Difference between revisions of "Huntington1911 - 2"

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[[Paper - The anatomy and development of the systemic lymphatic vessels in the domestic cat|'''Huntington 1911''']]: [[Huntington1911 - 1|Part I. The development of the systemic lymphatic vessels in their relation to the blood-vascular system]] | [[Huntington1911 - 2|Part II. The development of the preazygos and azygos segments of the thoracic ducts]] | [[Huntington1911 - Bibliography|Bibliography]]
 
=Part II. The development of the preazygos and azygos segments of the thoracic ducts=
 
=Part II. The development of the preazygos and azygos segments of the thoracic ducts=
  
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The thoracic duct approach of the left jugular lymph sac occupies sections 10 to 23 of slide vii in this embryo, and is shown in the dorsal view of the reconstruction, fig. 89, to which the individual sections following are referred for orientation.
 
The thoracic duct approach of the left jugular lymph sac occupies sections 10 to 23 of slide vii in this embryo, and is shown in the dorsal view of the reconstruction, fig. 89, to which the individual sections following are referred for orientation.
  
The process arises in the usual situation from the dorso-medial aspect of the left jugular lymph sac and trends dorso-mesad
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The process arises in the usual situation from the dorso-medial aspect of the left jugular lymph sac and trends dorso-mesad across the internal jugular vein (series 189, sUde vii, section 10, fig. 43). In reference to the level of the common jugular confluence, which occurs in slide vii, section 26, the thoracic duct approach in series 189 arises more anteriorly from the jugular lymph sac than in the preceding series 210. Its extent and course
 +
can be followed accurately in the successive sections shown in figs. 43 to 55 inclusive (series 189, slide vii, sections 10 to 22 inclusive).
  
  
 +
In section 11 (fig. 44) the thoracic duct approach is separated by a thin endothelial partition from the lumen of the main jugular lymph sac, which it rejoins in the following section (section 12, fig. 45), only to again separate in the next section (section 13, fig. 46). In the meanwhile, the thoracic duct approach has arched from the lateral to the medial side of the internal jugular vein {25), closely applied to its dorsal surface, between the same and the thyro-cervical artery (24) j and its blind terminal is beginning to bend ventro-mesad into the neuro-venous recess between the vagus-sympathetic strand {1 , 22) and the internal jugular vein {26). In the next three sections (sections 14, 15, 16, figs. 47, 48 and 49) the thoracic duct approach becomes slightly dilated and diverticular (sections 15 and 16, figs. 48 and 49), but remains detached from the jugular sac. In section 15 (fig. 48) it appears subdivided, by endothelial partition, into two, and in section 16 (fig. 49) into three components. The protrusion which the common jugular confluence sends towards the jugular approach of the lymph sac and which forms the point of the subsequent invagination of the venous angle by the lymphatic tap, begins to appear in section 16 (fig. 49), and can be traced through the succeeding sections as a derivative of the lateral aspect of the main internal jugular vein, between it and the median surface of the jugular lymphatic sac.
  
 
Subclavian V.
 
 
 
 
across the internal jugular vein (series 189, sUde vii, section 10, fig. 43). In reference to the level of the common jugular confluence, which occurs in slide vii, section 26, the thoracic duct approach in series 189 arises more anteriorly from the jugular lymph sac than in the preceding series 210. Its extent and course
 
can be followed accurately in the successive sections shown in figs. 43 to 55 inclusive (series 189, slide vii, sections 10 to 22 inclusive).
 
 
In section 11 (fig. 44) the thoracic duct approach is separated by a thin endothelial partition from the lumen of the main jugular lymph sac, which it rejoins in the following section (section 12, fig. 45), only to again separate in the next section (section 13, fig. 46). In the meanwhile, the thoracic duct approach has arched from the lateral to the medial side of the internal jugular vein {25), closely applied to its dorsal surface, between the same and the thyro-cervical artery (24) j and its blind terminal is beginning to bend ventro-mesad into the neuro-venous recess between the vagus-sympathetic strand {1 , 22) and the internal jugular vein {26). In the next three sections (sections 14, 15, 16, figs. 47, 48 and 49) the thoracic duct approach becomes slightly dilated and diverticular (sections 15 and 16, figs. 48 and 49), but remains detached from the jugular sac. In section 15 (fig. 48) it appears subdivided, by endothelial partition, into two, and in section 16 (fig. 49) into three components. The protrusion which the common jugular confluence sends towards the jugular approach of the lymph sac and which forms the point of the subsequent invagination of the venous angle by the lymphatic tap, begins to appear in section 16 (fig. 49), and can be traced through the succeeding sections as a derivative of the lateral aspect of the main internal jugular vein, between it and the median surface of the jugular lymphatic sac.
 
  
 
In the six following sections, 17 to 22 inclusive, (figs. 50 to 55 inclusive) the proximal end of the process, connected with the sac, gradually recedes, while the blind distal end {12') conti. ues to develop in the ventro-medial direction between the main vein {25, in fig. 55) and the dorsal and dorso-medial venous tributaries {16 and 17, fig. 55), dipping ventrad and approaching the interval between the internal jugular vein {25) and the sympathetic nerve (i), the site of its subsequent junction with the independently developed perivenous lymphatics of the ventral mediastinal and dorsal prevertebral regions, which together constitute the anlages of this cephalic portion of the future thoracic duct, (cf. series 143, slides x and xi, figs. 172 to 183). The thoracic duct approach ends blindly in section 26 of slide vii. The
 
In the six following sections, 17 to 22 inclusive, (figs. 50 to 55 inclusive) the proximal end of the process, connected with the sac, gradually recedes, while the blind distal end {12') conti. ues to develop in the ventro-medial direction between the main vein {25, in fig. 55) and the dorsal and dorso-medial venous tributaries {16 and 17, fig. 55), dipping ventrad and approaching the interval between the internal jugular vein {25) and the sympathetic nerve (i), the site of its subsequent junction with the independently developed perivenous lymphatics of the ventral mediastinal and dorsal prevertebral regions, which together constitute the anlages of this cephalic portion of the future thoracic duct, (cf. series 143, slides x and xi, figs. 172 to 183). The thoracic duct approach ends blindly in section 26 of slide vii. The
 
course and the relations to surrounding structures of the thoracic duct approach of this embryo are shov^rn in the dorsal view of the reconstruction (fig. 89), containing the 50 sections of slide vii in a magnification of 225 diameters.
 
course and the relations to surrounding structures of the thoracic duct approach of this embryo are shov^rn in the dorsal view of the reconstruction (fig. 89), containing the 50 sections of slide vii in a magnification of 225 diameters.
 +
  
 
This view of the reconstruction presents from the lateral to the mesal border the following structures.
 
This view of the reconstruction presents from the lateral to the mesal border the following structures.
  
1. Caudal end of left jugular lymph sac, with the jugular (13) and subclavian {14) approaches, between which passes the ventral branch of the thyro-cervical artery (24) The fifth spinal nerve (54) penetrates the lymph sac, while the sixth nerve lies on its lateral aspect with the primitive ulnar veno-lymphatic arching over it to enter the lymph sac.
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# Caudal end of left jugular lymph sac, with the jugular (13) and subclavian {14) approaches, between which passes the ventral branch of the thyro-cervical artery (24) The fifth spinal nerve (54) penetrates the lymph sac, while the sixth nerve lies on its lateral aspect with the primitive ulnar veno-lymphatic arching over it to enter the lymph sac.
 
+
# The dorsal aspect of the internal jugular vein (25).
2. The dorsal aspect of the internal jugular vein (25).
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# The main trunk and branches of the thryo-cervical artery (24) .
 
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# The left superior intercostal vein (46), with the dorsal {16) and dorso-medial or prevertebral {17) tributaries, enters thedorsomedial angle of the internal jugular vein between the thyrocervical artery {24) and
3. The main trunk and branches of the thryo-cervical artery (24) .
+
# the double strand of the vagus {22) and sympathetic (/) nerves.
 
+
# The thymus {21).
4. The left superior intercostal vein (46), with the dorsal {16) and dorso-medial or prevertebral {17) tributaries, enters thedorsomedial angle of the internal jugular vein between the thyrocervical artery {24) and
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# The aortic arch (7), with the left carotid artery {23) and the origin of the innominate artery {45).
 
 
5. the double strand of the vagus {22) and sympathetic (/) nerves.
 
 
 
6. The thymus {21).
 
  
7. The aortic arch (7), with the left carotid artery {23) and the origin of the innominate artery {45).
 
  
 
The thoracic duct approach {12) arises by two branches from the dorso-medial aspect of the sac, turns caudo-mesad between the internal jugular vein and the ascending branch of the thyrocervical artery, in front of the left superior intercostal vein, and approaches the recess formed between the main vein and the bundle of the nerves.
 
The thoracic duct approach {12) arises by two branches from the dorso-medial aspect of the sac, turns caudo-mesad between the internal jugular vein and the ascending branch of the thyrocervical artery, in front of the left superior intercostal vein, and approaches the recess formed between the main vein and the bundle of the nerves.
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In fig. 60 (section 9 of slide x) a large dorso-medial venous tributary {16, left superior intercostal vein) empties into the dorso-medial angle of the common jugular trunk. The thoracic duct approach {12), occupying the same relative position as in the preceding figure, lies between this tributary and the thyro-cervical artery, dorsal to the main vein. Section 12 of slide x (fig. 61) shows the distal reduced end of the approach in this same situation. The subclavian approach of the jugular lymph sac (14) is reduced to a narrow longitudinal cleft on the dorso-lateral aspect of the main vein. This reduction is coincident with the derivation from the dorsal aspect of the subclavian approach of the primitive ulnar veno-lymphatic (31). This channel arches dorsad over the Vlth spinal nerve (30), and appears in this section in its typical relation to this nerve. Finally in the succeeding section, 13 of slide ix, (fig. 62), the thoracic duct approach ends blindly. The structures shown in the section dorsal to the common jugular trunk are now encountered in the following order: mesally the terminal of one of the branches of the dorsomedial venous plexus (16) ; next the thyro-cervical artery (24) ; then the Vlth segmental nerve (30), and, dorsal of this, the primitive ulnar veno-lymphatic (31),
 
In fig. 60 (section 9 of slide x) a large dorso-medial venous tributary {16, left superior intercostal vein) empties into the dorso-medial angle of the common jugular trunk. The thoracic duct approach {12), occupying the same relative position as in the preceding figure, lies between this tributary and the thyro-cervical artery, dorsal to the main vein. Section 12 of slide x (fig. 61) shows the distal reduced end of the approach in this same situation. The subclavian approach of the jugular lymph sac (14) is reduced to a narrow longitudinal cleft on the dorso-lateral aspect of the main vein. This reduction is coincident with the derivation from the dorsal aspect of the subclavian approach of the primitive ulnar veno-lymphatic (31). This channel arches dorsad over the Vlth spinal nerve (30), and appears in this section in its typical relation to this nerve. Finally in the succeeding section, 13 of slide ix, (fig. 62), the thoracic duct approach ends blindly. The structures shown in the section dorsal to the common jugular trunk are now encountered in the following order: mesally the terminal of one of the branches of the dorsomedial venous plexus (16) ; next the thyro-cervical artery (24) ; then the Vlth segmental nerve (30), and, dorsal of this, the primitive ulnar veno-lymphatic (31),
 +
  
 
The series of sections of this embryo shown here are interesting not only in respect to the behavior of the thoracix; duct approach, but also in reference to the mutual relations of this structure and the terminals of the dorso-medial tributary venous plexus. These relations are to be discussed subsequently in considering, as whole, the evidence upon which some investigators have based their conception of direct venous origin of the lymphatic system by fusion of a number of separate elements detached from the main venous trunks (cf. infra p. 153.)
 
The series of sections of this embryo shown here are interesting not only in respect to the behavior of the thoracix; duct approach, but also in reference to the mutual relations of this structure and the terminals of the dorso-medial tributary venous plexus. These relations are to be discussed subsequently in considering, as whole, the evidence upon which some investigators have based their conception of direct venous origin of the lymphatic system by fusion of a number of separate elements detached from the main venous trunks (cf. infra p. 153.)
 +
  
 
The variable appearance of the thoracic duct approach in the younger stages is perhaps best illustrated by the exaggerated development presented in a 13 mm. embryo (series 92), in which the process occupies the last thirteen sections of slide x, and the first fourteen sections of slide ix. Selected microphotographs of this series are shown in figs. 63 to 78. The embryo presents the largest and most dilated thoracic duct approach of any in my collection. Fig. 63 gives a topographical view of the region involved. The succeeding sections are cut down and confined to the immediate neighborhood of the thoracic duct approach. Fig. 63 shows the sections of the internal jugular vein {25) and jugulo-cephalic trunk {29), approaching their point of union in the common jugular confluence. A large process of the jugular lymph sac {IS) extends, as the jugular approach, into the interval between the main veins. Along the mesal aspect of the internal jugular vein {25) are placed the sympathetic (i), the vagus {22), the left common carotid artery {23), and, further ventrad, the thymus {21 ) . The dorsal portion of the sac is about to form the subclavian approach {11, 14) In fig. 64, the jugular approach {13) has entered more deeply into the narrowing interval between the internal jugular {26) and jugulo-cephalic trunks {29), and the sharply marked constriction between it and the remainder of the sac (subclavian approach, 14) foreshadows its coming separation from the latter. A protrusion of the dorso-medial angle of the jugular approach, adjacent to the internal jugular vein, indicates the beginning derivation of the thoracic duct approach {12).
 
