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Grosser O. Lewis FT. and McMurrich JP. The Development of the Digestive Tract and of the Organs of Respiration. (1912) chapter 17, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

XVII. The Development of the Digestive Tract and of the Organs of Respiration: Introduction | Early Entodermal Tract | Mouth and Its Organs | Oesophagus | Stomach | Small Intestine | Large Intestine | Literature | Liver | Pancreas | Pharynx and its Derivatives | Respiratory Apparatus | Figures | Literature
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The Development of the Pharynx and of the Organs of Respiration

File:Otto Grosser
Otto Grosser (1873 - 1951)

By Otto Grosser, of Prague.

Online Editor pages 
Prof. Otto Grosser (1873 - 1951) was an Austrian anatomist and embryologist. In 1907 he was appointed extraordinary professor at the German University in Prague, becoming ordinarius of anatomy there in 1909.
  • Anatomie und Entwicklungsgeschichte der Eihäute und der Placenta. Wien, 1909.
  • Grosser O. Lewis FT. and McMurrich JP. The Development of the Digestive Tract and of the Organs of Respiration. (1912) chapter 17, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.
  • Entwicklungsgeschichte des Menschen. Vergleichende und menschliche Plazentationslehre. In Josef Halban (1870-) and Ludwig Seitz (1872-): Biologie und Pathologie des Weibes. Volume 6, 1. Berlin and Wien, 1925.
  • Frühentwicklung, Eihautbildung und Plazentation des Menschen und der Säugetiere. Deutsche Frauenheilkunde. Volume 5. München, 1927.

A. The Pharynx and its Derivatives

The formation of the anterior part of the digestive tract has already been described at the beginning of this chapter, and it will be necessary to consider here only its further differentiation.

At first it is merely a short, tubular outgrowth of the yolksack, somewhat flattened dorsoventrally ; the oral portion of its ventral wall, in the region of the pharyngeal membrane, rests on the ectoderm, and its rostral end projects somewhat beyond this membrane as Seessel's pouch (Figs. 314-316; see also the section of this chapter dealing with the development of the mouth cavity). Whether it presents further differentiations at the time of its first formation cannot be stated with certainty, but in the youngest human embryos that have been studied and in which it is already present the anlage of the first pharyngeal pouch is apparent (embryos of Kromer-Pfannenstiel and Dandy, Fig. 314).

In all Craniota there are formed bilaterally symmetrical lateral diverticula of the anterior portion of the digestive tract, which, pressing aside the lateral mesoderm of the head, come into apposition with corresponding invaginations of the ectoderm; the endodermal diverticula are termed pharyngeal pouches (also pharyngeal grooves, or inner branchial grooves or pouches), while the ectodermal invaginations are known simply as branchial grooves, or as ectodermal or outer branchial grooves (outer pharyngeal grooves, Hammar). By pressing aside the mesoderm the ectoderm and endoderm for a time come into contact and fuse, forming the epithelial closing membrane, which breaks through in all forms that have a branchial respiration; the branchial grooves and pharyngeal pouches thus become continuous and together form the branchial clefts. The formation of open branchial clefts occurs also in reptiles and birds, but not, under normal conditions, in mammals (see below). The pharyngeal pouches and branchial grooves are later again separated by the ingrowth of mesoderm or (sinus cervicalis) by the constriction of the branchial grooves from the surface and the subsequent modification of their epithelium. The number of pharyngeal pouches that are formed in succession on either side varies in the gnathostomatous Craniota from nine to five, the number in general diminishing with an increasing degree of organization.

Between the branchial clefts — that is to say, between the pharyngeal pouches and branchial grooves — are the branchial or visceral arches, each of which contains a skeletal rod, the cartilaginous branchial arch, its musculature, an aortic arch, and a nerve-trunk. The branchial arches are named in succession the mandibular, hyoid, and branchial arches proper, these last being numbered in succession from before backward. Behind the last branchial cleft lies the last branchial arch, the number of arches being one more than that of the clefts, an arrangement determined by the formation of branchial leaflets on both walls of the clefts. The first branchial cleft is also known as the hyomandibular cleft.

I. General Morphology of the Pharyngeal Pouches

The anlagen of the pharyngeal pouches appear in succession and are formed earlier than the corresponding external branchial grooves, as was observed by Eiickert and Piersol. In man the external grooves first become evident when the pharyngeal pouches have come into contact with the ectoderm, — that is to say, when the lateral cranial mesoderm has been pressed aside and the closing membrane formed (Fig. 316). At about the time of the formation of the two first membranes the closure of the neural canal in the brain region occurs, and there is an increase in the amount of the cranial mesoderm, whereby for the first time the transverse diameter of the skull notably surpasses that of the pharyngeal tube and opportunity is afforded for the formation of the external grooves (developmental period between the formation of the 10th and the 15th pairs of primitive segments).

Keibel Mall 2 314.jpg Keibel Mall 2 315.jpg
Fig. 314. — Pharynx of the embryo Klb (Kromer-Pfannenstiel ; Normentafel, No. 3; 5-6 primitive segments, length, determined from the number of sections, 1.38 mm.). Ds., yolk-sack; Rh., pharyngeal membrane; 1., 2., etc., St., first, second, etc., pharyngeal pouch; Thyr. or Th., thyreoid; V. S., ventral pharyngeal groove. X 150. In all the models the epithelial lining of the cavity is represented, not the cavity itself. Fig. 315. — Pharynx of the embryo Rob. Meyer No. 335 (9-10 pairs of primitive segments, length, determined from the number of sections, 1.70 mm.). Ka., doubtful branchial anlage. The remaining lettering as in Fig. 314. X 150.

The pharyngeal pouches grow out from the anterior part of the digestive tract not only directly laterally but also somewhat dorsally; moreover, a groove, known as the ventral pharyngeal groove, extends along the ventral wall of the pharynx from each pouch, and is recognizable even at its first formation (Figs. 314 to 317). While the surfaces of contact with the ectoderm, i.e., the closing membranes, have at first a somewhat circular outline (Fig. 316), they later elongate to a long, narrow strip, almost perpendicular to the axis of the pharynx (Figs. 317 and 318). By an increase in depth — that is to say, a lateral extension of the pouches (a process that depends upon an increase in the thickness of the intervening branchial arches)— these structures become sharply marked off from the principal lumen of the pharynx (compare Figs. 316 and 317). In a pouch which has thus become relatively narrow and deep (Figs. 317 and 318) there is to be distinguished a cranial, a caudal, and an indistinctly delimited dorsal surface, a lateral edge, a dorsal and a ventral angle, and a ventral pharyngeal groove extending toward the mid-ventral line. By the formation of these grooves the branchial arches become marked out upon the ventral wall of the pharynx as elevations projecting into its lumen. The grooves (with, perhaps, the exception of the first) do not, however, at first reach the mid-line, and accordingly leave an area in that situation, known as the area mesobranchialis (His, 1885) (Figs. 317 and 319). In the caudal part of this lies the heart swelling (Fig. 317), but slightly marked in the human embryo. In the three anterior pouches, and later on also in the fourth, the dorsal angle grows dorsally a little beyond the region of the closing membrane and so forms the dorsal prolongation (dorsal diverticulum) of the pouch (Born, Piersol, Hammar, 1902; Tandler, 1909 ).[1] Piersol terms the ventral angle, together with the lateral edge and the ventral groove, the wing of the pouch. By the continued deepening of the lateral parts of the ventral grooves a ventral prolongation of the pouch is formed, most distinct in the second and third pouches (Fig. 319; indicated in Fig. 317) ; this is the ventral prolongation of Hammar (1902) and the ventral diverticulum of Fox (1908).

Keibel Mall 2 316.jpg

Fig. 316. — Pharynx of the embryo Hah in the collection of the First Anatomical Institute, Vienna (about 15 pairs of primitive segments, length about 3 mm.). Ect., ectoderm; Ka., doubtful branchial anlage; Rh., pharyngeal membrane, broken through in two spots. The remaining lettering aa in Fig. 314. X 150.

Keibel Mall 2 317.jpg

Fig. 317. — Pharynx of the embryo Rob. Meyer No. 300 (Normentafel Xo. 7, 23 pairs of primitive segments, 2.5 mm. vertex-breech length) seen from the ventral surface. The closing membranes of the gillclefts are outlined in black. At. mes., the somewhat depressed oral end of the area mesobranchialis ; Hw., heart swelling; Lar., laryngotracheal groove; V.V., ventral prolongations of the pharyngeal 'pouches. Other lettering as in Fig. 314. X 150.

Keibel Mall 2 318.jpg

Fig. 318. — The model shown in Fig. 317 from the lateral surface. Ka., doubtful branchial anlage; L., liver; Lar., laryngotracheal groove; Lu., lung anlage; Mw., angle of mouth. Other lettering as in Fig. 314. X 150.

As a result of the elongation of the closing membranes and the formation of the dorsal and ventral diverticula, the dorsoventral diameter of the pharyngeal pouches increases much more rapidly in their lateral than in their medial portions, and a distinct delimitation of the pouches medially now becomes possible (Figs. 319 and 320). One may, with H. Rabl (1907 and 1909), term the at first uniformly broad pouches the primary pouches and the later stages, with the lateral portions broadened, the secondary pouches; the latter are connected with the pharynx by narrow connecting portions, the ductus pharyngo-branchiales. These ducts later become verv distinct in the caudal pouches (third to the fifth) (Fig. 325).

