Paper - The Development of the Nose and of the Pharynx and its Derivatives in Man
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Sudler MT. The development of the nose and of the pharynx and its derivatives in man. (1902) Amer. J Anat. 1:391–416.
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The Development of the Nose and of the Pharynx and its Derivatives in Man
Mervin T. Sudler, Ph.D., M. D.
From the Anatomical Laboratory of the Johns Hopkins University.
(1902) With 13 Text figures.
The presence of visceral folds homologous with the branchial arches of the fishes and lower vertebrates, the progressively changing circulatory system, the origin of the vertebrate mouth and its relation to the head and brain have combined to make the study of this region one of the most interesting and important divisions of mammalian embryology. In fact a knowledge of the development of this region not only aids us in understanding the adult anatomy but has been of the greatest service in giving us facts upon which to base definite ideas in regard to the evolution of these structures. There is scarcely a series of phenomena anywhere in the whole science of embryology that better illustrates the idea that “ontogeny repeats phylogeny” than the development of this region in the mammalia.
Most of the work done on this region has dealt with the visceral folds, the circulation and the origin of the thyroid and thymus glands, and the exact form of the pharyngeal cavities have had comparatively little attention, especially in man. The work of His is especially valuable. Hammer has also studied this region in man. He reconstructed the cavities and his work on the fate of the first branchial pocket and the origin of the Eustachian tube is complete and exhaustive. Piersol has modeled the pharyngeal clefts in a complete series of rabbit embryos.
It seemed desirable to have a knowledge of the exact form of the cavity of the human mouth and nose in the early stages of development, and hence at the suggestion of Dr. Mall I have made a series of models of the cavities of the mouth and nose of the valuable series of human embryos contained in his collection. The wax plate method of Born was used throughout the work. The embryos used represent the first seven weeks of foetal development. The magnifications were varied to suit the thickness and size of the sections of the different embryos used. In all the figures the cavities are represented as solid models and hence it is necessary to bear in mind that one is looking at a negative picture and not a positive one. The reverse is true, however, for the thyroid, thymus and salivary glands as these are solid objects and are so represented by the solid models. In nearly all of the models the mucous membrane was included. In the model of embryo XII and in one of the two made of embryo CLXIII only the cavity was modeled. For the model of embryo XII I am indebted to Dr. Mall. In making the illustrations to this paper an effort has been made to preserve enough of the embryo to make the relations of the model clear. At the same time details have‘~been omitted in order to avoid making the whole appear too complicated and obscure the most important structures. For pictures of the entire embryos the reader is referred to the beautiful illustrations of Bardeen and Lewis, where embryos CLXIII, CIX, and XXII are figured. These authors also refer to the articles previously written describing the embryos used for this paper. The following table will show the comparative size and age of the embryos used for these models:
This embryo is 2.1 mm. long and about 13 days old ; and hence it represents a late two weeks’ or early three weeks’ embryo. The entire embryo has been modeled by Dr. Mall and I have used his model and the figure of it. In this embryo the pharynx is still in a very simple condition. The stomodaeum has not yet broken through to form the mouth, the endodermal cavity shows Seesel’s pocket and diverticula corresponding to the 1st and 2nd visceral pouches. The general structures and relations can be seen in fig. 1. To describe the modelin detail from points of View not shown in the figure: Seen from above it presents a ﬂattened slightly curved plate; the oral end resembling an ordinary cloverleaf in outline. The middle leaf represents Seesel’s pocket, while the two lateral prominences correspond to the first pair of visceral pouches. Directly behind these are two smaller lateral swellings which fit into the 2nd pair of visceral pouches. These are not exactly symmetrical as the right is caudal to the left one. Behind these distinct prominences two pairs of waves can be discovered in the outline of the edge of this flattened surface. These are the first traces of the developing 3rd and 4th visceral pouches. In the figure they have been almost completely lost. This surface is curved convexly between the prominences of the 2nd pair of visceral pouches and gradually ﬂattens oralwards and caudalwards. Viewed from the side (fig. 1) the same structures as noted from above, as well as some others on the caudal side of the plate, are visible. Directly under the prominence representing Seesel’s pocket there is another of similar shape, but smaller, for the mouth. Just in front (ventral) of this the stomodaeum appears as a decided pouch or pit.
Fig. 1. Lateral View of the model of embryo XII (after Mall). Br.’ and Br./’, ﬂrst and second visceral pouches; M, month just behind the stomodaeum; L, rudiment of the liver; T, median thyroid rudiment.
On the under side of the model, on each side a ridge runs toward the middle line from the first pair of visceral pouches. At their point of junction in the middle line the median thyroid rudiment shows as a rounded eminence. This rudiment looked at directly from below is broader laterally than it is dorsoventrally.
The lack of bilateral symmetry of the pharynx and of the oesophagus is very marked. The position of the 2nd pair of visceral pouches has already been mentioned; in addition the left side of the (esophagus is thicker and heavier. At its caudal end it curves ventrally and ends in an eminence representing the rudiment of the liver. On the right side the oesophagus comes to a rounded edge. A cross-section would be wedge-shaped with the base of the wedge toward the left and the apex toward the right.
This embryo has a vertex-breach length of 6 mm. and a neck-breach length of 7 mm. It is about 24 days old or, in round numbers, it is an embryo of the 4th week. The model of this region shows that marked changes occur in the period intervening between this and the preceding stage. The various regions are easily recognized. Seen from above this model is nearly an isosceles triangle, The base represents the mouth and hypophysis and the rounded apex the dorsal wall of the oesophagus. The sides of the triangle show three rounded swellings (see fig. 3) corresponding to the 1st, 2nd and 3rd visceral pouches. The last named is the least prominent from this View since it is on a lower level and partly hidden by the projection of the 2nd. At this stage the 4th is well developed but much lower (caudal) and hidden from view by the 3rd.
