a ductless gland and is much larger in human embryos.

The hearty as may be seen by comparing Figs. 96 and 147, although at first pressed against the tip of the head, shifts caudally until, in the 35 mm. embryo, it lies in the thorax opposite the first five thoracic nerves. Later it shifts even further caudad. The same is true of the other internal organs, the metanephros excepted. As the chief blood vessels are connected with the heart and viscera, profound changes in the positions of the vessels are thus brought about, for the vessels must shift their positions with the organs which they supply.

Ventral Dissections. — Ventral dissections of the viscera arc very easily made. With the safety razor blade, start a cut in a coronal plane through the caudal end of the embryo and the lower limb buds (Fig. 148). Extend this cut laterad and cephalad through the body wall and the upper limb bud. The head may be cut away in the same plane of section, and the cut continued through the body wall and upper limb bud of the opposite side back caudally to the starting point. Section the embryo in a coronal plane, parallel with the first section and near the back, so that the embryo will rest upon the flattened surface. With forceps now remove the ventral body wall. By tearing open the wall of the umbilical cord along one side it may be removed, leaving the intestinal loop intact. Pull away the heart, noting its external structure. The liver may also be removed, leaving the stomach and intestine uninjured. A portion of the septum transversum covering the lungs may be carefully stripped away and the lungs thus laid bare.

Dissections made in this way show the trachea and lungs, the esophagus, stomach and dorsal attachment of the septum transversum, the course of the intestinal canal, and also the mesonephroi and their ducts. Favorable sections through the caudal end of the body may show the urogenital sinus, rectum, and sections of the umbilical arteries and allantois (Figs. 97, 124 and 148). In late stages, by remo\-ing the digestive organs, the urogenUal duels and glands are beautifully demonstrated (Figs. 223 and 224).

Fig. 148. — Ventral dissection of a 13 mm. pig embryo, showing lungs, digestive canal and mesonephmi. The ventral body wall, heart and li\'er have been remoi'ed and the limb buds cut across. X 6.

Development of the Face

The heads of pig embryos have long been used for the study of the development of the face. The heads should be removed by passing the razor blade between the heart and adjacent surface of tbe head, thus severing the neck. Next cut away the dorsal part of the head by a section parallel to the ventral surface, the razor blade passing dorsal to tbe branchial clefts and eyes. Mount, ventral side up, three stages from embryos 6, 12. and 14 mm- long, as shown in Figs. 97 and 149.

In the early stages (Figs. 97 and 124) the four branchial arches and clefts are seen. The third and fourth arches soon sink into the cer\ncal sinus, while the mandibular processes of the first arch are fused early to form the lower jaw. Laterally the frontal process of the head is early divided into laltrul and median nasal processes by the development of the olfactory pits. The processes are distinct and most prominent at 12 mm. (Fig. 149 A). Soon, in 13 to 14 mm. embryos

, the median nasal processes fuse with the maxillary processes of the first arch and constitute the upper jaw (Fig. 149 B). The lateral nasal processes fuse with the maxillary processes and form the cheeks, the lateral part of the lips, and the ala of the nose. Later, the median nasal processes unite and become the median part of the upper lip. Meanwhile the mesial remainder of the original frontal process (Fig. 149 A) is compressed and becomes the septum and dorsum of the nose. The development of the olfactory organ will be traced on p. 371.

Flg. 149. — Two stages showing the development of the face in pig embrj'os, X 7. A, \'entral view of face of a 12 mm. embiyo; B, of a 14 mm. embijo.

The early development of the face is practically the same in human embryos (Figs. 150 and 370). In embryos of 8 mm. the lateral and median nasal processes have formed. The maxillary processes next fuse with the nasal processes, after which the median nasal processes unite. Coincident with these changes the mandibular processes fuse and from them a median projection is developed which forms the anlage of the ckin.

Epithelial ingrowths begin to form the lips at the fifth week (Fig. 159). As the median nasal processes and the maxitlary processes take part in their development, the failure of these parts to fuse may produce hare lip. The line of fusion of the median nasal piocesses is evident in the adult as the pkUtrum. The lips of the newborn child arc peculiar in that their proximal surfaces are covered with numerous villi, finger-like processes which may be a milliroeter or more in length.