Keibel Mall 2 319.jpg

Fig. 319. — Pharyngeal pouches of the embryo Hah of the First Anatomical Institute, Vienna (5.2 mm., about the same stage as No. 16 of the Normentafel). The closing membranes of the left side are outlined in black. The thyreoid has been omitted. (From a model in the Institute.) Lor., anlage of larynx. Other letterings in Fig. 314. X 50.

Just as the anlagen of the pouches appear in a cranio-caudal series, sc. too, their enlargement takes place in succession in the same direction. Associated with this there is a dorsoventral flattening and a rather considerable lateral broadening of the pharyngeal lumen (Figs. 317 and 319), in such a way that the maximum broadening occurs opposite the first pouch, and caudal to the last pouch there is at first a diminution of the gut. The region in which the pouches occur practically represents the anlage of the pharynx (including the floor of the mouth). At the height of the development of the pouches the embryonic pharynx is strongly flattened dorsoventrally and convex dorsally and has a somewhat triangular outline, the base being directed orally aud the apex at the point of union of the air- and food-passages.

Altogether five pharyngeal pouches are formed in the human embryo (Hammar, 1904; in 1889 His speaks of a rudimentary fifth pouch), and all of these reach the ectoderm.

According to Tourneux and Soulie (1907), a sixth pouch also occurs; this discovery has not, however, been thoroughly described. Tandler (1909) has described a diverticulum caudal to .the fifth pouch (embryo shown in Fig. 320; indicated by the letters Div.), but he has left it open whether or not it is to be regarded as a rudimentary sixth pouch. Opposed to such an identification is the fact that the ultimo-branchial bodies are associated with the fifth pouch, whereas in general they belong to the last formed pouch (the sixth in birds, according to H. Rabl, 1907). The diverticulum seen in Fig. 320 appears to be identical with that figured by Soulie and Bardier (1907) for an embryo of 6 mm. and designated as the fifth pouch, but it is certainly not identical with the fifth pouch of other authors.[2]

Keibel Mall 2 320.jpg

Fig. 320. — Pharyngeal region of the embryo BR (Normentafel No. 42, 9.75 mm. vertex-breech measurement), from the ventral surface. (From a model in the First Anatomical Institute, Vienna.) D. br. II and IV, ductus branchialis from the second and fourth branchial grooves; D. c, ductus cervicalis; Div., diverticulum (see text); Ect., ectoderm; 1., 2. St., first and second pharyngeal pouch; Thym. III, thymus anlage of the third pouch; Thyr., thyreoid; ub. K., ultimobranchial body; V. c, vesicula cervicalis. The approximate limit between ectoderm and entoderm is shown by a broken line. X 40.

The available data regarding the appearance of the pouches are as follows: The first pouch appears shortly after or simultaneously with the separation of the anterior part of the digestive tract from the yolk-sac (Fig. 314, p. 447) ; it reaches the ectoderm at a stage in which there are ten primitive segments. In the embryo from which Fig. 315 was constructed the mesoderm has been pressed laterally by the pouch, but the ectoderm and endoderm have not fused to form a closing membrane. The second pouch, which is indicated quite early (Figs. 314 and 315), has formed its closing membrane at a stage with thirteen or fourteen primitive segments (compare also Fig. 316). At this time the third pouch is formed; it has reached the ectoderm in an embryo with twenty-three primitive segments (Figs. 317 and 318). Such an embryo also shows the anlage of the fourth pouch; this shows a certain amount of variability in its development, but it reaches the ectoderm in embryos of about 4 mm. in greatest length (thirty-five primitive segments). The rudimentary fifth pouch is perhaps already formed in the stage shown hi Figs. 317 and 318 (p. 461) ; a closing membrane for the pouch is twice shown in the Normentafel in embryos of 5 mm. The pouch appears to be an appendage of the fourth (for details see later on). — In correspondence with the number of pouches, six branchial arches are to be recognized in the human embryo, of which, however, only four are visible from the surface. The fifth, in correspondence with the incomplete development of the fifth pouch, is at first very indistinct; its aortic arch, the fifth, is rudimentary and its nerve can only be distinguished transitorily (Tandler, Elze, Grosser). Its skeletal portion, which is included in the thyreoid cartilage, is, however, later of considerable size. The sixth arch is not bounded caudally by a pouch (see p. 446) ; its participation in the formation of the larynx is doubtful (see later).

Almost contemporaneously with the formation of the first pharyngeal pouch or only a little later there appears the anlage of the thyreoid gland, usually termed the anlage of the median thyreoid; the word median, however, now seems to be superfluous, since it probably represents the only anlage of the thyreoid tissue (see later, p. 468). The anlage is recognizable before the first pharyngeal pouch has come into contact with the ectoderm, as a prominence in the ventral wall of the pharynx (Figs. 314 and 315) ; it appears, therefore, much earlier than is shown in the Normentafel. It then becomes constricted to form a stalked vesicle (Figs. 317 and 318), and its stalk, whose lumen becomes obliterated, persists for some time as an epithelial cord. The thyreoid anlage belongs primarily to the medial region between the first two ventral pharyngeal grooves, that is to say, to the oral portion of what is later the area mesobranchialis. It is at first anterior to, not in, the region of the second branchial arch. The hollow stalk of the vesicle is the thyreoglossal duct (His).

Even before the obliteration of this duct the first ventral pharyngeal groove becomes prolonged medially and divides into two limbs, which unite in the median line with the corresponding ones of the other side and so enclose a median elevation, the tuberculum impar (Fig. 317). The opening of the thyreoglossal duct is situated at first upon the summit of the tubercle, but later it becomes shifted into the posterior boundary furrow or, according to Ingalls (1907), in an embryo of 4.9 mm., into "the region of the second arch, immediately aboral to the tuberculum impar." Since the tuberculum belongs to the medial region (His, 1885), it is evidently not a derivative of a branchial arch.[3]When the arches are formed they are connected behind the tuberculum impar by a transverse elevation, a copula. This and the tubercle are for the most part taken into the anlage of the tongue, whose formation has been described in the portion of this chapter dealing with the development of the mouth. Behind the copula the mesobranchial area remains for some time but little altered; only a median groove, which replaces the heart swelling, becomes more marked (Fig. 319), and later the area is employed in the formation of the larynx (p. 476).

In addition to the embryos figured above, which have been kindly contributed by their owners, there are certain others that have been described which have important bearings on the early stages of the pharynx; these are the embryo Dandy (1910) with seven pairs of primitive segments, the embryo Pfannenstiel III (Normentafel, No. 6), which has been described by Low (1908), with thirteen or fourteen segments, and the embryo XII of Mall's collection, which has been described by Sudler (1901-02) and has fourteen pairs of segments. The embryos Klb and R. Meyer 300 have already been reconstructed by Kroemer (1903) and by Thompson (1907), but on a small scale and with very cursory descriptions. The embryo Dandy corresponds almost exactly with the embryo Klb, and the Pfannenstiel III and Mall XII embryos are almost exactly equivalent to the embryo HaL. In this the closing membrane of the first branchial cleft of the left side is not yet complete, but is divided into two parts by a strip in which there is no mesoderm, but where the two epithelia have not yet fused (Fig. 316). A similar condition occurs in the embryo Pfannenstiel III. Low has assigned the two portions of the closing membrane to two successive pharyngeal pouches, which is clearly an error and has led him to describe the first pouch as lying originally dorsal to the second, when, as the later development shows, he was describing merely the two angles of the first pouch. — Occasionally small irregular evaginations occur in connection with the pharyngeal pouches and the branchial grooves, as in the embryo shown in Fig. 319, on the dorsal side; they have also been observed and figured by Ingalls. Such are perhaps comparable to the embryonic intestinal diverticula described by F. T. Lewis and Thyng (Amer. Journ. Anat., vol. 7, 1907-8).

A remarkable observation has been made by the author in all young embryos with the first pharyngeal pouches well developed; these are the embryos R. Meyer 335, HaL, Pfannenstiel III (loaned for this purpose), R. Meyer 330, and also a somewhat pathological, young embryo from the collection of R. Meyer. In the region of the first pouch there projects ventrally (Figs. 315 and 316) or caudally (Fig. 318) from the closing membrane into the pharyngeal lumen an irregularly knobbed process filled with mesoderm. That it is an accidental structure or due to post-mortem changes seems to be excluded by the regularity of its occurrence (Low has figured, but not described it). It disappears quite early (in the oldest embryo examined, Fig. 318, it is present only on the left side and is greatly reduced in size; in embryos of 4.25, 5.0, and 5.8 mm. and in those still older, it is wanting), and may perhaps be interpreted as a rudimentary internal gill. It would not be the first instance of a very ancient rudiment well developed in the human embryo. Similar structures have not yet been observed in other amniote embryos.