The variable appearance of the thoracic duct approach in the younger stages is perhaps best illustrated by the exaggerated development presented in a 13 mm. embryo (series 92), in which the process occupies the last thirteen sections of slide x, and the first fourteen sections of slide ix. Selected microphotographs of this series are shown in figs. 63 to 78. The embryo presents the largest and most dilated thoracic duct approach of any in my collection. Fig. 63 gives a topographical view of the region involved. The succeeding sections are cut down and confined to the immediate neighborhood of the thoracic duct approach. Fig. 63 shows the sections of the internal jugular vein {25) and jugulo-cephalic trunk {29), approaching their point of union in the common jugular confluence. A large process of the jugular lymph sac {IS) extends, as the jugular approach, into the interval between the main veins. Along the mesal aspect of the internal jugular vein {25) are placed the sympathetic (i), the vagus {22), the left common carotid artery {23), and, further ventrad, the thymus {21 ) . The dorsal portion of the sac is about to form the subclavian approach {11, 14) In fig. 64, the jugular approach {13) has entered more deeply into the narrowing interval between the internal jugular {26) and jugulo-cephalic trunks {29), and the sharply marked constriction between it and the remainder of the sac (subclavian approach, 14) foreshadows its coming separation from the latter. A protrusion of the dorso-medial angle of the jugular approach, adjacent to the internal jugular vein, indicates the beginning derivation of the thoracic duct approach {12).
 +
  
 
In the succeeding section (fig. 65) the jugular approach {IS) has separated from the subclavian approach {14) of the jugular lymph sac and thus cleared the path for the passage of the thyrocervical artery {24) between the two cardinal divisions of the lymph sac, whose caudal terminations are represented by the jugular and subclavian approaches. The origin of the thoracic duct approach {12) from the dorso-medial angle of the jugular approach -is now defined.
 
In the succeeding section (fig. 65) the jugular approach {IS) has separated from the subclavian approach {14) of the jugular lymph sac and thus cleared the path for the passage of the thyrocervical artery {24) between the two cardinal divisions of the lymph sac, whose caudal terminations are represented by the jugular and subclavian approaches. The origin of the thoracic duct approach {12) from the dorso-medial angle of the jugular approach -is now defined.
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In fig. 73 (series 92, slide ix, section 9) the subclavian approach [14) is much reduced, and lies along the lateral aspect of the common jugular vein (26). Its connection with the thoracic duct approach (12) is plainlj' visible, passing between the main vein (26) and the thyro-cervical arterj' (24)- Dorsal somatic and prevertebral venous tributaries (16), approaching the dorsomedial angle of the common jugular vein (26), intersect the course of the thoracic duct approach, and begin to divide the same into a medial and a lateral element. This division is completed in the following section (fig. 74) in which the united dorsal and prevertebral tributaries form a venous trunk of considerable size (16) which is approaching its confluence with the comimon jugular vein by passing between the lateral and medial components of the thoracic duct approach. In other words, the appearance of the thoracic duct approach in this and in the following sections is the expression of the ventro-mesal arched course of this structure across the fork or angle between the common jugular vein (26) and the terminal of its ventro-medial tributary plexus (16). The reconstructions shown in figs. 89, 90 and 91 indicate this topographical relationship clearly. The blind mesal end of the arched process (12) now lies mesal to the tributary vein (16) and dorsal to the sympathetic nerve (7). In the following section (section 11 of slide ix, fig. 75), the dorso-medial venous tributary (16) is elongated and close to the main vessel, while the reduced sections of the thoracic duct approach lie on either side of the same, as indicated by the forked leader (12), The subclavian approach (14)y still prominent and connected with the medial section of the thoracic duct approach in the preceding section 10 (fig. 74), appears now in section 11 reduced to a detached small blind end (not labelled in the figure), close to the lateral wall of the main vein and ventral to the thyro-cervical artery (24)- In the succeeding section 12 (fig. 76) the dorso-medial venous tributary (superior intercostal vein) enters the common jugular trunk. The lateral portion of the thoracic duct approach (not labelled in the figure) is seen in the deep recess between this entrance, the main vein {S6) and the thyro-cervical artery (24) . The mesal end of the thoracic duct approach terminates blindly in this section just dorsal to the sympathetic nerve (1). In section 13 (fig. 77) the lateral terminal of the thoracic duct approach (IS) can still be clearly seen in the interval between the wide dorsal opening of the combined somatic and prevertebral (17) tributary stem into the main vein (26) and the thyro-cervical artery {S4) . It appears very much reduced in the same situation in section 14 (fig. 78), and ends blindly in the following section. The last three sections of this series (figs, 76, 77 and 78) show the beginning of the jugulo-subclavian confluence.
 
In fig. 73 (series 92, slide ix, section 9) the subclavian approach [14) is much reduced, and lies along the lateral aspect of the common jugular vein (26). Its connection with the thoracic duct approach (12) is plainlj' visible, passing between the main vein (26) and the thyro-cervical arterj' (24)- Dorsal somatic and prevertebral venous tributaries (16), approaching the dorsomedial angle of the common jugular vein (26), intersect the course of the thoracic duct approach, and begin to divide the same into a medial and a lateral element. This division is completed in the following section (fig. 74) in which the united dorsal and prevertebral tributaries form a venous trunk of considerable size (16) which is approaching its confluence with the comimon jugular vein by passing between the lateral and medial components of the thoracic duct approach. In other words, the appearance of the thoracic duct approach in this and in the following sections is the expression of the ventro-mesal arched course of this structure across the fork or angle between the common jugular vein (26) and the terminal of its ventro-medial tributary plexus (16). The reconstructions shown in figs. 89, 90 and 91 indicate this topographical relationship clearly. The blind mesal end of the arched process (12) now lies mesal to the tributary vein (16) and dorsal to the sympathetic nerve (7). In the following section (section 11 of slide ix, fig. 75), the dorso-medial venous tributary (16) is elongated and close to the main vessel, while the reduced sections of the thoracic duct approach lie on either side of the same, as indicated by the forked leader (12), The subclavian approach (14)y still prominent and connected with the medial section of the thoracic duct approach in the preceding section 10 (fig. 74), appears now in section 11 reduced to a detached small blind end (not labelled in the figure), close to the lateral wall of the main vein and ventral to the thyro-cervical artery (24)- In the succeeding section 12 (fig. 76) the dorso-medial venous tributary (superior intercostal vein) enters the common jugular trunk. The lateral portion of the thoracic duct approach (not labelled in the figure) is seen in the deep recess between this entrance, the main vein {S6) and the thyro-cervical artery (24) . The mesal end of the thoracic duct approach terminates blindly in this section just dorsal to the sympathetic nerve (1). In section 13 (fig. 77) the lateral terminal of the thoracic duct approach (IS) can still be clearly seen in the interval between the wide dorsal opening of the combined somatic and prevertebral (17) tributary stem into the main vein (26) and the thyro-cervical artery {S4) . It appears very much reduced in the same situation in section 14 (fig. 78), and ends blindly in the following section. The last three sections of this series (figs, 76, 77 and 78) show the beginning of the jugulo-subclavian confluence.
 +
  
 
In the later stages the thoracic duct approach has extended further ventro-mesad into the recess between the common jugular vein and the sympathetic nerve, and in this situation it eventually makes its connection with the two independently developed lymphatic channels of the anterior mediastinal region, viz., the preazygos segment of the thoracic duct dorsally, and the bronchomediastinal trunk ventrally. The mode of this union is discussed below under a separate heading (cf. infra, p. 111). Figs. 79 to 82 show transverse sections of the left side of the lower cervical region in a 15 mm. cat embryo (series 245, slide xi, sections 24 to 27, X 225), and fig. 90 gives the dorsal view of the reconstruction on the same scale of the thoracic duct approach and the related structures in the same embryo (slide xi, sections 1-44). In fig. 79 (sUde xi, section 24) the terminal extremity of the thoracic duct approach (12) is seen between the common jugular vein {S6) and the sympathetic nerve (1), having reached this situation by extending ventro-mesad across the dorsal tributary 16 (left superior intercostal vein), between this vessel and the main venous trunk.
 
In the later stages the thoracic duct approach has extended further ventro-mesad into the recess between the common jugular vein and the sympathetic nerve, and in this situation it eventually makes its connection with the two independently developed lymphatic channels of the anterior mediastinal region, viz., the preazygos segment of the thoracic duct dorsally, and the bronchomediastinal trunk ventrally. The mode of this union is discussed below under a separate heading (cf. infra, p. 111). Figs. 79 to 82 show transverse sections of the left side of the lower cervical region in a 15 mm. cat embryo (series 245, slide xi, sections 24 to 27, X 225), and fig. 90 gives the dorsal view of the reconstruction on the same scale of the thoracic duct approach and the related structures in the same embryo (slide xi, sections 1-44). In fig. 79 (sUde xi, section 24) the terminal extremity of the thoracic duct approach (12) is seen between the common jugular vein {S6) and the sympathetic nerve (1), having reached this situation by extending ventro-mesad across the dorsal tributary 16 (left superior intercostal vein), between this vessel and the main venous trunk.
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The section is taken below the common jugular confluence, consequently the jugular approach is no longer seen, and the blind end of the thoracic duct approach appears isolated. The only other portion of the jugular lymph sac carried caudad to this level is the subclavian approach (14), seen on the lateral aspect of the main vein. The sequence of structures now successively encountered along the mesal and dorsal aspects of the main common jugular vein (26) are the following, proceeding meso-laterad :
 
The section is taken below the common jugular confluence, consequently the jugular approach is no longer seen, and the blind end of the thoracic duct approach appears isolated. The only other portion of the jugular lymph sac carried caudad to this level is the subclavian approach (14), seen on the lateral aspect of the main vein. The sequence of structures now successively encountered along the mesal and dorsal aspects of the main common jugular vein (26) are the following, proceeding meso-laterad :
  
1 . Thymus (21 ) , with sub- and suprathymic venous tributaries entering the main vein.
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# Thymus (21 ) , with sub- and suprathymic venous tributaries entering the main vein.
 
+
# Left common carotid artery (23),
2. Left common carotid artery (23),
+
# Vagus nerve (22).
 
+
# Sympathetic nerve (i).
3. Vagus nerve (22).
+
# Thoracic duct approach (12),
 
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# Dorsal somatic tributary of main vein (16). (Anlage of left superior intercostal vein).
4. Sympathetic nerve (i).
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# Thyro-cervical artery (24) 8. Subclavian approach of jugular lymph sac (14), on the dorso-lateral aspect of the common jugular vein.
 
 
5. Thoracic duct approach (12),
 
 
 
6. Dorsal somatic tributary of main vein (16). (Anlage of left superior intercostal vein).
 
  
7. Thyro-cervical artery (24) 8. Subclavian approach of jugular lymph sac (14), on the dorso-lateral aspect of the common jugular vein.
 
  
 
This order and relationship is maintained uniformly and clearly in the three succeeding sections shown in f gs. 80, 81 and 82.
 
This order and relationship is maintained uniformly and clearly in the three succeeding sections shown in f gs. 80, 81 and 82.
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We are dealing therefore apparently with two potential paths which the blind terminal of the thoracic duct approach may take in its passage from the jugular lymph sac mesad beyond the vagussympathetic line to the upper mediastinal region, where it establishes eventually its secondary connections with the preazygos segment of the thoracic duct and with the broncho-mediastinal trunk. In one type (illustrated by the sections and reconstructions of series 245 (figs. 79 to 82, fig. 90) the process dips, after passing between the left superior intercostal and main jugular vein at an acute angle, caudad on the lateral side of the sympathetic strand (fig. 90).
 
We are dealing therefore apparently with two potential paths which the blind terminal of the thoracic duct approach may take in its passage from the jugular lymph sac mesad beyond the vagussympathetic line to the upper mediastinal region, where it establishes eventually its secondary connections with the preazygos segment of the thoracic duct and with the broncho-mediastinal trunk. In one type (illustrated by the sections and reconstructions of series 245 (figs. 79 to 82, fig. 90) the process dips, after passing between the left superior intercostal and main jugular vein at an acute angle, caudad on the lateral side of the sympathetic strand (fig. 90).
  
 
 
76 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
  
 
In the other type, illustrated by series 142 (figs. 83 to 88; fig. 91) the thoracic duct approach, maintaining the identical relation to left superior intercostal and common jugular veins, gains the ventro-medial aspect of the sympathetic cord, by passing obliquely between the same and the vagus.
 
In the other type, illustrated by series 142 (figs. 83 to 88; fig. 91) the thoracic duct approach, maintaining the identical relation to left superior intercostal and common jugular veins, gains the ventro-medial aspect of the sympathetic cord, by passing obliquely between the same and the vagus.
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In regard to the remaining structures fig. 87 (series 142, slide X, section 22) shows, by comparison with the preceding figure 86, the tap of the jugular approach into the internal jugular vein (25) just above the confluence with the jugulo-cephalic trunk (29), and the prolongation of its ventral process {IS) over the ventral aspect of the common jugular angle. Fig. 88 (series 142, slide X, section 24) shows the confluence of the internal jugular (S5) and jugulo-cephalic (^9) veins fully established, and gives a very clear picture of the relation which the ventral prolongation (IS) from the jugular approach of the lymph sac bears to the common jugular angle. In both figs. 87 and 88 the dorsal venous tributary (sup. intercostal vein anlage) is indicated by the leader 16. It enters the main vein in section 3 of the following slide (xi). Fig. 91 gives the dorsal view of the reconstruction of the vascular and nervous structures of the left side in slides
+
In regard to the remaining structures fig. 87 (series 142, slide X, section 22) shows, by comparison with the preceding figure 86, the tap of the jugular approach into the internal jugular vein (25) just above the confluence with the jugulo-cephalic trunk (29), and the prolongation of its ventral process {IS) over the ventral aspect of the common jugular angle. Fig. 88 (series 142, slide X, section 24) shows the confluence of the internal jugular (S5) and jugulo-cephalic (^9) veins fully established, and gives a very clear picture of the relation which the ventral prolongation (IS) from the jugular approach of the lymph sac bears to the common jugular angle. In both figs. 87 and 88 the dorsal venous tributary (sup. intercostal vein anlage) is indicated by the leader 16. It enters the main vein in section 3 of the following slide (xi). Fig. 91 gives the dorsal view of the reconstruction of the vascular and nervous structures of the left side in slides X and xi of this embryo, carried caudad to the level of the vertebral vessels Wl, 4^). Comparison with fig. 90 will show the correspondence of the two preparations in all essential points and will accentuate the above described difference in the course of the thoracic duct approach (12) and tjie preazygos segment of the thoracic duct (35) in relation to the vagus-sympathetic strand.
  
