Paper - The development of the human pharyngeal tonsil
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- 1 The Development of the Human Pharyngeal Tonsil
The Development of the Human Pharyngeal Tonsil
Laboratory of Histology and Embryology, Cornell Umwersity, Ithaca, New Y ark
One Text Figure And Three Plates (Thirty Figures)
The factors determining the appearance of large and small accumulations of lymphoid tissue in the tunica propria of the mucosa of the digestive and respiratory passageways have in the past received only scant attention. In more recent years, attempts have been made to ascertain What factors might be responsible for the formation of the tonsils making up the well-known Waldeyer’s ring, the Peyer’s patches of the ileum, the nodules of the vermiform process, and the numerous isolated nodules and diﬁuse lymphatic tissue scattered throughout the tracts immediately under the epithelium.
Of the older views, that of Stohr (1891) has been widely quoted. It Was his belief that accumulations of lymphocytes outside of lymphatic nodes and lymphatic and blood Vessels are agents of resorptive processes utilized in the removal of structures which have entered upon a partial or complete regression and illustrated by the collections of lymphocytes in degenerating organs, such as the pronephros of lower vertebrates, gills of Anura, thymus, vermiform process, and palatine tonsils; the last were then believed to be derived from the ‘degenerating’ second pharyngeal pouch. In this connection, Kingsley (’24) found that the true gland of the nictitating membrane of the cat and rabbit is a regressive structure and that this bears a close relation to the lymphoid tissue present.
That the regressive nature of the tonsils is the explanation of the lymphocytopoietic reaction of the mesenchyme at those sites seem improbable. In this connection, Kingsbury ( ’32 a) states:
- It is obvious, however, that as an interpretation of the intimate relationship of lymphocyte formation and epithelium such as occurs in the tonsils, the suggestion is not thoroughly adequate in itself, since it covers, in any event, but a superﬁcial aspect of the matter. The developmental factors responsible for the differentiation must lie deeper.
A second interpretation was proposed by Schaffer (1898), who believed that the formation of lymph nodules was bound causally with the presence of gland ducts, there being a positive chemotactic action involved. A chemical stimulation was again offered by Loeb (’15) as an explanation for the close relation between regressive organs (transplanted tissue) and lymphocyte production. He concluded “that the metabolic changes taking place in tissues after homoio-transplantation stimulate the activity of the lymphocytes and cause the altered function of the fibroblasts which now produce ﬁbrous tissues in great quantities.”
Kingsbury and Rogers (’27) and Kingsbury (’32 a) analyzed carefully the development of the palatine tonsil of the calf and pharyngeal tonsil of the cat, respectively. They reached the same conclusion from each investigation, viz., that, as a result of the developmental mechanics of the regions in question, areas of negative tension were formed which were particularly favorable for lymphocyte proliferation. Besides stress, another factor closely linked with this free cell proliferation was found to be the vasculogensis of the region, i.e., the production of lymphocytes is closely linked with the vascular endothelium.
The pharyngeal tonsil of the cat was selected by Kingsbury (’32 a) for analysis because of its relatively simple structure; i.e., it is not complicated as in man by the presence of longitudinal folds and the bursa pharyngea (50 per cent of the cases). It was one purpose of the present investigation to analyze the development of the human pharyngeal tonsil from the standpoint of the growth tensions involved, in order to see if it conformed to the relations found in the cat, the ultimate aim being to discover the reasons for the lymphocytic differentiation there existent. Furthermore, the need was felt for checking the statements relevant; to the embryology of that organ since numerous contradictions and misstatements are found in the literature.
Materials and Methods
Sections of 221 human embryos, ranging in size from 7 mm. C.R. length to newborn, were examined. These included 140 series and 12 dissected tonsillar regions from the Carnegie Institution, Department of Embryology, and 50 series and 19 dissected tonsillar regions from the Cornell collection. The dissected regions for the most part included the basis cranii and were ﬁrst examined under the dissecting microscope and photographed. Most of the material was formalin ﬁxed and stained in toto with carmine, Lyon’s blue being used as a counterstain in some cases. The dissected regions of two fresh fetuses were ﬁxed in Helly and stained with azure eosin.
