Paper - The development of the thymus in the pig 1 (1915): Difference between revisions
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| [[File:Mark_Hill.jpg|90px|left]] This historic 1915 paper by Badertscher describes the development of the {{thymus}} in the {{pig}}. | |||
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See also: {{Ref-Badertscher1915a}} | |||
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'''Modern Notes:''' {{thymus}} | {{pig}} | |||
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PubMed Search: [http://www.ncbi.nlm.nih.gov/pubmed/?term=pig+thymus+development ''pig thymus development''] | [http://www.ncbi.nlm.nih.gov/pubmed/?term=thymus+development ''thymus development''] | |||
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=The Development of the Thymus in the Pig I. Morphogenesis= | |||
J. A. Badertscher | |||
From the Department of Histology and Embryology, Cornell University, Ithaca, N. Y. | |||
Twelve Figures — Two Plates | |||
==Historical== | |||
It has been definitely established by many investigators that the thymus of all mammals is of epithelial origin. More recent investigations have shown, however, that the epithelial anlage of the thymus is not derived from the same germ layer in all mammals. Investigators are agreed on the point that the thymus of mammals, when it is of a purely entodermal origin, is a derivative of the ventrally directed epithelial diverticulum of the third pharyngeal pouch. It has also been quite definitely settled that in some mammals (mouse, Roud '00) the thymus is entirely of ectodermal origin. The mixed (ectodermal-entodermal) origin of the thymus in some mammals has not yet been generally accepted. In pig embryos it is the close topographical relation that exists between the cervical vesicle and the third pharyngeal pouch that makes a mixed origin of the thymus possible. | It has been definitely established by many investigators that the thymus of all mammals is of epithelial origin. More recent investigations have shown, however, that the epithelial anlage of the thymus is not derived from the same germ layer in all mammals. Investigators are agreed on the point that the thymus of mammals, when it is of a purely entodermal origin, is a derivative of the ventrally directed epithelial diverticulum of the third pharyngeal pouch. It has also been quite definitely settled that in some mammals (mouse, Roud '00) the thymus is entirely of ectodermal origin. The mixed (ectodermal-entodermal) origin of the thymus in some mammals has not yet been generally accepted. In pig embryos it is the close topographical relation that exists between the cervical vesicle and the third pharyngeal pouch that makes a mixed origin of the thymus possible. | ||
thymus which is derived from the third pharyngeal pouch is a comparatively long mid-ventrally directed epithelial tube before the cervical sinus is fused to it. He also makes no mention of the XII cranial nerve which plays an important part in modifying the topographical relations of the anterior portion of the thymus to surrounding structures. Basing his conclusions on inaccurate observations, his views in regard to a mixed origin of the thymus have now only an historical value. | Among some of the workers on the early development of the thymus of the pig may be mentioned Fischelis, Kastschenko, Zotterman, Born, Bell, and Fox, the first thi'ee of whom attribute to the thymus an ectodermal-entodermal origin. Fischelis ('85) derived the thymus from the third pharyngeal pouch and the third branchial groove. According to this investigation these two fuse, and from then- point of fusion each contributes about one-half to a ventrally directed downgrowth, the anlage of the thymus. This conclusion is erroneous, for that portion of the thymus which is derived from the third pharyngeal pouch is a comparatively long mid-ventrally directed epithelial tube before the cervical sinus is fused to it. He also makes no mention of the XII cranial nerve which plays an important part in modifying the topographical relations of the anterior portion of the thymus to surrounding structures. Basing his conclusions on inaccurate observations, his views in regard to a mixed origin of the thymus have now only an historical value. | ||
The first detailed study of the early development of the thymus in the pig was made by Kastschenko ('87). He describes the mesial portion of the sinus cervicalis, which he calls the 'vesicula thymica,' as fusing with the anterior end of the epithehal anlage of the thymus, which is derived from the third pharyngeal pouch. In the shif tings of some of the structures in the neck, that occur in young embryos during growth, the lateral portion of the cervical vesicle is separated mechanically from its mesial portion by the h3^oglossal nerve. The free lateral portion of the cervical vesicle gives rise to the 'thymus superficialis' which is necessarily of ectodermal origin. He claims that the superficial thymus is not a constant structure, for, in a 30 mm. embryo that he examined, it was not present. The anterior end of the thymus to which the mesial portion of sinus cervicalis has fused, plus the parathyroid, that lies close to it, he designates the thymus head; while the large remaining portion of the thymus which is of a purely entodermal origin, plus the thymus head, he calls the 'thymus profunda.' The largest embryo examined by him was 82 mm. in length. | The first detailed study of the early development of the thymus in the pig was made by Kastschenko ('87). He describes the mesial portion of the sinus cervicalis, which he calls the 'vesicula thymica,' as fusing with the anterior end of the epithehal anlage of the thymus, which is derived from the third pharyngeal pouch. In the shif tings of some of the structures in the neck, that occur in young embryos during growth, the lateral portion of the cervical vesicle is separated mechanically from its mesial portion by the h3^oglossal nerve. The free lateral portion of the cervical vesicle gives rise to the 'thymus superficialis' which is necessarily of ectodermal origin. He claims that the superficial thymus is not a constant structure, for, in a 30 mm. embryo that he examined, it was not present. The anterior end of the thymus to which the mesial portion of sinus cervicalis has fused, plus the parathyroid, that lies close to it, he designates the thymus head; while the large remaining portion of the thymus which is of a purely entodermal origin, plus the thymus head, he calls the 'thymus profunda.' The largest embryo examined by him was 82 mm. in length. | ||
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Zotterman ('11) also made a detailed study of the morphogenesis of the thymus of the pig. Her conclusions are in accord with those of Kastschenko with the exception that in about one-half of the specimens examined the superficial thymus was connected with the thymus head by a cord of cells that looped over the hypoglossal nerve. The superficial thymus was found in all the specimens examined and in the largest (105 mm.) investigated aU the features common to the thymus (cortex, medulla, Hassall's corpuscles, etc.) were present. | Zotterman ('11) also made a detailed study of the morphogenesis of the thymus of the pig. Her conclusions are in accord with those of Kastschenko with the exception that in about one-half of the specimens examined the superficial thymus was connected with the thymus head by a cord of cells that looped over the hypoglossal nerve. The superficial thymus was found in all the specimens examined and in the largest (105 mm.) investigated aU the features common to the thymus (cortex, medulla, Hassall's corpuscles, etc.) were present. | ||
Fox ('08) agrees with Kastschenko that the superficial thymus | Fox ('08) agrees with Kastschenko that the superficial thymus of the pig arises by a constriction of the fundus praecervicaHs, but claims that in embryos up to 35 mm. in length, the oldest stage he examined, its histological structure does not resemble that of the thymus more than any other branching epithelial mass. | ||
of the pig arises by a constriction of the fundus praecervicaHs, but claims that in embryos up to 35 mm. in length, the oldest stage he examined, its histological structure does not resemble that of the thymus more than any other branching epithelial mass. | |||
Born ('83) derived the thymus anlage in pig embryos from the third pharyngeal pouch, while Bell ('06) also is inclined to believe that the ectoderm takes no part in the formation of the thymus. | Born ('83) derived the thymus anlage in pig embryos from the third pharyngeal pouch, while Bell ('06) also is inclined to believe that the ectoderm takes no part in the formation of the thymus. | ||
==Material and Methods== | |||
For investigation of the early stages of the morphogenesis of the thymus, the excellent collection of pig embryo series, 3 to 42 mm. in length, in the Department of Histology and Embryology of Cornell University, proved very helpful. In addition to these, five embryos ranging from 9 to 21.5 mm. in length, and the neck and upper thoracic region of eight embryos ranging from 32 to 95 mm. in length, were sectioned transversely. These sections, 10 microns in thickness, were stained with hematoxylin and eosin. From series of this group reconstructions of the pharyngeal region were made. Many dissections exposing the thymus were made of the neck and upper thoracic region of embryos from 100 to 280 mm. in length (full term). The thymus of a pig one day old was also examined. | For investigation of the early stages of the morphogenesis of the thymus, the excellent collection of pig embryo series, 3 to 42 mm. in length, in the Department of Histology and Embryology of Cornell University, proved very helpful. In addition to these, five embryos ranging from 9 to 21.5 mm. in length, and the neck and upper thoracic region of eight embryos ranging from 32 to 95 mm. in length, were sectioned transversely. These sections, 10 microns in thickness, were stained with hematoxylin and eosin. From series of this group reconstructions of the pharyngeal region were made. Many dissections exposing the thymus were made of the neck and upper thoracic region of embryos from 100 to 280 mm. in length (full term). The thymus of a pig one day old was also examined. | ||
==Morphogenesis== | |||
In the investigation on the morphogenesis of the thymus special attention was constantly directed toward the development of the superficial thymus because its existence is not yet generally accepted and, since the latest developmental stage in which it was investigated by Zotterman was only 105 mm. in length, its fate is not definitely known. | In the investigation on the morphogenesis of the thymus special attention was constantly directed toward the development of the superficial thymus because its existence is not yet generally accepted and, since the latest developmental stage in which it was investigated by Zotterman was only 105 mm. in length, its fate is not definitely known. | ||
An 11 mm. embryo was the developmental stage chosen as the starting point for the study of the morphogenesis of the thymus. At this stage the ectodermal and entodermal parts of the branchial grooves and pharyngeal pouches can still be | An 11 mm. embryo was the developmental stage chosen as the starting point for the study of the morphogenesis of the thymus. At this stage the ectodermal and entodermal parts of the branchial grooves and pharyngeal pouches can still be distinguished from each other without difficulty. The sinus cervicahs, formed by the rapid growth in a caudal direction of the mandibular and hyoid arches and the more retarded growth of the branchial arches proper, is already well mapped out. As this stage shows well accepted relations and developmental steps it needs no further description. | ||
