Difference between revisions of "Paper - The fate of the ultimobranchial bodies in the pig (1918)"

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| [[File:Mark_Hill.jpg|90px|left]] This historic 1918 paper by Badertscher describes the fate of the ultimobranchial bodies in the {{pig}}. The embryonic ultimobranchial body gives rise to the parafollicular cells (C cells) of the {{thyroid}} gland.
 
| [[File:Mark_Hill.jpg|90px|left]] This historic 1918 paper by Badertscher describes the fate of the ultimobranchial bodies in the {{pig}}. The embryonic ultimobranchial body gives rise to the parafollicular cells (C cells) of the {{thyroid}} gland.
 
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See also by this author: {{Ref-Badertscher1919}}
 
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Badertscher JA. The fate of the ultimobranchial bodies in the pig (Sus scrofa). (1918) Amer. J Anat. 23: 89-131.

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This historic 1918 paper by Badertscher describes the fate of the ultimobranchial bodies in the pig. The embryonic ultimobranchial body gives rise to the parafollicular cells (C cells) of the thyroid gland.


See also by this author: Badertscher JA. The ultimobranchial bodies in postnatal pigs (Sus scrofa). (1919) Amer. J Anat. 25: 13-26.

Modern Notes: thyroid | pig

Endocrine Links: Introduction | BGD Lecture | Science Lecture | Lecture Movie | pineal | hypothalamus‎ | pituitary | thyroid | parathyroid | thymus | pancreas | adrenal | endocrine gonad‎ | endocrine placenta | other tissues | Stage 22 | endocrine abnormalities | Hormones | Category:Endocrine
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1903 Islets of Langerhans | 1904 interstitial Cells | 1908 Pancreas Different Species | 1908 Pituitary | 1908 Pituitary histology | 1911 Rathke's pouch | 1912 Suprarenal Bodies | 1914 Suprarenal Organs | 1915 Pharynx | 1916 Thyroid | 1918 Rabbit Hypophysis | 1920 Adrenal | 1935 Mammalian Hypophysis | 1926 Human Hypophysis | 1927 Adrenal | 1927 Hypophyseal fossa | 1930 Adrenal | 1932 Pineal Gland and Cysts | 1935 Hypophysis | 1935 Pineal | 1937 Pineal | 1938 Parathyroid | 1940 Adrenal | 1941 Thyroid | 1950 Thyroid Parathyroid Thymus | 1957 Adrenal



Pig Links: Introduction | Estrous Cycle | 1897 Pig Embryo Development Plates | 1951 Pig Embryology | Category:Pig

  Historic Papers: 1894 Blastodermic Vesicle | 1903 12mm Pig | 1903 Adrenal | 1905 Thymus | 1908 Pancreas | 1908 Pharyngeal Pouches | 1908 Intestinal Diverticula | 1910 Hypoglossal Ganglia | 1911 Prenatal Growth | 1911 Embryo 7.8 mm | 1916 Colon | 1916 Yolk Sac | 1918 Wolffian body | 1919 Corpus Luteum | 1919 Postnatal Thyroid | 1919 Placental Cord | 1921 Estrous and Implantation | 1922 Limb Arteries | 1924 Pig | 1937 Coronary Circulatory | 1938 Abnormal Brain

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The Fate of the Ultimobranchial Bodies in the Pig

J. A. Badertscher

Deparlmenl of Anatomy, Indiana University School of Medicine, Bloomington, Indiana

Four Plates

I. Introduction

The fate of the ultimobranchial bodies is one of the many unsettled questions associated with the development of the thyroid gland. While most of the results of the considerable amount of investigation that has been done in recent years on these structures in various mammalian species have led to the interpretation that they do not contribute to the structural elements of the thyroid gland, there is still a diversity of views as to their actual fate. The variety of views expressed in the literature in regard to their fate in mammals is apparently due to several factors among which may be mentioned: (1) the possibility of a variable developmental behavior of these structures in different mammalian types, (2) inadequate series of successively older embryonic stages (especially embryos of larger mammals) , and (3) faulty technique (principally poor fixation of the thyroid gland in the older embryos of the larger mammals, especially man).

1 Some of the younger stages used in this study were prepared during the summer of 1914 while a guest in the Department of Histology and Embryology in Cornell University. I wish to express my appreciation to Prof. B. F. Kingsbury for the facilities so generously extended to me during that time.


A study of the ultimobranchial bodies in a wide range of successively older developmental stages of pig embryos of which the thyroid gland was well fixed, has resulted in bringing to light some interesting and important developmental features of these structures in this mammalian type.^

II. Historical

A brief historical outline representing in a general way the different views in regard to the fate of the ultimobranchial bodies is here given. The works of Verdun ('98) and Grosser ('12) render an extensive bibliography in this article unnecessary.

Born ('83) claims for the thyroid in the pig a triple origin. In a 21 mm. embryo he finds that the nuclei and cytoplasm of the cells composing the lateral thyroids stain more intensely than the cells of the median thyroid, but in a 37 mm. embryo in which the lateral thyroids have become imbedded in and fused with the median thyroid there is no histological difference between these structures and the median thyroid.

According to Moody ('12) in pig embryos 100 mm. in length no difference is to be observed between the central and lateral parts of the gland in vascularity, colloid formation or connective tissue development." He believes that the ultimobranchial bodies contribute to the structural elements of the thyroid gland.

Simon ('96) claims that in mammals (guineapig, rabbit, cat, calf, sheep, and pig) the lateral thyroids do not actually fuse with the elements of the median thyroid, although they become entirely inbedded in the latter. The lateral thyroids in early developmental stages show signs of growth and further development. During this period, which he designates the periode d'activite, the lateral thyroids are broken up into cell cords and cell masses. This is brought about in an entirely passive way, by the ingrowth of vascular tissue and of elements from the median thyroid. These cell cords and cell masses stain more intensely than do the elements of the median thyroid. Traces of the lumen persist mainly in the more central part of the lateral thyroids. These structures in later developmental stages undergo degenerative changes. This period, which he designates the periode de survivance, is characterized by a disappearance of the cell cords and the degeneration of the more centrally located epithelial cells, forming cysts lined with cuboidal or columnar epithelium which may or may not be ciliated. He also claims that the formation of cysts in the lateral thyroids of pig embryos is not a constant occurrence. Cysts in these structures in the pig were found only in five out of eleven embryos which he examined. In a 210 mm. embryo, the largest examined, no traces of the lateral thyroids were found. He is of the opinion that these structures in the pig disappear entirely.


  • No consideration was given to the origin of the structure variously termed 'ultimobranchial body,' 'postbranchial body,' 'suprabranchial body,' 'telqbranchial body,' and 'lateral thyroids.' The morphological value of these terms have been discussed by Greil ('05), Rabl ('09), Kingsbury' ('14), and others. Throughout the descriptive part of this work the term 'ultimobranchial body' will l^e used.


Rabl ('08) finds that in the older mole embryos the lateral thyroids are reduced to insignificant structures, being represented by cell cords and cysts.

Verdun ('98) finds that in birds (chicken and duck) the postbranchial bodies remain independent structures of a glandular character but do not produce colloid. He regards these structures as special glands for birds. In the thyroid of mammals (rabbit, cat, dog, mole) the postbranchial bodies are represented by cysts and cell cords. The cysts vary greatly in size in the different species studied. Neither during the embryonic nor the postnatal life of these mammals was he able to demonstrate the transformation of the epithelial cords of the postbranchial bodies into thyroid follicles. He beheves that the cysts and cell cords represent atrophied vestiges of the special gland in birds.

According to Tourneaux and Verdun ('97) the lateral thyroids in human embryos can for some time be recognized as a rather densely staining mass on the posterior surface of the lateral lobes of the thyroid gland. They undergo the same structural changes as the median thyroid, but more slowly. From the median thyroid anlage, however, is derived the larger part of the structural elements of the thyroid gland.

According to Christiani, in rodents (rat) the lateral thyroid gives rise to an epithehal body.^

Maurer ('99) finds that in the Echidna the postbranchial bodies do not fuse with the median thyroid anlage. In the adult condition the thyroid lies posterior to the postbranchial bodies. The latter are represented by two alveolar structures which developed colloid (judged by staining reaction). The first traces of colloid formed in the postbranchial bodies appears, however, in later developmental stages than does that of the median thyroid.

Prenant ('94) finds that in sheep embryos the lateral thyroid develops into a central canal with an irregular lumen from the walls of which cell cords and cell masses (recognized by their dense structure) extend into the substance of the median thyroid. An intimate fusion takes place between the lateral and medial elements. In later developmental stages the tissue, which in earlier stages can be recognized as derived from the lateral thyroids, disappears. He was unable to determine whether or not the lateral thyroids contribute to the structural elements of the gland.

In this brief historical sketch the following views as to the fate of the ultimobranchian bodies were brought out : (1) They contribute to the structural elements of the thyroid gland; (2) They develop into cysts; (3) They develop into a gland of a different structure from that of the thyroid gland; (4) They develop into epithehal bodies, and (5) They disappear entirely.