- The work was carried out under the direction of Prof. B. F. Kingsbury, to whom I am grateful for his kind assistance. I also wish to acknowledge the generous help of Dr. G. L. Streeter, Carnegie Institution of Embryology, in placing at my disposal abundant material.
External anatomy of the tonsillar region
In human fetuses aged 3 months or over, that part of the nasal pharynx which is termed the vault, fornix, or roof is very clearly delimited from the posterior and lateral pharyngeal walls. It has been described (Poirier and Charpy, ’01) as a roughly quadrilateral area bounded rostrad by the superior border of the choanae and nasal. septum; posteriorly by a line which cuts the pharyngeal tubercle and the beginning of the posterior wall; laterally by the lateral recess of Rosenmiiller and the pharyngeal opening of the auditory tube (of Eustacheus). Since the pharyngeal tonsil (adenoids) eventually occupies the entire vault, it is proper to name it the tonsillar region.
In the younger fetuses the region is distinctly more triangular in sha.pe with the apex at the junction of vault and posterior wall. The bursa pharyngea (embryonalis) when present is always found at this point.
A system of folds running longitudinally was clearly discernible in the youngest specimens examined from the surface. In older fetuses the folds become more numerous and distinct, tending to radiate rostrally from the apex of the bursa to reconverge around the nasal septum (Snook, ’34, ﬁgures 1 to 10). In fetuses under 100 mm. no lymphatic tissue is present within the tonsillar area. In the late fetus and newborn_ the folds found in the posterior half of the vault are densely ‘packed’ with lymphatic tissue. Rounded eminences appear along these folds, which are centers of lymphocyte formation.
This system of folds and grooves, the median recess in particular, has been regarded by some as the first indication of the pharyngeal tonsil, (Schwabach, 1887; Testut, ’12). However, since well-developed folds appear before any lymphatic tissue is formed, they must be the expression of the developmental transformations of the region. It will similarly be shown that the appearance ofilymphatic tissues bears a deﬁnite relation to the developmental tensions in the pharyngeal Vault.
Growth tensions in the tonsillar region
Certain growth tendencies in the tonsillar region can readily be followed; others are more obscure. The anterior part of the pharyngeal roof early becomes involved in the formation of the nasal septum. The down-growth carries with it as indicator of the extent of growth the hypophyseal stalk which is destined to diﬂerentiate into the glandular pharyngeal hypophysis. In every case examined, the pharyngeal (proximal) extremity of the stalk was found to have been drawn rostrad ; in some cases the stalk was found to lie nearly parallel with the epithelium. Cases have been reported in which the pharyngeal hypophysis was found far down on the septum (Oitelli, ’11; see also ﬁgs. 1 and 3). The evidence indicates that the region rostrad of the tonsillar area is one subjected to active growth stresses in a longitudinal direction, and that this stress is transmitted ‘over’ the tonsillar region proper as indicated by the extent of fascia pharyngobasilaris. In the septal region the entire space between epithelium and cartilage is ﬁlled with dense connective tissue continuous with the fascia. The vomer ossiﬁes "from this dense membrane. In the vault proper, the fascia is separated from the epithelium by an area of looser tissue which is destined to form tunica propria (and submucosa) (ﬁgs. 1 and 2). In the posterior third of the Vault the m. longus capitis inserts on the side of the fascia and basilar periosteum (figs. 7 and 21).
Fig. 1 Median plane reconstruction of a 55-mm. human fetus series (somewhat diagrammatic) with the fascia pharyngobasilaris lgiotted in from paras:1gittal section. A, pharyngeal aponeurosis; BP, bursa pharyngea embryonalis; FP, fascia pharyngobasilaris; H, hypophysis cerehri in sellaturcici; L, median suspensory ligament (raphe); N, notochord; 0, occipital ossiﬁcation; P, soft palate; PH, pharyngeal hypophysis; V, vomer. The tonsil forms in the looe tissue rostrad of the bursa and between the fascia pharyngobasilaris and the epithelium. X 8.5.