Embryo of lJf-.5 mm. {figures 1 and 2). This is the youngest developmental stage from which a reconstruction of one side of the neck was made. Since the determination of the real origin of the thymus was one of the crucial points in this investigation only that part of the neck containing the anterior portion of the thymus. anlage and the vesicula cervicalis was modelled. | |||
The posterior edge of the hyoid arch has grown over the opening of the sinus cervicalis, shutting it off from the exterior. The cavity thus formed is the vesicula cervicalis (F.c.) or the 'vesicula thymica' of Kastschenko.^ The vesicula cervicalis, now widely separated from the ectoderm (Ect.), is still connected with it by a heavy cord of cells, the ductus cervicalis (D.c). Only in places through its entire extent are traces of a lumen left. To the outer end of the vesicula cervicalis is attached a cord of cells that runs in an antero-ventral and mesial direction and connects with the second pharyngeal pouch. This is the ductus branchialis (D.b.). The anterior one-fifth of this structure possesses a lumen which is continuous with that of the second pouch. At this stage it is impossible to determine the extent of the part that is of entodermal origin and the extent that is of ectodermal origin. The boundary line between the two has disappeared through the obliteration of the lumen. Fox ('08) was unable to find the ductus branchiahs in pig embryos but demonstrated a long diverticulum — 'filiform process' — arising from the ventrolateral angle of the second pouch and connected with the ectoderm. This process was not present in any of the embryos I examined. | |||
The posterior edge of the hyoid arch has grown over the opening of the sinus cervicalis, shutting it off from the exterior. The cavity thus formed is the vesicula cervicalis (F.c.) or the 'vesicula thymica' of Kastschenko.^ The vesicula cervicalis, now widely separated from the ectoderm (Ect.), is still connected with it by a heavy cord of cells, the ductus cervicalis (D.c). Only in places through its entire extent are traces of a lumen left. To the outer end of the vesicula cervicalis is attached a cord of cells that runs in an antero-ventral and mesial direction and connects with the second pharyngeal pouch. This is the ductus branchialis (D.b.). The anterior one-fifth of this structure possesses a lumen which is continuous with that of the second pouch. At this stage it is impossible to determine the extent of the part that is of entodermal origin and the extent that is of ectodermal origin. The boundary line between the two has disappeared through the obliteration of the lumen. Fox ('08) was unable to find the ductus branchiahs in pig embryos but demonstrated a long diverticulum — 'filiform process' — arising from the ventrolateral angle of the second pouch and connected with the | |||
1 "According to H. Ilabl ('09) the term 'vesicula cei'vicalis' is to be applied to the entire complex, including the two ductus branchiales; Hammar uses the term 'vesicula praecervicalis' only for the vesicular portion that is associated with the third pharyngeal pouch, this portion being approximately identical with the 'fundus praecervicalis' (cervicalis) of His and H. Rabl, as well as with the 'vesicula thymica' of Kastschenko and the 'sinus vesicle' of Zuckerhandl." (Quoted from Keibel and Mall's Human embryology, vol. 2, p. 456). | |||
The vesicula cervicahs (fig. 2, V.c.) hes lateral to the third pharyngeal pouch, between the cephalo-dorsal portion of the parathyroid {Pt. 3) and the caudo-ventral part of the ganglion nodosum (Gji.) and a short distance anterior to the hypoglossal nerve (N.XII). Its general shape is fusiform, with its long axis almost perpendicular to the surface ectoderm. The middle third is solid, but each extremity contains a cavity. The expansion of the central portion is due to a proliferation of the cells of its anterior wall which presses tightly against the dorsal surface of the parathyroid. The pressure against the parathyroid has apparently caused the obliteration of the cavity of the vesicula cervicalis in its central portion. Its inner third lies closely along the ventral side of the ganglion nodosum into which its curved end projects and with which it apparently is fused. | The vesicula cervicahs (fig. 2, V.c.) hes lateral to the third pharyngeal pouch, between the cephalo-dorsal portion of the parathyroid {Pt. 3) and the caudo-ventral part of the ganglion nodosum (Gji.) and a short distance anterior to the hypoglossal nerve (N.XII). Its general shape is fusiform, with its long axis almost perpendicular to the surface ectoderm. The middle third is solid, but each extremity contains a cavity. The expansion of the central portion is due to a proliferation of the cells of its anterior wall which presses tightly against the dorsal surface of the parathyroid. The pressure against the parathyroid has apparently caused the obliteration of the cavity of the vesicula cervicalis in its central portion. Its inner third lies closely along the ventral side of the ganglion nodosum into which its curved end projects and with which it apparently is fused. | ||
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The parathyroid {Pt. 3) is now a massive structure lying lateral to the third pharyngeal pouch {S.h.3), anterior to the hypoglossal nerve with which it is in contact, ventral to the ganglion nodosum and dorsal to the carotid artery (A.c). Its general shape is that of a hemisphere with its flat side turned toward the vagus nerve and the vesicula cervicalis. A depression in both the vesicula cervicalis and the parathyroid mark the points of most intimate contact between the two structures. | The parathyroid {Pt. 3) is now a massive structure lying lateral to the third pharyngeal pouch {S.h.3), anterior to the hypoglossal nerve with which it is in contact, ventral to the ganglion nodosum and dorsal to the carotid artery (A.c). Its general shape is that of a hemisphere with its flat side turned toward the vagus nerve and the vesicula cervicalis. A depression in both the vesicula cervicalis and the parathyroid mark the points of most intimate contact between the two structures. | ||
The entodermal anlage of the thymus (T.e.) is now, throughout its greatest extent, a solid cord of cells, and still attached to the third pharyngeal pouch. Its anterior end lies closely against the parathyroid and the hypoglossal nerve. From its point of origin it extends caudally, and, with the exception of about onefourth of the posterior portion, lies ventro-laterally to the carotid artery and the vagus nerve. The caudal portion makes a rather sharp turn in a ventro-mesial direction and occupies the upper part of the pericardial region. The diameter of the caudal part is considerably greater than that of the remaining portion. This is due to the presence of a large lumen and thick walls in this region. In the central and anterior portions only a few slight traces of a lumen persist. The vesicula cervicalis and the | The entodermal anlage of the thymus (T.e.) is now, throughout its greatest extent, a solid cord of cells, and still attached to the third pharyngeal pouch. Its anterior end lies closely against the parathyroid and the hypoglossal nerve. From its point of origin it extends caudally, and, with the exception of about onefourth of the posterior portion, lies ventro-laterally to the carotid artery and the vagus nerve. The caudal portion makes a rather sharp turn in a ventro-mesial direction and occupies the upper part of the pericardial region. The diameter of the caudal part is considerably greater than that of the remaining portion. This is due to the presence of a large lumen and thick walls in this region. In the central and anterior portions only a few slight traces of a lumen persist. The vesicula cervicalis and the anterior portion of the entodermal thymus do not come in contact with each other in this developmental stage. | ||
Embryo of 17.5 mm. {figures 3 and I^. During the interval between this and the previous stage shiftings in the pharyngeal region have taken place that have changed the relation of some of the parts to each other. The ductus cervicalis {D.c), now a solid cord of cells, is still connected with the ectoderm. The ductus branchialis on the left side, which was not modelled, has lost its connection with the outer end of the vesicula cervicalis. No traces of it in this region can be seen. It is, however, still connected with the second branchial pouch from which it extends for a short distance toward the point of its former attachment. ^ On the right side it is still a continuous solid cord of cells extending from the second pouch to the vesicula cervicalis. In a 21 mm. pig embryo Zotterman ('11) had demonstrated the ductus branchialis as a continuous cord of cells while the ductus cervicalis as being broken. In the embryos which I examined the ductus branchialis was always the first to become discontinuous. | |||
The vesicula cervicalis {V.c.) no longer lies perpendicular to the ectoderm. The vesicula cervicalis medialis {V.c.vi.y extends from its point of attachment to the ganglion nodosum in an antero-lateral direction to the hypoglossal nerve (N.XII) around which it forms an acute angle. Fr©m the nerve the vesicula cervicalis lateralis {v.c.l.) extends for a short distance in a caudolateral direction. This is the same general direction taken by the ductus cervicalis which is connected to the vesicula cervicalis laterahs and the ectoderm. The vesicula cervicalis medialis is tightly wedged in between the vagus on its dorsal side and the parathyroid gland and a small portion of the thymus on its ventral aspect. Its caudal portion is greatly flattened but as it approaches the nerve it gradually assumes a cylindrical form which also is the shape of the vesicula cervicaUs lateralis. A part of its flattened caudal portion dips into the ganglion nodosum while a portion lies in close contact with the thymus. Fusion between the thymus and the vesicle has apparently not yet taken place, for the boundary of both can still be clearly deterr mined. The lumen of the vesicula cervicalis is for the most part obliterated. Only slight traces here and there in its course persist. It is largest in the portion that dips into the ganglion nodosum. Here the lumen is large and the wall of this portion of the vesicle is no thicker than that of earlier stages. Apparently no cell proliferation takes place in this region. The surface of the entire vesicula cervicalis is more or less irregular. An idea of its shape can best be obtained by referring to figure 4 in which the hypoglossal nerve and a part of the ganglion nodosum were removed, thus almost entirely exposing it. | |||
2 A reconstruction of this remnant was deemed unnecessary since it takes no part in the formation of the thymus and would have needlessly increased the size of the model. | 2 A reconstruction of this remnant was deemed unnecessary since it takes no part in the formation of the thymus and would have needlessly increased the size of the model. | ||
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The parathyroid {Pt. 3) is an elongated and very irregular mass of cells that is tightly packed in between the cervical vesicle and vagus nerve on its dorsal aspect, and the carotid artery on its mesial surface. Its caudo-mesial and caudo-lateral portions are in contact with the thymus while its anterior portion is on a Ifevel with the arch of the vesicula cervicalis over the hypoglossal nerve. | The parathyroid {Pt. 3) is an elongated and very irregular mass of cells that is tightly packed in between the cervical vesicle and vagus nerve on its dorsal aspect, and the carotid artery on its mesial surface. Its caudo-mesial and caudo-lateral portions are in contact with the thymus while its anterior portion is on a Ifevel with the arch of the vesicula cervicalis over the hypoglossal nerve. | ||