Fig. I50. — Development of the face of the human embryo (His). A, Embiyo of 8 mm. (X 7.5); the median frontal process differentiating into median nasal processes or processus globularcs, towaid which the mamillary of the hrst visceral arch are extending. B, Embryo of 13.7 mm. (X 5); the glohular, lateral nasal and maxillary processes are in apposition; the primitive nans is now better defined. C, Embryo of 17 mm. (X 5); immediate boundaries of mouth are more definite and the nasal orifices are partly formed, external ear appearing. D, Embryo of neariy eight weeks (X 5).

The external ear is developed around the first branchial cleft by the appearance of small tubercles which form the auricle. The cleft itself becomes the exlerttal auditory meatus and the concha of the ear. (For the development of the external ear see Chapter XIII.)

Development of the Palate

This may be studied advantageously in pig embryos of two stages: (a) 20 to 25 mm. long; (b) 28 to 35 mm. long. Dissections may be made by carrying a shallow incision from the anlage of ihc mouth bark to the external ear on each si<le (Fig. 152). The incisions are then continued through the neck in a plane parallel to the hard palate. Before mounting the preparation, remove the top of the head by a section cutting through the eyes and nostrils parallel to the first plane of section. Transverse sections through the snout may also be prepared to show the positions of tongue and palatine folds before and after the fusion of the latter (Fig. 151).

In pig embryos of 20 to 25 mm. the jaws are close together and the mandible usually rests against the breast. Shelf-like folds of the maxillae, the lateral palatine processes, are separated by the tongue and are directed ventrad (Figs. 151 i4 and 152 A). The median nasal processes also give rise to a single heart-shaped structure, the median palatine process (Fig. 152). In embryos of 26 to 28 mm. the mandible drops, owing to growth changes, and the tongue is withdrawn from between the palatine processes (Fig. 151 5). With the withdrawal of the tongue the palatine folds bend upward to the horizontal plane, approach each other and fuse, thus cutting off the nasal passages from the primitive oral cavity (Fig. 152 5)The primitive choanw (cf. Fig. 153), formed by rupture of the membrane separating the olfactory pits from the oral cavity, now lead into the nasal passages, which in turn communicate with the pharynx by secondary permanent choame. At the point in the median line where the lateral and median palatine processes meet, fusion is not complete, leaving the incisive fossa, and laterad between the two processes openings persist for some time, which are known as the incisive canals (of Stenson).

Fig. 151. — Sections through the jaws of pig embryos to show development of the hard palate. A, 22 mm.; B, 34 mm. X 8.

Fig. 152.— Dlsscci ions to show the development of the hard palate in pig embryos. X 5. A, Ventrjl vkw of palatine processes of a 22 mm. pig embr>'o, the mandible having been removed; B, Same of ,15 mm. embrj'o showing fusion of palatine processes.

In human embrj'os these changes are essentially identical (Fig. 153). The lateral palatine processes begin to fuse cranio-caudally at about the end of the second month. At the same time ffaiatine hotifs first appear in the lateral palatine folds and thus form (he AurJ {nihif. Camially the bones do not develop and this portion of the folds forms the soft palate and the uvvla (Fig. 152). The unfused backward prolongations of the palatine folds give rise to the pkaryngopalatine arches, which are taken as the boundary line between the oral cavity proper and the pharynx in adult anatomy.

Fig. I53. — The Poof of the mouth of a human embnn sbout two and a half months old, tbowing the dci-vlopmcnt of the palate (after HisV X ". p.g.. Pfocessus globularis: ^.j.', palatine process of prvy-es-sus fElobularis: iii.v. niaxillai>' jirwms: «.r', pabtine fold of maiillan- piTms& Close lo the angle hetircen this aitd the palatine process of the proces^s globularis on each side ai« the primiihe chimiiP.