The differentiation of the posterior part of tonsillar area and posterior pharyngeal wall is more complex. There are involved in its transformation the m. longus capitis, m. superior constrictor, fascia pharyngobasilaris, pharyngeal aponeurosis, a.nd median raphe. From the undifferentiated condition shown in ﬁgure 11, the fascia appears as two dense bundles of ﬁbrous differentiation and inserts on the lateral borders of the fascia, thus introducing a tension factor over the posterior part of the vault. These two bundles of fascia retain their identity in this region, but blend rostrad with each other and with the mucous membrane.
The m. superior constrictor typically extends to a point slightly caudal to the pharyngeal angle and is separated from the epithelium by the aponeurosis, which occasionally extends to the pharyngeal angle (ﬁgs. 1 and 10).
The tonsillar area is thus interpolated between the differentiation of the nasal septum rostrally and the formation of the pharyngeal musculature posteriorly. The stresses in the Vault are longitudinal as evidenced by the formation and relations of the fascia pharyngobasilaris. The fascia, originating from the perichondrium of the basioccipital cartilage near the foramen magnum, runs diagonally toward the pharyngeal epithelium, making contact with it near the region of the pharyngeal hypophysis (ﬁgs. 1 and 3). A median and more caudal portion of the fascia bends more sharply, and, blending with the median raphe, reaches the mucosa just back of the pharyngeal angle, where it blends with the aponeurosis (ﬁgs. 1 and 10, and Snook, ’34, ﬁg. 26). Thus, the vault is characteristic in that its mucosa is lacking the dense connective tissue and muscular support found completely surrounding it. In this region, it is the looser tissue between the epithelium and the fascia which forms reticular tissue with lymphocytopoietic potencies.
The ‘neutrality’ of the region is shown in one other respect; viz., it is located immediately ‘under’ the occipital-sphenoid synchondrosis. It is in relation to two opposing growth tendencies; elongation of the occipital bone posteriorly, and the growth of the sphenoid rostrad. The cells of the synchondrosis are often seen to be aligned as they are in the epiphyseal cartilage (ﬁg. 2).
Histogenesis of the tonsil
Within the tonsillar area one would expect to ﬁnd quite early indications of the transformation of the mesenchym.e between fascia and epithelium into reticular tunica propria in which lymphocytes woud presently appear.
An early stage before any diﬁerentiation has taken place is shown in ﬁgure 11. In ﬁgure 12 the fascia pharyngebasilaris has formed, and the mesenchymal cells between it and the epithelium show a slightly more ‘reticular’ nature than before. It is not until the 39-mm. stage is reached that deﬁnitely marked out areas of ‘reticular tissue’ appear within the looser tissue of the sub—epithelial zone in small laterally placed ‘islands’. These areas are larger and more distinct in the 55-mm. embryo. Figure 14 is from a section through such an area. The area of ‘reticular’ tissue blends with the surrounding ﬁbers. Figure 13 is from the same section, but more rostrad, and at about the level of the pharyngeal hypophysis, showing the ﬁbers of the fascia running close and parallel to the epithelium.
In both of the above specimens, the areas of reticular tissue were found in parasagittal sections. In median sections a raphe usually exists (ﬁgs. 6 and 21). A similar area from an 80-mm. fetus is shown in ﬁgure 15. A few scattered ‘free’ cells appear at about this stage, but the stain (carmine and Lyon’s blue) does not permit of a detailed cell study. More medially, the tissue between the fascia and epithelium is loose, but the ﬁbers still run parallel to those of the fascia, but in the meshes are large numbers of extravasated erythrocytes (ﬁg. 16). Extravasation (hemorrhage) is very common in this area in Homo, just as Kingsbury (’32 b) found in the eat, Where he interpreted it as an expression of “a faulty vasculogenesis in the region.” We are here dealing with a highly vascular area, especially rich in small vessels, with a very loose connective tissue support. A 1531-mm. specimen also shows hyaline degeneration of the developing tonsillar tissue alone.