The thymus {T.e.) is considerably longer than in the preceding stage. Its cephahc and caudal ends have about the same relative position to the other structures as in the 14.5 mm. embryo. Its greater length at this stage is due to growth which has kept pace with the growth of the pharynx. It is still connected with the third pharyngeal pouch by a greatly attenuated cord of cells. Its anterior portion (figs. 3-4) is fused to the caudal aspect of the parathyroid from which it extends caudally. As in the preceding stage, the caudal portion makes a sharp turn in a ventromesial direction and lies over the upper portion of the pericardium. The caudal portion of the right thymus extends across the mesial plane while the same region of the left thymus lies to the left of the mesial plane and extends farther caudally than | The thymus {T.e.) is considerably longer than in the preceding stage. Its cephahc and caudal ends have about the same relative position to the other structures as in the 14.5 mm. embryo. Its greater length at this stage is due to growth which has kept pace with the growth of the pharynx. It is still connected with the third pharyngeal pouch by a greatly attenuated cord of cells. Its anterior portion (figs. 3-4) is fused to the caudal aspect of the parathyroid from which it extends caudally. As in the preceding stage, the caudal portion makes a sharp turn in a ventromesial direction and lies over the upper portion of the pericardium. The caudal portion of the right thymus extends across the mesial plane while the same region of the left thymus lies to the left of the mesial plane and extends farther caudally than the right one. The anterior and central portions are cyUndrical in outhne, having an almost uniform diameter. In the left thymus the lumen of the anterior and central regions has entirely disappeared, while in the right thymus only a trace of it persists in the central portion. The caudal portion in the pericardial region is greatly enlarged. The lumen in this region is broken but in places is quite large in diameter. The walls are very thick and irregular, no longer retaining their cylindrical shape. | ||
the right one. The anterior and central portions are cyUndrical in outhne, having an almost uniform diameter. In the left thymus the lumen of the anterior and central regions has entirely disappeared, while in the right thymus only a trace of it persists in the central portion. The caudal portion in the pericardial region is greatly enlarged. The lumen in this region is broken but in places is quite large in diameter. The walls are very thick and irregular, no longer retaining their cylindrical shape. | |||
The anterior portion of the thymus also extends for a short distance along the ventro-lateral aspect of the parathyroid. It thus has two prongs between which lies the epithelial body. This condition is not present on the right side and was not observed in other specimens of about the same developmental stage. In stages earlier than this the parathyroids are anterior to the hypoglossal nerve. The anterior portion of the thymus is in close contact with both, as shown in figure 2. In the shiftings that occurred during the interval between this and the previous stage it appears that a portion of the thymus was carried along by the nerve and strung along the parathyroid thus bringing about the split condition of its anterior end. | The anterior portion of the thymus also extends for a short distance along the ventro-lateral aspect of the parathyroid. It thus has two prongs between which lies the epithelial body. This condition is not present on the right side and was not observed in other specimens of about the same developmental stage. In stages earlier than this the parathyroids are anterior to the hypoglossal nerve. The anterior portion of the thymus is in close contact with both, as shown in figure 2. In the shiftings that occurred during the interval between this and the previous stage it appears that a portion of the thymus was carried along by the nerve and strung along the parathyroid thus bringing about the split condition of its anterior end. | ||
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Embryo of 21.5 mm. (figure 5). In this stage the vesicula cervicalis has lost its connection with the ectoderm. The ductus cervicalis has entirely disappeared. The vesicula cervicalis lateralis {T.s.-V.c.l.) lies lateral to the hypoglossal nerve (N.XII). It is a large fusiform shaped mass of cells containing in its anterior portion a narrow tortuous lumen. This structure is of a purely ectodermal origin and represents the 'thymus superficialis' of Kastschenko. It is connected to the vesicula cervicalis medialis by the pars intermedia or connecting band (P.i.) that loops over the hypoglossal nerve. This band was not observed by Kastschenko, hence he held that the superficial thymus remained free from the remaining portion of the thymus. The vesicula cervicalis medialis, also greatly expanded, has lost its connection with the ganglion nodosum, possesses no lumen, and lies along the antero-lateral side of the massive parathyroid (Pi. S) where it is fused with the anterior portion of the thymus. | Embryo of 21.5 mm. (figure 5). In this stage the vesicula cervicalis has lost its connection with the ectoderm. The ductus cervicalis has entirely disappeared. The vesicula cervicalis lateralis {T.s.-V.c.l.) lies lateral to the hypoglossal nerve (N.XII). It is a large fusiform shaped mass of cells containing in its anterior portion a narrow tortuous lumen. This structure is of a purely ectodermal origin and represents the 'thymus superficialis' of Kastschenko. It is connected to the vesicula cervicalis medialis by the pars intermedia or connecting band (P.i.) that loops over the hypoglossal nerve. This band was not observed by Kastschenko, hence he held that the superficial thymus remained free from the remaining portion of the thymus. The vesicula cervicalis medialis, also greatly expanded, has lost its connection with the ganglion nodosum, possesses no lumen, and lies along the antero-lateral side of the massive parathyroid (Pi. S) where it is fused with the anterior portion of the thymus. | ||
The anterior portion of the thymus (T.e.) has lost its connection with the pharynx and hes on the dorso-lateral side of the parathyroid and is fused with the vesicula cervicalis medialis. In the region of the fused portion it contains a cavity of considerable size while the remaining portion along the epithelial body is without a lumen. From the epithelial body the thymus extends in a caudal and a slightly medial and ventral direction as a solid cord of cells. Just anterior to its entrance into the thoracic cavity it is slightly enlarged. The extreme caudal portion which lies within the thoracic cavity turns abruptly in a ventral direction, is greatly flattened, and in contact with the pericardium. The thoracic segments of the right and left thymus at this stage lie closely together but are not fused. Bell, however, in a 20 mm. embryo, describes them as being fused. | The anterior portion of the thymus (T.e.) has lost its connection with the pharynx and hes on the dorso-lateral side of the parathyroid and is fused with the vesicula cervicalis medialis. In the region of the fused portion it contains a cavity of considerable size while the remaining portion along the epithelial body is without a lumen. From the epithelial body the thymus extends in a caudal and a slightly medial and ventral direction as a solid cord of cells. Just anterior to its entrance into the thoracic cavity it is slightly enlarged. The extreme caudal portion which lies within the thoracic cavity turns abruptly in a ventral direction, is greatly flattened, and in contact with the pericardium. The thoracic segments of the right and left thymus at this stage lie closely together but are not fused. Bell, however, in a 20 mm. embryo, describes them as being fused. | ||
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The hypoglossal nerve now forms an acute angle with that portion of the vagus lying immediately posterior to it. In the two preceding stages that were modelled, the corresponding angle formed by these two nerves was obtuse instead of acute. This change in the form of angle between the earlier and later stages apparently is due to shiftings — a consideration of which is to follow — that take place in the neck during the growth of young embryos by which a stress appears to be exerted on the hypoglossal nerve by the cervical vesicle. | The hypoglossal nerve now forms an acute angle with that portion of the vagus lying immediately posterior to it. In the two preceding stages that were modelled, the corresponding angle formed by these two nerves was obtuse instead of acute. This change in the form of angle between the earlier and later stages apparently is due to shiftings — a consideration of which is to follow — that take place in the neck during the growth of young embryos by which a stress appears to be exerted on the hypoglossal nerve by the cervical vesicle. | ||
The thymus at this early stage (21.5 mm.) can be divided into seven regions, most of which in the later stages become very pronounced. They are: (1) The 'superficial thymus' which represents the vesicula cervicalis lateralis and is of a purely ectodermal origin; (2) the 'thymus head' which represents the structure formed by the fusion of the vesicula cervicalis medialis and the anterior portion of the entodermal anlage of the thymus; (3) the 'connecting band' which loops over the hypoglossal nerve and connects the superficial thjanus with the thymus head and is of a purely ectodermal origin; (4) the 'mid-cervical segment' which is an enlargement of the thymus between the intermediary and cervico-thoracic cords; (5) the 'intermediary cord' which connects the thymus head with the mid-cervical segment; (6) | The thymus at this early stage (21.5 mm.) can be divided into seven regions, most of which in the later stages become very pronounced. They are: (1) The 'superficial thymus' which represents the vesicula cervicalis lateralis and is of a purely ectodermal origin; (2) the 'thymus head' which represents the structure formed by the fusion of the vesicula cervicalis medialis and the anterior portion of the entodermal anlage of the thymus; (3) the 'connecting band' which loops over the hypoglossal nerve and connects the superficial thjanus with the thymus head and is of a purely ectodermal origin; (4) the 'mid-cervical segment' which is an enlargement of the thymus between the intermediary and cervico-thoracic cords; (5) the 'intermediary cord' which connects the thymus head with the mid-cervical segment; (6) the 'thoracic segment' which lies in the anterior portion of the thorax and is spread over a portion of the pericardium; and (7) the 'cervico-thoracic cord' which unites the mid-cervical segment to the thoracic segment. This system of nomenclature, for which we are indebted to Kastschenko, Zotterman, and Bell, will be used in the discussion of all the later developmental stages. | ||
the | |||
The different regions of the thymus described in the 21.5 mm. embryo were examined microscopically in the following five stages. These will be briefly described in order to present a more complete developmental history up to a 95 mm. embryo, which was the oldest stage in which a part of the pharyngeal region was reconstructed. | The different regions of the thymus described in the 21.5 mm. embryo were examined microscopically in the following five stages. These will be briefly described in order to present a more complete developmental history up to a 95 mm. embryo, which was the oldest stage in which a part of the pharyngeal region was reconstructed. | ||