  
X and xi of this embryo, carried caudad to the level of the vertebral vessels Wl, 4^). Comparison with fig. 90 will show the correspondence of the two preparations in all essential points and will accentuate the above described difference in the course of the thoracic duct approach (12) and tjie preazygos segment of the thoracic duct (35) in relation to the vagus-sympathetic strand.
+
It is readily seen, in comparing the reconstructions of the two older stages just described (figs. 90 and 91) with that of the earlier embryo shown in fig. 89, that the latter, in the construction of the terminal of its thoracic duct approach, combines potentially the ability of developing into either one of the two more advanced conditions just described. The process which the thoracic duct approach turns nearly horizontally mesad into the interval between vagus and sympathetic is present in all three reconstructions. In many embryos, as will appear presently in describing the critical stages in detail, the thoracic duct approach appears to make its connection with the independently developed anlages of the preazygos portion of the thoracic duct in this situation, and continued development will lead to the condition seen in series 142 (fig. 91). If, on the other hand, the caudal extremity of the thoracic duct approach of the 13.5 mm. embryo (fig. 89) joins the independently developed lymphatic anlages at the root of the left superior intercostal vein and along the common jugular lateral to the sympathetic line, the resulting relation will be as seen in series 245 (fig. 90).
  
It is readily seen, in comparing the reconstructions of the two older stages just described (figs. 90 and 91) with that of the earlier embryo shown in fig. 89, that the latter, in the construction of the terminal of its thoracic duct approach, combines potentially the ability of developing into either one of the two more advanced conditions just described. The process which the thoracic duct approach turns nearly horizontally mesad into the interval between vagus and sympathetic is present in all three reconstructions. In many embryos, as will appear presently in describing the critical stages in detail, the thoracic duct approach appears to make its connection with the independently developed anlages of the preazygos portion of the thoracic duct in this situation, and continued development will lead to the condition seen in series 142 (fig. 91). If, on the other hand, the caudal extremity of the thoracic duct approach of the 13.5 mm. embryo (fig. 89) joins the independently developed lymphatic anlages at the root of the left superior intercostal vein and along the common jugular lateral to the sympathetic line, the resulting relation will be as seen in series 245 (fig. 90).
 
  
 
These observations are further supported by comparison of the preparations just described with the corresponding portions of the two reconstructions shown in figs. 170 and 171.
 
These observations are further supported by comparison of the preparations just described with the corresponding portions of the two reconstructions shown in figs. 170 and 171.
  
Fig. 170 (series 218) shows the stage in a 15 nam. embryo just prior to the junction of the thoracic duct approach with the independently formed preazygos segment of the thoracic duct (35). The latter is in its greater part already a distinct and considerable segment of the future continuous duct channel. Between its blind cephalic extremity and the blind caudal end of the thoracic dUct approach (12) are a number of scattered and still separate lymphatic anlages along the common jugular and innominate veins on each side of the sympathetic line. By continued exten
 
  
 +
Fig. 170 (series 218) shows the stage in a 15 mm. embryo just prior to the junction of the thoracic duct approach with the independently formed preazygos segment of the thoracic duct (35). The latter is in its greater part already a distinct and considerable segment of the future continuous duct channel. Between its blind cephalic extremity and the blind caudal end of the thoracic dUct approach (12) are a number of scattered and still separate lymphatic anlages along the common jugular and innominate veins on each side of the sympathetic line. By continued extension of these as yet isolated lymphatic anlages the still lacking intermediate bondpiece will be produced, which, by union of its cephaUc extremity with the thoracic duct approach of the jugular lymph sac and junction of its caudal end with the cephali? termnal of the preazygos segment of the thoracic duct {35), will produce the continuous and uninterrupted lymph channel of the succeeding stage (se.ies 143, 15.5 mm, embryo, fig. 171). A study of the reconstruction of series 218 makes it evident that the opportunity still exists of developing this final junction either on the dorsolateral or ventro-medial side of the sympathetic cord, for the lymphatic anlages exist in both situations. The end of the thoracic duct approach of this embryo rides with a well marked lateral and medial prolongation in the fork formed by the entrance of the left superior intercostal vein (16-46) into the common jugular. The lateral process, carrying the leader 12 in the figure, is in position to join the line of separate lymphatic anlages extending caudad on the lateral side of the sympathetic, and a prominent cephalic process of the already established segment of the preazygos lymph channel extends cephalo-laterad across the dorsal surface of the nerve, as if to meet the lymphatic line just described. If development had proceeded along this line in this embr^^o, the condition seen in series 245 might have resulted, with the modification of swinging the thoracic duct approach to the lateral instead of to the medial side of the left superior intercostal vein, but carrying the thoracic duct along the dorso-lateral aspect of the sympathetic nerve. In this case the large separate lymphatic anlage labelled 35' in fig. 170 would take an important share in building up the resulting lymph channel. On the other hand, the definite connection with the preazygos segment of the thoracic duct might have been established through the ventromedial terminal prolongation of the thoracic duct approach. A piece of the sympathetic nerve has been removed in the reconstruction in order to show the position of this structure, in the interval between sympathetic nerve (1) and vagus (22), The long slender prolongation extending forward between vagus (22) and common carotid arter\^ (23) is the path of junction for the ventral or broncho-mediastinal lymphatic trunk. I am under the strong impression that the observed variations in the relation of the embryonic thoracic duct approach to the sympathetic nerve hinge largely for their production upon the share which the broncho-mediastinal duct is to take in individual cases in the establishment of the preazygos segment of the thoracic duct channel. (Compare relations of 35 and ^7 in the schematic cross sections shown above in the text figures.)
  
78 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC* VESSELS
 
 
sion of these as yet isolated lymphatic anlages the still lacking intermediate bondpiece will be produced, which, by union of its cephaUc extremity with the thoracic duct approach of the jugular lymph sac and junction of its caudal end with the cephali? termnal of the preazygos segment of the thoracic duct {35), will produce the continuous and uninterrupted lymph channel of the succeeding stage (se.ies 143, 15.5 mm, embryo, fig. 171). A study of the reconstruction of series 218 makes it evident that the opportunity still exists of developing this final junction either on the dorsolateral or ventro-medial side of the sympathetic cord, for the lymphatic anlages exist in both situations. The end of the thoracic duct approach of this embryo rides with a well marked lateral and medial prolongation in the fork formed by the entrance of the left superior intercostal vein (16-46) into the common jugular. The lateral process, carrying the leader 12 in the figure, is in position to join the line of separate lymphatic anlages extending caudad on the lateral side of the sympathetic, and a prominent cephalic process of the already established segment of the preazygos lymph channel extends cephalo-laterad across the dorsal surface of the nerve, as if to meet the lymphatic line just described. If development had proceeded along this line in this embr^^o, the condition seen in series 245 might have resulted, with the modification of swinging the thoracic duct approach to the lateral instead of to the medial side of the left superior intercostal vein, but carrying the thoracic duct along the dorso-lateral aspect of the sympathetic nerve. In this case the large separate lymphatic anlage labelled 35' in fig. 170 would take an important share in building up the resulting lymph channel. On the other hand, the definite connection with the preazygos segment of the thoracic duct might have been established through the ventromedial terminal prolongation of the thoracic duct approach. A piece of the sympathetic nerve has been removed in the reconstruction in order to show the position of this structure, in the interval between sympathetic nerve (1) and vagus (22), The long slender prolongation extending forward between vagus (22) and common carotid arter\^ (23) is the path of junction for the ventral or broncho-mediastinal lymphatic trunk. I am under
 
  
 +
If now this ventro-medial end piece of the thoracic duct approach had effected its junction with the preazygos segment of the thoracic duct along the line of the lymphatic anlages on the medial side of the sympathetic and in the interval between this strand and the vagus, then the individual condition already described in series 142 (fig. 91) would have resulted.
  
 
DEVELOPMENT OP THE THORACIC DUCT 79
 
 
the Strong impression that the observed variations in the relation of the embryonic thoracic duct approach to the sympathetic nerve hinge largely for their production upon the share which the broncho-mediastinal duct is to take in individual cases in the establishment of the preazygos segment of the thoracic duct channel. (Compare relations of 35 and ^7 in the schematic cross sections shown above in the text figures.)
 
 
If now this ventro-medial end piece of the thoracic duct approach had effected its junction with the preazygos segment of the thoracic duct along the line of the lymphatic anlages on the medial side of the sympathetic and in the interval between this strand and the vagus, then the individual condition already described in series 142 (fig. 91) would have resulted.
 
  
 
This same topographical relationship of sympathetic nerve and lymph channel is also seen in the 15.5 mm. embryo, series 143, shown in reconstruction from the right side in Fig. 171. The thoracic duct approach (12) intersects the sympathetic nerve {1) on its ventro-medial aspect at an acute angle to establish its connection with the cephalic end of the preazygos segment of the thoracic duct (36). This t>T)e appears the prevalent one in the embryos in my collection.
 
This same topographical relationship of sympathetic nerve and lymph channel is also seen in the 15.5 mm. embryo, series 143, shown in reconstruction from the right side in Fig. 171. The thoracic duct approach (12) intersects the sympathetic nerve {1) on its ventro-medial aspect at an acute angle to establish its connection with the cephalic end of the preazygos segment of the thoracic duct (36). This t>T)e appears the prevalent one in the embryos in my collection.
Line 258: Line 236:
 
2. Intermediate stage. Embryos of 14 mm.
 
2. Intermediate stage. Embryos of 14 mm.
  
Series 210 — 14 mm. Series 212 — 14 mm. Series 122 — 14 mm. The sac, as a whole, and the thoracic duct approach in })articu
+
Series 210 — 14 mm. Series 212 — 14 mm. Series 122 — 14 mm. The sac, as a whole, and the thoracic duct approach in })articular, condenses and becomes more clearly defined against the surrounding mesoderm. The multilocular character due to remnants of the capillary walls in the interior, disappears and the approach obtains a clear and clean-cut lumen. The entire structure appears to contract somewhat on itself and develops more distinct walls. The recession, during which the thoracic duct approach occupies a relatively smaller territory is succeeded by 3. Final stage. Embryos of 15-17 mm.
 
 
 
 
80 DEVELOPMENT OT THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
lar, condenses and becomes more clearly defined against the surrounding mesoderm. The multilocular character due to remnants of the capillary walls in the interior, disappears and the approach obtains a clear and clean-cut lumen. The entire structure appears to contract somewhat on itself and develops more distinct walls. The recession, during which the thoracic duct approach occupies a relatively smaller territory is succeeded by 3. Final stage. Embryos of 15-17 mm.
 
  
 
Series 245 — 15 mm.
 
Series 245 — 15 mm.
Line 274: Line 247:
  
 
While I do not insist on the above outlined details in the chronological order of development of the thoracic duct approach, the embryos of my collection certainly tend to support the view expressed. Thus the 13 mm. embryo (series 92) offers the highest degree of expansion of the thoracic duct approach. The structure recedes and diminishes somewhat in extent in the 13.5 mm. stage (series 189), appears uniformly further reduced and shortened in the 14 mm. embryos (series 210, 212, 122), and finally, from the 15 and 15.5 mm. stages on, makes its definite connections with the preformed independently developed channels of the thoracic and broncho-mediastinal ducts.
 
While I do not insist on the above outlined details in the chronological order of development of the thoracic duct approach, the embryos of my collection certainly tend to support the view expressed. Thus the 13 mm. embryo (series 92) offers the highest degree of expansion of the thoracic duct approach. The structure recedes and diminishes somewhat in extent in the 13.5 mm. stage (series 189), appears uniformly further reduced and shortened in the 14 mm. embryos (series 210, 212, 122), and finally, from the 15 and 15.5 mm. stages on, makes its definite connections with the preformed independently developed channels of the thoracic and broncho-mediastinal ducts.
 +
  
 
In this entire genetic process there is not the slightest indication of an extension of lymphatic trunks from the jugular sac or from its thoracic duct approach toward the periphery. There is absolutely no suggestion of so-called ^'budding" or ^* sprouting, or of any other form of centrifugal extension of lymphatic vessels. On the contrary, the sac as a whole, and the processes therewith connected, recede actually for a time, and only then establish a secondary junction with the independently developed general systemic lymphatic channels. These have in the meanwhile formed along and around the temporarj"^ embryonic veins, as the result of the confluence of large numbers of extraintimal perivenous spaces in the sense previously defined in detail. Then- line of further growth, extension and union with each other is altogether centripetal^ from the outlying regions toward the jugular lymph sacs, and theh- union with the latter 's processes is the last and final step in the definite organization of the lymphatic sj^stem.
 
In this entire genetic process there is not the slightest indication of an extension of lymphatic trunks from the jugular sac or from its thoracic duct approach toward the periphery. There is absolutely no suggestion of so-called ^'budding" or ^* sprouting, or of any other form of centrifugal extension of lymphatic vessels. On the contrary, the sac as a whole, and the processes therewith connected, recede actually for a time, and only then establish a secondary junction with the independently developed general systemic lymphatic channels. These have in the meanwhile formed along and around the temporarj"^ embryonic veins, as the result of the confluence of large numbers of extraintimal perivenous spaces in the sense previously defined in detail. Then- line of further growth, extension and union with each other is altogether centripetal^ from the outlying regions toward the jugular lymph sacs, and theh- union with the latter 's processes is the last and final step in the definite organization of the lymphatic sj^stem.
Line 282: Line 256:
  
 
FIGURES 29 TO 91
 
FIGURES 29 TO 91
 
  
  
Line 401: Line 374:
 
11 Jugular lymph sac.
 
11 Jugular lymph sac.
  
12 Jugular l^Mnph sac, thoracic duct approach. 16 Dorso-metlial somatic venous tributaries. 22 Vagus.
+
12 Jugular lymph sac, thoracic duct approach. 16 Dorso-metlial somatic venous tributaries. 22 Vagus.
  