The thyreoid anlage in human embryos is at first exceptionally large, but seems to be subject to a certain amount of variation in form. Dandy describes a ventro-median pouch projecting from the union of the two first pharyngeal pouches — evidently the thyreoid anlage (compare Fig. 314), although he denies the occurrence of such a structure. In Fig. 316 it has a similar appearance, in Fig. 315 and also in the embryos Pfannenstiel III and Mall XII it has a much more distinct delimitation. The delimitation starts on the rostral side and appears later caudally; the separation from the obliterated thyreoglossal duct takes place, according to the Normentafel, in embryos of about 6 mm., occasionally, however, earlier or later. At this time the lumen of the anlage, which has usually become bilobed, has disappeared. For an account of the differentiation of the anlage see p. 468.

The closing membrane in the human embryo, as in those of mammals generally, remains imperforated (His) ; open branchial clefts do not occur. Perforation has, however, been frequently observed, most frequently in the case of the second pouch (Kolliker, Tettenhamer, and Hammar), which possesses the longest closing membrane (Hammar).[4]In this case perforation may be regarded as within the limits of variation, but in other pouches it is, as a rule, due to injury from handling, and such injuries assuredly also increase the percentage of cases of perforation of the second pouch.

After the closing membrane has become converted into an elongated strip (p. 448) the mesoderm again penetrates between the two epithelia of the membrane and the pouch once more becomes separated from the surface of the embryo. This process may be followed in its simplest form in the first pouch; in the more posterior ones it is combined with the formation and constriction off of the sinus cervicalis (C. Rabl, 1886 to 1887; sinus prcecervicalis, His, 1885; compare vol. I, p. 69, et seq.)[5] This is formed by the mandibular and hyoid arches growing more rapidly than the other arches in all dimensions, but especially the transverse, while the growth of the branchial arches proper lags behind that of their surroundings, so that they come to lie in the floor of a depression, the sinus cervicalis, which is open laterally. The caudal edge of the hyoid arch later grows backward over the month of the sinus, forming an indistinctly delimited operculum, which is less developed in man (Hammar) than in other mammals ; the sinus retains its connection with the exterior for a short time by means of the ductus cervicalis {prcecervicalis) , but finally becomes completely shut off so as to form the vesicula cervicalis {praecervicalis),[6] In man there persists at the surface only a shallow groove, the sulcus cervicalis {sulcus prcecervicalis, Hammar; cervical groove, H. Eabl), which at first marks the boundary line between the head and the thorax. The vesicula cervicalis lies lateral to the third pharyngeal pouch (Figs. 320, 321, 325, and 326), and is connected by diverticula, the former external branchial grooves, with the second and fourth (and for a short time also with the fifth) pouches; these diverticula become drawn out into long canals, the ductus branchiales (II and IV) (Figs 320 and 321). The vesicula and the ductus exist only for a short time ; their lumina vanish and the epithelial structure of the organs disappears.

With the sinus cervicalis are associated three cranial nerve placodes (branchial cleft organs of Froriep) ; they consist of intimate connections of the epithelium with the ganglia of the glossopharyngeus and vagus (see the chapter on the Nervous System). These placodes are also recognizable only for a short time and disappear with the other derivatives of the sinus.

From the time of His (1885) up to the present a number of authors have agreed in deriving the anlage of the thymus, either in whole or in part, from the epithelium of the sinus cervicalis. These results are hardly reconcilable, however, at least so far as man is concerned, with those of other authors (compare Hammar, 1910). — The fate of the sinus vesicle is, moreover, different in different mammals. While it disappears in man and in the cat at an early period and in the rabbit somewhat later, in the pig (Kastschenko 1887, Fox 1908) and in the sheep (Prenant) it persists for some time as a structure of considerable size, which Kastschenko has termed the thymus superficialis, but whose eventual fate is not yet certainly known. In the mole, according to the recent definite results of H. Rabl (1909), the thymus superficialis is actually formed from the vesicula cervicalis. — The third pharyngeal pouch seems also to vary in different species as to its lateral extension ventral to the sinus vesicle. — The duct-like remains of the second external branchial groove, Hammar (1903 and 1904), following C. Rabl (1886-7), has termed the ductus branchialis[7] while he names the corresponding duct of the fourth pouch the dtictus thyreo-cervicalis; since, however, this latter structure, in my opinion, has nothing to do with the actual thyreoid anlage, the term ductus branchialis has been used above for both duets, the numbers II and IV indicating the branchial grooves with which each corresponds.

The time of obliteration of the lumina of the ductus cervicalis and the ductus branchiales is subject to some variation (compare Figs. 320 and 321) ; according tc the Normentafel it occurs in embryos of about 9 mm., and in those of 11-14 mm. the epithelial cord formed from the ductus cervicalis has disappeared, as has also the vesicula cervicalis a little later. — The first pouch has separated from the ectoderm in an embryo of 14 mm. (Normentafel, No. 54).

  1. H. Rabl (1909) does not recognize a dorsal diverticulum as of general occurrence in mammals, and ascribes to it, in any event, no special significance (in the formation of the epithelial bodies).
  2. Such diverticula are probably without significance and very transitory. H. Rabl (1907) has shown, without description, something similar in the duck in Fig. 6 of his paper. See also p. 454.
  3. The account given above differs in many respects from that recently given by Kallius (Anat. Hefte, vol. 41, 1910) for the pig. Observations on a larger amount of human material than is at present available may show a necessity for some modifications of the statements made. Compare especially the development of the larvrix described below.
  4. The closing membrane of the first pouch is, however, in early stages by no means so short as Born and Hammar have imagined (compare Fig. 318). But the degeneration of the ventral part of the membrane takes place very early in this pouch.
  5. Rabl has altered the name proposed by His, on the ground that the term "praecervicalis " implies a structure situated anterior to the neck region ; His intended it to denote the ventral position of the sinus. Both terms are employed synonymously in the literature.
  6. According to H. Rabl (1909) the term vesicula cervicalis is to be applied to the entire complex, including the two ductus branchiales (see above) ; Hammar uses the term vesicula prcecervicalis only for the vesicular portion that is associated with the third pharyngeal pouch, this portion being approximately identical with the fundus prcecervicalis (cervicalis) of His and H. Rabl, as well as with the vesicula thymica of Kastschenko and the sinus vesicle of Zuekerkandl.
  7. Fox (1908) has not been able to find this branchial duct in the pig, but, on the other hand, demonstrates the occurrence of a long pouch-like diverticulum of the second pouch. Differences apparently occur in different species in this respect also. The diverticulum in the pig may correspond with the thymus anlage of the second pouch described by Piersol (1888) and others in the rabbit; this structure has not yet been observed in the human embryo.

II. The Differentiation of the Pharyngeal Pouches - the Second Pharyngeal Pouch and the Tonsils

Only for a short time do all the pharyngeal pouches have a relatively similar structure, differing essentially from one another only in that they diminish in size caudally (Fig. 319) ; they separate first into two groups, one of which consists of the first two pouches and the other of the remaining ones (Figs. 320 and 321). At the level of the first two pouches the pharynx rapidly increases in width, an increase that stands in relation to the development of the first arches as already described (p. 455). The succeeding pouches remain much less extensive, and from their epithelium a number of glandular organs develop.

By the increase in breadth of the pharynx the first and second pouches acquire a common pharyngeal opening (Fig. 320). The broadening occurs even before the separation of the pouches from the ectoderm; it practically corresponds to the primary tympanic cavity of Kastschenko or the pliaryngo-tympanic cleft (lateral pharyngeal enlargement) of Piersol. Nevertheless, as is shown by the thorough study of the later development by Hammar (1902), the entire complex is not concerned in the formation of the anlage of the middle ear. Only the first pouch becomes transformed into the primitive tympanic cavity ; its further development is described in another chapter. The second pouch gradually ceases to be an independent outpouching of the pharynx, its walls being taken up into the walls of that cavity, its dorsal angle only persisting as a slight evagination, which becomes pushed forward toward the point of connection of the first pouch with the pharynx, that is to say, toward the root of the primary tympanic cavity (Hammar) (Fig. 322). Between the derivatives of the first two pouches there now become interposed the palatine ridges, and they are thus definitely separated (Fig. 323), so that the fate of each may be readily determined. The dorsal angle of the second pouch becomes transformed into the palatine tonsil, and may, accordingly, at an early period, be termed the simis tonsillaris (Hammar).

The development of the tonsil, which has been thoroughly studied by Hammar (1903), is associated with the appearance of a small elevation, the tuberculum tonsillare, which is situated on the ventral wall of the pharynx and projects into the sinus tonsillaris, lying practically opposite it (Fig. 323). Both structures lie on the lateral edge of the pharynx, and their derivatives are therefore to be found later side by side on the lateral wall of the pharynx. After the appearance of the tuberculum the palatine arches become evident, and the arcus palatoglossi cannot, therefore, be derived directly from the hyoid arches, as His (1885) thought, even although they lie oral to the tonsils. The tuberculum quickly flattens to form a fold, which surrounds the sinus tonsillaris anteriorly and inferiorly and corresponds to the plica triangularis of the B.N.A. (His). The sinus itself becomes for a time divided by a plica intratonsillaris into two superposed recesses, and from the wall of the sinus tonsillaris epithelial processes grow out into the connective tissue of the mucous membrane; these processes are at first solid, but later become hollow by the degeneration of the central cells. Portions of the processes may be separated off, but these undergo degeneration. Around these epithelial processes there is formed, accompanied by abundant cell division, a lymphoid tissue, from which leucocytes penetrate into the epithelium. The plica triangularis, situated in front of the tonsil, is originally high, but it undergoes a progressive reduction which is continued even after birth and frequently results in the complete disappearance of the fold. The plica retrotonsillaris, which occasionally occurs in adults, belongs, according to Hammar, to relatively later developmental stages (fetuses of 190 mm. and upward). The fossa supratonsillaris (His) is formed from the upper recess of the sinus tonsillaris, with the assistance of the folds which surround the fossa.