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Embryo of 26 mm. The thymus as a whole is considerably larger than in the preceding stage. The surface of the superficial thymus, the thymus head, and the thoracic segment has become very irregular due to outgrowths of epithelial buds from the main stem. This budding represents the beginning of lobulation and had already started in the preceding stage. Lobulation of the mid-cervical segment has just begun. The superficial thymus extends only shghtly farther caudally from the hypoglossal nerve than in the preceding stage. The connecting band on the right side has disappeared but the superficial thymus has retained its usual topographical relation to the thymus head. From the parathyroid body the general direction of the thymus is in a caudo-mesia land ventral dhection. The intermediary cord on the right side is only a very slender cord of cells while that of the left side has a considerably greater diameter. The cervico-thoracic cords are short and have a uniform diameter of small dimension. The thoracic segments lie in contact with the anterior and ventral portion of the pericardium. The two segments lie close together and have fused in some places along their median sides. | Embryo of 26 mm. The thymus as a whole is considerably larger than in the preceding stage. The surface of the superficial thymus, the thymus head, and the thoracic segment has become very irregular due to outgrowths of epithelial buds from the main stem. This budding represents the beginning of lobulation and had already started in the preceding stage. Lobulation of the mid-cervical segment has just begun. The superficial thymus extends only shghtly farther caudally from the hypoglossal nerve than in the preceding stage. The connecting band on the right side has disappeared but the superficial thymus has retained its usual topographical relation to the thymus head. From the parathyroid body the general direction of the thymus is in a caudo-mesia land ventral dhection. The intermediary cord on the right side is only a very slender cord of cells while that of the left side has a considerably greater diameter. The cervico-thoracic cords are short and have a uniform diameter of small dimension. The thoracic segments lie in contact with the anterior and ventral portion of the pericardium. The two segments lie close together and have fused in some places along their median sides. | ||
Embryo of 32 mm. The connecting band on the left side is broken. No traces of it can be seen in connection with the thymus head but a remnant of it is still attached to the | Embryo of 32 mm. The connecting band on the left side is broken. No traces of it can be seen in connection with the thymus head but a remnant of it is still attached to the superficial thymus and extends as a greatly attenuated cord of cells toward the hypoglossal nerve. The general features of the entire thymus at this stage so closely resemble those of the 26 mm. embryo that a detailed description is unnecessary. The only difference of importance is the greater size of the organ as a whole and of the epithelial buds from the main stem. | ||
Embryo of 40 mm. The connecting band is continuous around the hypoglossal nerve on both sides. The parathyroids are slightly elongated and lie along the dorso-mesial side of the central third of the thymus head. Many of the primary epithelial buds of the enlarged segments (the superficial thymus, thymus heads, mid-cervical and thoracic segments) have sent out processes, thus marking the beginning of secondaiy lobulation. The intermediary cords are greatly attenuated and show no signs of budding. The transition from the thj^mus head to the intermediary cords and from the latter to the mid-cervical segment is very abrupt. The cervico-thoracic cords are short and lie near each other a short distance ventral to the trachear. The thoracic segments of both the right and left thymus are now fused along the greater part of their median plane. They are a little larger than those in the previous stage and have the same general position over the anterior and ventral portion of the pericardium. | Embryo of 40 mm. The connecting band is continuous around the hypoglossal nerve on both sides. The parathyroids are slightly elongated and lie along the dorso-mesial side of the central third of the thymus head. Many of the primary epithelial buds of the enlarged segments (the superficial thymus, thymus heads, mid-cervical and thoracic segments) have sent out processes, thus marking the beginning of secondaiy lobulation. The intermediary cords are greatly attenuated and show no signs of budding. The transition from the thj^mus head to the intermediary cords and from the latter to the mid-cervical segment is very abrupt. The cervico-thoracic cords are short and lie near each other a short distance ventral to the trachear. The thoracic segments of both the right and left thymus are now fused along the greater part of their median plane. They are a little larger than those in the previous stage and have the same general position over the anterior and ventral portion of the pericardium. | ||
Embryo of 52 mm. The connecting band on each side loops over the hypoglossal nerve and connects the thymus head with the superficial thymus. The one on the left side is a comparatively large and irregularly modelled cord of differentiated thymic tissue while the one on the right side is a slender and greatly attenuated cord of epithelial cells. The intermediary cords are still greatly attenuated cords of epithelial cells but are now studded here and there with small epithelial buds. The left cervico-thoracic cord is still slender with a nearly uniform diameter while the right one is much larger and has undergone lobulation. Both are now of differentiated thymic structure. The enlarged segments of the thymus are appreciably larger than those in the 40 mm. embryo. They have undergone extended | Embryo of 52 mm. The connecting band on each side loops over the hypoglossal nerve and connects the thymus head with the superficial thymus. The one on the left side is a comparatively large and irregularly modelled cord of differentiated thymic tissue while the one on the right side is a slender and greatly attenuated cord of epithelial cells. The intermediary cords are still greatly attenuated cords of epithelial cells but are now studded here and there with small epithelial buds. The left cervico-thoracic cord is still slender with a nearly uniform diameter while the right one is much larger and has undergone lobulation. Both are now of differentiated thymic structure. The enlarged segments of the thymus are appreciably larger than those in the 40 mm. embryo. They have undergone extended secondary lobulation the lobes of which on account of their large size lie in general more closely together than those in the previous stage. The parathyroid lies partly imbedded in the dorso-mesial aspect of the thymus head slightly anterior to its central portion. The superficial thymus lies closely along the antero-lateral aspect of the thymus head but is not fused to it. | ||
secondary lobulation the lobes of which on account of their large size lie in general more closely together than those in the previous stage. The parathyroid lies partly imbedded in the dorso-mesial aspect of the thymus head slightly anterior to its central portion. The superficial thymus lies closely along the antero-lateral aspect of the thymus head but is not fused to it. | |||
Embryo of 63 mm. The connecting band on both the right and left sides loops over the hypoglossal nerve. They are comparatively large and have an irregular surface similar to the left connecting band in the preceding stage. Aside from their greater size the large segments of the thymus in this stage present no striking morphological changes from those of the 52 mm. embryo. | Embryo of 63 mm. The connecting band on both the right and left sides loops over the hypoglossal nerve. They are comparatively large and have an irregular surface similar to the left connecting band in the preceding stage. Aside from their greater size the large segments of the thymus in this stage present no striking morphological changes from those of the 52 mm. embryo. | ||
Embryo of 95 mm. {figures 6 and 7). This is the oldest stage in which the anterior portion of the thymus was modelled. The left thymus was chosen for reconstruction although the right one would have done equally well. Figure 6 represents a lateral aspect of the thymus head {C.t.) and the superficial thymus (T.s.) The thymus head lies alongside the common carotid artery (A.c), its anterior end lying near the bifurcation into the external and internal carotid arteries. The parathyroid {Pt. 3) lies about midway between the two ends of the thymus head along its dorsal border and is closely attached to it. The superficial thymus {T.s) lies along the anterior half of the lateral aspect of the thymus head. Its dorsal and ventral borders are almost parallel to each other. The caudal border is rounding while its anterior part tapers irregularly into the slender connecting band {P.i.) which loops over the hypoglossal nerve {N.XII) and is connected with the thymus head. Figure 7 represents a ventral aspect of the same structures as seen in figure 6. The superficial thymus {T.s.) and the thymus head {C.t.) are flattened laterally. The anterior portion of the thymus head lies in contact with the dorsal border of the hypoglossal nerve. The superficial thymus lies closely against the thymus head but is not fused with it. Its posterior border on the right side gradually tapers down to a thin edge in contrast to the blunt posterior border of the left superficial thymus. The diameter of the | Embryo of 95 mm. {figures 6 and 7). This is the oldest stage in which the anterior portion of the thymus was modelled. The left thymus was chosen for reconstruction although the right one would have done equally well. Figure 6 represents a lateral aspect of the thymus head {C.t.) and the superficial thymus (T.s.) The thymus head lies alongside the common carotid artery (A.c), its anterior end lying near the bifurcation into the external and internal carotid arteries. The parathyroid {Pt. 3) lies about midway between the two ends of the thymus head along its dorsal border and is closely attached to it. The superficial thymus {T.s) lies along the anterior half of the lateral aspect of the thymus head. Its dorsal and ventral borders are almost parallel to each other. The caudal border is rounding while its anterior part tapers irregularly into the slender connecting band {P.i.) which loops over the hypoglossal nerve {N.XII) and is connected with the thymus head. Figure 7 represents a ventral aspect of the same structures as seen in figure 6. The superficial thymus {T.s.) and the thymus head {C.t.) are flattened laterally. The anterior portion of the thymus head lies in contact with the dorsal border of the hypoglossal nerve. The superficial thymus lies closely against the thymus head but is not fused with it. Its posterior border on the right side gradually tapers down to a thin edge in contrast to the blunt posterior border of the left superficial thymus. The diameter of the intermediary cords is considerably greater than in the preceding stages. Lobules are now present along their entire extent. They are a little shorter in this stage than in the 63 mm. embryo while the mid-cervical segment is somewhat longer. The cervico-thoracic cords are short and lie closely together. The thoracic segments are thin and flat and spread out over the pericardium to the left of the median line. They extend only a short distance to the right beyond the median line. | ||