Development of the Tongue

After the withdrawal of the tongue, the lateral palatine processes take up a horizontal position and their edges are approximated because the cells on the ventral sides of the folds proliferate more rapidly than those of the dorsal side (Schorr, Anat. Hette, Bd. 36, 1908). That the change in position of the palatine folds is not mechanical, but due to unequal growth, may be seen in Fig. 154, a section through the palatine folds of a pig embryo, which shows the right palatine fold in a horizontal position, although the left fold projects ventral to the dorsum of the tongue. A region of cellular proliferation may be seen on the under side of each process.

Fig. 154. — Section through the jaws of a 25 mm. pig embryo to show the change in the position of the palatine processes with reference to the tongue.

Anomalies. — The lateral palatine processes occasionally fail to unite in the middle line, producing a defect known as clefl palale. The extent of the defect varies considerably, in some cases involving only the soft palate, while in other cases both soft and hard palates are cleft. It may also be associated with hare lip.

The development of the tongue may be studied from dissections of pig embryos 6, 9, and 13 mm. long. As the pharynx is bent nearly at right angles, it is necessary to cul away its roof by two pairs of sections passing in different planes. The first plane of section cuts through the eye and first two branchial arches just above the cervical sinus (Fig, 155, I). From the surface, (he razor blade should be directed obliquely dorsad in cutting toward the median line., Cuts in this plane should be made from either side. In the same way make sections on each side in a plane forming an obtuse angle with the first section and passing dorsal to the cervical sinus (II), Now sever the remaining portion of the head from the body by a transverse section in a plane parallel to the first (III). Place the ventral portion of the head in a watch glass of alcohol, and, under the dissecting microscope, remove that part of the preparation cranial to the mandibular arches. Looking down upon the floor of the pharynx, remove any portions of the lateral pharyngeal wall which may still interfere with a clear view of the pharyngeal arches as seen in Figs. 98 and 156, Permanent mounts of the three stages mentioned above may be made and used for study by the student.



The tongue develops as two distinct portions, the body and the root, separated from each other by a V-shaped groove, the sulcus terminaiis. In both human and pig embryos the body of the tongue is developed from three anlages which are formed in front of the second branchial arches. These are the median, somewhat triangular tuberculum impar, and the paired lateral swellings of the first, or mandibular, arches, both of which are present in human

Fig. 155.— Uteral view of the head embryos of 5 mm. (Figs. 98 and 157 A). of a 7 mm. pig embryo The thr^ ^^ ^^ ^ ^^^^ ^^^^^^ elevation

hnes indicate the planes of sections to be " '

made in dissecting the tongue as deformed by the union of the second branchial sen in e text. arches (and, according to some workers, the

third as well) forms the copula. This, with the portions of the second arches lateral to it, forms later the base or root of the tongue. Between it and the

Branchial ardii Tubtrcutum impet Branchiai arch ^ Branchial arch Branchial arch ,

Laleral liltpfi atdoffi

Fig. 156.— Dissections showing the de\-eIopmcnt of the tongue in pig embryos. X 12- A, 9 mm. envbryo; S, 13 mm. embryo.

tuberculum impar is the point of evagination of the thyreoid gland. The copula also connects the tuberculum impar with a rounded prominence which is developed in the mid-ventral line from the bases of the third and fourth branchial arches. This is the anlage of the epiglottis. In later stages (Fig. 156 A and B) the lateral mandibular anlages, bounded laterally by the alveolo-lingual grooves, increase rapidly in size and fuse with the tuberculum impar, which lags behind in development and is said to form the median' septum of the tongue. According to Hammar, it atrophies completely. The epiglottis becomes larger and concave on its ventral surface. Caudal to it, and in early stages continuous with it, are two thick rounded folds, the arytenoid ridges. Between these is the slit-like glottis leading into the larynx (see p. 165).

Fig. 157.— The devetopmeDt of the tongue in human embryos. A, 5 mm.; B, 7 mm. (modified from Peters).