Keibel Mall 2 321.jpg

Fig. 321 Branchial derivatives of an embryo of 11.7 mm (nape-length), somewhat simplified. (After Hammar, 1903.) Sulc. c, sulcus cervicalis (pracervicalis) . The remaining lettering as in Fig. 320. X 21.

Hammar has not confirmed the view that the reeessus lateralis pharyngis (Roseninulleri) is derived from the second pharyngeal pouch (His, C. Rabl, Kastschenko). The recess appears relatively late, but definite observations upon it are wanting. — The reduction of the ventral part of the second pouch begins in embryos with a length of somewhat over 8 mm. (Hammar; compare also Figs. 319 and 320), and in an embryo of 17 mm. only the dorsal angle of the pouch, lying near the entrance into the primary tympanic cavity, is to be found. The same embryo also has a recognizable tuberculum tonsillare. In one of 24.4 mm. the plica intratonsillaris can be seen, in one of 70 mm. the budding out of the epithelial processes is beginning, and in one of 110 mm. the accumulation of cells in the connective tissue. Lymphocytes are first recognizable in embiyos of 140 mm. and secondary nodules in one of 235 mm. By this time the tonsil has acquired its characteristic features. — The plicaB triangularis and intratonsillaris are rudimentary structures in man, but may play a part in the manifold modifications of the tonsils which occur in the mammalian series. — As is the case with other adenoid organs, so in that of the tonsils all observers are not agreed as to the origin of the leucocytes; yet their derivation from the epithelium (Retterer) has been, probably correctly, opposed by Stohr, Kollmann, and Hammar. Stohr's view that they migrate into the tonsillar tissue from the blood-vessels is replaced by Hammar by the assumption that they are autochthonous structures of the mesoderm. — Griinwald (1910) derives the tonsil from the ventral portion of the second pouch and regards it as equivalent to a thymus metamere. He finds in fetuses cartilaginous outgrowths from the second and third branchial arches included in the tonsillar anlage and serving it for support. The adenoid tissue is of mesodermal origin.

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Fig. 322. Dorsal view of the left half of the pharynx of an embryo of 21 mm. (nape-length). (After Hammar, 1902.) Dors. II, dorsal diverticulum of the second pharyngeal pouch; Hyp., stalk of the hypophysis (cut); Pkh., tympanic cavity. X 21.
Fig. 323. Pharynx of an embryo of 24.4 mm (nape-length), from the left side, somewhat simplified. (After Hammar, 1903.) Gl., palatine ridge; Mh., mouth cavity; Oe., oesophagus; PI. itt., plica intratoneillaris; Tr., trachea; Tu.-ty. R., tubo-tympanic space; Tub. tons., tuberculum tonsillare. X 12.

III. The Third to the Fifth Pharyngeal Pouches - the Branchiogenic Organs

The importance of the three caudal pharyngeal pouches in the amniota lies mainly in the fact that their epithelium gives rise to a series of ductless glands; these are the thymus, the epithelial bodies or parathyreoids, and the ultimobranchial body (the so-called lateral thyreoid). Of these the first two are derived from the third and fourth pouches; the question of the development of the last is intimately connected with that as to the occurrence of a fifth pouch.

The thymus and epithelial bodies are formed in the lower vertebrates (with numerous pouches) from a series of pouches in succession; they are, accordingly, metameric (branchiomeric) organs, forming one of the characteristic products of a pharyngeal pouch and (together with the ultimo-branchial bodies) are collectively known as pharyngeal pouch or branchial cleft organs or as branchiogenic organs. Yet it must be left an open question, at least for the thymus (compare Hammar, 1910), whether this does not represent an originally unsegmented epibranchial organ. Epithelial bodies occur first in the tetrapodous vertebrates. Furthermore, they occur on all the pouches in none of the vertebrates, the anlagen being usually suppressed on certain of the pouches, namely the oral ones; in man they have not yet been described in the first two. According to my own observations, a circumscribed epithelial thickening may occur transitorily (embryo Hah of the First Anatomical Institute, Vienna, with about 15 primitive segments) in the region of the dorsal wall of the first pharyngeal pouch, opposite the previously described invagination of the ventral wall (p. 454) ; the significance of this hitherto unobserved structure is, however, still in doubt.

A discussion of the historical development of our knowledge regarding the origin of these organs would lead us too far; one may consult on this subject the accounts given by Kohn (1900) and Hammar (1910). The actual explanation of the developmental processes has been principally due to the work of Kohn (1896) and Groschuff (1896). The very complicated nomenclature of the parts bears witness to our knowledge of the actual relationships having been acquired step by step; the embryonic anlagen were known in part much earlier (Remak, 1855) than the corresponding definitive organs, and they therefore received at first for the most part erroneous interpretations. For instance, the glomus caroticum has repeatedly been included in the series of branchiogenic organs and derived from the third pouch; it is, however, a derivative of the chromaffin system, a paraganglion (A. Kohn; see the chapter on that system), and the anlage seen by various authors was that of an epithelial* body: Even in 1908 Fox, on historical grounds, termed the epithelial body of the third pouch the glandula carotica, although it has no other relation to the carotid than a transitory topographical one.[1] Such temporary topographic relations have also brought it about that the epithelial bodies derived from the third or fourth pharyngeal pouches have been termed glandules thymiques (Prenant) and glandules thyreoidiennes (Gley), as is usually done at the present time by French authors. The terms are historically intelligible, but are unfortunate, since, apart from the possibility of confusion with the main glands and disregarding the histological and physiological similarity of all the epithelial bodies, the topographical relations are characteristic only for a definite stage of development, and the epithelial bodies in a long series of mammals, including man, are, on the one hand (when in normal position), all attached to the thyreoid and, on the other, are throughout genetically connected with thymus anlagen and also are frequently accompanied by small thymus lobes, so that both names might be applicable to each body. A similar criticism applies to the names parathymus for the derivative of the third pouch and parathyreoid for that of the fourth, employed by Groschuff (1896) ; the same author in 1900 names the bodies parathymus and distinguishes them according to the pouch from which they are formed as parathymus III and parathymus IV, names which have embryologically a greater justification than the expression parathyreoid. Yet, at least for the adult condition in man, the name parathyreoid, proposed by the discoverer of the organs, Sandstrom, is quite characteristic, if one does not prefer the general name epithelial bodies, proposed by Kohn and borrowed from Maurer's description of the relations in amphibia. The very general term " glandules branchiales," used by Herrmann and Verdun (branchial glands, H. Rabl 1907), would also be strictly applicable to the other branchiogenic organs. The further classification of the epithelial bodies may probably be most satisfactorily based on the pouches from which they are formed, just as other metameric organs are grouped under a common designation and distinguished by numbers.

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Fig. 324. — Schema of the branchiogenic derivatives in man, adapted from the schemata of Groschuff and Kohn. D. ph.-br. Ill, ductus pharyngobranchialis of the third pharyngeal pouch; D. ph.-br. comm., ductus pharyngobranchialis communis of the fourth and fifth pouches (of the caudal pharyngeal pouch complex) ; D. th.-gl., ductus thyreoglossus; Ep. Ill and IV, epithelial bodies of the third and fourth pouches; Thym. Ill and IV, thymus anlagen of the third and fourth pouches; Tons., tonsil; ub. K., ultimobranchial body.

First of all, the development of the fifth pharyngeal pouch, which was long overlooked and whose existence was even denied, must receive some further consideration (see above, p. 453). A thorough exposition of the question has been given by Tandler (1909), who, however, had for study no material showing the pouch with an epithelial closing membrane (compare in this respect Harmnar in the Normentafel). The form of the pouch, as it makes its appearance, is extensively modified and complicated by the anlage of the ultimobranchial body. First of all (so far as our present knowledge extends) there appears on the fourth pouch, soon after its differentiation (embryo of about 3 mm., Hammar, 1902, Normentafel No. 11; compare also Ingalls, 1907, embryo of 4.9 mm., Normentafel No. 14), a process, directed ventrally and caudally,[2] which is longer than the ventral diverticulum of the third pouch of the same stage, but which, however, might readily be mistaken for such a diverticulum, as has apparently been done by earlier investigators and quite recently by Fox (1908). This process later becomes more distinctly separated from the fourth pouch, which then acquires a dorsal and a ventral diverticulum, the latter varying in extent (compare Tandler and Fig. 319). A lateral evagination of the caudoventral process reaches the ectoderm as a fifth pouch and forms a closing membrane ; this, however, is perhaps not always formed and is at all events very transitory. A dorsal diverticulum is apparently not formed,[3] and after the degeneration of the actual pharyngeal pouch, which is directed toward the ectoderm, the caudoventral process becomes converted into the ultimobranchial body. The fourth and fifth pouches are, accordingly, intimately associated genetically, and the ultimobranchial body is the sole derivative of the latter. Both pouches possess only a common communication with the pharynx, a ductus pharyngobranchialis communis (compare p. 451), and the intimate connection of these last two pouches may be expressed by uniting them in the term caudal pharyngeal pouch complex (Figs. 319, 320, and 324).