Except for the red cells in the previous specimen (80 mm.), very few free cells are encountered until the 100-mm. stage is reached, and those few which are present are widely scattered. A 100-mm. fetus shows a marked increase in the number of free cells, and two well-marked aggregations appear. (i‘he ‘reticular’ tunica propria is much more extensive and is particularly marked in the anterior lateral regions of the vault. Lymphatic channels are large and extensive, and numerous small blood vessels permeate each area. Each center of cell proliferation, as shown in ﬁgures 17 and 18, is closely related to the pharyngeal side of a large lymphatic channel and is penetrated by small blood vessels. These centers are on opposite sides of the median planes, 1.5 mm. apart and near the middle portion of the vault. A smaller, less dense area is still more laterad. In these three centers the cells are clearly lymphocytes. These relations uphold the statements made by Kingsbury (’32 a) that “there is much to suggest that vascular factors are intimately linked with cell proliferation”; and “It is apparent that the formation of this particular cell type (lymphocyte) bears a relation to the presence of lymphatic endothe1ium.” Isolated cells with myeoblastic and erythroblastic characteristics are found in the tunica propria and in the looser tissue between it and the dense fascia. Here, too, are found cells with histiocytic characteristics, viz., small deeply staining nucleus and a distinct vacuolated cytoplasm, irregular in shape. In other words, cellular differentiation is well under way in the tonsillar area.
This close relationship between lymphocyte proliferation and basal lymphatic channels within the tonsillar area is much in evidence in the smaller (younger) centers appearing in later stages, as illustrated in ﬁgures 2, 21, 24, and 25. At 120 mm., similar centers are present, but cell differentiation is occurring in all parts of the region, proceeding more rapidly within the tunica propria. Here the majority of the free cells are of the lymphocytic series, i.e., large and small lymphocytes and monocytes. An occasional eosinophile is also seen (ﬁgs. 19, 26, and 27). The tunica propria is now quite extensive and forms an especially thick layer in the region under the heads of the 111. longus capitis. It is nearly lacking in the more median plane sections, and the greatest number of differentiating cells are found laterad.
A second type of differentiation is found in the region bordering the tonsillar area. In the cat, Kingsbury (’32 b) found that the region between the tunica propria and the fascia (capsule) was one of myelo- and erythropoiesis. In certain human fetuses examined the regions of myelopoiesis were within the ﬁbres of the pharyngeal aponeurosis, i.e., between in. superior constrictor and epithelium, and in its lateral reaches (fig. 21, in lower left corner). Within these areas large numbers of eosinophilic and neutrophilic myelocytes and lymphocytes are formed (ﬁgs. 20, 22, 28, 29). In Homo, the regions of erythropoiesis were found to be interspersed between bundles making up the fascia pharyngobasilaris. Some centers were found very near the occipital periosteum. The smaller centers of normoblasts, etc. (ﬁgs. 30 and 31), are entirely lacking vascular connections. A few larger groups are found related to developing blood vessels. As another type of cellular differentiation might be incidentally mentioned the transformation of ﬁbrocytes into osteoblasts in the anterior part of the tonsillar area where the vomer is forming within the fascia pharyngobasilaris.
The above evidence would indicate that the diﬁerentiation of a particular cell type depends, to some extent, upon the type of tissue environment and the vascular supply, the former being determined, in the present case, by growth tensions to which the region is subjected. Thus lymphocyte proliferation is greatest in the region described above as the tonsillar region, anarea which has already been referred to being one of ‘negative’ growth tensions.
In a fetus of about the same age, 125 mm., the diﬁerentiation has progressed to a much greater extent. In the lateral regions there are many centers very similar to those of ﬁgures 17 and 18. Several centers are very large and form elevations into the pharyngeal cavity (ﬁg. 2). Between these various centers are wide areas with scattered lymphocytes. One of the largest accumulations is just rostrad of the bursa pharyngea, but not around it (Snook, ’34, ﬁg. 22). The youngest fetus which shows lymphatic tissue around the bursa and behind it is 143 mm. in length (Snook, ’34, ﬁg. 23), and here the lymphatic layer is much less developed than that which is rostrad of the bursa. Thus, contrary to the views of Schwabach (1888), the lymphoid proliferation does not begin immediately around the median recess, nor does it begin at the tip of the bursa as ﬁgured by Huber (’12). Lymphocytes do not appear in the tunica propria of the bursa until the differentiation is well advanced within the tonsillar area proper, especially in the posterior lateral regions where the centers of proliferation are largest, gradually decreasing in size in a rostral direction (ﬁgs. 4 and 7).