 
24 Thyrocervical artery.
 
24 Thyrocervical artery.
Line 577: Line 550:
 
26 Common jugular vein. 29 Jugulo-subclavian trunk«  
 
26 Common jugular vein. 29 Jugulo-subclavian trunk«  
  
 
 
\
 
  
  
Line 975: Line 945:
 
FIGURE 96
 
FIGURE 96
  
96 Dissection of main lymphatic vessels of neck and thorax in an adult cat
+
96 Dissection of main lymphatic vessels of neck and thorax in an adult cat (No. 29).
 
 
(No. 29).
 
  
 
1 External jugular vein and lymphatics.
 
1 External jugular vein and lymphatics.
Line 1,115: Line 1,083:
  
  
DEVELOPMENT OF THE THORACIC DUCT 91
+
==The Development of the Broncho-Mediastinal Trunk and of the Preazygos Segment of the Thoracic Duct==
 
 
The Development of the Broncho-Mediastinal Trunk and of the
 
 
 
Preazygos Segment of the Thoracic Duct
 
  
 
The area in which the development of the broncho-mediastinal and preazygos segments of the thoracic duct occurs is shown in fig. 100, which gives a topographical view of a transection of the upper thoracic region in a 12 mm. embryo (series 78, slide IX, section 5, X 50).
 
The area in which the development of the broncho-mediastinal and preazygos segments of the thoracic duct occurs is shown in fig. 100, which gives a topographical view of a transection of the upper thoracic region in a 12 mm. embryo (series 78, slide IX, section 5, X 50).
Line 1,127: Line 1,091:
 
The ventral portion of the field contains the coelom, with right {IfS) and left {J^O) auricles, right ventricle {50), ascending aorta (7) and ascending trunk of main pulmonarj'^ arter>' {10), Dorsal to this are seen the cross-sections of the right and left pulmonary arteries {10) descending to the lun^s. Between them ventrally, and trachea (9), aortae (7), vagi {22) and praecaval veins (S, 6) dorsally, is the area marked by the letter X, in which the development of the broncho-mediastinal trunk will proceed.
 
The ventral portion of the field contains the coelom, with right {IfS) and left {J^O) auricles, right ventricle {50), ascending aorta (7) and ascending trunk of main pulmonarj'^ arter>' {10), Dorsal to this are seen the cross-sections of the right and left pulmonary arteries {10) descending to the lun^s. Between them ventrally, and trachea (9), aortae (7), vagi {22) and praecaval veins (S, 6) dorsally, is the area marked by the letter X, in which the development of the broncho-mediastinal trunk will proceed.
  
A. The developynent of the broncho-ynediastinal or ventral mediastinal lymphatic channeh, which drain subsequently caudo-sinistrad into (lie left thoracic duct, or cephalo-sinistrad into the subclavian approach of the jugular lymph sac, or into both.
 
 
In the earlier stages (embrj^os of 11-14 mm.), an extensive ventral venous capillary network develops between the main
 
  
  
 +
A. The development of the broncho-mediastinal or ventral mediastinal lymphatic channel, which drain subsequently caudo-sinistrad into (lie left thoracic duct, or cephalo-sinistrad into the subclavian approach of the jugular lymph sac, or into both.
  
92 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
+
In the earlier stages (embrj^os of 11-14 mm.), an extensive ventral venous capillary network develops between the main venous trunks of the right and left sides, involving the distal part of the internal jugular, the common jugular and the innominate veins.
 
 
venous trunks of the right and left sides, involving the distal part of the internal jugular, the common jugular and the innominate veins.
 
  
 
Out of this general antecedent sinistro-dextral ci^illary reticulum the definite retro- or suprathymic innominate anastomosis develops subsequently in normal individuals, but in the earlier stages this plexus extends both ventral and dorsal to the thymus anlage.
 
Out of this general antecedent sinistro-dextral ci^illary reticulum the definite retro- or suprathymic innominate anastomosis develops subsequently in normal individuals, but in the earlier stages this plexus extends both ventral and dorsal to the thymus anlage.
Line 1,152: Line 1,112:
  
 
4. Ventral mediastinal branch, draining, as "suprathymic plexus," the area between the thymus {21), carotid {23) and vagus {22) . This is the line of the future typical permanent left innominate anastomosis dorsal to the thymus (fig. 101, 19).
 
4. Ventral mediastinal branch, draining, as "suprathymic plexus," the area between the thymus {21), carotid {23) and vagus {22) . This is the line of the future typical permanent left innominate anastomosis dorsal to the thymus (fig. 101, 19).
 
 
 
DEVELOPMENT OP THE THORACIC DUCT 93
 
  
 
5. Subthymic plexus, passing ventral to the thymus anlage (fig. 101, SO). This forms, if retained and further developed in the adult, the occasionally observed '*prethymic" left innominate vein ^'*\
 
5. Subthymic plexus, passing ventral to the thymus anlage (fig. 101, SO). This forms, if retained and further developed in the adult, the occasionally observed '*prethymic" left innominate vein ^'*\
Line 1,167: Line 1,123:
 
Thus, if the ventral portion of this venous plexus is followed caudad into the upper thoracic region, into the area of the developing broncho-mediastinal trunk marked X in fig. 100, the following observations can be made in stages of the proper length, and adequately fixed and stained:
 
Thus, if the ventral portion of this venous plexus is followed caudad into the upper thoracic region, into the area of the developing broncho-mediastinal trunk marked X in fig. 100, the following observations can be made in stages of the proper length, and adequately fixed and stained:
  
1. In embryos between 11 and 12 nmi. only venous capillaries are found. Thus figures 103 and 104 show transverse sections of this part of the upper thoracic region in a 12 nmi. embryo (series 217, slide viii, sections 21 and 29, X 200). The field, caudal to oesophagus and aorta (7), contains in the middle hne the trachea (S), the vagi {22), and further ventrad, the pulmonary
+
1. In embryos between 11 and 12 nmi. only venous capillaries are found. Thus figures 103 and 104 show transverse sections of this part of the upper thoracic region in a 12 nmi. embryo (series 217, slide viii, sections 21 and 29, X 200). The field, caudal to oesophagus and aorta (7), contains in the middle hne the trachea (S), the vagi {22), and further ventrad, the pulmonary arteries (10). The branches of the peritracheal venous plexus (18) are well developed. There is as yet no indication of lymphatic development.
 
 
  
  
•• Geo. S. Huntington : Contribution to the topographical anatomy of the thorax in the foetus at term and the new-bom child." Rep. Soc. N. Y. Lying-inHospital, 1897, p. 343, fig. xxxi.
+
* Geo. S. Huntington : Contribution to the topographical anatomy of the thorax in the foetus at term and the new-bom child." Rep. Soc. N. Y. Lying-inHospital, 1897, p. 343, fig. xxxi.
  
  
 
94 DEVELPPMENT OF THE STSTEBilC LYMPHATIC VESSELS
 
 
arteries (10). The branches of the peritracheal venous plexus (18) are well developed. There is as yet no indication of lymphatic development.
 
  
 
2. In 13 mm. embryos certain of the venous radicles entering into this plexus, are partly surrounded and enveloped by extraintimal lymphatic spaces, which arise independently of the venous system, as the first anlages of the future ventral mediastinal lymphatic channel. Figures 105 and 106 show transverse sections of this region in a 13 mm. cat embryo (series 92, slide vii, sections 30 and 38) in a magnification of 225 diameters. Ventral to trachea (9), aorta (7) and left vagus {2£) are seen sections of a number of venous radicles (4) of the ventral mediastinal plexus, some of which are partially surrounded by very smiall extraintimal perivenous lymphatic anlages (5). Only a few of the most prominent of these are indicated in the figures by leaders, but they exist in many other parts of the field.
 
2. In 13 mm. embryos certain of the venous radicles entering into this plexus, are partly surrounded and enveloped by extraintimal lymphatic spaces, which arise independently of the venous system, as the first anlages of the future ventral mediastinal lymphatic channel. Figures 105 and 106 show transverse sections of this region in a 13 mm. cat embryo (series 92, slide vii, sections 30 and 38) in a magnification of 225 diameters. Ventral to trachea (9), aorta (7) and left vagus {2£) are seen sections of a number of venous radicles (4) of the ventral mediastinal plexus, some of which are partially surrounded by very smiall extraintimal perivenous lymphatic anlages (5). Only a few of the most prominent of these are indicated in the figures by leaders, but they exist in many other parts of the field.
Line 1,187: Line 1,138:
 
Other 13.5 mm. embryos show a rapid advance in the perivenous Ijinphatic development.
 
Other 13.5 mm. embryos show a rapid advance in the perivenous Ijinphatic development.
  
Fig. 110 shows a transverse section of the upper thoracic region of another 13.5 mm. embryo (series 189, slide viii, section 35, X 225). Just ventro-mesad of the left vagus nerve and its
+
Fig. 110 shows a transverse section of the upper thoracic region of another 13.5 mm. embryo (series 189, slide viii, section 35, X 225). Just ventro-mesad of the left vagus nerve and its encircling vein is a venous radicle (4) almost completely surrounded by an extraintimal lymphatic space (5) in the process of replacing the atrophying vein with which it is so closely associated. The same structures (4, 5) are seen still more clearly defined in the corresponding position on the right side of the embryo.
 
 
 
 
 
 
DEVELOPMENT OF THE THORACIC DI3CT 95
 
 
 
encircling vein is a venous radicle (4) almost completely surrounded by an extraintimal lymphatic space (5) in the process of replacing the atrophying vein with which it is so closely associated. The same structures (4, 5) are seen still more clearly defined in the corresponding position on the right side of the embryo.
 
  
 
Comparison with tl\e corresponding sections just given in figures 103 to 109 show that the extraintimal anlage of the earlier stages, developed along the identical venous radicle, has increased in the 13.5 nrni. stage, so as to nearly envelop the vein, and that the latter, if followed cephalad and caudad, is now separated from the functional venous channels of this region and is in the process of further recession and degeneration as the lymphatic perivenous space enlarges and more and more replaces the antecedent venous channel. The vein in question in the older embryo (series 189) appears collapsed and shrunken, and contains only a few degenerating red blood cells. In the same situation on the right side of fig. 110 (series 189, slide viii, section 35), the section has cut the corresponding vein and the enveloping extraintinr al space at right angles, so that the central kernel of the shrinking vein (4), still containing a few degenerating red blood cells, is nearly completely surrounded by the replacing extraintimal lymphatic (5).
 
Comparison with tl\e corresponding sections just given in figures 103 to 109 show that the extraintimal anlage of the earlier stages, developed along the identical venous radicle, has increased in the 13.5 nrni. stage, so as to nearly envelop the vein, and that the latter, if followed cephalad and caudad, is now separated from the functional venous channels of this region and is in the process of further recession and degeneration as the lymphatic perivenous space enlarges and more and more replaces the antecedent venous channel. The vein in question in the older embryo (series 189) appears collapsed and shrunken, and contains only a few degenerating red blood cells. In the same situation on the right side of fig. 110 (series 189, slide viii, section 35), the section has cut the corresponding vein and the enveloping extraintinr al space at right angles, so that the central kernel of the shrinking vein (4), still containing a few degenerating red blood cells, is nearly completely surrounded by the replacing extraintimal lymphatic (5).
  
Fig. Ill shows the following section of the same embryo (series 189, slide viii, section 36), magnified 225 diameters, in a larger field which gives the topographical relations and may serve in in the orientation of the succeeding plates. The ventral circumference of the oesophagus iS) and the aorta (7) are seen dorsally. The trachea (9), with the vagi {22) on either side, occupies the central area. Ventrally are the two pulmonary arteries {10). The section is a most important one in interpreting the early histogenetic stages of the broncho-mediastinal lymphatic trunk and its relation to the embryonic mediastinal venous plexus. The same venous radicle (4), ventral to the vagus {22), already noted in the preceeding section (fig. 110), is seen on each side. On the left side it has been cut obliquely by the plane of the section, and hence shows a segment of considerable length, as a practically empty and partially collapsed endothelial bag (4),
+
Fig. Ill shows the following section of the same embryo (series 189, slide viii, section 36), magnified 225 diameters, in a larger field which gives the topographical relations and may serve in in the orientation of the succeeding plates. The ventral circumference of the oesophagus iS) and the aorta (7) are seen dorsally. The trachea (9), with the vagi {22) on either side, occupies the central area. Ventrally are the two pulmonary arteries {10). The section is a most important one in interpreting the early histogenetic stages of the broncho-mediastinal lymphatic trunk and its relation to the embryonic mediastinal venous plexus. The same venous radicle (4), ventral to the vagus {22), already noted in the preceeding section (fig. 110), is seen on each side. On the left side it has been cut obliquely by the plane of the section, and hence shows a segment of considerable length, as a practically empty and partially collapsed endothelial bag (4), surrounded by the extraintimal lymphatic space (6). On the right side of the embryo the corresponding venous radicle (4) and the perivenous lymphatic anlage (6) have been again cut more at right angles, so that the envelopment of the former by the latter can be clearly traced nearly throughout the entire circimiference.
 
 
 
 
 
 
96 DEVEIiOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
surrounded by the extraintimal lymphatic space (6). On the right side of the embryo the corresponding venous radicle (4) and the perivenous lymphatic anlage (6) have been again cut more at right angles, so that the envelopment of the former by the latter can be clearly traced nearly throughout the entire circimiference.
 