III. Material and Methods

The material used for this investigation was collected in great abundance at a packing house. The upper jaw, cranium, and thorax were removed from embryos ranging from 15 to 25 mm. in length. The part containing the thyroid was thus made comparatively small and fixed well. From embryos 26 to 75 mm. in length only the neck, from which the sides and the cervical vertebrae were removed, was reserved. From embryos 100 to 270 mm. in length (full term) only the thyroid with some of the surrounding structures — trachea, esophagus, a portion of the thymus, etc., was removed. The length in millimeters of the different developmental stages of which the thyroid was prepared for a study of the ultimobranchial bodies is as follows: 15, 16, 17, 17, 18, 18, 19, 19.5, 20, 20, 21, 21, 21.5, 22, 22, 23, 23, 23, 24, 24.5, 25, 25, 26, 27, 27, 28, 29.5, 30, 33, 35, 37.5, 38, 40, 48, 53, 60, 65, 65, 75, 100, 100, 111, 125, 125, 145, 150, 160, 175, 225, 245, 270, and 270. These figures represent the length of the embryos while in a fresh condition.

» Cited from Zuckerlandl ('03).


The fixing fluids employed were Zenker's fluid, Zenker-formol, and Picro-aceto-formol. The materia) was imbedded in paraffi.n. The earlier embryos were cut transversely in sections 5 microns thick, while those of later stages in sections 8 to 10 microns thick. Various stains were used. For embryos 15 to 65 mm. in length, iron hematoxylin gave the best results. The thyroid gland of later developmental stages was stained with Chloral hematoxylin and eosin, and eosin-methylene blue.

IV. Description of Stages

The earliest stage chosen for description is one just before the ultimobranchial bodies have fused with the thyroid gland. 'These structures will be described in two embryos of the same size only when there is a marked contrast in their size, structure, or position in the thyroid in the two embryos.

Embryo of 18 mm

(fig. 1)

The ultimobranchial bodies have lost their connection with the fourth (?) pharyngeal pouch and extend cephalad beyond the anteror margin of the thyroid gland. Their anterior portion is in form a slender tube, ovaF in cross section, and with wall two to three layers of cells (nuclei) thick. Caudalward the walls of these structures gradually becomes thicker. In the portion in relation to the thyroid gland the lumen in places is obliterated and the remnants persist as mere slits. Anteriorly the surface of these tubules is quite smooth, while caudal] y irregularities occur on their surface. Their caudal halves lie at varying distances dorsal to the lateral margin of the thyroid gland which has the general shape of a crescent with its horns directed anteriorly and dorsally. In a few sections they are separated from the thyroid only by a very thin layer of connective tissue iU). They extend almost to the caudal margin of the thyroid gland. Their caudal ends lie more closely together than their anterior ends.

The ultimobranchial bodies at this stage are composed of a syncytium. No cell walls are present. Vacuoles are found throughout their entire extent, although their distribution is not uniform. In places they can be found throughout an entire cross section of an ultimobranchial body while in other places they are confined to its more central portion. The vacuoles vary in size, the largest being almost as large as some of the nuclei.

The nuclei vary somewhat in size and in shape. Some are oval, some round, while others are irregular in outline. They contain from one to three nucleoli and a rather generous amount of chromatin which is in the form of granules and threads. The more centrally located nuclei have no regular arrangement while those near the periphery are in places quite regularly arranged. They are more closely packed together in the nonvacuolar than in the vacuolar portions of the ultimobranchial bodies. A consideration of this feature is of particular importance in stages in which the ultimobranchial bodies have fused with the thyroid gland. Mitoses of the nuclei can readily be found, especially in the larger more caudal part of the bodies, thus indicating a growth tendency of these structures. Neither blood vessels nor connective tissue are present in the ultimobranchial bodies at this stage.

The thyroid {T) is composed of nonvacuolar cell masses and cell cords^ the latter of which are for the most part transversely arranged. No cell walls can be demonstrated, hence the cell cords and cell masses have a syncytial structure. The nuclei vary in shape and somewhat in size but their form, average size, and structure in this stage is the same as in the ultimobranchial bodies. The nuclei of the thyroid are more closely packed together than in the vacuolar portions of the ultimobranchial bodies, but when a nonvacuolar portion of the latter is brought into the same microscopic field wdth a portion of the thyroid gland, no difference in structure can be seen between them even under high magnification (1500 diameters). Some of the spaces between the cords of cells are lined with endothelium and contain blood.


  • Norris ('16) finds that in early developmental stages of human embiyos the cell cords seen in cross sections of the thyroid gland represent in reality sections of fenestrated epithelial plates. As I have not made a careful study of the formation of the follicles in the thyroid gland I shall use the term 'cell cords' which is the microscopic picture presented in cross sections of the gland.


Embryo of 19.5 mm

(figs. 2 a and 2 b)

The ultimobranchial bodies extend slightly farther cephalad than the thyroid gland. Only slight traces of their lumen still persist. They lie along the dorsal surface of the thyroid gland but are located nearer the mesial plane than those of the preceding stage. In some places there is actual fusion between these structures and the thyroid gland (fig. 2 a, right side), while in other places a thin layer of connective tissue intervenes (fig. 2 a, left side). The ultimobranchial body on the left side extends almost to the caudal margin of the thyroid gland (fig. 2 b), w^hile on the right side it terminates twelve sections (5 microns in thickness) earlier. The shape and orientation of the thyroid gland is similar to that in the preceding stage.

In this stage, as in the preceding one, both vacuolar and nonvacuolar areas are found in places along the periphery of the ultimobranchial bodies. In some places where actual fusion has taken place wdth the thyroid gland it is impossible to tell w'here the two striictures meet. Fusion with the thyroid gland has taken place along the ventro-lateral surface of the ultimobranchial bodies. The dorso-medial surface of these structures is in places studded with epithelial buds (fig. 2 a) .

In the ultimobranchial bodies of this developmental stage are found a few nuclei in which the nucleoplasm stains quite deeply in comparison with that in the large majority of nuclei present. In some of these nuclei the chromatin is more abundant than in the more numerous and more lightly stained ones but in both types of cells it is distributed in the form of threads and granules. With comparatively low magnification they appear as dark specks among the other nuclei (fig. 2 a). Since these have apparently been regarded by some investigators as degenerating nuclei, they deserve special attention in successively older developmental stages.

Embryo of 20 mm

(figs. 3 a, 3 b, and 3 c)

The uLtimobranchial bodies are small anteriorly, and extend slightly farther cephalad than the thyroid gland. The one on the left side is separated for a short distance from the extreme anterior part of the thyroid (fig, 3 a). Caudal ward these structures rapidly become larger and form the greater portion of the horns of the crescent shaped tripartite complex. The one on the left side is slightly larger, and eleven sections (5 microns in thickness) longer than the one on the right side and extends as far caudally as does the thyroid gland. The extreme caudal portion of these structures is not fused with the thyroid gland. Remnants of the lumen are present in two places in the anterior third of the left one.

A feature quite noticeable in the ultimobranchial bodies of this developmental stage is the presence of unusually small nuclei which are found in small groups and promiscuously scattered among those of usual size. From their similarity in structure to the larger nuclei they seem to be normal. Deeply stained nuclei, which are somewhat more numerous throughout these structures than in the preceding stage, are also present in these groups.

It is impossible to determine definitely the exact place of fusion between the ultimobranchial bodies and the thyroid gland. Judging, however, from the uniformity of the distribution of the deeply stained nuclei, from the absence of cell cords along the greater portion of their dorso-mesial free border, from the manner in which they terminate, as stated above, from the absence of blood vessels, and from the distribution of the small nuclei and vacuoles, it seems that the cell masses labeled ultimobranchial bodies in the figures 3 a, 3 b, and 3 c represent exclusively the ultimobranchial bodies.


Epithelial buds, as represented in figure 3 b (Ep.B.), are present in various places along the free border of these structures. These buds are fused to the more or less vacuolar mass of cells. Mitosis can be found without much searching in both the ultimobranchial bodies and the thyroid gland.

Embryo of 21 mm

(fig. 4)

Both ultimobranchial bodies are as long as the thyroid gland. They are fused to the latter along their entire extent excepting the extreme caudal end of the left one which is separated from the gland by a thin layer of connective tissue. The one on the right side has a comparatively regular outline and makes up nearly all of the lateral portion of the tripartite complex ([/). The ultimobranchial body on the left side is more deeply embedded in the thyroid gland than the right one which makes it difficult to follow its extent in transverse sections. In places blunt and both vacuolar and nonvacuolar epithelial buds are attached to these structures.

Groups of small nuclei in the ultimobranchial bodies are present but they are not as mumerous as in the preceding stage. The darkly stained nuclei are no more numerous than in the previous stage. A few darkly stained nuclei were found in the cell cords of the thyroid gland. These have a structure similar to the darkly stained nuclei of the ultimobranchial bodies but are not nearly as numerous and can be found only after prolonged searching. Mitoses can readily be found in all the different components of the tripartite complex.