H. Rabl (1909) finds in the mole a common anlage for the two pouches and names it the caudal pharyngeal diverticulum. In this case the fourth poueh is more rudimentary than the fifth. — That the ultimobranchial body does not make its appearance behind the series of pharyngeal pouches (as a postbranchial body according to Maurer), but is derived throughout the whole vertebrate series from what is in each case the last pouch, which has become rudimentary, was insisted upon by Geil, who is responsible for the term here employed for the structure. The body is identical with the suprabranchial body of van Bemmelen. The expression telobranchial body, which has been frequently employed recently, has been rejected by H. Rabl, since it does not express the relation of the structure to the last pharyngeal pouch.

In the third branchial pouch the formation of the thymus is preceded by an elongation of the ventral diverticulum, which extends ventrally and medially (Figs. 319 and 320), and whose epithelium, consisting of closely packed cells, increases in height on the aboral wall of the diverticulum (Fig. 326). This thickening of the epithelium extends also upon the aboral and dorsal walls of the pouch itself (Figs. 324 and 325), and simultaneously there begins on the oral and lateral walls of the dorsal diverticulum and of the pouch itself (Figs. 324 and 325) a proliferation of the epithelium, which very early shows itself, by its histological differentiation, to be the anlage of an epithelial body. The cells appear to be vacuolated, their plasma reticular and refractory to stains (chromatophobe). The cell boundaries are at first indistinct (compare also Maximow, 1909, p. 538).

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Figs. 325 and 326. — Two sections through the embryo BR (compare Fig. 320). Fig. 325 is through the point of communication of the third pouch with the pharynx, Fig. 326 through the sinus and ductus cervicalis. Ao.-B., aortic arch; Ao. d. and Ao. v., aorta dorsalis and ventralis; Aw., arytenoid swelling; Ch., chorda dorsalis; D. br. II, IV, ductus branchialis of the second and fourth external branchial grooves; D. c, ductus cervicalis; D. ph.-br. Ill, ductus pharyngobranchialis of the third pharyngeal pouch; D. ph.-br. c, ductus pharyngobranchialis communis; Ep. Ill, IV, epithelial bodies of the third and fourth pharyngeal pouches; Hy., hyoid arch (operculum); Lar., anlage of the larynx; Per., pericardial cavity; S. c, sinus cervicalis; Sy., sympathicus; Thym. Ill, thymus anlage of the third pharyngeal pouch; Thyr., thyreoid; Tr., trachea; ub. K., ultimobranchial body; V.j., vena jugularis; X., vagus; XII. hypoglossus. X 40.

Keibel Mall 2 327.jpg Keibel Mall 2 328.jpg

Figs. 327 and 328. — Two sections through the embryo BR (compare Fig. 320). Lettering as in Figs. 325 and 326. X 40.

At the same time the medial portion of the pouch narrows to become the ductus pharyngobranchialis HI (connecting piece or duct, ductus thymopharyngeus of Hammar), while the lateral portion becomes the secondary pouch of H. Eabl or, if the anlagen of the thymus and epithelial body be disregarded, the remains of the pharyngeal pouch. The formation of a large vesicle from the secondary pouch does not occur in the human embryo. The ductus pharyngobranchialis soon atrophies completely, so that the derivatives of the pouch become free. The thymus anlage becomes first of all a thick-walled cylinder, at whose cranial end the cavity of the remains of the pharyngeal pouch is visible, while the sinus vesicle, which was in relation to the pouch laterodorsally, vanishes. Soon, however, the lumen of the thymus anlage disappears[4] and the thymus cord is formed. This thickens at its caudal end and so forms the body of the thymus (thoracic portion), while the uppermost portions become gradually thinner (cornu of the thymus, GroschufT; cervical portion) and come into connection with the epithelial body (Fig. 329). The entire anlage migrates caudally, the body more rapidly than the cranial end and the epithelial body, so that the cervical portion becomes more and more drawn out and finally vanishes. The migration usually takes place in front of (ventral to) the vena anonyma sinistra, frequently, however, behind it (Tourneux and Verdun). The epithelial body, which as a rule becomes quite separated from the thymus cord, normally halts in its caudal regression at the lower pole of the thyreoid. If the cranial end of the thymus cord does not disappear completely, there occurs beside the epithelial body an accessory thymus lobe (A. Kohn). Frequently in older embryos there is associated with the epithelial body a cyst, probably derived from the remains of the pharyngeal pouch (Kiirsteiner, 1899).

Keibel Mall 2 329.jpg

Fig. 329. — The branchiogenic organs of an embryo of 26 mm., somewhat simplified. (After Verdun, 1898.) Lob. pyr., lobus pyramidalis; V. scl., vena subclavia. The remaining lettering as in Figs. 325 and 330. X 20.

The thymus is from the beginning a paired organ and it remains so permanently ; neither fusion of the two anlagen by transverse connections nor a complete division into several lobes occurs, according to Hammar (1910). — The thymus is first represented in the Normentafel as a short cylinder in an embryo of 5 mm. ; the epithelial body, according to Tourneux and Verdun (1S97) and Tandler (1909), appears as an epithelial thickening in embryos of S mm., and Tandler finds cell differentiation in the einbryo represented in Fig. 325, Hammar, however, finding it earlier in an embryo of 8.3 mm. Nevertheless it is in some instances stated expressly in the Normentafel that the epithelial body is wanting in older embryos, up to 11 nun. According to the Nomientafel, the ductus pharyngobranchialis has disappeared in an embryo of 14.0 mm., but Hamrnar (1904) finds that this happens sometimes earlier and sometimes later (in embryos between 11 and 19 mm.). In an embryo of 12.5 mm. in the Normentafel the thymus has reached the pericardial cavity, and in one of 15 mm. the lumen is limited to the uppermost portion. The remains of the pharyngeal pouch are still to be seen, according to Hamrnar (1904), in an embryo of 24.4 mm.

According to Hamrnar (1911), there is a definite thickening of the wall of the thymus diverticulum while it is still cylindrical, situated dorsally in the more proximal portion and laterally in the more distal portion (compare Figs. 325 and 326) ; the thickening does not extend, however, to the tip of the diverticulum and accordingly cannot be regarded as the sole anlage of the thymus. The ends of the anlagen remain at first in close topographic relationship to the aortic arches, and their elongation corresponds to the increase in the distance between the third and fourth aortic arches and to the formation of the anterior surface of the neck. The elongation depends mainly on the stretching and not on the growth of the anlagen. During the elongation the anlagen, which have at first a more transverse position, come to occupy a position more nearly parallel to that of the body axis (as may be seen from Figs. 320 and 329). By the resulting rotation the dorsolateral anlagen of the epithelial bodies are brought to the ventral side of the remains of the pharyngeal pouches. At the junction of the cervical and thoracic portions the aperture bend of the thymus anlage is formed, and, in addition, a heart bend and an aortic bend may also be distinguished.

The epithelial character of the thymus anlage continues plainly evident for a considerable time. In fetuses of about 50 mm. vertex-breech length one finds at the cranial end of the thymus anlagen and also in the cornna numerous closed vesicles with a high cubical epithelium (compare also Kursteiner, 1899) and transitions from these to epithelial cords without a lumen. Cells arranged in an epithelium-like manner are generally distributed over the surfaces of the lobes of the body of the thymus. In fetuses of about the same stage of development (from 42 mm. onward), the centre of the anlage is beginning to appear clearer in sections — the differentiation of cortex and medulla is beginning. From now on the organ gradually assumes its characteristic appearance of being formed of small ("lymphoid") cells. Hassall's corpuscles make their appearance in it in fetuses of from 60 to 70 mm. (Hamrnar, 1910). — The thymus grows not only throughout the entire period of embryonic life, but also through childhood, until puberty, and only at that time does its involution begin, a process which goes on but slowly in individuals with perfect health ; during illnesses, however, an accidental involution may supervene and this explains the widely divergent statements of various authors concerning the condition of the postfetal thymus (Hamrnar, 1910).

According to Hamrnar (1911), the beginning of the "leucocyte infiltration" may be seen in fetuses of from 30 to 40 mm., and the formation of the medulla in those of 50 mm. The medulla appears at first in the central portions in the form of a longitudinal cord, extending later into the superficial boss-like anlagen of the follicles, which have appeared in the meantime ; the formation of the medulla is, accordingly, a continuous process, and for the most part it continues to be so later. In this manner the tract us centralis is formed, in whose composition, however, the cortical substance takes part later on. Since the boundary between the cervical and medullary portions is subject to functional modifications, the tractus centralis should not be termed the medullary cord, but preferably the parenchyme cord.