Seen from the side, as in fig. 2, the model bends almost at right angles. At the bend there is a rounded prominence extending dorsally. Opposite to this prominence the 3rd and 4th visceral pouches project at right angles to the line of the oesophagus and trachea. The 2nd projects downward and is curved oralward. The 1st visceral pouch shows on ‘its cephalic angle a greater degree of prominence than the other part of it. This is the first indication of the outgrowth of this part of the pouch to form the Eustachian tube. In this view there is much to suggest the model of the rabbit embryo shown by Piersol° in fig. 8 of his paper.
Fig. 2. Lateral view of the model of the pharynx of the model of embryo II. The embryo is represented as transparent. Magnified 25 diameters. Hyp., hypophysis; 0e., oesophagus; Tr., trachea, V. PJ, V. P.’/, V. P!” and V. P.", first, second, third and fourth visceral pouches.
From below the general shape is the same as when seen from above, but the picture is very much complicated by the projection of the visceral pouches, median, thyroid, rudiment, etc.
From the base of the prominence representing the 1st pouch a ridge runs backward on each side and terminates in the middle line. From the dorsal edge of this angle the median thyroid rudiment arises. It is still connected with the endoderm by a solid stalk (fig. 3). Directly in front of this angle there is a rounded depression corresponding to the tuberculum impar. The walls of this depression unite in the middle line and form a ridge running oralwards in the middle line. On either side of this the depressions correspond to the mandibular processes.
Fig. 3. Ventral view of the model of the pharynx of embryo II. Hyp, hypophysis, Oe., oesophagus ; T. i., depression of the tuberculum impar; Thyr. m., median thyroid rudiment; Tr., trachea; Tr., trachea; V. P.I, V. P.II, V. P.IIIand V. P.IV, ﬂrst, second, third and fourth visceral pouches.
Directly back of the ridges connecting the first visceral pouches there are two deep rounded grooves; of which the two prominences of the 2nd visceral pouches form the dorsal boundaries. The model gradually narrows toward the oesophagus, and the two successive ﬂat ridges, following one another and almost at right angles to the rest, represent the 3rd and 4th visceral pouches. The latter pair is not bilaterally symmetrical as the one on the right side is aboral to the one on the left side. This is not so decided in this embryo as in embryo XII just described. The left is also decidedly larger than the right one, and both show a tendency to become double at their extremities. This does not seem true in the next stage, represented by embryo CLXIII, although it is distinctly true for later stages. The disappearance and reappearance of this condition is not understood and in the absence of sufficient material it is suggested that there may be an individual variation in embryo CLXIII.
in this embryo as in embryo XII just described. The left is also decidedly larger than the right one, and both show a tendency to become double at their extremities. This does not seem true in the next stage, represented by embryo CLXIII, although it is distinctly true for later stages. The disappearance and reappearance of this condition is not understood and in the absence of sufficient material it is suggested that there may be an individual variation in embryo CLXIII.
In relation to the fate of the 2nd visceral pouch, His first stated that it formed the fossa of Rosenmiiller and the tonsil. This is the description that is also given in text-books. Kastschenko believes the fossa of Rosenmiiller arises from a furrow between the inner border of the 2nd and 3rd arches. His,‘ in a recent article, agrees that it does not come from the 2nd visceral pouch.
In the middle line between the prominences of the 4th visceral pouches there is a thin ﬂat projection, the beginning of the larynx.
In this embryo the mouth is a transverse slit with only the hypophysis to break the lines. In none of my models does it appear as a five—sided object usually described since His first mentioned it as such in human embryos of the 3rd week. Hammar also disagrees with His and finds it always as a transverse slit.
Fig. 4. Lateral View of the model of the pharynx of embryo CLXIII. Magnified 15 diameters. Hyp., hypophysis; N., outline of the side of the nasal depression; 0e., (esophagus; Thyr. m., median thyroid rudiment; Tr., trachea; V. P.’, V.P.”, V. P./” and V. P.", ﬂrst, second, third and fourth visceral pouches; A. B., line of section seen, fig. 6.
This embryo has a vertex-breach of 9 mm. and a neck-breach 9 mm. in length. It is about 30 days old or an embryo of the early 5th week of development. Two models were made of the pharyngeal cavity of this embryo. The first represents the cavity simply, while the second one includes also the mucous membranes lining it. The description will apply to the model of the cavity alone and the second model will be
mentioned only where it is markedly different. The same structures are easily recognized here that were prominent in the embryo just described. However, the shape, size and relative proportions have changed markedly. Seen from the side (fig. 4) another sharp bend has appeared at the oral end in addition to that at the aboral end already noted in embryo II. The part below this ventral (oral) end corresponds more nearly to the adult buccal cavity, especially the roof of it, than any structures that we have yet encountered. Just below this angle the hypophysis joins the mouth. From this angle the model runs dorsally in almost a straight line. This line is broken by the 1st visceral pouch and the flattened process of the 2nd. Then there is another sharp turn caudalward with a slight dorsal prominence at the angle already mentioned in the description of embryo II. The line then bends slightly ventrally and shows the prominences of the 3rd and 4th visceral pouches.
Fig. 5. Ventral view of the model of the pharynx of embryo CLXIII. D. Th. g1., ductus thyreoglossus; Hyp., hypophysis; 0e., (esophagus; Tr., trachea; V. P.’ V. P.’’, V. P./” and V. PJV, ﬂrst, second, third and fourth visceral folds.