Embryo of 21.5 mm

(fig. 5)

The ultimobranchial bodies lie along the entire extent of the dorso-medial margin of the thyroid gland and compose the largest portion of the tripartite complex. The anterior extremity of both ultimobranchial bodies and the posterior extremity of the left one are not fused with the thyroid gland. Their largest diameter (U) is about midway between their extremities from which they gradually taper to blunt points. Their greater portion is vacuolar but nonvacuolar areas are present in their deeper parts as well as along their periphery. Large blunt epithelial buds, some of which are vacuolar, are present in various places along their free border. The darkly stained nuclei and groups of small nuclei are more numerous than in the preceding stage.

Embryo of 22 mm

(figs. 6 a and 6 b)

The tripartite complex presents extremely varied pictures. Its anterior fourth is composed entirely of typical thyroid cell-cords while its caudal portion is composed chiefly of the ultimobranchial bodies (fig. 6 b) . The caudal portion of each ultmiobranchial bodj^ is composed of a cell mass of irregular outline in which remnants of the lumen, lined with columnar epithelium, still persist (fig. 6 b, L). Anteriorly, they are largely broken up into coarse cell cords which process marks the beginning of important developmental features in these structures (fig. 6 a, U). Nonvacuolar areas can be found throughout their entire extent. The deeply stained nuclei in the coarse cell cords, in the more central unbroken masses, and in the epithelial buds are more numerous than in any of the preceding stages. They can be quite readily found in the cell cords of the thyroid gland (figs. 6 a and 6 b), but are not nearly as numerous as in the ultimobranchial bodies. No degenerating nuclei, such as pyknotic or fragmented nuclei, were found. Small nuclei, in groups and diffusely scattered, are more numerous in the ultimobranchial bodies of this stage than in the previous one. In another 22 mm. embryo the size, shape, and extent of the ultimobranchial body along the thyroid gland is quite similar to that of the 21.5 mm. embryo described above.

Embryo of 23 mm

(figs. 8 a, 8 b, and 8 c)

The ultimobranchial bodies are small anteriorly and extend slightly farther cephalad than the thyroid gland. For a short distance anteriorly the ultimobranchial bodies and the thyroid are not fused. From their point of fusion with the thyroid they rapidly become larger so that the caudal portion of the tripartite complex is largely composed of the ultimobranchial bodies (figs. 8 a, 8 b, and 8 c, U). The epithelial buds attached to the ultimobranchial bodies are in general not as large as those in the 22 mm. embryo. The darkly stained nuclei and groups of small nuclei are also less numerous than in the preceding stage. Only an occasional darkly stained nucleus can be found in the cell cords of the thyroid gland. A few blood vessels of a capillary character are found in the larger portion of the ultimobranchial bodies.

Embryos of 24 to 30 mm

During this developmental period quite marked changes occur in the ultimobranchial bodies, the most pronounced of which is a breaking up of their major portion into cell cords which, when first formed, are usually larger than those of the thyroid gland. Two factors are apparent during the formation of cell cords, namely, a continued growth and division of the epithelial buds, and their invasion by mesenchymal and vascuolar connective tissue. The extent to which this process occurs during this developmental period varies. In some embryos these structures are almost entirely broken up into cell cords while in others a centrally located, more or less vacuolar and irregularly outlined core, variable in size, persists for some time longer. This process is illustrated in figure 9 (U), which represents a section through almost the middle portion of the tripartite complex in a 27 mm. embryo. In most stages of this developmental period (24 to 30 mm.), and even in some later stages, the caudal portion of the ultimobranchial bodies is for a time less broken up into cell cords than their more anterior part. Also, these structures never become entirely vacuolar. Some of the coarse cell cord^ are composed of a nonvacuolar syncytium. Nonvacular areas are also present in the more centrally located syncytial mass and in the larger and less unbroken caudal portion of these structures. Groups of small nuclei which appear normal in structure are preseilt in both vacuolar and non vacuolar parts. In places, instead of being arranged in groups, the small nuclei are quite uniformly scattered among the larger nuclei. Mitoses in both the thyroid gland and ultimobranchial bodies can readily be found.

It is during this developmental period and also in somewhat earlier and later stages that the darkly stained nuclei are most numerous. In only two developmental stages, namely, in a 23 mm, embryo (not the one described above), and in a 24 mm. embryo (fig. 7, D.N.), were degenerated (pyknotic and fragmented) nuclei found in sufficient numbers to .suggest a general degeneration of these structures. The degenerated nuclei in these stages were not generally distributed throughout the ultimobranchial bodies but were found in localized areas. A few darkly stained nuclei can be found in the cell cords of the thyroid gland during this developmental period. In the thyroid, however, they are never very numerous and in some they are found only after prolonged searching.

Embryo of 29.5 mm

(figs. 10 a, 10 b, and 10 c)

The tripartite complex in this embryo is of interest in that a large portion of it is asymmetrical in shape, due to the unequal length of the ultimobranchial bodies. Nearly all of the anterior fourth of the complex is symmetrical and is composed of cell cords of the thyroid gland only (fig. 10 a). The greater portion of the middle two-fourths of the tripartite complex is characterized by the presence of massive cell cords of the left ultimobranchial body and the entire absence of the right ultimobranchial body (fig. 10 b). Along the posterior fourth of the thyroid gland both ultimobranchial bodies are present. The left one terminates rather abruptly thirty sections (150 microns) anterior to the caudal end of the thyroid while the right one tapers to a point and extends as far caudally as the thyroid gland. The extreme caudal portion of each ultimobranchial body is less broken up into cell cords than is represented in figure 10 c. Small disconnected vacuolar areas are present in the more caudal portion of both branchial bodies. The large cell cords are almost entirely free from vacuoles but are characterized by a comparatively large number of small nuclei. The deeply stained nuclei, which are comparatively few in number, are most confirfed to the ultimobranchial bodies. Only a few are present in the cell cords of the thyroid gland. Only a very few degenerated nuclei were found.

In embryos from about 30 mm. in length to stages in which colloid is first present in the folUcles of the thyroid gland (75 mm.), the ultimobranchial bodies present a varied appearance. They are largely broken up into cell cords and in the progress of development the cell cords of the thyroid gland and usually those of the ultimobranchial bodies have become closely packed together so that a sharp demarcation between the median and lateral elements of the tripartite complex is not always evident. A description of a few stages will suffice to bring out the general character of the ultimobranchial bodies during this developmental period. Since the thyroid gland in previous stages is free from vacuoles, it is, I believe, safe to assume that the vacuolar areas found in the succeeding stages represent portions of the ultimobranchial bodies.

Embryo of 33 mm

The ultimobranchial bodies are limited to the posterior half of the tripartite complex. They are located on each side of the median plane, deeply buried beneath the dorsal surface of the thyroid gland and are represented by disconnected vacuolar areas the majority of which are not sharply circumscribed but gradually give place to the compactly arranged cell cords of the thyroid gland with which they are fused. The thyroid terminates posteriorly in two short blunt processes. In these processes small vacuolar areas are promiscuously scattered among the cell cords. A few" small vacuolar areas which are round in cross section and sharply demarcated by connective tissue from the surrounding cell cords were also found. Only a few darkly stained nuclei are present. No degenerated nuclei were found.

Embryo of 35 mm

The only traces of the ultimobranchial bodies are small disconnected vacuolar areas on each side of the median plane of the thyroid gland. The gland terminates posteriorly in two short blunt processes of nearly equal length, both of which are partly vacuolar. Only a few deeply stained nuclei are present.

Embryo of 37.5 mm

The anterior portion of the tripartite complex is very large and strongly crescent in outline. Caudalward it gradually loses its crescent outline and ends in a single blunt cone-shaped process. The ultimobranchial bodies lie in the posterior four-fifths of the thyroid gland. Their anterior ends lie imbedded beneath the dorsal surface of the thjrroid lateral to its median plane. Caudal ward they rapidly increase in size and shift in position so that in places they extend to the free surface on the lateral margin of the thyroid gland. Their posterior ends are fused and compose by far the largest part of the caudal one-fifth of the tripartite complex. The greater portion of the ultimobranchial bodies are in the form of vacuolar syncytial cores which give off coarse cell cords. Some of the coarse cell cords are vacuolar and many are fused to the cell cords of the thyroid gland. The central core is more or less' invaded with mesenchymal and vascular connective tissue. Deeply stained nuclei are quite numerous in the. ultimobranchial bodies and a few are found in the cell cords of the thyroid gland. No degenerated nuclei were found. The variableness in the size of the nuclei in the ultimobranchial bodies is more marked than in the nuclei of the thyroid gland, the former having a proportionally larger number of small nuclei. The extent to which the transformation of the ultimobranchial bodies has taken place in this stage is about equal to that in the 29.5 mm. embryo.