From 150 mm. on, the lymphatic tissue ‘spreads’ in all directions particularly rostrad and laterad and to a lesser degree over the posterior wall. The advancing borders have the same conﬁguration as the earliest lymphatic tissue to appear, viz., rounded centers of cell proliferation within the tunica propria and nearly always clearly associated with lymphatic channels and blood vessels (ﬁgs. 6, 7, and 25). The more central parts of the tonsillar mass unite to form a continuous sheet which increa.ses in thickness and density (ﬁg. 5).
At 195 mm. (ﬁg. 6), the lymphatic tissue has spread to the lateral parts of the vault and is invading the tunica ‘propria of the lateral recesses (of Rosemiiller). At 285 mm. (ﬁg. 7) a large amount of lymphatic tissue has been formed and the tonsillar folds are very distinct. Transverse sections of the tonsil at birth are shown in ﬁgures 8 and 9.
From the earliest appearance of lymphatic tissue in the tonsillar area, deﬁnite centers of proliferation are present. These centers are recognized by their greater density and rounded shape and often by a deﬁnite bulging into the pharynx (ﬁgs. 2, 4, and 25). When the mass of lymphocytes has become very dense throughout the va.ult, these centers are less distinct, but are still recognizable under higher powers. However, in the newborn specimens examined, very welldeﬁned rounded centers are scattered throughout the lymphatic tissue (figs. 8 and 9). Each has a very rich blood supply and is composed of closely massed lymphocytes, small and medium sized, reticular cells, an occasional monocytic cell and many erythrocytes. From their structures and relations these centers must be considered as primary nodules.
This brings up the question of the earliest appearance of lymphatic nodules. According to Ganghofner (1879), nodules are lacking in the foetus and the newborn, but appear during the ﬁrst and second years. Wex (1898) states that nodules can sometimes be recognized in the newborn. Prenant (1896) places the ﬁrst appearance of nodules at the end of the sixth month of uterine life. In the material at hand, germinal centers, or secondary nodules, are lacking up to the time of birth. Whether the primary nodules of the newborn have come directly from the centers seen in the younger fetuses or are formed directly in the diffuse lymphatic tissue of the later stage is a diﬁicult question to decide because of the incompleteness of the series for the older stages. The evidence indicates that the former is the case.
The numerous gland ducts of this region are secondarily surrounded by lymphatic tissue (ﬁgs. 7 and 8), which sometimes even forms a sheath around the longer ducts for some distance. This fact may be of importance in the formation of cysts in the vault. Lindt (’07) claimed that these cysts came from ‘tied off’ excretory glandducts; the lymphatic tissue would then be instrumental in causing the obstruction, with the subsequent retention of secretion. Cysts in the pharyngeal vault have also been shown to result from the closure of the mouth of the bursa pharyngea embronalis (Snook, ’34).
Summary and Conclusions
Statements of the earliest appearance. of lymphatic tissue in the tunica propria of the pharyngeal vault vary from 3 to 6 months (Ganghofner, 1878, and Killian, 1888). Hellman, (’27) states that the tonsil is laid down in the fourth month. This agrees with the ﬁndings of the present investigation in which the earliest accumulations of lymphocytes were observed at 100 mm. (3 months). Germinal centers (secondary nodules) were not present in the fetuses examined, two newborn included. This is to be expected if germinal centers represent a reaction against bacteria, etc., which would only be present after birth.