  
 
Fig. 1 11 A shows the decadent vein and the replacing extraintimal lymphatic on the left side of this section in a higher magnification, ( X 300). The structure of the collapsed venous endothelial tube, and the beginning development of endothelial character in the mesodermal cells limiting the perivenous extraintimal lymphatic anlage, can be here clearly made out.
 
Fig. 1 11 A shows the decadent vein and the replacing extraintimal lymphatic on the left side of this section in a higher magnification, ( X 300). The structure of the collapsed venous endothelial tube, and the beginning development of endothelial character in the mesodermal cells limiting the perivenous extraintimal lymphatic anlage, can be here clearly made out.
Line 1,215: Line 1,154:
 
Sections 38 and 39 (figs. 113 and 114) are particularly instructive inasmuch as the detached central vein (4) on the left side still contains a few degenerating red blood cells, while its former connection with the vahd and functional venous plexus around the left vagus nerve can still be faintly traced.
 
Sections 38 and 39 (figs. 113 and 114) are particularly instructive inasmuch as the detached central vein (4) on the left side still contains a few degenerating red blood cells, while its former connection with the vahd and functional venous plexus around the left vagus nerve can still be faintly traced.
  
In section 40 (fig. 115) the atrophying vein (4) and the surrounding lymphatic (5) of the left side appear divided into two
+
In section 40 (fig. 115) the atrophying vein (4) and the surrounding lymphatic (5) of the left side appear divided into two components. On the right side the corresponding structures end blindly.
 
 
 
 
 
 
DEVELOPMENT OP THE THORACIC DUCT 97
 
 
 
components. On the right side the corresponding structures end blindly.
 
  
 
In the following section (41, fig. 116) the left venule and lymphatic also terminate almost entirely in the indifferent mesodermal tissue, only a small renmant (4, 5) persisting between trachea and left vagus. This disappears entirely in the following section.
 
In the following section (41, fig. 116) the left venule and lymphatic also terminate almost entirely in the indifferent mesodermal tissue, only a small renmant (4, 5) persisting between trachea and left vagus. This disappears entirely in the following section.
Line 1,229: Line 1,162:
 
Thus figs. 117 to 122 show transverse sections of the ventral thoracic region in a 14 mm. cat embryo (series 212, slide x, sections 4, 5, 6, 7, 10 and 11, X 225). In figs. 117 and 118 the developing lymphatic anlages (5) of the broncho-mediastinal trunk can ke readily recognized and present the same relation to the atrophying venules, which they surround-, as in the sections of the preceding series just described and figured. In contradistinction to these structures the components of the permanent functional venous plexus of this region {32) form well-differentiated vascular channels.
 
Thus figs. 117 to 122 show transverse sections of the ventral thoracic region in a 14 mm. cat embryo (series 212, slide x, sections 4, 5, 6, 7, 10 and 11, X 225). In figs. 117 and 118 the developing lymphatic anlages (5) of the broncho-mediastinal trunk can ke readily recognized and present the same relation to the atrophying venules, which they surround-, as in the sections of the preceding series just described and figured. In contradistinction to these structures the components of the permanent functional venous plexus of this region {32) form well-differentiated vascular channels.
  
In fig. 119 (series 212, slide x, section 6) the scattered lymphatic anlages occupying in the two preceding sections the space between the left pulmonary artery {10) and the left vagus {22) j with its accompanying ventral mediastinal vein {32) j are gathered together into a more extensive elongated space (5). In the interior of this is seen the endothelial bag of the empty and atrophying venule (4) which the lymphatic is about to replace. The vein renmant is almost completely surrounded by the lymphatic space and only connected to the latter's endothelial wall by a bridge at one point. Other areas of similar lymphatic devel
+
In fig. 119 (series 212, slide x, section 6) the scattered lymphatic anlages occupying in the two preceding sections the space between the left pulmonary artery {10) and the left vagus {22) j with its accompanying ventral mediastinal vein {32) j are gathered together into a more extensive elongated space (5). In the interior of this is seen the endothelial bag of the empty and atrophying venule (4) which the lymphatic is about to replace. The vein renmant is almost completely surrounded by the lymphatic space and only connected to the latter's endothelial wall by a bridge at one point. Other areas of similar lymphatic development are seen on the opposite side {4-5) and (not labelled) in the center of the field, but the structures on the left side are, as in the earlier stafres (cf. series 189) better developed and larger.
 
 
 
 
98 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
opment are seen on the opposite side {4-5) and (not labelled) in the center of the field, but the structures on the left side are, as in the earlier stafres (cf. series 189) better developed and larger.
 
  
 
The succeeding figures, 120, 121 and 122 (sections 7, 10 and 11) show the same structures, which can be distinctly followed for some distance caudad of the last section figured. In fig. 122 the original connection of the large upper venous kernel of the left side (4) with one of the components of the peritracheal venous plexus (18) is still suggested By the arrangement of the adjacent cells, although the degenerating vein has been cut off from all actual communication with the permanently valid channels of the mediastinal venous plexus.
 
The succeeding figures, 120, 121 and 122 (sections 7, 10 and 11) show the same structures, which can be distinctly followed for some distance caudad of the last section figured. In fig. 122 the original connection of the large upper venous kernel of the left side (4) with one of the components of the peritracheal venous plexus (18) is still suggested By the arrangement of the adjacent cells, although the degenerating vein has been cut off from all actual communication with the permanently valid channels of the mediastinal venous plexus.
Line 1,240: Line 1,168:
 
The 14 mm. stage is so important, and the results gained by the examination of a laige number of embryos so conclusive in respect to the true ontogenesis of the systemic lymphatic channels, that I have selected a well marked example and pubUsh here eighteen consecutive, or nearly consecutive, sections from the same individual. I do this in order to show the extent to which extraintimal perivenous lymphatic development has proceeded in this stage, in respect to the length of the resulting segments of the future lymphatic channel, still separated from each other, and to accentuate, by a large number of successive illustrations from the same embryo, the histogenesis of the lymphatic anlages in their relation to the decadent embryonic venules which they eventually entirely replace. The conception involved by the theory of extraintimal systemic lymphatic development has proved apparently, to judge from published comments, difiScult to understand in some laboratories from the histo-mechanical standpoint. The following eighteen reproductions of sections of a 14 mm. embryo (series 214, slide xiii) surely explain, without detailed description, the principles underlying lymphatic ontogenesis in the mammalian embryo. I very gladly welcome the opportunity, which this series of Wistar publications offers, for presenting them.
 
The 14 mm. stage is so important, and the results gained by the examination of a laige number of embryos so conclusive in respect to the true ontogenesis of the systemic lymphatic channels, that I have selected a well marked example and pubUsh here eighteen consecutive, or nearly consecutive, sections from the same individual. I do this in order to show the extent to which extraintimal perivenous lymphatic development has proceeded in this stage, in respect to the length of the resulting segments of the future lymphatic channel, still separated from each other, and to accentuate, by a large number of successive illustrations from the same embryo, the histogenesis of the lymphatic anlages in their relation to the decadent embryonic venules which they eventually entirely replace. The conception involved by the theory of extraintimal systemic lymphatic development has proved apparently, to judge from published comments, difiScult to understand in some laboratories from the histo-mechanical standpoint. The following eighteen reproductions of sections of a 14 mm. embryo (series 214, slide xiii) surely explain, without detailed description, the principles underlying lymphatic ontogenesis in the mammalian embryo. I very gladly welcome the opportunity, which this series of Wistar publications offers, for presenting them.
  
The plates comprised in this group are shown in order cephalocaudad in figures 123 to 141 inclusive. In all of them the ex
+
The plates comprised in this group are shown in order cephalocaudad in figures 123 to 141 inclusive. In all of them the extraintimal lymphatic anlages of the broncho-mediastinal trunk are indicated by the leader 5, while the included remnant of the atrophying embryonic vein is labelled 4, as in the preceding and following figures.
 
 
 
 
DEVELOPMENT OP THE THOHACIC DUCT 99
 
 
 
traintimal lymphatic anlages of the broncho-mediastinal trunk are indicated by the leader 5, while the included remnant of the atrophying embryonic vein is labelled 4, as in the preceding and following figures.
 
  
 
Figs. 123, 124, 125 and 126 (sections 4, 5, 6 and 7) show well the relation of the venule (4), undergoing atrophy and replacement by the extraintimal lymphatic space (5), to the permanent venous channels of the mediastinal plexus. Thus in fig. 123 the dorsal large lighter area marked 4 is the blind cephalic end of a venule which is nearly detached from the permanent peritracheal (18) and ventral mediastinal (32) venous channels, and which in the succeeding sections is seen to be partially surrounded by the extraintimal lymphatic space (5) eventually designed to replace it entirely.
 
Figs. 123, 124, 125 and 126 (sections 4, 5, 6 and 7) show well the relation of the venule (4), undergoing atrophy and replacement by the extraintimal lymphatic space (5), to the permanent venous channels of the mediastinal plexus. Thus in fig. 123 the dorsal large lighter area marked 4 is the blind cephalic end of a venule which is nearly detached from the permanent peritracheal (18) and ventral mediastinal (32) venous channels, and which in the succeeding sections is seen to be partially surrounded by the extraintimal lymphatic space (5) eventually designed to replace it entirely.
Line 1,251: Line 1,174:
 
In figure 124 (section 5) this perivenous space (5) has developed. The included vein (4) is still partially connected with the functional peritracheal plexus (18). Further ventrad is another portion of this same plexus. The venous kernel here still contains a number of red blood-cells. These characteristic relations of developing lymphatic and receding vein can be followed clearly throughout the succeeding sections. The contrast between the well filled permanent veins {e.g.j 18, 32 in figs. 123, 124, 125 and 126), with walls fitted closely into the surroimding mesenchyme, and the atrophying venule surrounded by the extraintimal lymphatic anlage, is striking throughout the series. Some of the sections show in certain regions good pictures of nearly complete reduction of the decadent vein to an insignificant central kernel of connective tissue surrounded by the extraintimal lymphatic space; thus, e.g., figs. 137 to 141 in the region ventro-mesad to the left vagus {22).
 
In figure 124 (section 5) this perivenous space (5) has developed. The included vein (4) is still partially connected with the functional peritracheal plexus (18). Further ventrad is another portion of this same plexus. The venous kernel here still contains a number of red blood-cells. These characteristic relations of developing lymphatic and receding vein can be followed clearly throughout the succeeding sections. The contrast between the well filled permanent veins {e.g.j 18, 32 in figs. 123, 124, 125 and 126), with walls fitted closely into the surroimding mesenchyme, and the atrophying venule surrounded by the extraintimal lymphatic anlage, is striking throughout the series. Some of the sections show in certain regions good pictures of nearly complete reduction of the decadent vein to an insignificant central kernel of connective tissue surrounded by the extraintimal lymphatic space; thus, e.g., figs. 137 to 141 in the region ventro-mesad to the left vagus {22).
  
The 15 nma. stage shows the developing lymphatic plexus of the broncho-mediastinal trunk further advanced. Figs. 142 to 145 show four successive transverse sections through the ventral part of the upper thoracic region in a 15 mm. cat embryo (series 216, slide x, sections 32, 33, 34 and 35, X 225). The central decadent venule in the interior of the developing perivenous lymphatic space has in many places disappeared entirely, leav
+
The 15 nma. stage shows the developing lymphatic plexus of the broncho-mediastinal trunk further advanced. Figs. 142 to 145 show four successive transverse sections through the ventral part of the upper thoracic region in a 15 mm. cat embryo (series 216, slide x, sections 32, 33, 34 and 35, X 225). The central decadent venule in the interior of the developing perivenous lymphatic space has in many places disappeared entirely, leaving only the clear lymphatic lumen (5). In other places remnants of the replaced vein (4) can still be distinctly determined. This clearance has resulted in a relative diminution of the actual area of a cross section of the meshes of the lymphatic plexus, since now the entire lumen is given over to the newly formed lymphatic channel, whereas in the earlier stages its central area was still largely filled by the degenerating veil. The lymphatic spaces in these sections have increased considerably in number. (Only a few of the more prominent vessels are indicated m the figures by leaders.)
 
 
 
 
100 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
ing only the clear lymphatic lumen (5). In other places remnants of the replaced vein (4) can still be distinctly determined. This clearance has resulted in a relative diminution of the actual area of a cross section of the meshes of the lymphatic plexus, since now the entire lumen is given over to the newly formed lymphatic channel, whereas in the earlier stages its central area was still largely filled by the degenerating veil. The lymphatic spaces in these sections have increased considerably in number. (Only a few of the more prominent vessels are indicated m the figures by leaders.)
 
  
 
A transverse section of this region in a 16.^ mm. cat embryo (series 215, slide xiv, section 31, X 226) is shown in a fig.146. The preceding more diffuse lymphatic plexus of the earlier stages is condensed to a much smaller mesenchymal network. In a few of the larger meshes a remnant of the former central vein (4) can still be traced as a solid plug of mesoderm surrounded by the lymphatic channel (5).
 
A transverse section of this region in a 16.^ mm. cat embryo (series 215, slide xiv, section 31, X 226) is shown in a fig.146. The preceding more diffuse lymphatic plexus of the earlier stages is condensed to a much smaller mesenchymal network. In a few of the larger meshes a remnant of the former central vein (4) can still be traced as a solid plug of mesoderm surrounded by the lymphatic channel (5).
Line 1,262: Line 1,180:
 
In general the 15 and 15.5 mm. embryos of the cat represent the stages in the development of the broncho-mediastinal trunk in which the histological pictures of the lymph channels are least distinct aod striking. This is due to the fact that on the one hand they have lost the central venous kernel around which they were originally developed through confluence of extramthnal spaces, and which gave to the combined structures their area on section and their distinctive appearance in the earlier (13.5 and 14 mm.) stages, while on the other hand the independent growth and enlargement of the lymph channels proper begins in the 16 mm. stage. In some embryos, measuring 16 mm., the appearances characteristic of the 15 mm. and 15.5 mm. stages are still encountered. Thus fig. 147 shows a transverse section through the ventral area of the upper thoracic region in a 16 mm. embryo (series 230, slide xii, section 25, X 225). The correspondence of the histological picture to that presented by the 15 mm. or 15.5 mm. embryo is seen by comparison with series 215 and 216 (figs. 142 to 146).
 