Embryo of 38 mm

The ultimobranchial bodies are limited to the posterior two-thirds of the tripartite complex. Their anterior ends are small and entirely imbedded in the thyroid gland near its dorsal surface. Caudalward they rapidly increase in size. The tripartite complex ends in two blunt cone-shaped processes the greater portion of which are composed of the ultimobranchial bodies. The ultimobranchial bodies are composed of irregularly outlined syncytial cores which gradually merge into the compactly arranged cell cords, of the thyroid gland. Only a few capillaries are found in them. Mitoses in the vacuolar areas as well as in the cell cords of the thyroid are quite numerous. Only a few deeply stained nuclei are present. No degenerated nuclei were found.

Embryo of 40 mm

The ultimobranchial bodies lie in the posterior half of the thyroid gland. Their anterior parts are represented by small disconnected vacuolar areas which lie deeply buried in the thyroid gland lateral to its median plane. Caudalward these areas become large and branched so that the caudal fifth of the thyroid gland is largely invaded by a vacuolar syncytial mass which is not sharply demarcated from the closely packed cell cords of the thyroid gland. The tripartite complex ends in two blunt and slightly vacuolar processes of unequal length. A small number of deeply stained nuclei are present throughout the entire complex. Only a few degenerated nuclei were found in the vacuolar areas. There are realtively more small nuclei present in the vacuolar areas than in the non vacuolar portions and mitoses are more numerous in the latter than in the former areas.

Embryo of 48 mm

(figs. 11a and lib)

Both ultimobranchial bodies extend anteriorly as far as the thyroid gland. The one on the right side is isolated from the anterior fourth while the one on the left side is isolated from the anterior third of the thyroid gland (fig. 11a, U). Excepting a vacuolar area present in the anterior end of the left one and traces of the lumen caudal to the vacuolar area, the isolated portions of the ultimobranchial bodies have a structure identical to that of the thyroid gland along which they lie. The right ultimobranchial body near its fusion with the thyroid is quite large (fig. 11a, U). The part fused to the thyroid gland is difficult to follow, yet traces of it may be seen in the form of small vacuolar areas that are promiscuously scattered beneath the dorso-lateral margin of the tripartite complex. Some of these areas are sharply outlined while others gradually merge into the compactly arranged cell cords.

The left ultimobranchial body caudal to its most anterior point of fusion with the thyroid gland is characterized in some places by very irregular vacuolar areas, while in other places by large, closely packed cell cords. In some places also it is only partially fused to the thyroid gland while in other places it is entirely separated from it by connective tissue (fig. 11 b, U.) Traces of the lumen still persist (L). The tripartite complex ends in two large conical processes which have, excepting a small vacuolar area found in each, a typically thyroid structure. Only a few darkly stained nuclei were found. Mitoses throughout the entire complex can be found without much searching.

Embryo of 53 mm

(fig. 12)

Traces of the ultimobranchial bodies are present in the caudal half of the tripartite complex. The anterior end of each is represented by a small irregularly outlined vacuolar area which lies lateral to the median plane just below the dorsal surface of the gland. For some distance caudalward these areas become larger, in places very irregular in outline, in places broken up with typical thyroid structures, and are located more deeply in the lateral halves of the thyroid gland. The thyroid ends in a single blunt process that has a typically thyroid structure. In a few places the ultimobranchial bodies are unusually vacuolar. In these places the nuclei do not stain deeply (U). Similar lightly stained areas were observed by Kingsbury ('14) in the thyroid gland of human embryos. Also, a few groups or nests of small, closely packed nuclei were found. In some of these groups the nuclei had a normal structure, while in others they were only slightly stained. A few degenerated nuclei were found in the vacuolar areas and in their immediate neighorhood. Deeply stained nuclei are present in small numbers in both vacuolar and nonvacuolar parts.

Embryo of 60 mm

(fig. 13)

The ultimobranchial bodies are limited to the posterior third of the thyroid gland. The right one is fused to the dorso-lateral margin of the gland and along its greater extent is composed of loosely arranged cell cords (U). Near the caudal end of the thyroid it merges into a vacuolar area which is interspersed with typical thyroid structures. The nuclei of the cell cords are on an average smaller than those found in the thyroid gland but, excepting a few darkly stained nuclei, they have a normal structure. The left ultimobranchial body lies just lateral to the median plane and is more deeply imbedded in the thyroid than the right one. It is largely composed of loosely arranged cell cords. Small vacuolar areas are present throughout its entire extent. The tripartite complex ends in a single process which is partly vacuolar. Mitoses can readily be found in the loosely arranged cell cords.

Embryos of 65 mm

The ultimobranchial bodies in two embryos of this developmental stage are described in order to contrast the structure of these bodies in two embryos of the same age. In one embryo these structures are represented by small disconnected vacuolar areas which are promiscuously scattered in the caudal half of the tripartite complex. In the other embryo the ultimobranchial bodies are located in the posterior third of the thyroid gland, and each one is characterized by a large and very irregularly outlined and continuous vacuolar mass to which coarse and loosely arranged cell cords, some of which are vacuolar, are attached. In the extreme caudal portion of the thyroid gland these structures fuse with each other and make up a large portion of its blunt termination. In each embryo a few deeply stained nuclei and a few degenerated nuclei were found.

Embryo of 75 mm

This is the youngest developmental stage in which colloid is found in the thyroid gland (picro-aceto-formol and hematoxylin and eosin). The follicles containing colloid are not numerous but are quite uniformly distributed throughout the anterior portion of the gland. The ultimobranchial bodies are limited to the posterior two-thirds of the gland lateral to the median plane and bordering the dorsal surface of the gland. They are represented by a continuous area of cell cords w^hich contains no colloid. Within these areas are found small, irregularly outlined, and disconnected syncytial masses which contain an unusually large number of small nuclei. These nuclei have the same structure as those found in the cell cords of the thyroid gland. Many of the cell cords which do not contain colloid are fused to these syncytial masses. Vacuoles are almost entirely lacking. A few^ degenerated nuclei are present, found only after prolonged searching. The tripartite complex ends in two blunt processes which have a typical thyroid structure.

Embryos of 100 mm

The tripartite complex of two embryos deserves notice. In both the colloid is more abundant than in the previous stage.

Embryo No. 1 (fig. 14). The ultimobranchial bodies are limited to the middle tw^o-fourths of the thyroid gland. The right one lies partially exposed on the dorsal surface of the thyroid lateral to its medial plane. In some places it is composed of coarse and loosely arranged cell cords (U), while in other parts the cell cords merge into a large syncytial mass. In places the connection between it and the thyroid is more intimate than is shown in figure 14. In the syncytial masses the nuclei are on an average a little smaller than those in the cell cords of the thyroid, but in both their structure is the same. No vacuoles are present. A few deeply stained nuclei are present. Some are also found in the cell cords of the thyroid gland. A few degenerated nuclei were found. The ultimobranchial body on the left side has a similar structure to the one on the right side but lies deeply buried below the dorsal surface of the gland. The portion of the tripartite complex which can be distinctly recognized as a derivative of the ultimobranchial bodies, and the cell cords in their immediate neighborhood contain no colloid although the cell cords have a typical thyroid structure of somewhat earlier stages.


Embryo No. 2. The ultimobranchial bodies cannot be identified structurally. However, it is to be noted that on the right side, lateral to the median plane and along the dorsal border in the middle third of the gland is an area which contains no colloid. This area is composed of closely packed cell cords which have the same structure as the cell cords of the thyroid just before the appearance of colloid, such as in a 60 mm. embryo. On the left is an area similar in structure to the one on the right side only its cephalo-caudal extent is considerably less. These areas which are free from colloid correspond favorably in position to that of the ultimobranchial bodies in some of the previous stages. The thyroid terminates in two rather blunt processes the extreme caudal portions of which contain no colloid.

Embryo of 111 mm

The ultimobranchial body on the right side of the gland is represented by two small and widely separated syncytial masses which extend through a series of ten and six sections respectively (10 microns in thickness), and on the left side it is represented by a syncytial mass extending through a series of eleven sections. These syncytial masses are quite vacuolar and the nuclei are comparatively small and clear and many are irregular in outline. Only a few darkly stained and degenerated nuclei are present.

It is of importance to note that on the right side lateral to the median plane in the middle two-fourths of the gland and in line with the syncytial masses described above is an irregularly outlined area of closely packed cell cords. This area is quite large in cross section and is free from colloid. A similar area of about the same length is present in the left side of the thyroid but it extends farther caudally and not so far anteriorly as the right one. These areas which are free from colloid correspond favorably in position to that of the ultimobranchial bodies in some earlier developmental stages.

Embryos of 125 mm

The ultimobranchial bodies in two embryos of this developmental stage were examined.