Embryos of 105, I40, 170 and 280 mm., and pig 1 day post partem (text figures A, B, C, D and E, respectively). In these stages the structures covering the thymus were removed and the entire organ on the left side, undisturbed, was exposed to view. Diagrammatic drawings, representing accurately the outline of the lateral aspect of the different regions of the thymus, were made. In all cases specimens were selected in which the connecting band on both sides looped over the hypoglossal nerve and connected the superficial thymus with the thymus head. | Embryos of 105, I40, 170 and 280 mm., and pig 1 day post partem (text figures A, B, C, D and E, respectively). In these stages the structures covering the thymus were removed and the entire organ on the left side, undisturbed, was exposed to view. Diagrammatic drawings, representing accurately the outline of the lateral aspect of the different regions of the thymus, were made. In all cases specimens were selected in which the connecting band on both sides looped over the hypoglossal nerve and connected the superficial thymus with the thymus head. | ||
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Text figs. A, B, C, D Outline drawings of the exposed left thymus of embryos respectively 105, 140, 170 and 280 mm. (full term) in length; natural size. C.ct., cervico-thoracic cord; C.i., intermediary cord; C.t., caput thymus = thymus head; N.XII, hypoglossal nerve; S.m., mid-cervical segment; S.th., thoracic segment; T.S., thymus superficialis. | |||
Text fig. E Outline drawing of the exposed left thymus of a 'runty' pig, one day old and only 240 mm. in length; natural size. The thymus in this specimen was a few millimeters shorter than that in the full-term embryo; this is perhaps due to the fact that the specimen was a 'runt'. C.ct., cervico-thoracic cord; C.i., intermediary cord; C.t., caput thymus = thymus head; N.XII, hypoglossal nerve; S.m., mid-cervical segment; S.th., thoracic segment; T.s., thymus superficialis. | Text fig. E Outline drawing of the exposed left thymus of a 'runty' pig, one day old and only 240 mm. in length; natural size. The thymus in this specimen was a few millimeters shorter than that in the full-term embryo; this is perhaps due to the fact that the specimen was a 'runt'. C.ct., cervico-thoracic cord; C.i., intermediary cord; C.t., caput thymus = thymus head; N.XII, hypoglossal nerve; S.m., mid-cervical segment; S.th., thoracic segment; T.s., thymus superficialis. | ||
From the thymus head the intermediary cord {C.i.) and the mid-cervical segment (S.m.) extend in a meso-ventral direction to the anterior aperture of the thorax ventral to the trachea. In comparatively early stages they are more or less tortuous as represented in figure A, while in later stages their course is nearly straight. The mid-cervical segment in early stages is short and lies immediately anterior to the thorax while the intermediary cord is comparatively long as represented in figure A. As development proceeds the mid-cervical segment gradually becomes longer while the intermediary cord becomes shorter as represented in the figures. | From the thymus head the intermediary cord {C.i.) and the mid-cervical segment (S.m.) extend in a meso-ventral direction to the anterior aperture of the thorax ventral to the trachea. In comparatively early stages they are more or less tortuous as represented in figure A, while in later stages their course is nearly straight. The mid-cervical segment in early stages is short and lies immediately anterior to the thorax while the intermediary cord is comparatively long as represented in figure A. As development proceeds the mid-cervical segment gradually becomes longer while the intermediary cord becomes shorter as represented in the figures. | ||
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The connecting band was present on both sides in the majority of specmiens examined. It may, however, be absent either on one or on both sides. Its rupture is apparently due to the growth in length of the thymus not keeping pace with the growth in length of the neck. The expanded caudal portion of the thymus being firmly anchored in the anterior portion of the thoracic cavity, on account of the unequal rate of growth between the neck and the thymus, will exert a pull on that portion of the organ in the neck and thus greatly attenuate or tear the connecting band. Also there is thus a stress exerted on the hypoglossal nerve which apparently tends slightly to change its direction, as stated in the description of the thymus in 21.5 mm. embryo (p. 325). | The connecting band was present on both sides in the majority of specmiens examined. It may, however, be absent either on one or on both sides. Its rupture is apparently due to the growth in length of the thymus not keeping pace with the growth in length of the neck. The expanded caudal portion of the thymus being firmly anchored in the anterior portion of the thoracic cavity, on account of the unequal rate of growth between the neck and the thymus, will exert a pull on that portion of the organ in the neck and thus greatly attenuate or tear the connecting band. Also there is thus a stress exerted on the hypoglossal nerve which apparently tends slightly to change its direction, as stated in the description of the thymus in 21.5 mm. embryo (p. 325). | ||
A microscopical examination was made of the superficial thymus in various developmental stages, including that of two fullterm embryos. It was found that the histogenetic processes of this segment kept pace with those in the segments of the thymus which have a purely entodermal origin. | A microscopical examination was made of the superficial thymus in various developmental stages, including that of two fullterm embryos. It was found that the histogenetic processes of this segment kept pace with those in the segments of the thymus which have a purely entodermal origin. | ||
==Conclusions== | |||
The thymus of the pig has an ectodermal-entodermal origin. The respective origin of each segment is as follows: | The thymus of the pig has an ectodermal-entodermal origin. The respective origin of each segment is as follows: | ||
# The superficial thymus, which is a derivative of the cervical vesicle, has a purely ectodermal origin. It is a constant structure and, therefore, forms an integral part of the organ. | |||
# The connecting band is also a derivative of the cervical vesicle and has, therefore, a purely ectodermal origin. In the majority of embryos it persists to birth but may be absent either on one or on both sides. | |||
# The thymus head, in which is lodged the parathyroid III, is formed by a fusion of a portion of the cervical vesicle to the anterior end of the epithelial diverticulum derived from the third pharyngeal pouch. It has, therefore, an ectodermal-entodermal origin. | |||
# The intermediary and cervico- thoracic cords, and the midcervical and thoracic segments are derived wholly from the epithelial diverticulum of the third pharyngeal pouch and have, therefore, a purely entodermal origin. | |||
I wish to thank Dr. B. P. Kingsbury for the aid given me in this work. I am also indebted to Dr. David Marine, of the Western Reserve University, for sending me many formalinpreserved embryos of various sizes from which most of the drawings of the exposed thymus were made. | I wish to thank Dr. B. P. Kingsbury for the aid given me in this work. I am also indebted to Dr. David Marine, of the Western Reserve University, for sending me many formalinpreserved embryos of various sizes from which most of the drawings of the exposed thymus were made. | ||
==Literature Cited== | |||
Bell, E. T. 1906 The development of the thymus. Amer. Jour. Anat., vol. 5. | |||
Born, C. 1883 Uber die Derivate der embryonalen Schlundbogen und Schlund spalten bei Saugetieren. Arch. f. mikr. Anat., Bd. 22. | |||
FiscHELis, P. 1885 Beitriige zur Kenntnis der Entwickelungsgeschichte der Gl. thyreoidea und (11. thjanus. Arch. f. mikr. Anat., Bd. 25. | |||
Fox, H. 1908 The pharyngeal pouches and their derivatives in the IVIammalia. Am. Jour. Anat., vol. 8. | |||
Kastschenko, N. 1887 Das Schicksal der embryonalen Schlundspalten bei Saugetieren. Arch. f. mikr. Anat., Bd. 30. | |||
RouD, A. 1900 Contribution a I'etude de Torigine et de revolution de la thy roide laterale et due thymus chez le campagnol. Bull. Soc. vaudoise | |||
des Sc. natur., T. 36 | des Sc. natur., T. 36. | ||
Anat. Anz., Bd. 38. | ZoTTERMAN, A. 1911 Die Schweinthymus als eine Thymus ecto-entodermalis. Anat. Anz., Bd. 38. | ||
==Plates== | |||
===Abbreviations=== | |||
A.c, carotid artery Pi. 3, parathyroid derived from third | A.c, carotid artery Pi. 3, parathyroid derived from third | ||
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V.c.m., vesicula cervicalis medialis | V.c.m., vesicula cervicalis medialis | ||
PLATE 1 | PLATE 1 | ||
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1 Drawing of a reconstruction of the ])harynx and derivatives of the second and third pharyngeal pouches of the right side, including portions of the structures closely associated with the pharyngeal derivatives and a portion of the ectoderm. Ventral aspect; pig embryo 14.5 mm. in length. Model X 92, reduced one-half. | 1 Drawing of a reconstruction of the ])harynx and derivatives of the second and third pharyngeal pouches of the right side, including portions of the structures closely associated with the pharyngeal derivatives and a portion of the ectoderm. Ventral aspect; pig embryo 14.5 mm. in length. Model X 92, reduced one-half. | ||
2 Drawing of a reconstruction of a portion of the pharynx, the third pharyngeal pouch, anterior portion of the thymus anlage, parathyroid 3, cervical vesicle, and associates of the above named structures. Caudo-ventral aspect. The model represented in this figure was made from the same side of the same embryo (14.5 mm.) from which the model represented in figure 1 was made. | 2 Drawing of a reconstruction of a portion of the pharynx, the third pharyngeal pouch, anterior portion of the thymus anlage, parathyroid 3, cervical vesicle, and associates of the above named structures. Caudo-ventral aspect. The model represented in this figure was made from the same side of the same embryo (14.5 mm.) from which the model represented in figure 1 was made. Model X 245, reduced one-half. | ||
3 Drawing of a reconstruction of the same structures as enumerated under figure 2. This model was made to show specially the relation of the cervical vesicle to the thymus anlage and the hypoglossal nerve after the shifting of the structures in the neck have become quite noticeable. Ventral aspect, left side; pig embryo 17.5 mm. in length. Model X 182, reduced one-half. | 3 Drawing of a reconstruction of the same structures as enumerated under figure 2. This model was made to show specially the relation of the cervical vesicle to the thymus anlage and the hypoglossal nerve after the shifting of the structures in the neck have become quite noticeable. Ventral aspect, left side; pig embryo 17.5 mm. in length. Model X 182, reduced one-half. | ||
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Badertscher JA. The development of the thymus in the pig. I. Morphogenesis. (1915) Amer. J Anat. 17(3): 317-337.
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The Development of the Thymus in the Pig I. Morphogenesis
J. A. Badertscher
From the Department of Histology and Embryology, Cornell University, Ithaca, N. Y.
Twelve Figures — Two Plates
Historical
It has been definitely established by many investigators that the thymus of all mammals is of epithelial origin. More recent investigations have shown, however, that the epithelial anlage of the thymus is not derived from the same germ layer in all mammals. Investigators are agreed on the point that the thymus of mammals, when it is of a purely entodermal origin, is a derivative of the ventrally directed epithelial diverticulum of the third pharyngeal pouch. It has also been quite definitely settled that in some mammals (mouse, Roud '00) the thymus is entirely of ectodermal origin. The mixed (ectodermal-entodermal) origin of the thymus in some mammals has not yet been generally accepted. In pig embryos it is the close topographical relation that exists between the cervical vesicle and the third pharyngeal pouch that makes a mixed origin of the thymus possible.