The histogenesis of the thymus is at present a very contentious question (compare Maximow, 1909; Hamiuar, 1910; and Stohr, 1910), and cannot therefore be described in detail. Authors are agreed only as to the epithelial origin of the cellular reticulum and of the Hassall corpuscles. The chief elements of the organ, however, the small thymus cells, have been regarded either as immigrated cells and therefore as thymus lymphocytes (more recently especially by Maximow and Hammar) or as modified epithelial elements (among recent authors chiefly by Stohr). According to Sehaffer (1909), they must be regarded as lymphocytes, even if their epithelial origin be admitted. From the histological conditions and from their development in mammals, conclusive evidence is not at present to be drawn (compare Stohr, 1910), but comparison and theoretical considerations undoubtedly speak in favor of their epithelial nature.

Little can be said concerning the histogenesis of the epithelial bodies. Even in the stage when they are epithelial thickenings their cells, as has been already mentioned, are characteristically differentiated ; then the anlagen become split up by the epithelium growing out in the form of cords and probably also by the penetration of connective tissue between the cells, and the cell boundaries become distinct. The formation of different kinds of cells (compare the summary by Getzowa, 1907) occurs ouly in postfetal life. Lumina are not distinguishable in the epithelial bodies during embryonic life, but they may appear later. They may then become filled with a secretion, which shows the staining reactions of colloid, and such structures have hitherto continually led authors astray by causing the epithelial bodies to be confused with thyreoid tissue and to be regarded as young stages of it. Yet the acidophilous nature of a secretion is no indication of its colloid uature (Kohn, 1896) ; according the Erdheim (1904), any albuminous secretion, contained within a closed cavity, may assume the appearance of colloid.

According to H. Rabl (1900). (he whole of the secondary pouch, except so much of it as is employed for the formation of the thymus anlage, is transformed into the epithelial body, even the wall opposite the original epithelial proliferation, since the connective tissue also penetrates the lumen of the pouch; at least this is the case in the mole. The other authors, who limit the extent of the anlage of the epithelial body to a greater degree than is done in the text, namely to the dorsal diverticulum or to the dorsal angle of the pouch, believe that the wall of the pouch itself degenerates; precise observations are. however, wanting.

The development of the fourth pharyngeal pouch follows essentially the same lines as that of the third ; the ventral diverticulum is, however, much more feebly developed (Tandler, 1909), and it only occasionally undergoes a development into thymus tissue ; when this tissue is formed, it remains in the neighborhood of the epithelial body, which is formed from the dorsal and lateral portion of the pouch, and, accordingly, again is not limited to the dorsal diverticulum. The development of the epithelial body resembles that of the third pouch; no difference in the structure of the two bodies is discernible. The anlage of the ultimobranchial body is a derivative of the fifth 'pouch, which projects caudoventrally from the fourth pouch in the form of a thick-walled cylinder (Figs. 320, 324, and 328). The common communication of the two pouches with the pharynx, the ductus pharyngobranchialis communis, diminishes in size and becomes constricted off from the caudal pharyngeal pouch complex, just as does the corresponding part of the third pouch. The complex then separates from the pharynx, the epithelial body becomes independent by the disappearance of the thymus anlage and of the remains of the fourth pouch, and the ultimobranchial body becomes an elongated vesicle with thick walls. The two structures (epithelial body and ultimobranchial body) usually remain close together, however, and migrate somewhat ventrally and caudally, thus coming into relation with the thyreoid anlage.

The thymus ruetarnere of the fourth pouch was discovered by Groschuff; according to Erdheiru (1904), its persistence is to be regarded as a rarity. — The ductus pharyngobranchialis communis, which has also been termed the lateral thyreoid anlage, has been named, with reference to its relation to the ultimobranchial body, the ductus Oiyreopharyngeus. This name must be rejected, for the reasons given with reference to the ductus thyreocervicalis (see p. 457) ; in addition it may be mentioned that an occasionally persistent thymus anlage is also present in connection with this duct, as well as with the ductus pharyngobranchialis III, the. ductus thymopharyngeus of Hammar.

The thyreoid anlage (the middle or anterior thyreoid anlage of various authors), before its separation from the pharynx (p. 453), becomes bilobed with a divided lumen; at about the time when the thyreoglossal duct becomes broken it loses its lumen, and, undergoing a continuous displacement caudally, it develops into a broad structure composed of irregular cords of cells, disposed for the most part transversely. The derivatives of the caudal pharyngeal pouch complex apply themselves to the somewhat dorsally bent lateral portions of the anlage and become partly enclosed by it. This is the case with the ultimobranchial bodies, which then lose their lumina, but further than this they apparently do not always behave in the same manner. While in some cases they appear as compact bodies, in others they separate into an irregular group of small cells with strongly staining nuclei (Grosser; see Fig. 330). In man, however, in normal development, no cell formations that can be referred to the ultimobranchial bodies are to be distinguished after a time (see also p. 471) ; up to the present no evidence has been advanced in favor of the widely accepted view that the bodies become converted into thyreoid tissue, and such a transformation is rendered highly improbable by the results of comparative investigation (see p. 471). The name lateral thyreoid anlage, which has been applied to the ultimobranchial bodies, is therefore to be rejected (Verdun).

Keibel Mall 2 330.jpg

Fig. 330. — Section through the laryngeal region of the embryo Nat2 of the First Anatomical Institute, Vienna (19.75 mm. vertex-breech length). Cr., cricoid; Ep. Ill, IV, epithelial bodies; Sy., sympathicus; Thym., thymus; Thym.-Cy., small cyst of the thymus; Thyr., thyreoid; ub. K., ultimobranchial body; Vag., vagus; W., vertebra. X 60.

Toward the centre of the lateral lobes of the thyreoid there is to be found for some time (in fetuses of about 50 mm. vertexbreech length), as an expression of the more rapid growth of the lobes, a closer grouping of the thyreoid cords with a slighter development of the connective tissue; the differentiation of the cords takes place principally at the periphery. In this region there occurs in the stage mentioned the formation of lumina in the cell cords, which consequently appear beaded, and then the cords become divided up into individual groups of cells, the anlagen of follicles, which in part possess a lumen before becoming constricted off, although for the most part the lumina appear later and successively, even forming to some extent in the first years of childhood.

According to the Normentafel, the (middle) anlage of the thyreoid shows indications of a bilobed condition in embryos of about 5 mm. ; the occurrence of a lumen is variable. The thyreoglossal duet loses its lumen at a slightly earlier stage. It becomes drawn out to a long solid cord, which is broken in embryos between 6 and 7 mm.; it is occasionally distinguishable in later stages and remains of it may be found in embryos of 14 mm. In those of 8 mm. the anlage begins to separate into cords. — The ductus thyreoglossus, or its remains, occurs ventral to the hyoid bone and therefore between the derivatives of the first and second arches (His). The ultimobranchial body makes its appearance, according to Hammar, in embryos of 5 mm. as a cylindrical transformation product of the fifth pouch, or, it might be said, as an appendage of the fourth; the epithelial bodies IV, as well as the thymus anlagen, are defined in embryos of 8 mm. (Tandler, 1909), occasionally perhaps not until somewhat later (see p. 465). The caudal pharyngeal pouch complex separates from the pharynx in embryos of about 14 mm. (frequently only later, according to Hammar, 1904, in embryos over 18.5 mm. in length), and applies itself directly to the thyreoid. A little later, in embryos of somewhat over 15 mm., the lumen of the ultimobranchial body disappears. The small-celled proliferation of the ultimobranchial body, mentioned and figured above, can be perceived in two embryos in the collection of the First Anatomical Institute, Vienna (Nat. 1, with a length of 19.75 mm., and T. 1, with a length of 23 mm.) ; it seems also to have been observed by Tourneux and Verdun (1897) in an embryo of 19 mm. The denser grouping of the embryonic thyreoid cells in the lateral lobes of somewhat later stages (see p. 469), which these authors have also noticed, is not to be referred to the ultimobranchial bodies, as they have supposed. The (unpaired, middle) thyreoid anlage has been known since Rathke's time. That a derivative of the pharyngeal pouch region becomes associated with the thyreoid anlage in mammals was first observed by Wolfler (1880), and firmly established by Stieda (1881) and Born (1883) ; from the latter comes also the term lateral or posterior thyreoid anlage, which has been applied to the untimobranchial body, but which is rejected in the account given above.

In man the formation of the branchial derivatives is less completely and less easily followed than in many other mammals, as, for instance, the cat, but, on the other hand, more completely than in such a form as the rat. The differences which have been found in different species are partly to blame for the confusion which has long prevailed with regard to the development of these structures. Even in man the development of the individual organs has never yet been systematically followed throughout.

In correspondence with the extensive dislocation of the thymus, the epithelial body of the third pouch undergoes a much greater migration than that of the fourth, passing beyond it to come to rest at the lower border of the thyreoid. Consequently it appears as the inferior epithelial body, in contrast to the superior body of the fourth pouch situated at about the middle of the posterior (dorsal) surface of the thyreoid. This latter, in correspondence with the fusion that occurs between the ultimobranchial body and the thyreoid, in certain animals (rabbit, cat) regularly and in man frequently becomes more or less enclosed within the thyreoid, and then appears as an internal epithelial body, with which the thymus IV may be associated as an internal thymus lobe, in contrast to the external one arising from the third pouch. Nevertheless all these conditions are very variable, and striking anomalies of position, as well as diminution and increase of number of the epithelial bodies, occur. Thus, that of the third pouch may remain even in man near its place of origin, not far from the division of the carotid (p. 460), or, on the other hand, it may descend into the thoracic cavity with the thymus. A diminution in the number of epithelial bodies is very difficult to demonstrate, on account of the possible occurrence of anomalies in position ; increase, probably by division of the anlagen, was first observed by Kursteiner and has since been repeatedly seen; Zuckerkandl has described a case in which there were eight, and Erdheim one with eight and one with twelve. The epithelial body III seems especially subject to division.