Embryo No. 1 (fig. 15). The ultimobranchial body on the right side lies in the middle two-fourths of the thyroid gland. Its anterior end is deeply imbedded beneath the dorsal surface of the thyroid and is composed of a very irregularly outlined and vacuolar syncytial mass in which the nuclei have about the same size as those in the follicular epithelium of the thyroid. A few pale nuclei are present. Cell cords, some of which are coarse and vacuolar, lead from the vacuolar area and are fused with the surrounding thja-oid structures. Slightly farther caudal it reaches the free surface on the ventro-lateral side of the gland and is composed of a loose network of cell cords some of which are vacuolar. From this place it gradually occupies a more dorsal position in the thyroid gland and is composed of closely packed cell cords, having a structure similar to that of the thyroid gland just before the appearance of colloid. Its more caudal portion reaches the free surface of the thyroid gland on its dorsal aspect (U) and contains a large cyst (C).

The ultimobranchial body on the left side is represented by a. series of six small disconnected, and irregularly outlined syncytial masses which lie just lateral to the mesial plane of the gland. These areas are more or less vacuolar and do not contain any colloid. The thyroid ends in a single process throughout which the colloid is cjuite uniformly distributed.


Embryo No. 2 (figs. 16 a and 1Gb). The tripartite complex of this embryo is of interest in that the ultimobranchial bodies are only partially imbedded in the thyroid gland. The ultimobranchial body on the right side lies along the lateral margin of about the middle two-fourths of the thyroid gland to which it is fused. It is fusiform in shape, with its greatest diameter about midway between its ends (fig. 16 a, U). The free portion along its entire extent is composed of syncytial masses and coarse and tortuous cell cords in both of which are found cystoid follicles which are lined with cuboidal and columnar epithelium. Some of the nuclei in the syncytial masses and in the coarse cell cords stain more deeply than others, and in general they are more variable in size than those in the follicular epithelium of the thyroid gland. The nuclei in the epithelial lining of the cystoid folicles lie closely together and stain uniformly. While the cystoid follicles are free from colloid in this developmental stage, small follicles containing colloid are thinly scattered throughout its free portion (fig. 16 a, Co). Along the line of fusion of the free portion of the ultimobranchial body to the thyroid gland there is in the latter an area composed of cell cords in which colloid is just beginning to form (fig. 16 a). The cell cords of this area have a structure similar to those in earlier stages in which colloid formation has just begun. In the free portion of this structure vacuoles are almost entirely lacking and only a few degenerated nuclei were found.

The length of the ultimobranchial body on the left side is equal to the length of the right one. It also lies along the lateral margin of the thyroid gland but is more deepl}^ imbedded in it (fig. 16 b, U). In cross section it is smaller than the right one but, excepting the absence of cystoid follicles, it has a similar structure. By referring to the figure it will be seen that it merges gradually into the thyroid gland. The follicles containing colloid gradually become smaller toward the more central portion of the ultimobranchial body in which only an occasional small follicle can be found.

Embryo of 145 mm

(fig. 17)

The ultimobranchial bodies on both sides are limited to the middle two-fourths of the thyroid gland just lateral to its median plane. The right one along nearly its entire extent is partly exposed to the free surface along the dorsal border of the gland. The portion most deeply imbedded in the thyroid gland is represented by an area of cell cords in which the follicles containing colloid are quite numerous but all very small (U). In places along its free margin are found cystoid follicles which also contain colloid {C.F.).^

  • The substance in the cystoid follicles is called 'colloid' on the ground that it has a staining reaction identical to that of the colloid in the follicles of the thyroid gland.


The ultimobranchial body on the left side is exposed to the free surface only in a few places. The larger portion lies immediately beneath the dorsal border of the thyroid gland. In one of its exposed parts are found cystoid follicles which contain colloid. The imbedded parts have the same structure as the imbedded portion of the right one. No darkly stained nuclei were found in either of the ultimobranchial bodies.

Embryo of 150 mm

The ultimobranchial bodies are located in the middle and a part of the posterior third of the thyroid gland near its lateral borders. They are represented by areas of typical thyroid structures in which the follicles containing colloid are small and not very numerous. In places they reach the free surface of the gland along its lateral border. No cystoid follicles or deeply stained and degenerated nuclei are present.

Embryo of 160 mm

(fig. 18)

The only structures present in the thyroid gland indicative of the presence of the ultimobranchial bodies are areas (C/), on each side lateral to the median plane along the dorsal surface of the gland. In these areas the follicles containing colloid are quite small in comparison to the large majority present in the thyroid gland, but appreciably larger than those found in corresponding areas in the thyroid of 145 and 150 mm. embryos. These areas extend from about the caudal end of the anterior third well into the posterior fourth of the thyroid gland which terminates in a rather blunt single process. In the caudal end are a very few large follicles containing colloid but it was impossible to determine whether or not they developed in connection with the ultimob ranchial bodies. No darkly stained or degenerated nuclei were found.

Embryo of 175 mm

The follicles in the thyroid gland are on an average considerably larger than those found in the preceding stage. They vary greatly in size but are uniformly distributed throughout the gland. No structures are present which can be interpreted as derivatives of the ultimobranchial bodies.

Embryo of 225 mm

(fig. 19)

The only apparent traces of the ultimobranchial bodies are areas of considerable extent in which the follicles are comparatively small ([/). These areas are located in the middle third on each side of the median plane and along the dorsal surface of the thyroid gland, and compare in position to the ultimobranchial bodies in some other comparatively late developmental stages. The areas of small follicles on the right side is a little shorter than that on the left side.

Embryo of 245 mm

(fig. 20)

On the right side along the lateral margin of the posterior two-thirds of the thyroid gland is an area containing many cystoid follicles which contain colloid and which are lined with cuboidal epithelium. This area, small anteriorly, gradually becomes larger and reaches its greatest cross-section area near the posterior fourth of the thyroid gland. From this position it decreases in size and near its termination it is almost separated from the thyroid gland. This area occupies a position similar to that of the ultimobranchial bodies in some earlier stages and apparently represents a partially imbedded ultimobranchial body similar to the right one in No. 2 of the 125 mm. embryo (fig. 16a).

On the left side lateral to the median plane and below the dorsal surface of the thyroid gland is an area in which the average size of the follicles is appreciably smaller than the large majority of follicles in other portions of the thyroid gland. This area lies in the posterior half of the thyroid gland but does not extend as far caudally as the area of large follicles on the right side. It also corresponds favorably in position to that most generally occupied by the ultimobranchial bodies in earlier stages.

Embryos of 270 mm (full term)

The thyroid glands of two full term embryos were examined.

Embryo No. 1. The follicles containing colloid are variable in size but uniformly distributed throughout the gland. The only portion of the gland which can be interpreted as a derivative of an ultimobranchial body is an area of only small follicles on the right side lateral to the median line in the posterior half of the gland. This area extends through a series of only sixty sections (10 microns in thickness) and lies hear the dorsal surface of the gland.


Embryo No. 2 (fig. 21). The thyroid gland extends through a series of 827 sections (10 micrxjns in thickness). The left ultimobranchial body is not completely transformed into typical thyroid structures. It lies in the posterior half of the gland and can be traced through a series of 234 sections (2.3 mm.). It is characterized by a small area of tortuous and nonvacuolar syncytial cords free from colloid which is eccentrically located in an area of small follicles (U). The nuclei in the syncytial cords correspond in size and structure to those in the follicular epithelium. A few nuclei in mitotic division are present. No deeply stained or degenerated nuclei are present.

The right ultimobranchial body extends through a series of 243 sections and is found in the middle third of the thyroid gland. It is characterized by an area of small follicles. In both ultimobranchial bodies from their more central portion toward their periphery the follicles gradually become larger. There is no sharp line of demarcation between these structures and the thyroid gland.

V. Summary and Discussion

By comparing the rate of growth of the ultimobranchial bodies and the thyroid gland, it is seen that a more uniform size ratio is maintained in early than in later developmental stages. During this 'periode d'activite' (Simon) of the ultimobranchial bodies, which extends from an 18 mm. or earlier developmental stage to about a 33 mm. stage, the cephalo-caudal extent of the ultimobranchial bodies is nearly or entirely equal to that of the thyroid gland. In later stages (33 mm. to full term) in which the ultimobranchial bodies can be recognized structurally, their cephalocaudal extent is generally much less . than that of the thyroid gland, which indicates that in later developmental stages the rate of growth of the thyroid exceeds that of the ultimobranchial bodies. In embryos from about 50 mm. in length to full term the ultimobranchial bodies are usually located in the posterior half of the thyroid gland. In a few stages they occur in the middle third or the middle two-fourths of the gland. Simon ('96) claims that during this period of retarded growth of the ultimobranchial bodies, which he calls the 'periode de survivance,' they undergo degenerative changes which is manifested principally by cystic formations (guinea-pig, rabbit, cat, calf, sheep) or their complete disappearance (pig) .