Seen from above the model is roughly square. The hypophysis being at the ventral angle, the curved dorsal wall of the pharynx at the dorsal end and the two prominences of the 1st pair of visceral pouches at the angles of the square towards the sides of the embryo. Just behind these there is a deep rounded fossa and then a decided process projecting outward and backward, representing the 2nd pair of visceral pouches. The 1st pair of visceral pouches are shown as prominent ridges running outward and backward. In the model of the cavity only, they project much farther free than in the one in which the model includes the mucous membrane. A para-sagittal section of this region of the embryo would show a relatively large opening, then a narrow one running dorsally and connecting with the cavity of the 2nd visceral pouch. This cavity (shown as a ridge on the model), representing the oral part of the 1st visceral pouch was called by Moldenhaur the sulcus tubo-tympanicus. Hammar uses the same term to mean the oral lengthening and development of the 1st visceral pouches as first used by Moldenhaur. Compare this structure in figs. 2, 3, 4 and 5. Seen directly from behind (dorsally) all four pairs of visceral pouches are visible. The 1st pair running slightly upwards (cephalic). The 2nd pair show as two ridges ﬂattened dorsoventrally and projecting downward. The 3rd pair stand out at right angles to the oesophagus and the cells have already begun to proliferate around it as a comparison of the two models will show. In the one in which the endoderm is included this pair of pouches is very much larger and exhibits processes which are not suggested by the model, showing only the cavity. The 4th pair are peculiarly shaped structures prodectmg at mght angles to superior maxillary process the oesophagus and then bending sharply dorsally on themselves. Their ends are enlarged and their outline ls visceral folds. Section on a line indinearly triangular. They are relatively much larger in the model that includes the mucous membrane than in the one of the cavity only. In this model, however, they do not show a tendency to divide into two prominences as in embryo II or in others to be described later. Just beneath the 4th visceral pouch the whole pharynx constricts notably and marks off sharply the beginning of the oesophagus. See fig. 4. A section of the pharynx taken at any point between the dorsal angle and the prominence of the 4th visceral pouch is crescentic in outline with a part projecting ventrally as shown in fig. 5. Seen from the inside (caudal surface) the model shows the two ridges running from the prominences of the 1st visceral pocket toward the middle line. They are not so pronounced in the model of embryo II and are less prominent in the model of this embryo (CLXIII) that includes the mucous membrane than in the one where only the cavity is modeled. At the point of junction of these ridges in the middle line a small cylindrical projection marks the remnant of the thyreoglossal duct. The median thyroid rudiment has separated completely and sunken to the level of the 3rd visceral cleft. In front of these ridges the tuberculum impar has made a large depression. The median ridge seen in the model of embryo II has disappeared. Behind these ridges are two depressions showing the position of the two dorsal tongue rudiments. Behind (dorsalward) is still another depression situated in the median line the significance of which is not clear. Two ridges run caudalward and unite in the middle line to form a ﬂattened body. This is the rudiment of the larynx.
Fig. 6. View of reconstruction of embryo CLXIII from a ventral-caudal view. B. v., blood vessel; CIL, notochord L. rudiment of larynx; Max. tossa; N. x., vagusnerve; Ph., pharynx;; N., nasal
This embryo has a neck-breach length of 10.5 mm. and a vertex-breach of 11 mm. It is about 33 days old and so would be an embryo of the latter part of the fifth week. In spite of the apparently short time between this stage and the one just -described very decided changes have occurred. The model includes the mucous membrane and is relatively large. In this model the angles are not so sharp and the various structures show a tendency to become rounded. Seen from the side as in fig. 7 the same prominent bends are recognized but they are not so pronounced as in the model of embryo CLXIII. The ventral bend is larger and is joined to the nasal cavity as the illustration shows. The hypophysis is relatively higher and nearer the angle of the bend. The nasal cavities join the oral cavity at its edge and the posterior nares are in a position similar to the permanent condition in the frog. The prominence of the 1st visceral pocket is more complicated and the ridge running in from it toward the median line noted in the other two models is somewhat broken and irregular. The 2nd visceral pouch shows as a small knob projecting caudalward. The 3rd visceral pouch has disappeared as such and the 4th is prolonged into a hollow tube with two knobs on its caudal end. These will be described later as the lateral rudiments of the thyroid. At the point where they are attached the pharynx abruptly constricts into the oesophagus. The oesophagus is shown as a rounded body and the cavity does not exhibit the typical Maltese cross shape as described by Minot.
Fig. 7. Lateral view of the model of the pharynx of embryo CIX. Magniﬁed 15 diameters. The embryo is represented as transparent. Hyp., hypophysis; L., rudiment of larynx; N., nasal cavity; Oe., oesophagus; Thym., thymus; Thyr. m., median thyroid rudiment; Tr., trachea; V. P.I, V. P.II, V. P.IIIand V. P.IV, ﬂrst, second, third and fourth visceral pouches.
The trachea can be seen between the oesophagus and the 4th visceral pouch. At this level a section of it resembles a rounded bean With the hilum facing dorsally. Seen from above (cephalic) the model resembles the one just preceding in general outline except that the anglesare decidedly rounded. The 1st visceral pouch appears as a prominentridge (salcus tubo—tympanicus) running outwardly and dorsally andending in a rounded free prominence. The rounded fossa just backof it in embryo CLXIII has become here a ﬂattened surface. Seenfrom the inside (caudal surface) there is a large central rounded cavityoccupied by the tongue. A prominent U—shaped ridge separates itfrom a similar shaped groove, marking the position of the mandible.At the dorsal end of the tongue cavity a small conical projection marksthe position of the thyreoglossal duct. Directly dorsal from that thelarynx arises and the cavity shows as a T-shaped body. The top ofthe T facing ventrally. Caudalward the shape of the cavity changesgradually until the trachea is reached, when it has assumed the shapealready described. Seen from the front (ventrally) the nasal cavitiesproject outward from a large elevated fold in the middle line. Theyproject away from the middle line at an angle of about 20°.