Schwabach believed the bursa pharyngea to represent the ﬁrst anlage of the pharyngeal tonsil and that lymphocytes first appeared around that depression. The present study supports the statement of Killian that the pharyngeal tonsil is developed independently of the bursa pharyngea. The ﬁrst adenoid tissue to appear was found rostrad and laterad of the bursa. The tunica propria of the bursa was found to be ‘inﬁltrated’ only after a well-developed layer of lymphatic tissue had formed in the vault (140 mm.).
The purpose of this study was to try to discover what factors favored the great proliferation of lymphocytes in the tunica propria of the tonsillar region. It has been shown above that the tonsil forms in a region which has little skeletal support. The connective tissue condensations (fascia pharyngobasilaris, aponeurosis, etc.) represent regions of growth tensions and are formed on all sides of the vault, but the tissue of the tunica propria within these bounds retains its loose embryonic character (ﬁg. 1). This same region receives a particularly rich vascular supply. The relation of the centers of cell proliferation to Vascular endothelium was pointed out above. Thus, it is believed with Kingsbury ( ’32 a), that the loose character of the tissue ( ‘negative’ tension) a.nd the rich vascular supply are factors responsible for the differentiation of the pharyngeal tonsil. These factors are reﬂected in the frequent hemorrhage and other pathological processes there existent.
The presence of haemopoiesis in the tissues bordering the tonsillar area suggests the close relationship of these cells to lymphocytes and also to the adequacy of the region for their proliferation.
CITELLI 1911 L’Ipofisi faringea nella prima e seconda infanzia. Suoi rap porti colla mucosa faringea e Coll’iopiﬁsi centrale. Anat. Anz. Bd. 38, S. 242-256; 279-302; 334-349.
GANGPIOFNER, FR. 1879 Ueber die Tonsilla und Bursa pharyngea. Sitzungsber d. k. Akad. d. Wiss. in Wein., Bd. 78, S. 182-212.
HELLMAN, '1'. 1927 Der lymphatische Rachenring. S. 245-283 in Handbuch d. mikr. Anat. des Menschen, Bd. 5, Teil 1, Ed. Wilh v. Miillendorﬂ’.
HUBER, G. C. 1912 On the relation of the chorda éiorsalis to the anlage of the pharyngeal bursa or the median pharyngeal recess. Anat. Rec., vol. 6, pp. 373--404.
KILLIAN, G. 1888 Ueber die Bursa und Tonsilla pharyngea. Eine entwiekel ungeschichtliche und vergleichen-anatomische Studie. Morphol. J ahrb., Bd. 14, S. 618-711. KINGSBURY, B. F. 1932 a The developmental signiﬁcance of the mammalian pharyngeal tonsil: cat. Am. J. Anat., v. 50, pp. 201-231. 1932 b The development of the pharyngeal tonsil (cat): cell types. Am. J. Anat., vol. 51, pp. 269-305.
KINGSBURY, B. E, AND W. M. ROGERS 1927 The development of the palatine tonsil: calf (Bos taurus). Am. J. Anat., vol. 39, pp. 379-435.
KINGSLEY, D. W. 1924 Regressive structures and the lymphocyte. The plasma cell; its origin and development. A study of the mammalian nicitating membrane. Anat. Rec., vol. 29, pp. 1-19.
LOEB, L. 1915 The inﬂuence of changes in the chemical environment on the life a11d growth in tissues. J. Am. Med. Assoc., vol. 64, pp. 726-728. Pommn,
R, AND A. CHARPY 1901 Thaité d’anatomie humaine. T. 4. p. 162-174.
PRENANT, A. 1896 Elements d’embryologie de l’homme et des vertébrés. Livre a pp. 164-166. SCHAFFER, J. 1898 Beitrage zur Histologie menschlieher Organe. IV. Zunge. V. Mundhiihle-Schlundkopf, usw. Sitzungsber. Akad. Wiss. Wein. Math.-nat. KL, Bd. 106.
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1888 Zur Entwickelung der Rachentonsille. Arch. f. mikr. Anat-., Bd. 32, S. 187-213.
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WEX, F. 1898 Beitriige zur normalen und pathologischen Histologie der Rachentonsille. Zeitchr. f. Ohrenheilk, Bd. 34., S. 207-240.