The ultimobranchial bodies first fuse with the thyroid gland along their ventro-lateral border. In early stages (19 mm. to about 27 mm.) they make up a considerable portion of the horns of the crescent-shaped tripartite complex. The extent of their fusion to the thyroid gland during their period of retarded growth (from about 33 mm. to full term) is variable. In the majority of late stages they are entirely imbedded in the thyroid gland while in some they are only partially imbedded. The latter condition is particularly the case in the following embryos; 48 mm. (figs. 11 a and 11 b); 60 mm. (fig. 13) ; 100 mm. (fig. 14); 125 mm. (fig, 16 a and 16 b); and 145 mm. (fig. 17). In the later stages they usually lie more or less deeply imbedded below the dorsal surface of the thyroid gland lateral to its medial plane, but occur less frequently along the lateral or dorso-lateral margin of the gland.

The formation of vacuoles in the ultimobranchial bodies begins before their fusion with the thyroid gland has occurred and continues after fusion. However, in the various stages examined no ultimobranchial body was found that is vacuolar throughout. In human embryos Kingsbury ('14) finds that vacuolation, 'reticulation,' continues until the entire structure is altered in this way. The extent to which vacuolation takes place varies in embryos of the same and different developmental stages. For example in No. 1 of the 125 mm. embryos the more central portion of these structures are quite vacuolar while in No. 2 of the 125 mm. embryos no vacuoles are present. Also no vacuoles are present in the left ultimobranchial body in No. 2 of the 27G mm. embryos. In early stages non-vacuolar portions are present along the periphery as well as in the deeper portions of these structures. In later stages in which the ultimobranchial bodies are largely broken up into cell cords the vacuoles are most numerous in their more central unbroken portion although vacuolated syncytial cords were found. In a few stages of which the embryo 53 mm. long is an example, the only part of the ultimobranchial body that can be recognized structurally as such are small vacuolar syncytial masses entirely surrounded by typical thyroid structures (fig. 12).

Up to about a 24 mm. stage a marked contrast exists in the structure of the ultimobranchial body and the thyroid gland, in that the former are largely unbroken syncytial masses, \\'hile the latter is broken up into cell cords (as seen in cross section) . Although epithelial buds produce irregularities on the surface of the ultimobranchial bodies even in a 21 mm. stage and indications of cell cord formation were found in one 22 mm. embryo, the process of extensive cell-cord formation in these structures is particularly active in stages ranging from 24 to 27 mm. in length. The larger caudal end becomes broken up somewhat later than the smaller anterior end. Usually, also, the more central portion breaks up into cell cords later than the periphery. The syncytial cords when first formed are usually larger or coarser than those of the thyroid gland. Many are vacuolar for some distance away from the central more or less vacuolar core to which they may be attached. The time of breaking up of the central core into cell cords is very variable. The extent to which the ultimobranchial bodies become invaded with vascular tissue corresponds closely to the extent of cell cord formation. The first blood vessels, which are of a capillary nature, are found in these structures in a 23 mm. embryo.

According to Simon ('96) the cell cores of the ultimobranchial bodies are formed in an entirely passive way, namely, by the ingrowth of vascular tissue and of structural elements of the median thyroid. That the former is a potent factor in this process is, I believe, beyond doubt. It appears to me, however, that he lays too much stress on the formation of cell cords by the ingrowth of thyroid structures which will be considered later. Another active factor in the process of cell cord formation is a continued growth and branching of the epithelial buds found on their surface in early stages. The buds by continued growth and branching take the form of coarse cell-cords which can in many instances be recognized structurally from the smaller cell cords of the thyroid gland by the larger proportion of small nuclei which they contain and by vacuoles which, when present, are found in their more proximal ends near their attachment to the more central unbroken portion of these bodies. Also, in stages in which the darkly stained nuclei are numerous many can usually be found in the coarse cell cords. The presence of nuclei in mitotic "division in these cords is further evidence that they really grow.

The cell cords of the ultimobranchial bodies when first formed are generally more loosely arranged than those of the thyroid gland (figs. 9, 10 c, and 13). The time at which they become more compactly arranged and resemble in appearance the thyroid gland previous to the appearance of colloid in the latter, varies greatly. For example, in embryos of 48 and 53 mm. in length, excepting the small vacuolar portions, they have a structure, similar to the thyroid gland, while in both 125 mm. embryos cell cords in portions of these structures have still a quite loose arrangement.

The deeply stained nuclei are most numerous in the ultimobranchial bodies in stages from 20 mm. to about 30 mm. in length. In the first half of this brief developmental period (20 to 30 mm.) the ultimobranchial bodies attain their largest size as unbroken or solid structures while in the latter half of this period the process of cell cord formation is very active. The deeply stained nuclei diminish in number in stages beyond 30 mm. in length and finally disappear altogether. Their decrease in number is, however, not uniform in successively older stages. For example, in a 35 mm. embryo in which the only structural traces left of the ultimobranchial bodies are small disconnected vacuolar areas, the darkly stained nuclei are comparatively few in number, while in a 37.5 mm. embryo in which these structures are still large and easily traceable, the darkly stained nuclei are quite numerous. In late developmental stages in which the ultimobranchial bodes can be structurally recognized as such the darkly stained nuclei have largely or entirely disappeared. For example in No. 2 of the 125 mm. embryos there are some present although not in large numbers, while in the ultimobranchial bodies in No. 2 of the full term embryos no darkly stained nuclei are present.


The deeply stained nuclei have been regarded by Simon ('96) as degenerating nuclei. In only two developmental stages (23 and 24 mm. embryos, fig. 7) were degenerated nuclei found in sufficient number to suggest a general degeneration of these structures. In some of the later developmental stages degenerated nuclei were also found but always in small numbers. It appears that the degenerated nuclei are derived from the darkly stained nuclei although I was unable to trace their source through intermediate forms directly to them. Some of the nuclei in connective tissue cells, in developing muscle fibers, in epithelial cells lining the esophagus, and also in some stages in the cell cords of the thyroid gland, stain deeply. This gives them a structural appearance quite similar to those found in the ultimobranchial bodies. The presence of these nuclei in various developmental structures suggested the probability that the dark nuclei in the ultimobranchial bodies are in a certain physiological state. This, however, is mere conjecture. If they represented a general degeneration of the ultimobranchial bodies one would naturally expect to find large numbers of degenerated nuclei in later developmental stages, but a contrary condition is the case. They gradually decrease in number while the ultimobranchial bodies continue in their growth. This fact seems to me to be strong evidence in favor of the persistence of these structures.

A feature quite noticeable in the ultimobranchial bodies in most of the earlier developmental stages and in some of the quite late stages is the small and variable size of some of the ultimobranchial nuclei. Grosser ('10) and Kingsbury ('14) also observed small ultimobranchial nuclei in human embryos. The small nuclei are very variable in number in stages of about the same age. Some of these nuclei also stain deeply in stages in which deeply stained nuclei are present, and in a few instances groups of pale small nuclei were found. However, the large majority of the small ultimobranchial nuclei have a normal structure, in all stages in which they occur. In late stages no small nuclei are present. Although the significance of the darkly stained and the small ultimobranchial nuclei are unknown to me I am convinced that they do not represent a general degeneration ot the iiltiniobranchial bodies.

Follicles containing colloid appear first in the thyroid gland in a 75 mm. embryo. In the ultimobranchial bodies the follicles containing colloid are first quite numerous, though small (excepting the cystoid follicles), in a 145 mm. embryo (fig. 19). A few small follicles containing colloid were found in these structures in the 125 mm. embryos. The retarded development of colloid in the ultimobranchial bodies in the pig corresponds with a similar retardation in its development in these structures in the Echidna in which, according to Maurer ('99), they remain independent structures. The time at which the transformation of the ultimobranchial bodies into typical thyroid structures is completed, that is, when they can no longer be distinguished from the derivatives of the median thyroid anlage, is variable. For example in a 175 mm. embryo their transformation is complete while in No. 2 of the full term embryos the left one is composed of an area of small follicles in which is located a small area of cell cords free from colloid (fig. 21). A comparison of the structure of the right ultimobranchial body, which is composed of an area of small follicles, and the left one in No. 2 of the full term embryos also shows that one ultimobranchial body may undergo a more rapid transformation into typical thyroid structures than the other in the same embryo.

Cell cords are formed from the periphery of the ultimobranchial bodies usually sooner than from their more central portion, as stated above. It is also in the cell cords of the peripheral portion of the ultimobranchial bodies that colloid is first formed, so that the older peripheral follicles of these structures in many stages are larger than the more centrally located ones. Figures 18, 19, and 21 show that the follicles containing colloid gradually decrease in size toward the more central portions of these structures. Since colloid appears first in the thyroid gland many of the follicles are quite large before colloid is first formed in the ultimobranchial bodies. It would thus seem that if the ingrowth of structural elements of the thyroid gland into the ultimobranchial bodies is a factor in breaking up the latter into cell cords, as claimed by Simon, there would be some quite large thyroid follicles found in the deeper portion of the ultimobranchial bodies among the smaller ultimobranchial follicles which begin to develop comparatively late. However, excepting the cystoid follicles in the ultimobranchial bodies in some of the later stages, this condition is not found. The follicles containing colloid gradually increase in size from the more central portion to the periphery of these structures. It therefore seems that the contention of k^imon is incorrect.