This embryo has a length of neck—breach 13 mm., vertex—breach 13 mm. It is an embryo of the early part of the 6th week. Thismodel also includes the mucous membrane and with a few interestingexceptions very much resembles the one just described. Looked at from the side the 1st visceral pouch has become somewhat complicated and altered. The most noticeable change being a prominent fold run- ning from the 1st visceral pouch ventrally. The most striking difference noted from the side view is the lack of symmetry of the two sides. On the right side the hollow stalk connecting the lateral thyroid rudiment to the pharynx still persists. On the left side it has separated completely and the enlarged double end is fused with the now enlarged horseshoe-shaped median thyroid rudiment. On the right side the enlarged end of the lateral rudiment is in contact with the median rudiment but has not fused with it. Seen from above, this model differs from that of CIX chiefly in the increased rounding of the angles. The 1st visceral pouch, however, has changed and the fold running ventrally is much more prominent. This stage resembles the one ﬁgured by Piersol in Fig. 13 of his article already referred to. The hypophysis has just separated, although a slight thickening of epithelial elements mark its position on the model.
Seen from below (the inside) the model presents an even rounded hollow with the horseshoe-shaped groove surrounding it. This horse-shoe-shaped depression marks the position of the mandible. The thyreoglossal duct is quite a large conical elevation. From its unequal sizes and characters in this series of models it seems to be a structure subject to considerable variation in its development. The larynx here is further developed and its cross-section is decidedly T-shaped. Just where the larynx arises there is a proliferation or overgrowth of epithelial cells. This is the beginning of a. process which eventually closes the whole larynx for a period of foetal life.
Fig. 8. Lateral view of the model of the nose and pharynx of embryo CXLIV. Magniﬁed 15 diameters. Hyp., hypophysis; N., nose ;‘ Oe., oesophagus; Sub. mx. gl., rudiment of the submaxillary gland; Thym., thymus; Thyr. l., lateral thyroid rudiment; T. p., depression caused by the inferior turblnate process; Thyr. m., median thyroid rudiment; Tr.,trachea; V. P.I, ﬂrst visceral pouch.
This embryo has a neck-breach length of 12 mm. and a vertex-breach of 14 mm. It is about 5.5 weeks old. This model also includes the mucous membrane except the large endothelial plug which has nearly closed the larynx at its point of juncture with the pharynx. The closure is characteristic of this and later stages. In this embryo it is still possible to trace the line between the two surfaces of the cells. Later even this is impossible.
Seen from the side, as in Fig. 8, this model presents a covered outline, almost U-shaped. The 1st visceral pouch (Eustachian tube) pro- jeots outward decidedly. It is elongated and thin rather than tubular. On its under side there is a ridge near the end and a complication of its outer end by a slight depression. What remains of the 4th visceral pouch is completely cut off from the cavity of the pharynx, but the large lateral thyroid rudiments are still easily recognized as such, although both have now fused with the median rudiment.
Fig. 9. Lateral view of the model of the nose and pharynx of embryo XLIII. Magniﬁed 15 diameters. I-Iyp., hypophysis; N.,. nasal cavity; Sub. mx. gl., rudiment of submaxillary gland; Thym., thymus; Thyr., thyroid gland; T. p.’, depression caused by the inferior turbinate process; T. p.”, depression ofthe middle turbinate process; V. P.. ﬂrst visceral pouch.
Seen from above there is still a slight elevation where the hypophysis joined the mouth. The large ridge running between the nasal cavities is not so prominent and the cavities themselves are relatively much nearer the middle line. From below, the cavity of the tongue is the largest feature. The larynx, as has already been stated, is plugged, but there is a large transverse projection corresponding to the arms of the mentioned in describing the preceding models.
This embryo has a neck-breach length of 14 mm. and a vertex-breach of 16 mm. It is an embryo of the 6th week. This model also includes the epithelium except in the larynx where the plug is left out in order to show its position. This model is so like the preceding one just described that there is no need of a detailed description. The differences most striking in a side View of the model, as seen in Fig. 9, are the complete closure of the larynx by the epithelial plug; the lowered position of the thyroid and thymus rudiments; and the appearance of the rudiment of the submaxillary glands. The 1st Visceral pouch (Eustachian tube) is bent upwards and the ridge running inward is still recognizable. In this view the nasal cavity is represented by an angular body with a prominent horseshoe—shaped depression which was very noticeable in the model of embryo CXLIV.
Fig. 10. Lateral view of the model of the mouth and nose of embryo XXII. Magnified 10 diameters. N., nasal cavity; Oe., oesophagus; Sub. mx. gl., rudiment of the submaxillary gland; Thym., thymus; Tr., trachea; Thyr., thyroid; T. p.', inferior turbinate process; T. p.", middle turbinate process.