In general the 15 and 15.5 mm. embryos of the cat represent the stages in the development of the broncho-mediastinal trunk in which the histological pictures of the lymph channels are least distinct aod striking. This is due to the fact that on the one hand they have lost the central venous kernel around which they were originally developed through confluence of extramthnal spaces, and which gave to the combined structures their area on section and their distinctive appearance in the earlier (13.5 and 14 mm.) stages, while on the other hand the independent growth and enlargement of the lymph channels proper begins in the 16 mm. stage. In some embryos, measuring 16 mm., the appearances characteristic of the 15 mm. and 15.5 mm. stages are still encountered. Thus fig. 147 shows a transverse section through the ventral area of the upper thoracic region in a 16 mm. embryo (series 230, slide xii, section 25, X 225). The correspondence of the histological picture to that presented by the 15 mm. or 15.5 mm. embryo is seen by comparison with series 215 and 216 (figs. 142 to 146).
  
The majority of 16 mm. embryos show, however, a marked advance in the independent growth of the lymphatic channels,
+
The majority of 16 mm. embryos show, however, a marked advance in the independent growth of the lymphatic channels, which have become fully established. Their lumen now appears for the most part entirely cleared of the remnants of the decadent venules originally contained therein. In only a few instances does the original vein kernel appear reduced to an insignificant rudiment. The resulting plexus of the broncho-mediastinal lymphatic duct fills the typical position occupied in the earlier stages by the extensive preceding perivenous lymphatic reticulum. Thus figs. 148-153 give transverse sections through the ventral part of the upper thoracic region in another 16 mm. cat embryo (series 222, sUde X, sections 1, 2, 3, 10, 15 and 16, X 225). This embryo affords remarkably clear pictures of the fully developed lymphatic plexus of the ventral or broncho-mediastinal trunk.
 
 
 
 
 
 
DEVELOPMENT OF THE THORACIC DUCT 101
 
 
 
which have become fully established. Their lumen now appears for the most part entirely cleared of the remnants of the decadent venules originally contained therein. In only a few instances does the original vein kernel appear reduced to an insignificant rudiment. The resulting plexus of the broncho-mediastinal lymphatic duct fills the typical position occupied in the earlier stages by the extensive preceding perivenous lymphatic reticulum. Thus figs. 148-153 give transverse sections through the ventral part of the upper thoracic region in another 16 mm. cat embryo (series 222, sUde X, sections 1, 2, 3, 10, 15 and 16, X 225). This embryo affords remarkably clear pictures of the fully developed lymphatic plexus of the ventral or broncho-mediastinal trunk.
 
  
 
Figs. 148 and 153 (sections 1 and 16) give the topographical pictures of the entire region involved. The broncho-media^^tinal lymphatic plexus {37) is fully developed in the typical area between the trachea (5) and vagi (32) dorsally and the pulmonary arteries (10) ventrally. The lymphatic spaces possess a clear lumen and distinct endothelial walls. The remnants of the degenerating ventral mediastinal venous plexus which they have replaced have almost entirely disappeared. Only slight traces of them are still to be seen here and there. In the main the lymphatic channels are clear of venous kernels and form a rich interlacing lymphatic plexus. The central field contains the oesophagus (5), aorta (7) and part of the medial circumference of the left praecava (6). Dorsally on the right side the right azygos vein (S) and sympathetic nerve (1) appear, while on the left side the section includes the cephalic part of the azygos segment of the left thoracic duct (36).
 
Figs. 148 and 153 (sections 1 and 16) give the topographical pictures of the entire region involved. The broncho-media^^tinal lymphatic plexus {37) is fully developed in the typical area between the trachea (5) and vagi (32) dorsally and the pulmonary arteries (10) ventrally. The lymphatic spaces possess a clear lumen and distinct endothelial walls. The remnants of the degenerating ventral mediastinal venous plexus which they have replaced have almost entirely disappeared. Only slight traces of them are still to be seen here and there. In the main the lymphatic channels are clear of venous kernels and form a rich interlacing lymphatic plexus. The central field contains the oesophagus (5), aorta (7) and part of the medial circumference of the left praecava (6). Dorsally on the right side the right azygos vein (S) and sympathetic nerve (1) appear, while on the left side the section includes the cephalic part of the azygos segment of the left thoracic duct (36).
Line 1,274: Line 1,186:
 
Sections 2, 3, 10 and 15 of the same slide are shown in their ventral portions in figs. 149 to 152. They all exhibit the extent and continuity of the broncho-mediastinal lymphatic channel complex in this stage. The figures are not leadered, but the individual structures can be readily identified by reference to the topographical figures 148 and 153.
 
Sections 2, 3, 10 and 15 of the same slide are shown in their ventral portions in figs. 149 to 152. They all exhibit the extent and continuity of the broncho-mediastinal lymphatic channel complex in this stage. The figures are not leadered, but the individual structures can be readily identified by reference to the topographical figures 148 and 153.
  
The succeeding stages are characterized by a condensation of the extensive lymphatic plexus of the typical 16 nmi. and 17 mm.
+
The succeeding stages are characterized by a condensation of the extensive lymphatic plexus of the typical 16 nmi. and 17 mm. embryos into the more definite channel of the broncho-mediastinal trunk with larger caliber.
  
 +
Figs. 154 to 158 show the region of the broncho-mediastinal channel in four successive sections in a 20 mm. cat embryo (series 241, slide xix, sections 19, 20, 21 and 22, X 225). Fig. 154 again gives a topographical view of the entire field, while the same is cut down to the ventral portion in the remaining plates. In all the multi-meshed lymphatic plexus of the preceding stage is replaced by the more definite and larger channel of the bronchomediastinal trunk proper (37) y in the classical situation between the pulmonary arteries (10) ventrally and the trachea (9), vagi (SS) and permanent ventral mediastinal veins (18) dorsally.
  
 
+
This stage marks the practical attainment of adult conditions for the lymphatic channel of the broncho-mediastinal trunk, and from this period on its further development is merely a question of continued growth and adaptation to the environment in establishing secondary connections with the tributary channels, developed independently, the preazygos segment of the thoracic duct, and (usually) the ventral prolongation of the subclavian approach of the jugular lymph sac.
102 DEVELOPMENT OP THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
embryos into the more definite channel of the broncho-mediastinal trunk with larger caliber.
 
 
 
Figs. 154 to 158 show the region of the broncho-mediastinal channel in four successive sections in a 20 mm. cat embryo (series 241, slide xix, sections 19, 20, 21 and 22, X 225). Fig. 154 again gives a topographical view of the entire field, while the same is cut down to the ventral portion in the remaining plates. In all the multi-meshed lymphatic plexus of the preceding stage is replaced by the more definite and larger channel of the bronchomediastinal trunk proper (37) y in the classical situation between the pulmonary arteries (10) ventrally and the trachea (9), vagi (SS) and permanent ventral mediastinal veins (18) dorsally.
 
 
 
This stage marks the practical attainment of adult conditions for the lymphatic channel of the broncho-mediastinal trunk, and from this period on its further development is merely a question of continued growth and adaptation to the environment in establishing secondary connections with the tributary channels, developed independently, the preazygos segment of the thoracic duct, and (usually) the ventral prolongation of the subclavian approach of the jugular lymph sac.
 
  
 
As will be seen from the foregoing analysis of the development of the broncho-mediastinal lymphatic trunk in embryos of the cat, the principles of mammalian systemic lymphatic ontogenesis defined in Part I of this memoir are consistently sustained by the actual evidence afforded. The development of the perivenous extraintimal anlages which form the basis for the subsequent evolution of the systemic lymphatic channels, is a uniform and remarkably consistent process.
 
As will be seen from the foregoing analysis of the development of the broncho-mediastinal lymphatic trunk in embryos of the cat, the principles of mammalian systemic lymphatic ontogenesis defined in Part I of this memoir are consistently sustained by the actual evidence afforded. The development of the perivenous extraintimal anlages which form the basis for the subsequent evolution of the systemic lymphatic channels, is a uniform and remarkably consistent process.
  
The existence of these spaces in this and other regions of the embryo has been so often denied by recent contributors to the subject, or, if admitted, explained in every possible way except through the correct- interpretation, that I pubUsh in this paper a series of micro-photographs of a considerable number of sections taken from embryos of all the important stages. Of course these observations could be indefinitely multiplied. The larger the embryological collection, and the better the individual preparations are fixed and stained, the more conclusive will be the
+
The existence of these spaces in this and other regions of the embryo has been so often denied by recent contributors to the subject, or, if admitted, explained in every possible way except through the correct- interpretation, that I pubUsh in this paper a series of micro-photographs of a considerable number of sections taken from embryos of all the important stages. Of course these observations could be indefinitely multiplied. The larger the embryological collection, and the better the individual preparations are fixed and stained, the more conclusive will be the results of a careful examination of lymphatic development along the lines here indicated.
 
 
 
 
 
 
DEVELOPMENT OP THE THOHACIC DUCT 103
 
 
 
results of a careful examination of lymphatic development along the lines here indicated.
 
  
 
No competent and impartial observer can mistake the significance of the conditions here shown. Every stage of the process can be followed in detail. The behavior of the decadent embrvonic vein, and its relation to the enveloping extraintimal lymphatic channel, are absolutely demonstrated. The endothelium of the shrinking, vein has no share in furnishing the independent lymphatic endothelimn of the replacing mesenchymal space, and nowhere, in the entire process, is there the faintest suggestion of an "out-bud" or of a "splitting ofif" from the circumference of an otherwise vaUd embryonic vein of "lymphatic or "venolymphatic anlages.
 
No competent and impartial observer can mistake the significance of the conditions here shown. Every stage of the process can be followed in detail. The behavior of the decadent embrvonic vein, and its relation to the enveloping extraintimal lymphatic channel, are absolutely demonstrated. The endothelium of the shrinking, vein has no share in furnishing the independent lymphatic endothelimn of the replacing mesenchymal space, and nowhere, in the entire process, is there the faintest suggestion of an "out-bud" or of a "splitting ofif" from the circumference of an otherwise vaUd embryonic vein of "lymphatic or "venolymphatic anlages.
Line 1,303: Line 1,203:
  
  
 
104 DEVELOPMENT OP THE SYSTEMIC LYMPHATIC VESSELS
 
  
 
Consequently, in the pictures of the earlier stages (13, 13.5 and 14 mm. embryos) the area occupied by the still capacious abandoned venous channel, combined with that of the relatively narrow enveloping lymphatic space, appears larger and more prominent than in the later periods (15, 15.5 mm.) in which the permanent lymphatic channel and its reduced venous kernel occupies less histological territory.
 
Consequently, in the pictures of the earlier stages (13, 13.5 and 14 mm. embryos) the area occupied by the still capacious abandoned venous channel, combined with that of the relatively narrow enveloping lymphatic space, appears larger and more prominent than in the later periods (15, 15.5 mm.) in which the permanent lymphatic channel and its reduced venous kernel occupies less histological territory.
Line 1,312: Line 1,210:
 
Thus in the region here imder discussion the pictures offered by the 13, 13.5 and 14 mm. embryos are more conclusive than those furnished bj' the 15 and 15.5 mm. stages in reference to the genesis of the Ijonphatic channels. In the former, the lymphatic anlages and the decadent venules around which they develop are taken together, relatively larger and more numerous, and hence easier to recognize. In the latter, this early redimdant perivenous lymphatic plexus has been replaced by a relatively smaller, but much more definite and distinct channel, occupjdng, however, always a part of the area filled in the earlier stages by the less circumscribed antecedent lymphatic plexus with its contained remnants of abandoned embryonic veins. I lay special stress on this fact, because it is absolutely neoessarj' to take it into account in comparing stages between 13.5 and 15.5 nmi. In the latter stage the final IjTiiphatic anlages have definitely formed and have reached the relation to adjacent main venous lines which they henceforth are to occupy. The further growth and enlargement of these chamiels begins in the 16 mm. embrj'o, and proceeds from this point on, in many regions at the expense of the adjacent embryonic vein undergoing atrophy.
 
Thus in the region here imder discussion the pictures offered by the 13, 13.5 and 14 mm. embryos are more conclusive than those furnished bj' the 15 and 15.5 mm. stages in reference to the genesis of the Ijonphatic channels. In the former, the lymphatic anlages and the decadent venules around which they develop are taken together, relatively larger and more numerous, and hence easier to recognize. In the latter, this early redimdant perivenous lymphatic plexus has been replaced by a relatively smaller, but much more definite and distinct channel, occupjdng, however, always a part of the area filled in the earlier stages by the less circumscribed antecedent lymphatic plexus with its contained remnants of abandoned embryonic veins. I lay special stress on this fact, because it is absolutely neoessarj' to take it into account in comparing stages between 13.5 and 15.5 nmi. In the latter stage the final IjTiiphatic anlages have definitely formed and have reached the relation to adjacent main venous lines which they henceforth are to occupy. The further growth and enlargement of these chamiels begins in the 16 mm. embrj'o, and proceeds from this point on, in many regions at the expense of the adjacent embryonic vein undergoing atrophy.
  
 
 
DEVELOPMENT OF THE THORACIC DUCT 105
 
  
 
The only conclusion which seems to me to be warranted by actual observation is that certain embryonic veins form, during the process of their atrophy and final elimination from the definite venous organization, the supporting lines along which certain of the perivenous extraintimal lymphatic anlages first develop.
 