Among some of the workers on the early development of the thymus of the pig may be mentioned Fischelis, Kastschenko, Zotterman, Born, Bell, and Fox, the first thi'ee of whom attribute to the thymus an ectodermal-entodermal origin. Fischelis ('85) derived the thymus from the third pharyngeal pouch and the third branchial groove. According to this investigation these two fuse, and from then- point of fusion each contributes about one-half to a ventrally directed downgrowth, the anlage of the thymus. This conclusion is erroneous, for that portion of the thymus which is derived from the third pharyngeal pouch is a comparatively long mid-ventrally directed epithelial tube before the cervical sinus is fused to it. He also makes no mention of the XII cranial nerve which plays an important part in modifying the topographical relations of the anterior portion of the thymus to surrounding structures. Basing his conclusions on inaccurate observations, his views in regard to a mixed origin of the thymus have now only an historical value.
The first detailed study of the early development of the thymus in the pig was made by Kastschenko ('87). He describes the mesial portion of the sinus cervicalis, which he calls the 'vesicula thymica,' as fusing with the anterior end of the epithehal anlage of the thymus, which is derived from the third pharyngeal pouch. In the shif tings of some of the structures in the neck, that occur in young embryos during growth, the lateral portion of the cervical vesicle is separated mechanically from its mesial portion by the h3^oglossal nerve. The free lateral portion of the cervical vesicle gives rise to the 'thymus superficialis' which is necessarily of ectodermal origin. He claims that the superficial thymus is not a constant structure, for, in a 30 mm. embryo that he examined, it was not present. The anterior end of the thymus to which the mesial portion of sinus cervicalis has fused, plus the parathyroid, that lies close to it, he designates the thymus head; while the large remaining portion of the thymus which is of a purely entodermal origin, plus the thymus head, he calls the 'thymus profunda.' The largest embryo examined by him was 82 mm. in length.
Zotterman ('11) also made a detailed study of the morphogenesis of the thymus of the pig. Her conclusions are in accord with those of Kastschenko with the exception that in about one-half of the specimens examined the superficial thymus was connected with the thymus head by a cord of cells that looped over the hypoglossal nerve. The superficial thymus was found in all the specimens examined and in the largest (105 mm.) investigated aU the features common to the thymus (cortex, medulla, Hassall's corpuscles, etc.) were present.
Fox ('08) agrees with Kastschenko that the superficial thymus of the pig arises by a constriction of the fundus praecervicaHs, but claims that in embryos up to 35 mm. in length, the oldest stage he examined, its histological structure does not resemble that of the thymus more than any other branching epithelial mass.
Born ('83) derived the thymus anlage in pig embryos from the third pharyngeal pouch, while Bell ('06) also is inclined to believe that the ectoderm takes no part in the formation of the thymus.
Material and Methods
For investigation of the early stages of the morphogenesis of the thymus, the excellent collection of pig embryo series, 3 to 42 mm. in length, in the Department of Histology and Embryology of Cornell University, proved very helpful. In addition to these, five embryos ranging from 9 to 21.5 mm. in length, and the neck and upper thoracic region of eight embryos ranging from 32 to 95 mm. in length, were sectioned transversely. These sections, 10 microns in thickness, were stained with hematoxylin and eosin. From series of this group reconstructions of the pharyngeal region were made. Many dissections exposing the thymus were made of the neck and upper thoracic region of embryos from 100 to 280 mm. in length (full term). The thymus of a pig one day old was also examined.
Morphogenesis
In the investigation on the morphogenesis of the thymus special attention was constantly directed toward the development of the superficial thymus because its existence is not yet generally accepted and, since the latest developmental stage in which it was investigated by Zotterman was only 105 mm. in length, its fate is not definitely known.
An 11 mm. embryo was the developmental stage chosen as the starting point for the study of the morphogenesis of the thymus. At this stage the ectodermal and entodermal parts of the branchial grooves and pharyngeal pouches can still be distinguished from each other without difficulty. The sinus cervicahs, formed by the rapid growth in a caudal direction of the mandibular and hyoid arches and the more retarded growth of the branchial arches proper, is already well mapped out. As this stage shows well accepted relations and developmental steps it needs no further description.
Embryo of lJf-.5 mm. {figures 1 and 2). This is the youngest developmental stage from which a reconstruction of one side of the neck was made. Since the determination of the real origin of the thymus was one of the crucial points in this investigation only that part of the neck containing the anterior portion of the thymus. anlage and the vesicula cervicalis was modelled.
The posterior edge of the hyoid arch has grown over the opening of the sinus cervicalis, shutting it off from the exterior. The cavity thus formed is the vesicula cervicalis (F.c.) or the 'vesicula thymica' of Kastschenko.^ The vesicula cervicalis, now widely separated from the ectoderm (Ect.), is still connected with it by a heavy cord of cells, the ductus cervicalis (D.c). Only in places through its entire extent are traces of a lumen left. To the outer end of the vesicula cervicalis is attached a cord of cells that runs in an antero-ventral and mesial direction and connects with the second pharyngeal pouch. This is the ductus branchialis (D.b.). The anterior one-fifth of this structure possesses a lumen which is continuous with that of the second pouch. At this stage it is impossible to determine the extent of the part that is of entodermal origin and the extent that is of ectodermal origin. The boundary line between the two has disappeared through the obliteration of the lumen. Fox ('08) was unable to find the ductus branchiahs in pig embryos but demonstrated a long diverticulum — 'filiform process' — arising from the ventrolateral angle of the second pouch and connected with the ectoderm. This process was not present in any of the embryos I examined.
1 "According to H. Ilabl ('09) the term 'vesicula cei'vicalis' is to be applied to the entire complex, including the two ductus branchiales; Hammar uses the term 'vesicula praecervicalis' only for the vesicular portion that is associated with the third pharyngeal pouch, this portion being approximately identical with the 'fundus praecervicalis' (cervicalis) of His and H. Rabl, as well as with the 'vesicula thymica' of Kastschenko and the 'sinus vesicle' of Zuckerhandl." (Quoted from Keibel and Mall's Human embryology, vol. 2, p. 456).
The vesicula cervicahs (fig. 2, V.c.) hes lateral to the third pharyngeal pouch, between the cephalo-dorsal portion of the parathyroid {Pt. 3) and the caudo-ventral part of the ganglion nodosum (Gji.) and a short distance anterior to the hypoglossal nerve (N.XII). Its general shape is fusiform, with its long axis almost perpendicular to the surface ectoderm. The middle third is solid, but each extremity contains a cavity. The expansion of the central portion is due to a proliferation of the cells of its anterior wall which presses tightly against the dorsal surface of the parathyroid. The pressure against the parathyroid has apparently caused the obliteration of the cavity of the vesicula cervicalis in its central portion. Its inner third lies closely along the ventral side of the ganglion nodosum into which its curved end projects and with which it apparently is fused.
The parathyroid {Pt. 3) is now a massive structure lying lateral to the third pharyngeal pouch {S.h.3), anterior to the hypoglossal nerve with which it is in contact, ventral to the ganglion nodosum and dorsal to the carotid artery (A.c). Its general shape is that of a hemisphere with its flat side turned toward the vagus nerve and the vesicula cervicalis. A depression in both the vesicula cervicalis and the parathyroid mark the points of most intimate contact between the two structures.
The entodermal anlage of the thymus (T.e.) is now, throughout its greatest extent, a solid cord of cells, and still attached to the third pharyngeal pouch. Its anterior end lies closely against the parathyroid and the hypoglossal nerve. From its point of origin it extends caudally, and, with the exception of about onefourth of the posterior portion, lies ventro-laterally to the carotid artery and the vagus nerve. The caudal portion makes a rather sharp turn in a ventro-mesial direction and occupies the upper part of the pericardial region. The diameter of the caudal part is considerably greater than that of the remaining portion. This is due to the presence of a large lumen and thick walls in this region. In the central and anterior portions only a few slight traces of a lumen persist. The vesicula cervicalis and the anterior portion of the entodermal thymus do not come in contact with each other in this developmental stage.
Embryo of 17.5 mm. {figures 3 and I^. During the interval between this and the previous stage shiftings in the pharyngeal region have taken place that have changed the relation of some of the parts to each other. The ductus cervicalis {D.c), now a solid cord of cells, is still connected with the ectoderm. The ductus branchialis on the left side, which was not modelled, has lost its connection with the outer end of the vesicula cervicalis. No traces of it in this region can be seen. It is, however, still connected with the second branchial pouch from which it extends for a short distance toward the point of its former attachment. ^ On the right side it is still a continuous solid cord of cells extending from the second pouch to the vesicula cervicalis. In a 21 mm. pig embryo Zotterman ('11) had demonstrated the ductus branchialis as a continuous cord of cells while the ductus cervicalis as being broken. In the embryos which I examined the ductus branchialis was always the first to become discontinuous.
The vesicula cervicalis {V.c.) no longer lies perpendicular to the ectoderm. The vesicula cervicalis medialis {V.c.vi.y extends from its point of attachment to the ganglion nodosum in an antero-lateral direction to the hypoglossal nerve (N.XII) around which it forms an acute angle. Fr©m the nerve the vesicula cervicalis lateralis {v.c.l.) extends for a short distance in a caudolateral direction. This is the same general direction taken by the ductus cervicalis which is connected to the vesicula cervicalis laterahs and the ectoderm. The vesicula cervicalis medialis is tightly wedged in between the vagus on its dorsal side and the parathyroid gland and a small portion of the thymus on its ventral aspect. Its caudal portion is greatly flattened but as it approaches the nerve it gradually assumes a cylindrical form which also is the shape of the vesicula cervicaUs lateralis. A part of its flattened caudal portion dips into the ganglion nodosum while a portion lies in close contact with the thymus. Fusion between the thymus and the vesicle has apparently not yet taken place, for the boundary of both can still be clearly deterr mined. The lumen of the vesicula cervicalis is for the most part obliterated. Only slight traces here and there in its course persist. It is largest in the portion that dips into the ganglion nodosum. Here the lumen is large and the wall of this portion of the vesicle is no thicker than that of earlier stages. Apparently no cell proliferation takes place in this region. The surface of the entire vesicula cervicalis is more or less irregular. An idea of its shape can best be obtained by referring to figure 4 in which the hypoglossal nerve and a part of the ganglion nodosum were removed, thus almost entirely exposing it.