It also appears that in a few stages by far the larger portion of the ultimobranchial bodies undergo a transformation into typical thyroid structures even before colloid is formed in the thyroid gland. For example in embryos of 35 and 53 mm. in length the only structural features of the tripartite complex that can be interpreted as derivatives of the ultimobranchial bodies are small vacuolar areas (fig. 12) in contrast with the loosely arranged cell cords of these structures as found in embryos 37.5 and 60 mm. in length. Since in early stages it is impossible to distinguish the minute structure of the nonvacuolar portions of an ultimobranchial body from that of the thyroid gland when both are seen in the same microscopic field under high magnification, I believe that the vacuolar areas in embryos of 35 and 53 mm. in length represent only the more central cores of ultimobranchial bodies of which their more peripheral portion has undergone an early transformation into typical thyroid structures. This interpretation is supported by the conditions presented in a 48 mm. embryo in which the anterior portion of each ultimobranchial body is isolated from the thyroid gland. Excepting a small vacuolar area and traces of a lumen found in the isolated portion of the left one, the isolated portion of each of these bodies has a structure similar to the thyroid gland along which it lies.

I am of the opinion that the so variable developmental behavior of the ultimobranchial bodies in pig embryos throws light on a disputed point in connection with the development of these structures in human embryos. Grosser ('10) writes of a 'dichtere Zellgruppierung' in the thyroid gland of a human embryo 50 mm. long. He however does not believe that this dense cell area is derived from an ultimobranchial body but that it is 'niir der Ausdruck intensivem Wachstums der ganzen anlage, wiihrend die Differenzierung der neugebildeten Strange mehr oberflachlich stattfindet; die Zellen sind durchwegs typische Thyreoideazellen." Kingsbury ('14) finds that a human embryo 25 mm. long is the last stage in which the ultimobranchial body is clearly outlined. Their position in succeeding stages up to 41 mm. is occupied by a poorly circumscribed area of denser tissue." He is of the opinion that this "inner condensation" marks the place of disappearance of the ultimobranchial body although it may also well be as Grosser has stated, a center of growth." He further states that in 41 mm. and later developmental stages the "condensation is no longer recognizable." Although he was unable to satisfy himself as to the actual fate of the ultimobranchial bodies, he is of the opinion that they disappear.

From a study of the material used in this investigation I feel confident that the structure described by Grosser represents an ultimobranchial body. The process of cell cord formation at the periphery of the 'dichtere Zellgruppierung,' as described by him, corresponds favorably to the process of their formation in the ultimobranchial l3odies in pig embryos. Both the 'inner condensation' (Kingsbury) and the 'dichtere Zellgruppierung' (Grosser) apparently represent the central core which in the ultimobranchial bodies of pig embryos is found in a very wide range of developmental stages, even in a full term embryo (fig. 21). It seems that if the 'dichtere Zellgruppierung' represented a proliferati\'e center for the thyroid gland one would expect to find a rather large number of mitotic figures in them as an expression of rapid tissue growth. This, however, is not the case. No more nuclei in division are found in these areas than in any other portion of the thyroid gland.

The stages in which a comparatively early transformation of the greater portion of the ultimobranchial bodies takes place are comparatively few in number. Also there are comparatively few stages before full term in which there are no areas of small follicles (ultimobranchial bodies) present. Judging, therefore, from the so variable developmental behavior of the ultimobranchial bodies it seems that the 175 mm. stage referred to above is one in which the ultimobranchial bodies underwent an early transformation into typical thyroid structures.

The portion of the structural elements of the thyroid gland at birth derived from the ultimobranchial bodies is small in comparison to the part derived from the median thyroid anlage. Owing to the variable de\'elopmental behavior of the former structures the comparative proportion contributed by them and the median thyroid anlage undoubtedly varies in different embryos. Figures 22 a, 22 b, and 22 c are diagrammatic representations of the portions derived from the median thyroid anlage and the ultimobranchial bodies in No. 2 of the 270 mm. (full term) embryos.

In the posterior portion of the right ultimobranchial body in No. 1 of the 125 mm. embryos is a cyst which extends through a series of sixty-seven sections (10 microns in thickness). It is lined with cuboidal epithelium the cytoplasm of which stained only very faintly. In one place in its lumen an isolated group of cells is found. The nature of its formation is unknown to me. According to Simon ('96) the formation of cysts in these structures is a regular occurrence during their 'periode de survivance' in all animals examined by him, excepting in the pig in which they occurred in five out of eleven specimens. Since cyst formation occurred in only one specimen out of those I studied, it seems to be an exceptional developmental feature in the pig.

VI. Conclusions

  1. The ultimobranchial bodies in the pig participate in the formation of thyroid follicles. However, the portion of the gland in full term embryos that is derived from these structures is small in comparison with the part derived from the median thyroid anlage.
  2. The cephalo-caudal extent of the ultimobranchial bodies is equal to or nearly equal to that of the thyroid gland in embryos up to about 33 mm. in length. From this stage on to full term the latter grows more rapidly in size than the former so that in stages from about 50 mm. in length to full term the ultimobranchial bodies usually lie in the posterior half of the thyroid gland but may be found in the middle third or in the middle two-fourths of the gland.
  3. The developmental stages in which the ultimobranchial bodies transform into typical thyroid structures (that is, when they can no longer be recognized structurally from the median thyroid anlage) , vary greatly. The transformation of the greater part of these structures may take place as early as in a 35 mm. stage, before colloid is present in the thyroid gland, but in the majority of stages examined it takes place in later stages. Even in full term embryos an entire ultimobranchial body may not be completely transformed.
  4. The ultimobranchial bodies in a thyroid gland may vary in size, in shape, in the degree of their transformation, and in their location in the lateral halves of the thyroid gland. This variability is particularly pronounced in some of the later developmental stages.
  5. Colloid first appears in the follicles of the thyroid gland in embryos of 75 mm. in length. A few small follicles containing colloid appeared first in the ultimobranchial bodies of a 125 mm. embryo. In a 145 mm. embryo the follicles containing colloid in these structures are quite numerous although on an a\'erage small in comparison with those in the thyroid gland.
  6. Large cystoid follicles containing colloid may develop in the ultimobranchial bodies.
  7. The ultimobranchial bodies usually become entirely imbedded in the thyroid gland. In a few developmental stages they were found to be only partially imbedded.
  8. The formation of cysts in the ultimobranchial bodies of pig embryos is of rare occurrence.


VIII. Bibliography

Born, G. 1883 Uebcr die Derivate dcr embryonalen Schlundbogen und Schlund spalten bei Sjiugctieren. Arch. f. mikr. Anat., Bd. 22. FiscHELis, P. 1885 Beitriige zur Kenntniss der Entwickelungsgeschichte der

Gl. Thyreoidea und Gl. Thymus. Arch. f. mikr. Anat., Bd. 25. Fox, H. 1908 The pharyngeal pouches and their derivatives in the mammalia,

Am. Jour. Anat., vol. 8. Getzowa, S. 1907 Ueber die glandula parathyreoidea, intrathyreoidale Zell haufen derselben und Reste des postbranchialen Korpers, Arch. f.

path. Anat., Bd. 188. Greil, a. 1905 Ueber die Aniage der Lungen, sowie der ultimobranchialen,

(postbranchialen, suprapericardialen) Korper bei anuren Amphibien,

Anat. Hefte, Bd. 29. Grosser, O. 1910 Zur Kenutnis des ultimobranchialen Korpers beim Mem chen, Anat. Anz., Bd. 37.

1912 The development of the pharynx and of the organs of respiration. Manual of Human Embryology, edited by F. Keibel and F. P. Mall, vol. 2.

Herrmann, G. and Verdun 1899 Persistance des corps post-branchiaux chez

I'homme. Remarques sur I'anatomie comparce des corps post-branchiaux. Comptes Rend. Soc. Biol. Paris.

1900 Note sur les corps post-branchiaux des Cameliens. Les corps

post-branchiaux et la thyroide; vestiges kystiques. Comptes. Rend.

Soc. Biol. Paris. Kast.schenko, N. 1887 Das Schicksal der embryonalen Schlundspalten bei

Saugetieren. Arch. f. mikr. Anat., Bd. 30. Kingsbury, B. F. 1914 On the so-called ultimobranchial body of the mammalian embryo: Man. Anat. Anz., Bd. 47. KoHN, A. 1897 Studien liber die Schilddriise, II. Arch. f. Mikr. Anat., Bd. 38. Maurer, F. 1899 a Die Schilddriise, thymus und andere Schlundspaltenderi vate bei der Eidechse. Morph. Jahrb., Bd. 27.

1899 b Die Schlundspalten-Derivate von Echidna. Anat. Anz.