The only conclusion which seems to me to be warranted by actual observation is that certain embryonic veins form, during the process of their atrophy and final elimination from the definite venous organization, the supporting lines along which certain of the perivenous extraintimal lymphatic anlages first develop.
Line 1,320: Line 1,215:
 
The fact that numerous early embryonic venous channels, large and small, atrophy and disappear during the normal course of subsequent development, appears to afiford a more favorable field for the greater development of the adjacent mesenchymal inintercellular spaces, so that these enlarge more rapidly, as the correlated vein recedes. This relationship appears, however, to be based exclusively on the physical and mechanical advantages which the abandoned and shrinking primary venous line affords to the adjacent mesenchymal spaces for more rapid enlargement, in the sense of replacing the disappearing vein and occupying secondarily the space formerly filled by the haemal channel. This is evidently an important factor in determining the size and extent of the final lymphatic channel resulting from the confluence of the originally separate and independent perivenous anlages. Consequently, in the adult, the largest and best defined systemic lymphatic vessels either accompany reduced adult remnants of a relatively larger embryonic venous channel, or, in case of the latter's entire default, topographically replace the same. Now, while this relation manifests itself strikingly in many parts of the body, it is quite evident that the development of lymphatic chaimels occurs in other parts independently of preceding veins, by the confluence of independent intercellular mesenchymal spaces.
 
The fact that numerous early embryonic venous channels, large and small, atrophy and disappear during the normal course of subsequent development, appears to afiford a more favorable field for the greater development of the adjacent mesenchymal inintercellular spaces, so that these enlarge more rapidly, as the correlated vein recedes. This relationship appears, however, to be based exclusively on the physical and mechanical advantages which the abandoned and shrinking primary venous line affords to the adjacent mesenchymal spaces for more rapid enlargement, in the sense of replacing the disappearing vein and occupying secondarily the space formerly filled by the haemal channel. This is evidently an important factor in determining the size and extent of the final lymphatic channel resulting from the confluence of the originally separate and independent perivenous anlages. Consequently, in the adult, the largest and best defined systemic lymphatic vessels either accompany reduced adult remnants of a relatively larger embryonic venous channel, or, in case of the latter's entire default, topographically replace the same. Now, while this relation manifests itself strikingly in many parts of the body, it is quite evident that the development of lymphatic chaimels occurs in other parts independently of preceding veins, by the confluence of independent intercellular mesenchymal spaces.
  
The cat may, and does, differ in its details of lymphatic development, and in its adult lymphatic organization, from the conditions obtaining in many other mammalian forms. In fact, even in the present very incomplete state of our knowledge of the comparative anatomy of the mammalian lymphatic system, we realize that what we regard as the mammaUan ground plan of lymphatic organization may in certain genera undergo great and far-reaching modifications, as for example the conditions deter
+
The cat may, and does, differ in its details of lymphatic development, and in its adult lymphatic organization, from the conditions obtaining in many other mammalian forms. In fact, even in the present very incomplete state of our knowledge of the comparative anatomy of the mammalian lymphatic system, we realize that what we regard as the mammaUan ground plan of lymphatic organization may in certain genera undergo great and far-reaching modifications, as for example the conditions determined by Silvester'^ for the whole group of the Cebidae, and those found by myself in Macropua rufus. And yet these diflferences, established and maintained within the natural limits of the mammalian class, cannot, in whatever degree they actually exist, in individual forms be basic. In any given mammalian type, the systemic lymphatic vessels, whatever their adult relation to and connection with the venous system may be, must develop in accordance with a genetic groimd plan common to all mammalia.
 
 
 
 
106 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
mined by Silvester'^ for the whole group of the Cebidae, and those found by myself in Macropua rufus. And yet these diflferences, established and maintained within the natural limits of the mammalian class, cannot, in whatever degree they actually exist, in individual forms be basic. In any given mammalian type, the systemic lymphatic vessels, whatever their adult relation to and connection with the venous system may be, must develop in accordance with a genetic groimd plan common to all mammalia.
 
  
  
Line 1,334: Line 1,224:
  
  
 
+
==Part II Plates==
PART II PLATES
 
  
 
FIGURES 100 TO 158
 
FIGURES 100 TO 158
 
  
  
Line 1,794: Line 1,682:
  
 
FIGURES 138 AND 139
 
FIGURES 138 AND 139
 
«
 
  
 
13S Same, section 22. 139 Same, section 23.
 
13S Same, section 22. 139 Same, section 23.
 
 
  
 
3 Right pre^aval vein.
 
3 Right pre^aval vein.
Line 1,833: Line 1,717:
 
6 Precardinal or precava of left side.
 
6 Precardinal or precava of left side.
  
 +
7 Aorta.
  
 +
8 Oesophagus.
  
7
+
9 Trachea.
  
 +
10 Pulmonary artery.
  
Aorta.
+
22 Vagus.
  
  
8
 
  
 +
FIGURES 142 AND 113
  
Oesophagus.
+
142 Transverse section through ventral part of upper thoracic region in a 15 mm. cat embr>'o (series 216, slide X. section 32), X 225.
  
 +
143 Same, section 33.
  
9
+
3 Right precaval vein.
  
 +
4 Atrophying embryonal vein, forming kernel in interior of developing
  
Trachea.
+
and replacing lymphatic space. 5» Extrainthnal or perivenous lymphatic space surrounding degenerating embryonal vein.
  
 +
7 Aorta.
  
10
+
8 Oesophagus.
  
 +
9 Trachea.
  
Pulmonary artery.
+
10 Pulmonary artery. 22 Vagus.
  
  
22
 
  
 +
FIGURES 144 and 145
  
Vagus.
+
144 Same, section 34.
  
 +
145 Same, section 35.
  
 
+
3 Right prccaval vein.
FIGUKES 142 AND 113
 
 
 
142 Transverse section through ventral part of upper thoracic region in a 15 mm. cat embr>'o (series 216, slide X. section 32), X 225.
 
 
 
143 Same, section 33.
 
 
 
3 Right precaval vein.
 
 
 
4 Atrophying embryonal vein, forming kernel in interior of developing
 
 
 
and replacing lymphatic space. 5» Extrainthnal or perivenous lymphatic space surrounding degenerating embryonal vein.
 
 
 
7 Aorta.
 
 
 
8 Oesophagus.
 
 
 
9 Trachea.
 
 
 
10 Pulmonary artery. 22 Vagus.
 
 
 
 
 
 
 
FKJURKS 144 and 145
 
 
 
144 Same, section 34.
 
 
 
145 Same, section 35.
 
 
 
3 Right prccaval vein.
 
  
 
4 Atrophying embryonal vein, forming kernel in interior of devel oping and replacing lymphatic space.
 
4 Atrophying embryonal vein, forming kernel in interior of devel oping and replacing lymphatic space.
Line 2,016: Line 1,879:
  
  
FIGURKS lol AND lo2
+
FIGURES 151 AND 152
  
  
Line 2,113: Line 1,976:
  
  
B. The development of the proximal portion of the thoracic duct proper J between the termination of thoracic duct approach of the jugular lymph sac and the beginning of the azygos segment of the thoracic du^ts, caudal to the level of the aortic arch, viz, the preazygos segment of the thoracic duct.
+
==The development of the preazygos segment of the thoracic duct==
 +
 
 +
B. The development of the proximal portion of the thoracic duct proper between the termination of thoracic duct approach of the jugular lymph sac and the beginning of the azygos segment of the thoracic ducts, caudal to the level of the aortic arch, viz, the preazygos segment of the thoracic duct.
  
 
In the earlier purely venous stages a venous plexus between oesophagus and vertebral column (i7, fig. 101), drains caudolaterad into the mesal surface of the main jugular and innominate trunks. This plexus continues the supracardinal venous line cephalad beyond the level of the azygos-Cuvierian junction. The terminals of this plexus are frequently joined by dorsal somatic venous tributaries {16, fig. 101), near their entrance into the main vein. These vessels have been previously discussed in their relation to the general tributary system of thejuguloinnominate lin-e (cf. supra p. (92), and their disposition is schematically shown in fig. 101.
 
In the earlier purely venous stages a venous plexus between oesophagus and vertebral column (i7, fig. 101), drains caudolaterad into the mesal surface of the main jugular and innominate trunks. This plexus continues the supracardinal venous line cephalad beyond the level of the azygos-Cuvierian junction. The terminals of this plexus are frequently joined by dorsal somatic venous tributaries {16, fig. 101), near their entrance into the main vein. These vessels have been previously discussed in their relation to the general tributary system of thejuguloinnominate lin-e (cf. supra p. (92), and their disposition is schematically shown in fig. 101.
Line 2,125: Line 1,990:
 
a. The development of independent mesenchymal intercellular lymphatic spaces, with well defined endothelial limiting walls.
 
a. The development of independent mesenchymal intercellular lymphatic spaces, with well defined endothelial limiting walls.
  
 
 
108 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
  
 
Such spaces {77) are seen in the two sections lateral to the oesophagus. They apparently develop from the beginning as intercellular clefts, without relation to embryonic veins. The preazygos region presents these spaces in almost all 13 and 14 mm. embryos. Their development in the mammalian embryo is of the utmost phylogenetic significance and affords important evidence in the interpretation of systemic lymphatic ontogenesis in the sauropsida, and especially in reptiles. The investigation of avian and reptilian lymphatic development has been carried on for the past year in the anatomical laboratory of Columbia University. These researches are now nearly completed, and some of the main results were presented at the 27th session of the American Association of Anatomists at Cornell University in December of last year. They establish a common genetic ground plan for the development and adult organization of the amniote lymphatic system. The avian and reptilian type only differs from that encountered in the mammal in respect to the higher degree of development of the jugular lymph sac, as an anterior or cervical veno-lymphatic heart, in the ontogenetic appearance of other areas of equal phylogenetic significance, as remnants of the multiple ancestral series of segmental veno-lymphatic hearts, and in the preponderance of peripheral lymphatic development by confluence of independent intercellular mesodermal spaces, not associated topographically, as in the mammal, with temporary embryonic venous channels. In other words, the extraintimal perivenous development of the majority of peripheral lymphatic channels described in this communication appears as a caenogenetic process in the mammalian embryo, whereas in the sauropsid amniotes most of the systemic lymphatic channels develop along more primitive phylogenetic lines, by the direct confluence of numerous intercellular mesodermal spaces.
 
Such spaces {77) are seen in the two sections lateral to the oesophagus. They apparently develop from the beginning as intercellular clefts, without relation to embryonic veins. The preazygos region presents these spaces in almost all 13 and 14 mm. embryos. Their development in the mammalian embryo is of the utmost phylogenetic significance and affords important evidence in the interpretation of systemic lymphatic ontogenesis in the sauropsida, and especially in reptiles. The investigation of avian and reptilian lymphatic development has been carried on for the past year in the anatomical laboratory of Columbia University. These researches are now nearly completed, and some of the main results were presented at the 27th session of the American Association of Anatomists at Cornell University in December of last year. They establish a common genetic ground plan for the development and adult organization of the amniote lymphatic system. The avian and reptilian type only differs from that encountered in the mammal in respect to the higher degree of development of the jugular lymph sac, as an anterior or cervical veno-lymphatic heart, in the ontogenetic appearance of other areas of equal phylogenetic significance, as remnants of the multiple ancestral series of segmental veno-lymphatic hearts, and in the preponderance of peripheral lymphatic development by confluence of independent intercellular mesodermal spaces, not associated topographically, as in the mammal, with temporary embryonic venous channels. In other words, the extraintimal perivenous development of the majority of peripheral lymphatic channels described in this communication appears as a caenogenetic process in the mammalian embryo, whereas in the sauropsid amniotes most of the systemic lymphatic channels develop along more primitive phylogenetic lines, by the direct confluence of numerous intercellular mesodermal spaces.
  
These relative conditions have been outlined in a previous publication on the phylogenetic relations of the lymphatic and blood-vascular systems in vertebrates.*^^ The results of the more
+
These relative conditions have been outlined in a previous publication on the phylogenetic relations of the lymphatic and blood-vascular systems in vertebrates.*^^ The results of the more recent detailed investigations of Ijonphatic development in birds and reptiles have fully sustained the interpretation given in the publication quoted.
  
Geo. S. Huntington, "The phylogenetic relations of the lymphatic and
+
Geo. S. Huntington, "The phylogenetic relations of the lymphatic and blood -vascular systems in vertebrates.*' Anat. Record, vol. IV, no. 1, January 1910.
blood -vascular systems in vertebrates.*' Anat. Record, vol. IV, no. 1, January 1910.
 
  
  
 
DEVELOPMENT OF THE THORACIC DUCT 109
 
 
recent detailed investigations of Ijonphatic development in birds and reptiles have fully sustained the interpretation given in the publication quoted.
 
  
 
b. The darkly stained mass of cells seen in the interval between 16 and i7 in both sections is the result of obliteration of early embrj^onic vascular channels, which have lost their connection with the vessels, and whose endotheUal walls are apparently in the process of reverting to indifferent mesodermal cells. Many of these degenerated vascular aggregations are formed in embryos between 13 and 15 mm. crown-rump measure.
 
b. The darkly stained mass of cells seen in the interval between 16 and i7 in both sections is the result of obliteration of early embrj^onic vascular channels, which have lost their connection with the vessels, and whose endotheUal walls are apparently in the process of reverting to indifferent mesodermal cells. Many of these degenerated vascular aggregations are formed in embryos between 13 and 15 mm. crown-rump measure.
Line 2,148: Line 2,005:
 
Fig. 162 shows a transverse section of the left side of the upper thoracic region in a 14 nron. cat embr^^o (series 210, slide IX, section 26, X 200). The prevertebral tributarj^ plexus (17) turns mesad of the sympathetic nerve (1) to enter the dorso-medial angle of the left innominate vein (40), Dorsad of the main vein is the ascending portion of the left subclavian artery (S3). Between aorta (7) and innominate vein (40) are other elements of the mediastinal venous plexus, in the interval between the sympathetic nerve (1) and the vagus {22) j with ventral branches (32) in front of the latter.
 