2 Homo; 125-mm. fetus. Parasagittal section through vault and posterior pharyngeal wall. From above downward are seen the basilar cartilage, fascia pharyngobasilaris, and mucosa with lymphatic channels and a protruding ‘nodule.’ Longus capitis muscle is seen at extreme right. X 12.
3 Homo; 135-mm. fetus. Median sagittal section through pharyngeal hypophysis. To the right (rostrad) the fascia blends with the mucosa; to the left, lymphatic tisue appears between facia and epithelium. x 24.
4 Homo; 50-mm. fetus. Parasagittal section; showing a large lymphatic nodule just rostrad of the pharyngeal angle. Fascia and occipital ossiﬁcation are above. X 12.
5 Homo; 180-mm. fetus. Sagittal section, showing a wide area of loose tissue between tonsil and fascia. X 60.
6 Homo; 195-mm. fetus. Transverse section through pharyngeal tonsil, showing denser center of proliferation and invasion of lateral recess. Fascia and m. longu capitis are above. X 8.
7 I-Iomo; 285-mm. fetus. Parasagittal section through the tonsil. Many gland ducts are seen passing through the lymphatic tissue. Fascia and muscle above. X 9.5.
8 Homo; newborn. Transection through tonsil and median recess. Deﬁnite primary nodules are seen in diffuse lymphatic tissue. X 12.
9 Same. Two primary nodules are seen. X 50.
10 Homo; 55-mm embryo. Nearly median section, showing the actual features from which ﬁgure 1 was traced. Superior constrictor muscle is seen in lower right-hand corner. Fascia extends from upper right to lower left. The side of the bursa is present in the lower left. X 76.
11 Homo; 11-mm. embryo. Undifferentiated stage. Basilar cartilage forming at left. Sagittal ection. X 280.
12 Homo; 18-mm. embryo. Sagittal section showing the condensation forming the fascia. pha.ryngohasi1aris. Basilar cartilage not shown. X 280.
13 Homo; 55-mm. embryo. Sagittal section showing fascia directly under the epithelium at the rostral end of the tonsillar region. X 280.
14 Same. From within the tonsillar area, i.e., posterior to the above, showing beginning of the formation of ‘reticular’ tunica propria. X 280.
15 Homo; 80—mm. fetus. Parasagittal section, showing ‘reticular’ tunioa propria from anterior portion of tonsillar area. Fascia above. X 150.
16 Same. Median section, showing hemorrhage within the loose tissue be tween fascia (above) and epithelium. X 150.
17 Homo; 100-mm. fetus. Parasagittal section, showing one of three centers of lymphocyte proliferation in this fetus. A large lymphocytic vessel is associated with the center. X 150.
18 Same. A center from the opposite side. X 280.
19 Homo; 120-mm. fetus. Lymphocytes are seen forming in the tuniea propria of the tonsillar area. Fascia above. X 300.
20 Same. Myelocyte formation from the region bordering the tonsillar area posteriorly. X 280.
21 Homo; 135-mm. fetus. Transverse section through tonsillar region. Several nodules are shown. Basilar cartilage above. X marks the site of myelopoiesis (ﬁg. 22). X 95.
22 Same. Myeloeyte formation in the region bordering tonsillar area. X 640.
23 Homo; 1-L3-mm. fetus. Sagittal section, showing the tunica propria ‘ﬁlled’ with lymphocytes. Fascia above. X 120_
24 Homo, 125-mm. fetus. Lymphocyte proliferation under a lymphatic vessel. X 48.
25 Homo; 180-mm. fetus. New eenters of lymphocyte formation from anterior margin of the pharyngeal tonsil. Relation to lymphatic is clearly shown. X 28.
26 and 27 Homo; 120»mm. fetus. Cells of the lymphocyte series forming within the tunica propria of the tonsillar area. Monocytes and an eosinophile are also shown. X 1216.
28 and 29 Same. Myelocyte formation from the region posterior to the tonsillar area. Some lymphocytes are also shown. X 1216.
30 and 31 Same. Erythroblasts and normoblasts from within the fascia pharyngobasilaris. X 1216.
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