This embryo has a neck—breach length of 18 mm. and a vertex—breach 20 mm. It is an embryo of the 7th Week. Seen from the side the nose is joined to the pharynx further dorsally and is relatively decidedly larger. The grooves representing the prominenees of the jaws are more distinct and the space between them shows as a prominent flattened and slightly curved body (Fig. 10). The Eustachian tube is more prominent and although still very much ﬂattened has assumed proportions nearer those of the adult. It is relatively more cephalic than in the other models. The thyroid and thymus rudiments are relatively further caudalwards. The epithelial plug of the larynx has disappeared and left the opening free. The rudiment of the sub-maxillary gland (Sub. max. gl., Fig. 10) is lobulated and more compli- cated than in the model just described. Seen from above the main body of the model is oval with the larger end ventralwards. The Eustachian tubes project laterally and dorsally. Ventrally the nasal cavities have approached one another and, instead of slanting away from one another, tend to approach as they leave the main body of the model. Seen from below the model shows a large rounded cavity for the tongue with a notch in the ventral border for the fraanum. All traces of the thyreoglossal duct as a free opening has disappeared. Taken as a whole, this model is decidedly more rounded and the proportions are nearer those of the adult than in any of the embryos studied.
The Nose and Mouth
In His’ article on the formation of the nose he introduces the subject by sayingthat the mouth and nose come originally from four separate sources, viz.:—the two nasal pits, the stomodaeum and the pharynx (Kopfdarm). In regard to the nose he says the nasal pits develop as separate pouches above the stomodaeum and the story of their development is the story of the development of the frontal process.
In the model of embryo XII (a late two weeks’ embryo) two of these structures are present, the stomodaeum and pharynx, while the two nasal pits have as yet not made their appearance. In an embryo of the 4th week the stomodaeum and pharynx have united and the nose is represented by two olfactory plates barely outlined in Fig. 2.
It is only when an embryo of about 4% weeks old (embryo CLXIII) is studied that the olfactory organ has taken on the form of pits. Here they are two bean-shaped cavities (see Figs. 4 and 6). The caudal end is enlarged and a shallow groove, runs towards the mouth cavity. It is not yet connected with the mouth cavity so it is not shown in Fig. 4 which is drawn from the side. The ridge overlying it, however, is shown faintly outlined. At its outer angle the superior maxillary process stands out as a rounded eminence. Fig. 6 shows the frontal process and an idea can be gotten of the comparative distance of the olfactory pits from the middle line in an embryo of this stage. In an older stage (5 weeks), represented by the model of embryo CIX, the four parts mentioned by His as forming the nose and mouth have united. In Fig. 7, which shows this embryo from the side, the nasal cavity is represented by two ﬂattened processes attached to the buccal cavity by two slender stalks. These meet the mouth at an angle; that is, they diverge slightly as they leave the mouth cavity. These two ﬂattened surfaces bear suggestions of ridges which are developed in the later stages, the most prominent being a cephalic dorsal one running backward toward the mouth opening. Seen from the inside there is a decided prominence at the angle where the pedicle and main nasal cavity join one another. This is the beginning of Jacobson’s organ.
In an embryo slightly older, such as we have in the model of embryo CVI (this model is not ﬁgured), the plates seen from the side are no longer ﬂat and the ridges shown in embryo CIX have become developed to so much greater extent that they can be distinctly recognized. These probably mark the ﬁrst divisions into ridges and furrows that mark the rudiments of the turbinate processes. On the inner side the surface is also ﬂattened and there is a small rounded knob marking the position of J acobson’s organ as mentioned for embryo OIX. Considering this part of the nasal cavity as a whole, the model shows it to be decidedly three sided. A cross—section of it would represent a right angled triangle. The right angle being at the junction of the outer surface with the lower (caudal) surface. The inner surface slopes downward towards the middle line and completes the outline of the triangle. The anterior nares run up to join this structure as a rounded body and meet it at right angles to the remainder of the structure. The posterior nares are a little larger than the anterior and show on their outer dorsal surfaces ridges continuous with the ridges on the surface of the nasal cavity. The entire model looked at from above shows that there is no angle of divergence between the models of the nasal cavities. They meet the mouth at almost right angles. In an embryo of the 6th week (about 57} weeks old), such as represented by the model of embryo CXLIV, the nose seen from the side, as in Fig. 8, is a rather irregular six—sided ﬁgure in outline. The pedicle is relatively larger and the groove running down on its outer side divides it into a .horseshoe—shaped body. The triangular-shaped outline previously mentioned has been almost entirely lost, The anterior nares meet the main opening of the olfactory cavity, not at a right angle but almost in a straight line. In the model of embryo XLIII of the 6th week, but still older than the preceding .embryo, the nasal cavity is relatively larger and occupies more of the model. The same general shape is preserved, however, as can be seen by comparing Fig. 8 and Fig. 9. The posterior nares are wider and are closer to the Eustachian tube. The same horseshoe-shaped folds are also prominent. The angle at which the nasal processes meet the mouth is here reversed and instead of diverging they approach one another as they leave the mouth cavity. In the nose of the oldest embryo modeled, that of a 7 weeks’ embryo (embryo XXII), the nasal cavity is relatively much increased in size and is not so angular. The relative size of the posterior nares has increased constantly in relation to the size of the anterior nares. This enlargement seems to be a progressive growth in a cephalic and dorsal direction, and the result is to bring the rudiment of the Eustachian tube and the nose closer and closer as development proceeds. The folds on the lateral surface (see Fig. 10) have changed somewhat, but the tendency toward the horseshoe shape described in the other models is still recognizable here. From the inside, Jacobson’s organ appears as a small conical elevation looking dorsally, and it is located near the caudal angle of the juncture of the pedicle and the large nasal cavity. In this model the cavities are relatively nearer to one another than in any of the models described, and seen from above they tend to approach one another slightly as they leave the buccal cavity.