2 A reconstruction of this remnant was deemed unnecessary since it takes no part in the formation of the thymus and would have needlessly increased the size of the model.
^ From the reconstruction as represented in figures 3 and 4 it will be seen that the vesicula cervicalis now loops over the hypoglossal nerve. For the sake of simplicity as well as for clearness, that portion of the vesicula cervicalis lying between the nerve and the pharynx will be termed the 'vesicula cervicalis medialis' while the part lying between the nerve and the surface ectoderm will be termed the 'vesicula cervicalis lateralis.'
The parathyroid {Pt. 3) is an elongated and very irregular mass of cells that is tightly packed in between the cervical vesicle and vagus nerve on its dorsal aspect, and the carotid artery on its mesial surface. Its caudo-mesial and caudo-lateral portions are in contact with the thymus while its anterior portion is on a Ifevel with the arch of the vesicula cervicalis over the hypoglossal nerve.
The thymus {T.e.) is considerably longer than in the preceding stage. Its cephahc and caudal ends have about the same relative position to the other structures as in the 14.5 mm. embryo. Its greater length at this stage is due to growth which has kept pace with the growth of the pharynx. It is still connected with the third pharyngeal pouch by a greatly attenuated cord of cells. Its anterior portion (figs. 3-4) is fused to the caudal aspect of the parathyroid from which it extends caudally. As in the preceding stage, the caudal portion makes a sharp turn in a ventromesial direction and lies over the upper portion of the pericardium. The caudal portion of the right thymus extends across the mesial plane while the same region of the left thymus lies to the left of the mesial plane and extends farther caudally than the right one. The anterior and central portions are cyUndrical in outhne, having an almost uniform diameter. In the left thymus the lumen of the anterior and central regions has entirely disappeared, while in the right thymus only a trace of it persists in the central portion. The caudal portion in the pericardial region is greatly enlarged. The lumen in this region is broken but in places is quite large in diameter. The walls are very thick and irregular, no longer retaining their cylindrical shape.
The anterior portion of the thymus also extends for a short distance along the ventro-lateral aspect of the parathyroid. It thus has two prongs between which lies the epithelial body. This condition is not present on the right side and was not observed in other specimens of about the same developmental stage. In stages earlier than this the parathyroids are anterior to the hypoglossal nerve. The anterior portion of the thymus is in close contact with both, as shown in figure 2. In the shiftings that occurred during the interval between this and the previous stage it appears that a portion of the thymus was carried along by the nerve and strung along the parathyroid thus bringing about the split condition of its anterior end.
Embryo of 21.5 mm. (figure 5). In this stage the vesicula cervicalis has lost its connection with the ectoderm. The ductus cervicalis has entirely disappeared. The vesicula cervicalis lateralis {T.s.-V.c.l.) lies lateral to the hypoglossal nerve (N.XII). It is a large fusiform shaped mass of cells containing in its anterior portion a narrow tortuous lumen. This structure is of a purely ectodermal origin and represents the 'thymus superficialis' of Kastschenko. It is connected to the vesicula cervicalis medialis by the pars intermedia or connecting band (P.i.) that loops over the hypoglossal nerve. This band was not observed by Kastschenko, hence he held that the superficial thymus remained free from the remaining portion of the thymus. The vesicula cervicalis medialis, also greatly expanded, has lost its connection with the ganglion nodosum, possesses no lumen, and lies along the antero-lateral side of the massive parathyroid (Pi. S) where it is fused with the anterior portion of the thymus.
The anterior portion of the thymus (T.e.) has lost its connection with the pharynx and hes on the dorso-lateral side of the parathyroid and is fused with the vesicula cervicalis medialis. In the region of the fused portion it contains a cavity of considerable size while the remaining portion along the epithelial body is without a lumen. From the epithelial body the thymus extends in a caudal and a slightly medial and ventral direction as a solid cord of cells. Just anterior to its entrance into the thoracic cavity it is slightly enlarged. The extreme caudal portion which lies within the thoracic cavity turns abruptly in a ventral direction, is greatly flattened, and in contact with the pericardium. The thoracic segments of the right and left thymus at this stage lie closely together but are not fused. Bell, however, in a 20 mm. embryo, describes them as being fused.
The hypoglossal nerve now forms an acute angle with that portion of the vagus lying immediately posterior to it. In the two preceding stages that were modelled, the corresponding angle formed by these two nerves was obtuse instead of acute. This change in the form of angle between the earlier and later stages apparently is due to shiftings — a consideration of which is to follow — that take place in the neck during the growth of young embryos by which a stress appears to be exerted on the hypoglossal nerve by the cervical vesicle.
The thymus at this early stage (21.5 mm.) can be divided into seven regions, most of which in the later stages become very pronounced. They are: (1) The 'superficial thymus' which represents the vesicula cervicalis lateralis and is of a purely ectodermal origin; (2) the 'thymus head' which represents the structure formed by the fusion of the vesicula cervicalis medialis and the anterior portion of the entodermal anlage of the thymus; (3) the 'connecting band' which loops over the hypoglossal nerve and connects the superficial thjanus with the thymus head and is of a purely ectodermal origin; (4) the 'mid-cervical segment' which is an enlargement of the thymus between the intermediary and cervico-thoracic cords; (5) the 'intermediary cord' which connects the thymus head with the mid-cervical segment; (6) the 'thoracic segment' which lies in the anterior portion of the thorax and is spread over a portion of the pericardium; and (7) the 'cervico-thoracic cord' which unites the mid-cervical segment to the thoracic segment. This system of nomenclature, for which we are indebted to Kastschenko, Zotterman, and Bell, will be used in the discussion of all the later developmental stages.
The different regions of the thymus described in the 21.5 mm. embryo were examined microscopically in the following five stages. These will be briefly described in order to present a more complete developmental history up to a 95 mm. embryo, which was the oldest stage in which a part of the pharyngeal region was reconstructed.
Embryo of 26 mm. The thymus as a whole is considerably larger than in the preceding stage. The surface of the superficial thymus, the thymus head, and the thoracic segment has become very irregular due to outgrowths of epithelial buds from the main stem. This budding represents the beginning of lobulation and had already started in the preceding stage. Lobulation of the mid-cervical segment has just begun. The superficial thymus extends only shghtly farther caudally from the hypoglossal nerve than in the preceding stage. The connecting band on the right side has disappeared but the superficial thymus has retained its usual topographical relation to the thymus head. From the parathyroid body the general direction of the thymus is in a caudo-mesia land ventral dhection. The intermediary cord on the right side is only a very slender cord of cells while that of the left side has a considerably greater diameter. The cervico-thoracic cords are short and have a uniform diameter of small dimension. The thoracic segments lie in contact with the anterior and ventral portion of the pericardium. The two segments lie close together and have fused in some places along their median sides.
Embryo of 32 mm. The connecting band on the left side is broken. No traces of it can be seen in connection with the thymus head but a remnant of it is still attached to the superficial thymus and extends as a greatly attenuated cord of cells toward the hypoglossal nerve. The general features of the entire thymus at this stage so closely resemble those of the 26 mm. embryo that a detailed description is unnecessary. The only difference of importance is the greater size of the organ as a whole and of the epithelial buds from the main stem.
Embryo of 40 mm. The connecting band is continuous around the hypoglossal nerve on both sides. The parathyroids are slightly elongated and lie along the dorso-mesial side of the central third of the thymus head. Many of the primary epithelial buds of the enlarged segments (the superficial thymus, thymus heads, mid-cervical and thoracic segments) have sent out processes, thus marking the beginning of secondaiy lobulation. The intermediary cords are greatly attenuated and show no signs of budding. The transition from the thj^mus head to the intermediary cords and from the latter to the mid-cervical segment is very abrupt. The cervico-thoracic cords are short and lie near each other a short distance ventral to the trachear. The thoracic segments of both the right and left thymus are now fused along the greater part of their median plane. They are a little larger than those in the previous stage and have the same general position over the anterior and ventral portion of the pericardium.
Embryo of 52 mm. The connecting band on each side loops over the hypoglossal nerve and connects the thymus head with the superficial thymus. The one on the left side is a comparatively large and irregularly modelled cord of differentiated thymic tissue while the one on the right side is a slender and greatly attenuated cord of epithelial cells. The intermediary cords are still greatly attenuated cords of epithelial cells but are now studded here and there with small epithelial buds. The left cervico-thoracic cord is still slender with a nearly uniform diameter while the right one is much larger and has undergone lobulation. Both are now of differentiated thymic structure. The enlarged segments of the thymus are appreciably larger than those in the 40 mm. embryo. They have undergone extended secondary lobulation the lobes of which on account of their large size lie in general more closely together than those in the previous stage. The parathyroid lies partly imbedded in the dorso-mesial aspect of the thymus head slightly anterior to its central portion. The superficial thymus lies closely along the antero-lateral aspect of the thymus head but is not fused to it.
Embryo of 63 mm. The connecting band on both the right and left sides loops over the hypoglossal nerve. They are comparatively large and have an irregular surface similar to the left connecting band in the preceding stage. Aside from their greater size the large segments of the thymus in this stage present no striking morphological changes from those of the 52 mm. embryo.
Embryo of 95 mm. {figures 6 and 7). This is the oldest stage in which the anterior portion of the thymus was modelled. The left thymus was chosen for reconstruction although the right one would have done equally well. Figure 6 represents a lateral aspect of the thymus head {C.t.) and the superficial thymus (T.s.) The thymus head lies alongside the common carotid artery (A.c), its anterior end lying near the bifurcation into the external and internal carotid arteries. The parathyroid {Pt. 3) lies about midway between the two ends of the thymus head along its dorsal border and is closely attached to it. The superficial thymus {T.s) lies along the anterior half of the lateral aspect of the thymus head. Its dorsal and ventral borders are almost parallel to each other. The caudal border is rounding while its anterior part tapers irregularly into the slender connecting band {P.i.) which loops over the hypoglossal nerve {N.XII) and is connected with the thymus head. Figure 7 represents a ventral aspect of the same structures as seen in figure 6. The superficial thymus {T.s.) and the thymus head {C.t.) are flattened laterally. The anterior portion of the thymus head lies in contact with the dorsal border of the hypoglossal nerve. The superficial thymus lies closely against the thymus head but is not fused with it. Its posterior border on the right side gradually tapers down to a thin edge in contrast to the blunt posterior border of the left superficial thymus. The diameter of the intermediary cords is considerably greater than in the preceding stages. Lobules are now present along their entire extent. They are a little shorter in this stage than in the 63 mm. embryo while the mid-cervical segment is somewhat longer. The cervico-thoracic cords are short and lie closely together. The thoracic segments are thin and flat and spread out over the pericardium to the left of the median line. They extend only a short distance to the right beyond the median line.