An internal epithelial body completely surrounded by the thyreoid is very rare in man, according to Getzowa (1907). According to the same authoress, cell cords of typical epithelial body tissue may occur in the interior of the thyreoid even when an external epithelial body IV is present. She is inclined to ascribe these cords to epithelial bodies of the fifth pouches, but embryological confirmation of this idea is as yet wanting. — The retrogression and atrophy of the cranial end of the thymus occasionally fails to take place, especially in connection with certain variations of the cervical nerves; a thymus lobe then occurs high up in the neck (Bien 1906 and 1907, Hammar 1910).

The ultimobranchial body in all vertebrates below the mammals is an independent structure which assumes a glandular character, produces alveoli and cell cords, but develops no colloid. In Echidna, according to Maurer, who has thoroughly studied the whole question, the body is also independent, but develops alveoli with colloid; nevertheless this material has been identified with that of the thyreoid only on the basis of its staining reactions. In all the higher mammals the body fuses with the middle thyreoid anlage, and its further history cannot then be followed with certainty. In many mammals a cyst can be found situated beside the internal epithelial body, surrounded by thyreoid tissue, and frequently finished with a ciliated epithelium and possessing mucous glands in its wall (central canal of the thyreoid of Prenant, vesicule postbranchiale of Herrmann and Verdun) ; furthermore there may be cell cords which extend into the interior of the thyreoid and vesicles, which are not always to be distinguished from undeveloped thyreoid tissue (Herrmann and Verdun 1899, Sehaffer 1909). Similar rudiments occasionally occur in older human fetuses, of 55 to 65 mm. vertex-breech length, according to Herrmann and Verdun (1899). The cysts and glands have been derived by most authors from the ultimobranchial bodies themselves, yet some of them at least may represent the remains of the fourth and even of the fifth pharyngeal pouch (Groschuff, 1896). If this be the case, then only the cell cords and the small vesicles can be ascribed to the ultimobranchial bodies, these, however, occurring distinctly only in a few species (dromedary, sheep, cow, hedgehog, mole), as well as cysts whose epithelium is in a state of proliferation and is producing the cell cords (Herrmann and Verdun 1900). These authors also describe a case of an ultimobranchial body remaining independent in a camel one year of age ; its structure was that of a gland, which was quite different from the thyreoid but rudimentary, and consisted of coiled tubules, closed vesicles, massive cell columns, and cell spheres.

In thyreo-aplasia, the defect of the (middle) thyreoid anlage, the ultimobranchial body gives rise to no thyreoid tissue (Maresch 1898, Peucker 1899, Erdheim 1904). In such cases one finds in addition to the epithelial body IV larger and smaller cysts, partly with contents which stain bright red with eosin; beside the cysts lie some lobes of serous or mucous gland tissue, and in one case Erdheim found immediately beside the cyst " a thin layer of small epithelial cells with dark nuclei." In any event these observations are opposed to the formation of thyreoid tissue from the ultimobranchial bodies; their significance is in harmony with the view stated above. The bodies are essentially rudiments, and one need not assume, as Erdheim has done, that only in thyreo-aplasia " the lateral thyreoid anlagen are also aplastic." In the atrophic thyreoids of cretins and idiots Getzowa (1907) observed cell masses and cords which likewise point to an occasional persistence of the ultimobranchial bodies in man. They correspond histologically with no other glandular tissue of the region, and are composed of large polyhedric cells rich in protoplasm and with large nuclei moderately rich in chromatin. In addition there were also small cysts which were not formed of thyreoid tissue. According to the same authoress, struma? may arise from the parathyreoids or from the ultimobranchial bodies as well as from the thyreoid, whence the form variability of these tumors.

In general the so variable behavior of the ultimobranchial bodies throughout the whole mammalian series may be explained on the supposition that in the mammalia they constitute functionless rudiments (Herrmann and Verdun). Groschuff (1896) rightly sees, in the union of the bodies with the thyreoid, a condition that is confined to the mammalia, a process which essentially corresponds to the formation of an internal epithelial body or thymus lobe, but does not justify the derivation of the thyreoid from different anlagen.

The thymus of the mammalia is not directly homologous with that of the lower vertebrates, since in the latter it owes its origin to dorsal and in the former to ventral diverticula of the pharyngeal pouches. A harmonizing of the relations seems to Maurer to be made possible by the conditions in Lacerta, in which a transitory slender ventral process of the third pharyngeal pouch is formed, which occupies the same position as the thymus anlage of the mammalia, but is not transformed into thymus tissue, but atrophies. Further the extension of the thymus anlage upor the dorsal wall of the pharyngeal pouch itself may be mentioned in this connection. — The sometimes occurring differentiation of the ventral diverticulum of the fourth pouch into thymus tissue has never been followed directly, but has been assumed on account of the occasional occurrence of thymus lobes on the epithelial body IV. — In the rabbit the second pouch also gives rise to a transitory ventral diverticulum, that is to say, to a thymus anlage (compare p. 456, footnote).

The persistence of portions of the branchial system of cavities may give rise to branchiogenic fistulae, cysts, and tumors (see especially Hammar, 1904). The region that corresponds to the outer opening of the ductus branchialis II and the ductus cervicalis is to be found at the anterior border of the m. sternocleidomastoideus. The second pharyngeal pouch corresponds to the tonsillar depression; the openings of the ductus pharyngobranchialis III and pharyngobranchialis communis are to be looked for near the larynx, about in the region of the sinus pyriformis, whence the n. laryngeus superior, as a branch of the fourth branchial arch, must pass between them. A fistula of the second cleft must lie, if the development of the vessels be normal, between the external and internal carotids and ventral to the glossopharyngeus and vagus; a fistula of the third cleft, between the common carotid and vagus, as well as between the glossopharyngeus and laryngeus superior; while a fistula of the fourth cleft must bend around the subclavian on the right and the arch of the aorta on the left, since these are derivatives of the fourth aortic arch. The occurrence of these fistulse is, therefore, very unlikely; the fistula of the second cleft is the only one that has hitherto been recognized with perfect certainty. — As to the persistence of the ductus tbyreoglossus and the formation of median cervical fistuhe from it, the resume of Erdheim (1904) may be consulted.

A. Literature on the Development of the Pharynx

Bien, G. : Ueber accessorische Thymuslappen imj trigonum caroticum, Anat. Anz., vol xxix, 1906.

Bien, G. : Ueber accessorische Thymuslappen im trigonum caroticum bei einem Embryo von 17 mm. grosster Lange, Anat. Anz., vol. xxxi, 1907.

Born, G. : Ueber die Derivate der embryonalen Schlundbogen und Schlundspalten bei Saugetieren, Arch, fiir mikr. Anat., vol. xxii, 1883.

Dandy WE. A human embryo with seven pairs of somites measuring about 2 mm in length. (1910) Amer. J Anat. 10: 85-109.

Elze, C. : Beschreibung eines raenschlichen Embryo von ea. 7 mm. grosster Lange, etc., Anat. Hefte, vol. xxxv, 1907.

Erdheim, J.: I. Ueber Sebilddriisenaplasie. II. Geschwiilste des ductus thyreoglossus. III. Ueber einige menschliche Kiemenderivate. Beitr. zur path. Anat. und allg. Path., vol. xxxv, 1904.

Fox H. The pharyngeal pouches and their derivatives in the mammalia. (1908) Amer. J Anat. 8(3): 187-250.

Getzowa, S. : Ueber die glandula paratbyreoidea, intrathyreoidale Zellhaufen derselben und Reste des postbranchialen Korpers, Arch, fur path. Anat., vol. clxxxviii, 1907.

Greil, A. : 1905. See p. 496.

Groschupp, K. : Bemerkungen zu der vorlaufigen Mitteilung von Jacoby : Ueber die Entwicklung der Nebendrusen der Sehilddruse und der Carotidendriise, Anat. Anz., vol. xii, 1896.

Groschupp, K. : Ueber das Vorkommen eines Tbymussegmentes der vierten Kiementasche beim Menschen, Anat. Anz., vol. xvii, 1900.

Grosser, O. : Zur Kenntnis des ultimobrancbialen Korpers beim Menchen, Anat. Anz., vol. xxxvii, 1910.

Grosser, O. : Der Nerv des funften Visceralbogens beim Menschen, Anat. Anz., vol. xxxvii, 1910.

Grosser, O. : Zur Entwicklung des Vorderdarmes menscblicher Embryonen bis 5 mm. grosster Lange. Sitzber. R. Akad. Wiss. Wien, vol. cxx, 1911.

Gruenwald, L. : Ein Beitrag zur Entstehung und Bedeutung der Gaumenmandeln, Anat. Anz., vol. xxxvii, 1910.