In reference to the time of appearance of the rudiments of the various salivary glands there is a difference of opinion among the different authors. Some of this discrepancy is no doubt due to uncertainty in regard to the exact age of the different embryos studied by different observers. Chievitz states that the submaxillary appears in an embryo of a neck-breach length of 14: mm. (six weeks). Hammar “ found it in an embryo 13.2 mm. long and His ” in one 13.8 mm. long. In regard to the parotid Hammar states that he has found it at the end of the ﬁrst month and that it does not appear as a solid rounded rod as generally described but as a groove which eventually closes off and forms the duct of the gland. His states that it appears at 7-} weeks and Chievitz found it -at the end of the 8th week. Hammar found the sublingual at 9 weeks and Chievitz states that he observed it a little before the rudiment of the parotid appeared, which would mean early in the 8th week.
In this series of models the submaxillary gland ﬁrst appears in that of embryo OXLIV, where it is simply a rounded rod of cells staining deeper than the surrounding tissues. This embryo has a neck-breach length of 14 mm. In the model of embryo XLIII it is quite a large rudiment, as seen in Fig. 9. It is a straight cylindrical shaft with a knob turned directly away from the middle line. It meets the mouth cavity at the angle where the tongue and the ridge separating it from the groove of the mandible meet. It is a solid object throughout. In the model of the oldest embryo studied, embryo XXII, it has grown larger and the bulbus end has grown into an oval-shaped bodv covered with rounded enlargements marking the future lobes of the gland. It is still solid and the connective tissue around it shows a slight condensation into a capsule. No other salivary gland was observed at this stage.
Stieda in 1881 was the ﬁrst to observe that the thymus gland originated from a visceral pouch. Then came Born’s important contribution stating that it arises from entoderm. This has since been conﬁrmed by Prenant, Mall and His.
Beard worked on a complete series of Raja embryos Where the thymus develops from the ﬁrst four branchial clefts, while a rudimentary thymus makes its appearance on the fifth, In this case the rudiments arise just at the junction between the “ epiblast and hypoblast,” and he suggests that some cells of both layers are included in the rudiments and that the two kinds have different fates; that in the adult the corpuscles of Hassal represent the remains of the epiblastic cells, while the lymphoid tissue is the transformation hypoblastic cells. From its point of origin he compares its function to that of the tonsil and thinks that it is an organ developed to guard the gills and pharyngeal region. Capobianco believes the lymphoid tissues are cells which have immigrated into the primary rudiment and that the lymphatic structure of the thymus was acquired secondarily. Kastchenko, who worked on pig embryos, described the various details of the development of the thymus with great care. He thought the greater part of it came from the epithelium of the 3rd visceral pouch but that part also came from the sinus precervicalis.
Harman describes the condition found in two babies at term where there were a soeia thymi cervicalis and a thymus aecessorius present, He refers to the condition in the sheep, where he says the 3rd and 4th Visceral pouches participate in the formation of the thymus, and suggests that where there is an accessory thymus in man there may have been a reversion and the 4th pouch had given rise to it. He quotes Sir Astley Cooper as saying that he has frequently observed that the cervical portion of the thymus is higher on the right than on the left side.
In the present study the model of embryo II shows the 3rd visceral pouch as a ridge with a ventral free end (V. P.”’, Fig. 2), but no differentiation of tissue to suggest a thymus. In the model of embryo CLXIII the prominent ridge has disappeared and the 3rd visceral pouch projects out directly from the pharynx. This pouch has a slightly enlarged end (V. P.” in Figs. 4 and 5). In the next stage, in an embryo of about 4; weeks of age, the thymus (Thym., Fig. 7) approaches very closely the condition ﬁgured by Born, on page 297 (ﬁg. d.), in which the thymus has completely lost its connection with the pharynx, and its original hollow is closed to a erescent—shaped opening that is quite characteristic and aids greatly in identifying the gland at this stage. It is a curved, elongated mass with an enlarged superior cephalic end. The lower and smaller end runs parallel with the thyroid rudiment until that bends abruptly and crosses the middle line. The two structures do not come into contact at any point, however.
In embryo CLXXV, which is only slightly older, practically the same condition prevails as in the ‘model just described. The differences are that the enlarged head is relatively smaller and the general curve of the whole rudiment is not so pronounced, as shown in Fig. 12. The small process that projects dorsally and laterally is still seen.
In the model of embryo CXLIV of the sixth week the relative position of the thymus rudiment is lower and the division into different areas and parts has disappeared. The head“ is barely recognizable as a slight enlargement. They have approached the middle line at the aboral end, although they still do not meet. In the model of embryo XLIII the thymus is still lower so that now the thyroid rudiment is above even the head of the thymus. The whole rudiment projects beyond the thyroid and is nearer the middle line. In the model of the oldest embryo studied, that of an embryo of the 7th week (embryo XXII), the thymus rudiment has sunken relatively lower. It.is in contact with the thyroid rudiment along half of its upper surface and the other end is free. In this lower free half the two rudiments approach one another and meet in the middle line where the ends are slightly swollen and bend ventrally. Apparently this is a beginning of the folding of the thymus rudiment found in the adult organ. In this model the thymus rudiment on the right side extends a little higher than on the left side. The same condition is true in that of embryo CXLIV. This will be referred to later in the discussion of the lack of bilateral symmetry in the development of the thyroid.
The thyroid gland has proven of great interest to morphologists on account of its origin and change of position and function. In the tunicates it secretes an adhesive material used in the capture of the food of the animal. In the higher mammals it has sunken to a lower functional plane and its function has proven to be one of the puzzles of the physiologist Remak discovered that it was of entodermal origin. Stieda and Wolﬂer discovered the lateral rudiments, independently, at the same time. Its origin is ably dealt with in Born’s  article. I give his exact words below because they have served as a basis for our knowledge of the subject since they were written.