Ergjinzungsheft, Bd. 16. Moody, R. M. 1912 Some features of the histogenesis of the thyroid gland in

the pig. Reprints of Papers from the Dept. of Anat. of the Univ. of

Cal., vol. 4. NoRRis, E. H. 1910 The morphogenesis of the follicles in the human thyroid

gland. Am. Jour. Anat., vol. 20. Prexant, a. 1894 Contribution a I'etude organique et histologique du thymus,

de la glande thja-oide et de la glande carotidienne. La Cellule, T. 10. Rabl, H. 1913 Die Entwicklung der Derivate des Kiemendarms beim Meer schweinchen. Arch. f. mikr. Anat., Bd. 82. Schapfer, J., AND Rabl, H. 1908 and 1909 Das thyreothymische System des

Maulwurfs und der Spitsmaus. I. Morphologic und Histologic by J.

Schaffer. II. Die Entwicklung des thyreothymischen System beim

Maulwurf by H. Rabl. Sitzber. kais. Akad. Wiss. Wien, vols. 117

and 118.


122 J. A. BADERTSCHER

Simon, Ch. 1896 'I'hyroide later;ilc et glandule thyroidienne chez les mam miferes, These de Xancy. Symington, J. 1897 t ber Tliyreoidea, Glandylae parathyreoidcae und Thymus beim dreizehigen Faulthier (Ai, Bradypus tridactylus). Arch. f.

Anat. u. Physiol. Supplement-Band zur Anat. Abt. TouRNEUX, F., and Verdun, P. 1897 Sur les premiers developpements de la

thyroide, du thymus et des glandules thyroidiennes Chez L'Homme.

Journ. de Anat. et de la Phys., T. 23. Verdun, P. 1898 Contribution a I'etude des derives Branchiaux chez les ver tcbres superieurs, These, Toulouse. ZucKERKANDL, E. 1903 Die EntM icklung dcr Schilddruse und der thymus bei

der Ratte. Anat. Hefte, Bd. 21.


Plates

Plate 1

PLATE 1

EXPLANATION OF FIGURES

1 From a photogi'aph of a transverse section of the ultimobranchial bodies and the thyroid gland about midway between the anterior and posterior ends of the latter. Before the fusion of the ultimobranchial bodies with the thyroid gland. From an embryo 18 mm. long. X 60.

2 a and 2 b From photographs of transverse sections of the ultimobranchial bodies and the thyroid gland taken respectively near the anterior and posterior ends of the latter. P'usion between the ultimobranchial bodies and the thyroid gland has in some places taken place. From an embryo 19.5 mm. long. X 60.

3 a, 3 b, and 3 c From photographs of transverse sections of the ultimobranchial bodies and the thyroid gland taken respectively near the anterior, middle, and posterior portions of the latter. The numerous deeply stained nuclei in the ultimobranchial bodies are represented by small black dots. From an embryo 20 mm. long. X 60.

4 From a photograph of a transverse section about midway between the two ends of the tripartite complex. The horns of the crescent are largely composed of the ultimobranchial bodies the left one of which is quite irregular along its dorso-mesial surface due to epithelial buds. From an embryo 21 mm. long, X 60.

5 From a photograph of a transverse section about mid.vay between the two ends of the tripartite complex showing the large size of the ultimobranchial bodies. From an embryo 21.5 mm. long. X 60.

6 a and 6 b From photographs of transverse sections through near the middle and caudal portions respectively of the tripartite complex, showing nvunerous deeply stained nuclei (small black dots in the figures) in the ultimobranchial bodies and a few in the cell cords of the thyroid gland. The figures also show that the caudal portion of the ultimobranchial bodies are less broken up into cell cords than their more anterior portion. From an embryo 22 mm. long. X 60.

7 From a photograph of a portion of an ultimobranchial body showing degenerated and deeply stained nuclei. From an embryo 23 mm. long. X 650.

D.N., degenerated nuclei T., thyroid gland

D.S.N. , deeply stained nuclei Tr., trachea

Ep. B., epithelial buds U., ultimobranchial body L., lumen


Plate 2

EXPLANATION OP FIGURES

8 a, 8 b, and 8 c From photographs of transverse sections through near the anterior, middle, and posterior portions respectively of the tripartite complex, showing the gradual enlargement of the ultimobranchial bodies from their anterior to their posterior ends. The posterior end of the tripartite complex is largely composed of the ultimobranchial bodies. From an embryo 23 mm. long. X 60.

9 From a photograph of a transverse section about midway between the two ends of the tripartite complex showing the ultimobranchial bodies largely broken up into coarse cell cords. From an embryo 27 mm. long. X 56.

10 a, 10 b, and 10 c From photographs of transverse sections through the anterior, middle and posterior portions respectively of the tripartite complex. The ultimobranchial body on the left side extends along the posterior threefourths of the thyroid gland while the right one along only its posterior fourth. The unecjual size of the two ultimobranchial bodies, which are largely broken up into coarse cell cords, produce the asymmetry of the tripartite complex. From an embryo 29.5 mm. long. X 56.

11 a and 11 b From photographs of sections through the anterior and nearly the middle portions respectively of the tripartite complex showing both ultimobranchial bodies separated from the anterior portion of the thyroid gland (fig. 11 a) and the left one with traces of the lumen also separated from the thyroid (fig. 11 b). From an embryo 48 mm. long. X 60.

Ep.B., epithelial buds Tr., trachea

L., lumen U., ultimobranchial body

T., thyroid gland


Plate 3

EXPLANATION OF FIGURES

12 From a photograph of a small portion of a section through an ultimobranchial ])ody (the light area in the figure) which merges gradually into typical thyroid structures. In the light area the majority of the ultimobranchial nuclei do not stain deeply. From an embryo 53 mm. long. X 337.

13 From a photograph of a portion of a transverse section of the tripartite complex showing the right ultimobranchial body which is largely composed of coarse and loosely arranged cell cords. From an embryo 60 mm. long. X 60.

14 From a photograph of a portion of a section of the tripartite complex showing the right ultimobranchial body which, in this particular place, is composed of loosely arranged cell cords in which no colloid is present. From an embryo 100 mm. long. X 56.

15 From a photograph of a portion of a transverse section through the posterior portion of the tripartite complex showing the compactly arranged cell cords of the right ultimobranchial body in which is located a cyst. The area inside the dotted circle is free from colloid. From an embryo 125 mm. long. X 45.

16 a and 16 b From photographs of portions of a transverse section of ths tripartite complex showing respectively the right and left ultimobranchial bodies. The right one is only partially imbedded in the thyroid gland and contains many cystoid follicles (C.i^.)'which do not contain colloid and a few small follicles which contain colloid (Co). The black dots in the portion of the figure labeled 'thyroid' represent colloid. The left ultimobranchial body is more deeply imbedded in the thj-roid gland. From an embryo 125 mm. long. X 38.

17 From a photograph of a portion of a transverse section of the trip:.rtite complex showing the right ultimobranchial body in which are found both small and cystoid follicles that contain colloid. From an embryo 145 mm. long. X 38.

18 From a photograph of a portion of a section of the tripartite complex showing the left ultimobranchial body which is represented by an area of small follicles. The black dots in the figure represent colloid. From an embryo 160 nun. long. X 38.

C, cyst T., thyroid gland

C.T., cystoid follicles U., ultimobranchial body

Co., colloid

Plate 4

EXPLANATION OF FIGURES

19 From a photograph of a portion of a section through the left ultimobranchial body and a portion of the thyroid gh\nd surrounding it. The ultimobranchial body is characterized by follicles which contain colloid and which are on an average appreciably smaller than the follicles of the thyroid gland. From an embryo 225 mm. long. X 38.

20 From a photograph of a portion of a section through the right ultimobranchial body and a portion of the thyroid gland surrounding it. The ultimobranchial body contains many cystoid follicles which contain colloid. The colloid dropped out from some of the follicles during the process of staining. From an embryo 245 mm. long. X 38.

21 From a photograph of a portion of a section through the left ultimobranchial body and a portion of the thyroid gland. The ultimobranchial body is characterized by an area of small follicles in which is located a small area free from colloid. The light dots represent follicles from which the colloid has fallen. This figure represents the ultimobranchial body at C in figure 22 a. From No. 2 of the embryos 270 mm. long (full term). X 38.

22 a, 22 b, and 22 c These figures show the relative size of the ultimobranchial bodies and the thyroid gland in No. 2 of the embryos 270 mm. long (full term). The extent of the ultimobranchial bodies is outlined by a dotted line. Inside the left ultimoljranchial body is a small area (X), also outlined by a dotted line, which is free from colloid (figs. 22 a and 22 c). The portion of the left ultimobranchial body outside the area X and all of the right one is characterized by follicles which are on an average appreciably smaller than those of the thyroid gland. Figures 22 b and 22 c represent cross sections through the tripartite complex at b and c respectively of the structures represented in figure 22 a. X 7.5.

T., thyroid U., ultimobranchial body




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