Embryos of 105, I40, 170 and 280 mm., and pig 1 day post partem (text figures A, B, C, D and E, respectively). In these stages the structures covering the thymus were removed and the entire organ on the left side, undisturbed, was exposed to view. Diagrammatic drawings, representing accurately the outline of the lateral aspect of the different regions of the thymus, were made. In all cases specimens were selected in which the connecting band on both sides looped over the hypoglossal nerve and connected the superficial thymus with the thymus head.
By referring to the figures cited above it will be seen that the comparative size of the superficial thymus {T.s.) and the thymus head {C.t.) vary somewhat in different developmental stages. In general, the proportional size of the former to the latter is greater in earlier than in later developmental stages. From numerous dissections that were made it was found that the comparative sizes of the two structures vary considerably in embryos of about the same developmental stage or even in those of the same litter. Figure A represents about the average comparative size of the superficial thymus and the thymus head in embryos of about 105 mm. in length, while the size of the superficial thymus of an 140 mm. embryo as represented in figure B is considerably larger than it ordinarily occurs in corresponding developmental stages. Variations in size of the superficial thymi in the same embryo also occur; e.g., the right one in a 170 mm. embryo was an oblong flap that covered the anterior one-fourth of the lateral surface of the thymus head, while the left one is much smaller as represented in figure C. In all the embryos examined the superficial thymus was always closely associated with the thymus head but never fused with it.
Text figs. A, B, C, D Outline drawings of the exposed left thymus of embryos respectively 105, 140, 170 and 280 mm. (full term) in length; natural size. C.ct., cervico-thoracic cord; C.i., intermediary cord; C.t., caput thymus = thymus head; N.XII, hypoglossal nerve; S.m., mid-cervical segment; S.th., thoracic segment; T.S., thymus superficialis.
Text fig. E Outline drawing of the exposed left thymus of a 'runty' pig, one day old and only 240 mm. in length; natural size. The thymus in this specimen was a few millimeters shorter than that in the full-term embryo; this is perhaps due to the fact that the specimen was a 'runt'. C.ct., cervico-thoracic cord; C.i., intermediary cord; C.t., caput thymus = thymus head; N.XII, hypoglossal nerve; S.m., mid-cervical segment; S.th., thoracic segment; T.s., thymus superficialis.
From the thymus head the intermediary cord {C.i.) and the mid-cervical segment (S.m.) extend in a meso-ventral direction to the anterior aperture of the thorax ventral to the trachea. In comparatively early stages they are more or less tortuous as represented in figure A, while in later stages their course is nearly straight. The mid-cervical segment in early stages is short and lies immediately anterior to the thorax while the intermediary cord is comparatively long as represented in figure A. As development proceeds the mid-cervical segment gradually becomes longer while the intermediary cord becomes shorter as represented in the figures.
The cervico-thoracic segment (C.ct.) in all stages is short and of a comparatively small diameter. It Ues in the extreme ventral portion of the anterior aperture of the thorax. In early stages the cords of the right and left thymus lie closely together and in later stages they fuse with each other.
The thoracic segment (S.th.) in later developmental stages is composed of the thoracic portions of both the right and left thymus which have fused in this region. In embryos about 105 mm. in length, and later stages, it is spread over the anteroventral surface of the left side of the pericardium. The swinging of the right segment toward the left side is already noticeable in a 42 mm. embryo. This segment is thickest along the median line (3.5 to 4 mm. in full term embryos) and gradually tapers down to a thin irregular edge.
The connecting band was present on both sides in the majority of specmiens examined. It may, however, be absent either on one or on both sides. Its rupture is apparently due to the growth in length of the thymus not keeping pace with the growth in length of the neck. The expanded caudal portion of the thymus being firmly anchored in the anterior portion of the thoracic cavity, on account of the unequal rate of growth between the neck and the thymus, will exert a pull on that portion of the organ in the neck and thus greatly attenuate or tear the connecting band. Also there is thus a stress exerted on the hypoglossal nerve which apparently tends slightly to change its direction, as stated in the description of the thymus in 21.5 mm. embryo (p. 325).
A microscopical examination was made of the superficial thymus in various developmental stages, including that of two fullterm embryos. It was found that the histogenetic processes of this segment kept pace with those in the segments of the thymus which have a purely entodermal origin.
Conclusions
The thymus of the pig has an ectodermal-entodermal origin. The respective origin of each segment is as follows:
- The superficial thymus, which is a derivative of the cervical vesicle, has a purely ectodermal origin. It is a constant structure and, therefore, forms an integral part of the organ.
- The connecting band is also a derivative of the cervical vesicle and has, therefore, a purely ectodermal origin. In the majority of embryos it persists to birth but may be absent either on one or on both sides.
- The thymus head, in which is lodged the parathyroid III, is formed by a fusion of a portion of the cervical vesicle to the anterior end of the epithelial diverticulum derived from the third pharyngeal pouch. It has, therefore, an ectodermal-entodermal origin.
- The intermediary and cervico- thoracic cords, and the midcervical and thoracic segments are derived wholly from the epithelial diverticulum of the third pharyngeal pouch and have, therefore, a purely entodermal origin.
I wish to thank Dr. B. P. Kingsbury for the aid given me in this work. I am also indebted to Dr. David Marine, of the Western Reserve University, for sending me many formalinpreserved embryos of various sizes from which most of the drawings of the exposed thymus were made.
Literature Cited
Bell, E. T. 1906 The development of the thymus. Amer. Jour. Anat., vol. 5.
Born, C. 1883 Uber die Derivate der embryonalen Schlundbogen und Schlund spalten bei Saugetieren. Arch. f. mikr. Anat., Bd. 22.
FiscHELis, P. 1885 Beitriige zur Kenntnis der Entwickelungsgeschichte der Gl. thyreoidea und (11. thjanus. Arch. f. mikr. Anat., Bd. 25.
Fox, H. 1908 The pharyngeal pouches and their derivatives in the IVIammalia. Am. Jour. Anat., vol. 8.
Kastschenko, N. 1887 Das Schicksal der embryonalen Schlundspalten bei Saugetieren. Arch. f. mikr. Anat., Bd. 30.
RouD, A. 1900 Contribution a I'etude de Torigine et de revolution de la thy roide laterale et due thymus chez le campagnol. Bull. Soc. vaudoise
des Sc. natur., T. 36.
ZoTTERMAN, A. 1911 Die Schweinthymus als eine Thymus ecto-entodermalis. Anat. Anz., Bd. 38.
Plates
Abbreviations
A.c, carotid artery Pi. 3, parathyroid derived from third
C.t., caput thymus = thymus head pharyngeal pouch
D.b., ductus branchialis S.fc. :?, sacculus branchialis II = second
D.c, ductus cervicalis pharyngeal pouch
Ect., ectoderm S.b.3, sacculus branchialis III = third
G.n., ganglion nodosum pharyngeal pouch
G.S.C., superior cervical ganglion S.b.4, sacculus branchialis IV = fourth
N'.X., vagus nerve ' pharyngeal pouch
N.XII., hypoglossal nerve T.e., entodermal thvmus
P., pharynx T.s., thymus superficialis
P.L, connecting band V.c, vesicula cervicalis
V.C.I., vesicula cervicalis lateralis
V.c.m., vesicula cervicalis medialis
PLATE 1
EXPLANATION OF FIGURES
Figures 1 to 7 were drawn by Miss Cora Whitman, from wax models which were made by the author. The text figures A to E were drawn by the author.
1 Drawing of a reconstruction of the ])harynx and derivatives of the second and third pharyngeal pouches of the right side, including portions of the structures closely associated with the pharyngeal derivatives and a portion of the ectoderm. Ventral aspect; pig embryo 14.5 mm. in length. Model X 92, reduced one-half.
2 Drawing of a reconstruction of a portion of the pharynx, the third pharyngeal pouch, anterior portion of the thymus anlage, parathyroid 3, cervical vesicle, and associates of the above named structures. Caudo-ventral aspect. The model represented in this figure was made from the same side of the same embryo (14.5 mm.) from which the model represented in figure 1 was made. Model X 245, reduced one-half.
3 Drawing of a reconstruction of the same structures as enumerated under figure 2. This model was made to show specially the relation of the cervical vesicle to the thymus anlage and the hypoglossal nerve after the shifting of the structures in the neck have become quite noticeable. Ventral aspect, left side; pig embryo 17.5 mm. in length. Model X 182, reduced one-half.
PLATE 2
EXPLANATION OF FIGURES
4 Drawing of the same model as represented in figure 3, with the hypoglossal nerve and a portion of the ganglion nodosum removed to expose the cervical vesicle and more clearly to show its relation to the thymus anlage. Dorso-lateral aspect; reduced one-half.
5 Drawing of a reconstruction showing the relation of the vesicula cervicalis lateralis (T.s.) to the thymus head (C.t.) and their topographical relation to neighboring structures. In this stage the thymus and the cervical vesicle, which have fused, have lost their connection respectively with the entoderm and ectoderm. Right side, lateral aspect; pig 21.5 mm. in length. Model X 182, reduced one-half.
6 Drawing of a reconstruction showing the topographical relation of the superficial thymus to the thymus head. These two structures, in the specimen from which the reconstruction of the thymus was made, are connected with each other by the connecting band (P.i.) which loops over the hypoglossal nerve. Left thymus, lateral aspect. X 30, reduced one-half.
7 Drawing of model represented in figure 6; ventral aspect.
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