The foregoing account applies to the early origin of the mucous membrane alone. The musculature of the tongue is supplied chiefly by the hypoglossal nene, and both nerve and muscles develop caudal to the branchial region in which the tongue develops. The musculature migrates cephalad and gradually invades the branchial region beneath the mucous membrane. At the same time, the tongue may be said (o extend caudad until its root is covered by the epithelium of the third and fourth branchial arches. This is shown by the fact that the sensory portions of the nn. trigeminus and faciaiis, the nerves of the first and second arches, supply the body of the tongue, while the rn. glossopharyngeus and vagus, the nerves of the third and fourth arches, supply the root and the caudal portion of the body of the tongue.

In fetuses of 50 to 60 mm. (C R) ihe fungiform and filiform papilla: may be distinguished as elevations of the epithelium. Taste buds appear in the fungiform papillae of 100 mm. (C R) fetuses and are much more numerous in the fetus than in the adult. The vallate papilla (Fig. 158 A) appear as a V-shaped epithelial ridge, the apex of the V corresponding to the site of the thyreoid vagination (Joramen cacum). At intervals along the epitheUal ridges circular epithelial downgrowths develop (85 mm. C R) which take the form of inverted and hollow truncated cones (Fig. 158). During the fourth month circular clefts appear in the epithelial downgrowths, thus separating the walls of the vallate papillx from

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Development of the Teeth

The enamel organs, which give rise to the enamel of the teeth and arc the moulds, so to speak, of the future teeth, are of ectodermal origin. There first appears in embryos of about 11 mm. an ectodennal downgrowth, the denial ridge or lamina, on the future alveolar portions of the upper and lower jaws {Fig, 159). These laminae are parallel and mesial to the labial grooves. At intervals, on each curved dental ridge or lamina a series of thickenings develop, the anlages of the enamel organs (Fig. 160). Soon the ventral side of each enamel organ becomes concave (fetuses of 40 mm. C H) forming an inverted cup and the concavity is occupied by dense mesenchymal tissue, the denial papilla (Figs. 159 B and 162). An enamel organ with dental papilla forms the anlage of each decidual or milk tooth. Ten such anlages are present in the upper jaw and ten in the lower jaw of a 40 mm. fetus. The connection of the dental anlages with the dental ridge is eventually lost. The position of the tooth anlage between the tongue and lip is shown in Fig. 163.

Fig. 159. — Early stages in the development of the teeth (Rose). ^,at 17 mm. (X 90); 41 mm. (X 45).

Dtntal lamina

Fig. 160. — Diagrams showing the early development of three teeth. One of the teeth is shown in section (Lewis and StOhr).

The anlages of those permanent leeth which correspond to the deddual, or milk teeth, are developed in another series along the free edge ol the dental lainina (Fig. 160 D) and come to lie mesad of the decidual teeth. In addition, the anlages of three permanent molars are developed on each side, both above and below, from a backward or aboral extension of the dental lamina, entirely free from the oral epithelium (Fig. t6I). The anlages of the first permanent molars appear at seventeen weeks (180 mm. C H), those of the second molars at m weeks after birth, while the anlages of the third permanent molars or wisdom teeth are not found until the fifth year. The permanent dentition of thirty-two teeth is then complete.

Fig. 161.— Dental

a ODd anlages of the milk teeth of the upper jaw fnm a fetus of 115 n (R6se in KoUmann).

Fig. 162.— Section through the upper fir^t decidual incisor tooth froma65 I. human fetus. X 70.

Fig. 163. — Parasagittal section through the mandible and tongue of a 65 mm. human fetus showing tlie poutioD of the anlage of the first incisor tootti. X 14.

Fig. 164. — Sectioo through a. portion o[ the crown of a developing tooth sbowiog the various layers (after Toumeux in Heider).

The internal cells of the enamel organs are at first compact, but later by the development of an intercellular matrix the cells separate, forming a reticulum resembling mesenchyme, termed the enamel pulp (Fig. 162). The outer enamel cells, at first cu!>oidal, flatten out and later form a fibrous layer. The inner enamel cells bound the cup-shaped concavity of the enamel organ. Over the crown of the tooth these cells, the ameloblasls, become slender and cxilumnar in form, producing the enamel layer of the tooth along their basal ends (Fig. 164). The enamel is laid down first an an uncalcified fibrillar layer which later becomes calcified in the form of enamel prisms one for each ameloblast. The enamel is formcif first at the apex of the cn.>wn of the totith and extends downward toward the root. The enamel cells about the future root of the tooth remain cuboidal or low columnar in form, come into contact with the outer enamel cells, and the two layers constitute the epithelial sheath of the root which does not produce enamel prisms (Fig. 165).