In this series of models the thyroid rudiment appears in the earliest one studied (embryo XII at the end of the 2nd week). In this embryo it is represented as a rounded eminence directly on the ridges uniting the ﬁrst pair of visceral pouches. It is broader from side to side than it is ventro—dorsally, making its outline elliptical. This is the rudiment of the thyroid gland and the ductus thyreoglossus.
In the model of the fourth week embryo (Thyr. m., Fig. 3, embryo II), it is a solid structure arising from the dorsal part of the apex of the angle formed by the junction of the ridges running inward from the 1st pair of visceral pouches. It is just oral to the hollow space representing the inner of the 2nd visceral arches. Directly oral to it is a hollow space representing the tuberculum impar. The thyroid rudiment consists of a slender stalk surmounted by a knob facing aborally (dorsally also in this embryo). Its shape might be compared to a pipe with a short stem and the bowl facing the back. The 4th visceral pouch (V. P.", Fig. 2) is a ﬂattened object with the part attaching it to the pharynx slightly smaller than the distal end. It shows no thickening or development into the lateral thyroid rudiment.
In the next stage, as seen in embryo CLXIII, the thyreoglossal duct remains as a small conical eminence on the ridge connecting the ﬁrst visceral pouches (see D. Th. gl., Fig. 5). It is at the junction of the tuberculum impar and the two dorsal rudiments of the tongue. The median thyroid rudiment has become entirely disconnected and sunken to the level of the 3rd visceral ‘fold. It has spread out laterally and has a bilobed structure. The left lobe is decidedly the longer and approaches closer to the floor of the mouth than the right. The 4th visceral pouches (V. P.,", Figs. 4 and 5) appear as two ventral projections with enlarged knobs bent sharply dorsally. In this embryo they show no tendency to become bilobed, as is seen in embryo II and in later stages. It has already been suggested that this may represent an individual variation.
In the model of embryo CIX the median thyroid rudiment is U- shaped with a more or less irregular outline. The right arm, however, is decidedly shorter than the left. The transverse part of the thyroid that runs across the middle line is on the level with the 4th visceral pouch. This part shows the usual network structure formed by cords of solid cells. The arms of the U are crescentic in cross-section with the hollow looking away from the middle line and embracing the thymus rudiment, although the two do not come in contact. The lateral rudiments are still connected with the pharynx. They consist of hollow tubes surmounted by solid expansions. These are unlike on the two sides. On the right side the ventral knob is smaller and less prominent than the dorsal, while on the left they are about equal in size and placed on the same level. This lack of bilateral symmetry seems an unimportant thing to note here, but that it is not simply an individual variation is shown by the fact that it occurs in the next stages.
Fig. 11. Ventral view of the dorsal portion of the model of embryo CIX. D. Th. gl., ductus thyreoglossus. Oe., oesophagus; Thym., thymus; Thyr. m., median thyroid rudiment.
In the model of embryo CLXXV the median U—shaped rudiment has sunken a little lower than in the preceding stage. The right limb is still smaller and not as long as the one on the left side. The lateral rudiments show a most interesting lack of bilateral symmetry (see Thyr. l., Fig. 12). On the right side the rudiment is still connected with the pharynx by a small hollow stalk. The ventral knob lies in contact with the middle of the right limb of the median rudiment but there is no histological continuity. On the left side, on the contrary, the connection between the rudiment and the pharynx has disappeared. At a point corresponding to the stalk on the left side there is a short duct pointing toward a similar short duct on the lateral thyroid rudiment. The Ventral knob of this rudiment is not only in Contact with the median thyroid rudiment at a point corresponding to the one on the right, but a microscopical examination shows that there is actual continuity of the two structures. As in the embryo described just before this, the lateral thyroid rudiments are unlike on the two sides. On the right side the ventral knob is much smaller than the dorsal one, while on the left side it is the same size or a little larger. In both of these models the median thyroid rudiment shows a dorsal enlargement at the right-hand angle of the U.
In the next stage, represented by embryo CXLIV, the median and lateral thyroid rudiments have united on both sides and all connection with the pharynx has been lost. They can still be recognised however (Thyr. 1., Fig. 8), as distinct rudiments. Under the microscope, however, there is structural continuity of the two parts.
FIG. 12. Dorsal View of the dorsal portion of embryo CLXXV, showing the lack of symmetry of the lateral thyroid rudiments. 0e.,oesophagus; Thym.. thymus; Thyr. m., median thyroid rudiment; Thyr. 1., lateral thyroid rudiment.
In a later stage, in embryo XLIII, the structures have begun to assume more nearly their adult relations to one another. The lateral rudiments are a part of the median, and a few prominent lobules at the point of their union is all that is left to suggest their existence. The loop of the U is smaller while the limbs have enlarged and show lobulated structures suggestive of the condition in the adult gland.
In embryo XXII (7th week) the shape of the thyroid is almost that of the adult, the small isthmus connecting the two lateral lobes. The two lateral lobes are pyriform in shape and attached to the isthmus at the smaller end.
Mall  has noted that in the chick and dog the branchial arches appear sooner on the left side than on the right and suggests that the development of the heart and bending of the head to the left is the cause of it. Attention has been already called to a similar condition in human embryos in this article, and it seems well to sum up the differences which seem distinctive of the two sides at the different stages.