Hammar, J. A.: Studien iiber die Entwicklung des Vorderdarmes und einiger angrenzender Organe. I. Abth. Allgemeine Morphologie der Schlundspalten beim Menschen. Entwicklung des Mittelohrraumes und des ausseren Gehorganges, Arch, fur mikr. Anat., vol. lix, 1902. — II. Abth. Das Schicksal der zweiten Schlundspalte. Zur vergleichenden Embryologie und Morphologie der Tonsille, Arch, fiir mikr. Anat, vol. Ixi, 1903.

Hammar, J. A. : Ein beachtenswerter Fall von kongenitaler Halskiemenfistel nebst einer Uebersicht iiber die in der normalen Ontogenese des Menschen existierenden Vorbedingungen solcber Missbildungen, Beitr. zur path. Anat. und allg. Path., vol. xxxvi, 1904.

Hammar, J. A.: Ueber die Natur der kleinen Thymuszellen, Arch, fiir Anat. u. Phys. Anat. Abth. 1907.

Hammar, J. A. : Fiinfzig Jahre Thymusf orschung. Kritische Uebersicht der normalen Morphologie, Ergebn. der Anat. u. Entwicklungsgesch., vol. xix, 1910.

Hammar, J. A.: Zur groberen Morphologie und Morphogenie der Menschen thymus. Anat. Hefte, vol. xliii, 1911.

Herrmann, G., and Verdun, P. : Notes sur l'anatomie des eorps post-branchiaux. Miseellanees biologiques dediees au professeur Alfred Giard. Paris 1899.

Herrmann, G., and Verdun, P. : Persistance des corps post-branchiaux chez l'homme. Remarques sur l'anatomie comparee des corps post-branchiaux. Comptes Rend. Soc. Biol. Paris, 1899.

Herrmann, G., and Verdun, P. : Note sur las corps post-branchiaux des Cameliens.— Les corps post-branchiaux et la thyroide; vestiges kystiques. Comptes Rend. Soc. Biol. Paris, 1900.

His, W. : Anatomie menschlicher Embryonen, Heft 3, Leipzig, 1885.

His, W. : 1. Ueber den sinus praecervicalis und iiber die Thymusanlage. 2. Nachtrag zu vorstehender Abhandlung. Arch, fiir Anat. u. Phys. Anat. Abth. 1886.

His, W. : Schlundspalten und Thymusanlage, Arch, fiir Anat. u. Phys. Anat. Abth. 1889.

Ingalls, N. W. : Beschreibung eines menschlichen Embryo von 4.9 mm. Lange, Areb. fiir mikr. Anat., vol. lxx, 1907.

Kastschenko, N. : Das Scbicksal der embryonalen Scblundspalten bei Saugetieren, Arcb. fiir mikr. Anat., vol. xxx, 1887.

Kohn, A.: Studien iiber die Scbilddriise, I and II, Arch, fiir mikr. Anat., vols. xliv and xlviii, 1895 and 1896.

Kohn, A.: Die Epithelkorperchen, Ergebn. der Anat. u. Entwicklungsgescb., vol. ix, 1899.

Kroemer: Wacbsmodell eines jimgen menschlichen Embryo, Verb. d. Deutsch. Ges. fiir Gynak., 1903.

Kuersteiner, W. : Die Epithelkorperchen des Menschen in ibrer Beziehung zur thyreoidea und thymus, Anat. Hefte, vol. xi, 1898.

Low A. Description of a human embryo of 13-14 mesodermic somites. (1908) J Anat Physiol. 42(3): 237-51. PMID 17232769 | PMC1289161

Mabesch, R. : Kongenitaler Defekt der Schilddriise bei einem elf jahrigen Madchen mit vorhandenen " Epitbelkorpercben," Zeit. fiir Heilk., vol. xix, 1898.

Maurer, F. : Die Schilddriise, thymus und andere Schlundspaltenderivate bei der Eidechse, Morph. Jahrb., vol. xxvii, 1899.

Maurer, F. : Die Entwicklung des Darmsystems, Hertwig's Handb. der vergl. u. exp. Entwicklungslehre, vol. ii, 1902.

Maximow, A. : Untersuchungen iiber Blut und Bindegewebe. II. Ueber die Histogenese der thymus bei Saugetieren, Arch, fiir mikr. Anat., vol. lxxiv, 1909.

Meyer, R. : Ueber Bildung des Recessus pharyngeus medius s. Bursa pharyngea im Zusammenhang mit der Chorda dorsalis bei menschlichen Embryonen. Anat. Anzeiger. Bd. 37. 1910.

Peuker, H. : Ueber einen neuen Fall von kongenitalem Defekt der Schilddruse mit vorhandenen " Epithelkorperchen," Zeitschr. f . Heilk., vol. xx, 1899.

Piersol, G. A. : Ueber die Entwicklung der embryonalen Scblundspalten und ihre Derivate bei Saugetieren, Zeit. fiir wiss. Zool., vol. xlvii, 1888.

Prenant, A. : Contribution a l'etude organique et histologique du thymus, de la glande thyroide et de la glande carotidienne, La Cellule, vol. x, 1894.

Prenant, A. : Sur le developpement des glandes accessoires de la glande thyroide et celui de la glande carotidienne, Anat. Anz., vol. xii, 1896.

Rabl, C. : Zur Bildungsgeschichte des Halses, Prager ined. Wochenschr., vols, xi and xii, 1886 and 1887.

Rabl, H. : Ueber die Anlage der ultimobranchialen Korper bei den Vogeln, Arch. fur mikr. Anat., vol. lxx, 1907.

Schaffer, J., and Rabl, H. : Das thyreothymische System des Maulwurfs und der Spitzmaus. I. Morphologie und Histologic by J. Schaffer. II. Die Entwicklung des thyreo-thymischen Systems beim Maulwurf by H. Rabl. Sitzber. kais. Akad. Wiss. Wien, vols. 117 and 118, 1908 and 1909.

Simon, Ch. : Thyroide laterale et glandule thyroidienne chez les mammif eres, Nancy, 1896.

Stteda, L. : Untersuchungen iiber die Entwicklung der glandula thymus, glandula thyreoidea und glandula carotica. Leipzig, 1881.

Stoehr, P. : Die Entwicklung des adenoiden Gewebes, der Zungenbalge und der Mandeln des Menschen, Festschr. z. fiinfzig jahrigen Doktorjubilaum von Nageli und Kolliker, 1891. (Author's abstract in Anat. Anz., vol. vi, 1891 1892.)

Stoehr, P. : Ueber die Natur der Thymuselemente, Anat. Hefte, vol. xxxi, 1906.

Stoehr, P. : Ueber die Abstammung der kleinen Thymusrindenzellen, Anat. Hefte, vol. xii, 1910.

Sudler MT. The development of the nose and of the pharynx and its derivatives in man. (1902) Amer. J Anat. 1:391–416.

Tandler, J. : Ueber die Entwicklung des V. Aorteubogens und der V. Schlundtasche beirn Menschen, Anat. Hefte, vol. xxxviii, 1909.

Tettenhamer, E. : Ueber das Vorkommen offener Seblundspalten bei einem mensch lieben Embryo, Miincbener med. Abhandl., Series 7, part 2, 1892.

Thompson P. Description of a human embryo of twenty-three paired somites. (1907) J Anat Physiol, 41(3):159-71. PMID 17232726

Thompson P. A note on the development of the septum transversum and the liver. (1908) J Anat Physiol. 42(2): 170-5. PMID 17232762

Tourneux, F., and Soulie, A.: Sur l'existence d'une V. et d'une VI. poche endodermique ehez l'embryon humain, Comptes Rendus Soe. Biol. Paris, .1907.

Tourneux, F., and Verdun, P. : Sur les premiers developpements de la thyroide, du thymus et des glandules thyroidiennes, Journ. de l'Anat. et de la Phys., vol. xxxiii, 1897.

Verdun, P. : Derives branchiaux chez les Vertebres superieurs, These, Toulouse, 1898.

Woeleler, A. : Ueber die Entwicklung und den Bau der Schilddruse mit Riicksicht auf die Entwicklung der Kropfe. Berlin, 1881.

Zuckere:andl, E. : Die Epithelkorperchen von Didelphys azara nebst Bemerkungen iiber die Epithelkorperchen des Menscben, Anat. Hefte, vol. xix, 1902.

Zuckerkandl, E. : Die Entwicklung der Schilddruse und der thymus bei der Ratte, Anat. Hefte, vol. xxi, 1903.


  1. In some mammals an epithelial body is actually situated at the bifurcation of the carotid, as, for instance, in Echidna (Maurer), the sheep (Prenant) and Didelphys azara (Zuckerkandl) ; in the last the glomus caroticum is also recognisable.
  2. The structure in Figs. 317, 318, and 331 marked as a doubtful fifth pouch is perhaps merely an analogue of the diverticulum described on p. 452.
  3. Compare, however, p. 471 (Getzowa) as regards the occurrence of a corresponding epithelial body.
  4. The thymus canal, described by older authors as occurring in later stages, does not exist.

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Grosser O. Lewis FT. and McMurrich JP. The Development of the Digestive Tract and of the Organs of Respiration. (1912) chapter 17, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

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