Fig. 165. — LoDf^tudinal section of a deciduous tooth of a newborn dog. X 42. The while spaces bclwecn the inner enamel cells and the enamel are artificial and due lo shrinkage (Lewis

The Dental Papilla. — The outermost cells of the dental papilla at the end of the fourth month arrange themselves as a definite layer of columnar epithelium. Since they produce the dentine, or dental bone, these cells are known as odontoblasts (Fig. 165). When the dentine layer is developed, the odontoblast cells remain internal to it, but branched processes from them (the dentinal fibers of Tomes) extend into the dentine and form the dental canali-culi. Internal to the odontoblast layer, the mesench)mial cells differentiate into the dental pulp, popn ularly known as the "nerve" of the tooth. This is composed of a framework of reticular tissue in which are found blood vessels, lymphatics, and nerve fibers. The odontoblast layer persists throughout life and continues to secrete dentine, so that eventually the root canal may be obliterated.

Dental Sac. — The mesenchymal tissue surrounding the anlage of the tooth gives rise to a dense outer layer and a more open inner layer of fibrous connective tissue. These layers form the defital sac (Fig. 165). Over the root of the tooth a layer of osteoblasts or bone forming cells develops, and, the epithelial sheath formed by the enamel layers having disintegrated, these osteoblasts deposit about the dentine a layer of bone which is known as the substantia ossea or cement. The cement layer contains typical bone cells but no Haversian canals. As the teeth grow and fill the alveoli, the dental sac becomes a thin vascular layer, continuous externally with the alveolar periosteum, internally with the periosteum of the cement layer of the tooth.

When the crown of the tooth is fully developed the enamel organ disintegrates, and, as the roots of the teeth continue to grow, their crowns approach the surface and break through the gums. The periods of eruption of the various milk or decidual teeth vary with race, climate, and nutritive conditions. Usually the teeth are cut in the following sequence:

Decidual or Milk Teeth

Median Incisors sixth to eighth month.

Lateral Incisors eighth to twelfth month.

First Molars twelfth to sixteenth month.

Canines seventeenth to twentieth month.

Second Molars twentieth to thirty-sixth month.

The permanent teeth are all present at the fifth year. They are located mesial to the decidual teeth (Fig. 166), and, before the permanent teeth begin to erupt, the roots of the milk teeth undergo partial resolution, their dental pulp dies, and they are eventually shed. Toward the sixth year, before the shedding

Fic. 166. — The skull of a five-year-old child showing position {Sobot ta-M cMumch ) .

Permanent tnciiors of the decidual and peimanent teeth

of the deciduous teeth begins, each Jaw may contain twenty-six teeth. The permanent teeth are "cut" as follows (McMurrich in Keibel and Mall, vol. 2);

First Molars seventh j-ear.

Median Incisors eighth year.

Lateral Incisors ninth year.

First Premolars tenth year.

Second Premolars eleventh year.

Sw HMIa J thirteenth to fourteenth yew.

Third Molars (Wisdom Teeth) seventeenth to fortieth year.

The teeth o( vertebrates are homologues of the placoid scales of elasmobranch fishes (sharks and skates). The teeth of the shark resemble enlarged scales, and many generations of ici-th are produced in the adult fish. In some mammaUan embryos three or even four dentitions are present. The primitive teeth of mammals are of the canine type, and from this conical tooth the incisors and molars have been differentiated.

Anomalies. — Dental anomalies are frequent and may consist in the congenital absence of some or all of the teeth, or in the production of more than the normal number. Defective teeth are frequently associated with hare lip. Cases have been noted in which, owing to defect of the enamel organ, the enamel was entirely wanting. Many cases in which a third dentition occurred have been recorded and occasionally fourth molars may be developed behind the wisdom teeth.