The statement of Sir Astley Cooper in regard to the thymus usually being higher on the right side than on the left has been referred to before. This same condition has shown itself in these models. The thyroid, however, is the most marked example. In all of the models old enough to show a division of the median rudiment into the two lobes, the right one has been the shorter and the left one the longer, except in the embryo of the 7th week where the two were about equal in this respect. The lateral thyroid rudiments are unlike and their differences are most striking in embryo CLXXV where the right is still attached to theppharynx and the left one has lost its connection. The relative positions and differences of the visceral pouches of embryos XII and II have already been explained.
FIG~13- Ventral View of the dorsal part of the model of embryo XXII. L, rudiment of larynx; 0e, oesophagus;Thym., thyymus; Thyr-, thyroid; Tr, trachea.
Taking these differences together they show that in the human embryo the left side of the pharynx develops slightly more rapidly than the right side in the first few weeks of development. In the oldest embryo (embryo XXII of the 7th week) these differences seem to have disappeared and the two sides are symmetrical or very nearly so.
The shape of the human pharynx changes from a more or less rounded cavity and one without many distinguishing characters, at the end of the 2nd week, to an angular, much differentiated cavity, during the 4th week. After that time it gradually loses its angularity, until it is a curved and well rounded cavity in the embryo of the 7th week.
The angle at the aboral end of the pharynx gradually shifts its position from a point opposite the 3rd visceral pouch in an embryo of the 3rd week, where it is most pronounced, until it is just oral to the 2nd visceral arch. As an angle it h.as completely disappeared in an embryo of the early part of the 6th week.
The Eustachian tube is the result of an extension of the cephalic angle and ridge of the 1st visceral pouch and the narrowing of the buccal cavity.
The sulcus tubo-tympanicus of Moldenhauer is a cephalic extension of the groove running toward the middle line from the 1st pharyngeal pouch. A part of it is converted into the mesial portion of the Eustachian tube by the conversion of the sulcus into a tube through the gradual narrowing of the buccal cavity.
The relative position of the posterior nares changes by gradually moving backward and becoming larger. The posterior nares and the Eustachian tube in the embryos of the ﬁrst few weeks are widely separated but this shifting of the position of the posterior nares brings them into the relative positions which they occupy in the adult.
The turbinate processes show themselves as elevations quite early in the 6th week. The inferior is the most distinct. These gradually undergo differentiation and separation, but even in the 7th week they are still much unlike the adult condition.
In man the thyroid arises from the union of a median rudiment situated at the point of junction of the tuberculum impar and the two dorsal rudiments‘ of the tongue with a paired rudiment arising as a differentiation of the lining of the -1th visceral pouch.
In man the rudiment of the thymus arises from the endothelium of the 3rd visceral pouch.
In the development of the pharynx the left side develops more rapidly than the right side, thus causing a lack of bilateral symmetry. This difference gradually decreases until by the 7th week the two sides have become about equalized. The cause of this difference is unknown but probably the bending of the head, as Mall suggests, may have some inﬂuence in determining it.
Note.—The original drawings to illustrate this article were made by Mr. George T. Kline and the reproduction of them superintended by Mr. F. S. Lockwood. I am greatly indebted to them for their care and interest in the work throughout.
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- Ang. Hemmer: Notiz fiber die Entwicklung der Zunge und Mundspeicheldriisen beim Menschen. Anat. Anz., Bd. XIX, 1901.
- Minot CS. A Laboratory Text-Book Of Embryology. (1903) Philadelphia:P. Blakiston's Son & Co.
- J. H. Chievitz: Beitriige zur Entwickelungsgeschichte der Speicheldriisen. Arch. 1. Anat-. u. Physiol., Anat. Abth., 1885.
- A. Prenant: Contribution a 1’ étude du development organique et histologie du thymus, etc. La Cel1u1e,.189-1.
- F. P. Mall: The branchial clefts of the dog with a. special reference to the origin of the thymus gland. Studies from the Biological Laboratory, Johns Hopkins Universlty, Vol. 4, No. 4, 1888.
- J. Beard: The Development and Probable Function of the Thymus. Anat. Anz., Bd IX, 1394.
- Capobianco: Contribution ala morphol. du thymus. Arch. ital. de biol., XVII, 1892.
- Kastchenko: Das Schlundspsltensystem des Huhnchens. Arch. f. Anat. 11. Phys, Anat. Abth., 1887.
- N. Bishop Harman: “Socia thymi cervicalis” and Thymus accessorius. The Journal of Anatomy and Physiology, vol. XXXVI, Pt. 1, 1901.
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- Loc. cit. Es ist also ohne Zweifel, dass der epitheliale Thei1,der Thyroides. dutch Verschmelzung zweler urspriinglich riiumlich getrennter und histologisch verschieden gebauter Bestandtheile hergestellt wird; der eine von diesen, der unpaare, waehst in der Medianlinie aus dem Epithel der Vereiniguugsstelle der zweiten Kiemenbogen aus, derselbe nimmt frijhzeitig die bekannte Netzstructur an, der andre paarige Theil wird durch zwei schlauchartige, ventrale Austiilpungen der vierten Kiemenspalten, die etwas naeh innen convergiren, dargestellt; Ietztere verlieren erst naeh der Verschmelzung mit dem medianen Antheil, der sieh allmahlich näh hinten verschiebt, ihre einfache Schlauchform und wandeln sieh in ein Netz V011 Zellbalken um.
- Mall: The brsnchiai clefts of the dog with special reference to the origin of the thymus gland. Also, Development of the Eustachian tube, middle ear, tympanic membrane and meatus in the chick. Studies from the Biological Laboratory of the Johns Hopkins University, vol. 4, No. 4, 1888.
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