Fig. 162 shows a transverse section of the left side of the upper thoracic region in a 14 nron. cat embr^^o (series 210, slide IX, section 26, X 200). The prevertebral tributarj^ plexus (17) turns mesad of the sympathetic nerve (1) to enter the dorso-medial angle of the left innominate vein (40), Dorsad of the main vein is the ascending portion of the left subclavian artery (S3). Between aorta (7) and innominate vein (40) are other elements of the mediastinal venous plexus, in the interval between the sympathetic nerve (1) and the vagus {22) j with ventral branches (32) in front of the latter.
  
Between the oesophagus and the prevertebral plexus (17) is an area in which venous elements (4), formerly connected with the same, are undergoing replacement by the extraintimal lymphatic anlages (5) of the preazygos segment of the thoracic duct. These structures cannot be distinctly made out in fig. 162, but they are shown in a higher magnification in detail in fig. 163, which includes the region of the same section dorsal to the oesoph
+
Between the oesophagus and the prevertebral plexus (17) is an area in which venous elements (4), formerly connected with the same, are undergoing replacement by the extraintimal lymphatic anlages (5) of the preazygos segment of the thoracic duct. These structures cannot be distinctly made out in fig. 162, but they are shown in a higher magnification in detail in fig. 163, which includes the region of the same section dorsal to the oesophagus magnified 225 diameters. The prevertebral venous plexus (17) extends in an arch concentric with the dorsal circumference of the oesophagus (5), in front of the vertebral centers. Between it and the oesophagus, mesal to the sympathetics (i), are seen on each side former elements of the venous plexus (4) surrounded by the extraintimal lymphatic anlages (5) of the preazygos segment of the thoracic duct.
 
 
 
 
110 DEVELOPMENT OP THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
agus magnified 225 diameters. The prevertebral venous plexus (17) extends in an arch concentric with the dorsal circumference of the oesophagus (5), in front of the vertebral centers. Between it and the oesophagus, mesal to the sympathetics (i), are seen on each side former elements of the venous plexus (4) surrounded by the extraintimal lymphatic anlages (5) of the preazygos segment of the thoracic duct.
 
  
 
Figs. 164, 165 aiid 166 show the same structures in the next three sections (25, 24 and 23), proceeding cephalad. Fig. 166 is especially clear. In it the original line of connection of the atrophying venous kernel (4) with the prevertebral plexus (17) can still be made out as a band of condensed mesoderm, and the endothelium of the perivenous lymphatic space (5) is clearly marked.
 
Figs. 164, 165 aiid 166 show the same structures in the next three sections (25, 24 and 23), proceeding cephalad. Fig. 166 is especially clear. In it the original line of connection of the atrophying venous kernel (4) with the prevertebral plexus (17) can still be made out as a band of condensed mesoderm, and the endothelium of the perivenous lymphatic space (5) is clearly marked.
Line 2,159: Line 2,011:
 
Figs. 167 and 168 show this area in two successive sections of the same embryo further caudad (series 210, slide ix, sections 43 and 44, X 150), and fig. 169 gives the last section magnified 200 diameters. The empty central venous remnant (4), with the enveloping extraintimal lymphatic anlage (5), and the still functional elements of the prevertebral plexus (17), can be seen in all of the sections. They also show, nearer to the dorsolateral wall of the oesophagus, one of the collections of darkly stained mesodermal cells above referred to as representing remnants of formerly patent blood-vessels.
 
Figs. 167 and 168 show this area in two successive sections of the same embryo further caudad (series 210, slide ix, sections 43 and 44, X 150), and fig. 169 gives the last section magnified 200 diameters. The empty central venous remnant (4), with the enveloping extraintimal lymphatic anlage (5), and the still functional elements of the prevertebral plexus (17), can be seen in all of the sections. They also show, nearer to the dorsolateral wall of the oesophagus, one of the collections of darkly stained mesodermal cells above referred to as representing remnants of formerly patent blood-vessels.
  
The series of microphotographs shown in figs. 160 to 169 inclusive again conclusively demonstrates the origin of this portion of the lymphatic channel by confluence of independent extraintimal mesenchymal spaces surrounding and replacing degenerating embryonic venules. The reproductions only give relatively faint and indistinct views of the actual conditions observed under the microscope. The general mesenchymal tissue in this region is extremely loose and the area is very difficult to photograph sharply and distinctly. The actual sections, however, offer histological pictures identical with those observed as above described in the development of the ventral mediastinal lymphatic duct. The extraintimal lymphatic spaces are either closely applied to part of the circumference of the wall of the
+
The series of microphotographs shown in figs. 160 to 169 inclusive again conclusively demonstrates the origin of this portion of the lymphatic channel by confluence of independent extraintimal mesenchymal spaces surrounding and replacing degenerating embryonic venules. The reproductions only give relatively faint and indistinct views of the actual conditions observed under the microscope. The general mesenchymal tissue in this region is extremely loose and the area is very difficult to photograph sharply and distinctly. The actual sections, however, offer histological pictures identical with those observed as above described in the development of the ventral mediastinal lymphatic duct. The extraintimal lymphatic spaces are either closely applied to part of the circumference of the wall of the shrinking venule, or they occasionally completely surround the same. In the former case the contained venous remnant is connected at part of its periphery with the wall of the enveloping lymphatic space. In the latter and more exceptional case, the free ventral nucleus of the lymphatic space is formed by the atrophied vein. In either case the lumen of the' latter may contain a few degenerated red blood cells. These pictures are again constant in embryos of the appropriate stages. The lymphatic anlage can be accurately traced from its indefinite beginning among the perivenous mesenchymal intercellular clefts through a number of successive sections to its similar distal terminations in the same intercellular plexus. Following the sections from this point caudad through a varying intervening area in which no distinct lymphatic channel appears, the same line will sooner or later reveal the repetition of the same process, and the formation of another link in the still disjointed chain of primitive lymphatic aniages. It is again to be noted here that in the earlier stages (13, 13.5,
 
 
 
 
 
 
DEVELOPMENT OF THE THORACIC DUCT 111
 
 
 
shrinking venule, or they occasionally completely surround the same. In the former case the contained venous remnant is connected at part of its periphery with the wall of the enveloping lymphatic space. In the latter and more exceptional case, the free ventral nucleus of the lymphatic space is formed by the atrophied vein. In either case the lumen of the' latter may contain a few degenerated red blood cells. These pictures are again constant in embryos of the appropriate stages. The lymphatic anlage can be accurately traced from its indefinite beginning among the perivenous mesenchymal intercellular clefts through a number of successive sections to its similar distal terminations in the same intercellular plexus. Following the sections from this point caudad through a varying intervening area in which no distinct lymphatic channel appears, the same line will sooner or later reveal the repetition of the same process, and the formation of another link in the still disjointed chain of primitive lymphatic aniages. It is again to be noted here that in the earlier stages (13, 13.5,
 
  
 
14 mm.), the mdividual spaces, plus thek kernel of atrophied vein, are relatively larger than in the later (15-15.5 mm.) embryos. In the earlier stages (13 nrni., 13.5 mm. and 14 mm. embryos) these separate and still isolated segments of the future continuous lymphatic channel begin and end blindly. In the
 
14 mm.), the mdividual spaces, plus thek kernel of atrophied vein, are relatively larger than in the later (15-15.5 mm.) embryos. In the earlier stages (13 nrni., 13.5 mm. and 14 mm. embryos) these separate and still isolated segments of the future continuous lymphatic channel begin and end blindly. In the
Line 2,314: Line 2,160:
  
 
5 Kxtraintimal or perivenous lymphatic space surroumiing degener ating embryonal vein. 8 Oesophagus. 40 Innominate vein.
 
5 Kxtraintimal or perivenous lymphatic space surroumiing degener ating embryonal vein. 8 Oesophagus. 40 Innominate vein.
 
 
 
•J
 
  
  
Line 2,664: Line 2,506:
  
  
 
138 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
  
 
(f)) is larger and still contains a number of red blood cells. In the four succeeding sections, 19, 20, 21 and 22 (figs. 248, 249, 250 and 251) the former line of communication of the decadent venule (4) with the functional azygos channel can still be distinctly traced as a strand of connective tissue cells joining it to the beginning of the right interazygos plexus. The separation of the venous radicle undergoing extraintimal lymphatic replacement has just been accomplished in this area.
 
(f)) is larger and still contains a number of red blood cells. In the four succeeding sections, 19, 20, 21 and 22 (figs. 248, 249, 250 and 251) the former line of communication of the decadent venule (4) with the functional azygos channel can still be distinctly traced as a strand of connective tissue cells joining it to the beginning of the right interazygos plexus. The separation of the venous radicle undergoing extraintimal lymphatic replacement has just been accomplished in this area.
Line 2,673: Line 2,513:
 
Figs. 244 and 245 give two successive sections from the caudal part of the thoracic region in series 34 (slide xxxi, sections 18 and 19, X 225) in which the earlier conditions, preceding the first appearance of the lymphatic anlages of the thoracic duct in the azygos segment, are shown.
 
Figs. 244 and 245 give two successive sections from the caudal part of the thoracic region in series 34 (slide xxxi, sections 18 and 19, X 225) in which the earlier conditions, preceding the first appearance of the lymphatic anlages of the thoracic duct in the azygos segment, are shown.
  
Between the right azygos vein {3) and the aorta (7) are seen
+
Between the right azygos vein {3) and the aorta (7) are seen the components of the ventro-medial azygos plexus {4)f partially separated from the main venous trunks, but not, as yet, surrounded by extraintimal lymphatic anlages, as, further cephalad, '(slide xxviii) in the sections just described, (figs. 246 to 251).
 
 
 
 
 
 
DEVELOPMENT OF THE THORACIC DUCT 139
 
 
 
the components of the ventro-medial azygos plexus {4)f partially separated from the main venous trunks, but not, as yet, surrounded by extraintimal lymphatic anlages, as, further cephalad, '(slide xxviii) in the sections just described, (figs. 246 to 251).
 
  
 
The 15 mm. and 15.5 mm. embryos show, in the majority of instances, an advance over the preceding stage in two directions :
 
The 15 mm. and 15.5 mm. embryos show, in the majority of instances, an advance over the preceding stage in two directions :
Line 2,699: Line 2,533:
 
IV. This topographical rearrangement has forced the ventromedial azygos tributaries, where they still persist {34 in figs. 252, 253, 257), and the thoracic duct imlage (36) to pass from the ventro-medial to the ventral aspect of the main azygos trunks.
 
IV. This topographical rearrangement has forced the ventromedial azygos tributaries, where they still persist {34 in figs. 252, 253, 257), and the thoracic duct imlage (36) to pass from the ventro-medial to the ventral aspect of the main azygos trunks.
  
V. Both azygos veins lie in the region ventro-lcUeral to the intersegmental arteries. The supra-aortic area between them is still
+
V. Both azygos veins lie in the region ventro-lcUeral to the intersegmental arteries. The supra-aortic area between them is still occupied by the interazygos (supra-cardinal) anastomosis (cf. infra, figs. 261 and 262).
 
 
 
 
 
 
140 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
occupied by the interazygos (supra-cardinal) anastomosis (cf. infra, figs. 261 and 262).
 
  
 
VI. The sections taken from the caudal portion of the thoracic region (slides xvii and xviii) show the right and left thoracic duct anlages (36) following the ventral aspect of the respective azygos veins (cf. fig. 192). They are not yet united into a continuous channel, but their lumen is clear, without trace of the venous kernel around which they developed.
 
VI. The sections taken from the caudal portion of the thoracic region (slides xvii and xviii) show the right and left thoracic duct anlages (36) following the ventral aspect of the respective azygos veins (cf. fig. 192). They are not yet united into a continuous channel, but their lumen is clear, without trace of the venous kernel around which they developed.
Line 2,721: Line 2,549:
 
Figs. 257 and 258 (sections 31 and 32 of slide xvii) show on alternating sides small thoracic duct anlages (36), and in the root of the dorsal mesogastrium sections of the well-developed anterior mesenteric lymphatics (61). Some of the later stages (series 253, figs. 265 to 270) will show the full development of these mesenteric channels and their confluence with the thoracic duct. Tliis area, is, however, responsible for the development of what I have above defined as the post-azygos segment of the thoracic duct and will hence be considered in detail in the second volume of this series.
 
Figs. 257 and 258 (sections 31 and 32 of slide xvii) show on alternating sides small thoracic duct anlages (36), and in the root of the dorsal mesogastrium sections of the well-developed anterior mesenteric lymphatics (61). Some of the later stages (series 253, figs. 265 to 270) will show the full development of these mesenteric channels and their confluence with the thoracic duct. Tliis area, is, however, responsible for the development of what I have above defined as the post-azygos segment of the thoracic duct and will hence be considered in detail in the second volume of this series.
  
 
 
DEVELOPMENT OF THE THORACIC DUCT 141
 
  
 
In comparison with the series just considered figs. 259 and 260 show transverse sections of the lower thoracic region of series 258, slide xviii, sections 1 and 9, X 225. This embryo is rated as 17 mm. in crown-rump length, but the lymphatic development of the azygos region Ls relatively retarded and only slightly in advance of the conditions presented by the 15.5 mm, series 143. Direct comparison of figs. 252 to 258 with figs. 259 and 260 will show this correspondence, both at the lev^l of the intersegmental arteries {2 in fig. 260), and in the intervals between them (fig. 259) . In the older embrj'^o the right and left thoracic duct anlages (36) are somewhat larger and form longer channel segments, but otherwise the two series belong practically to the same developmental stage, in spite of their difference in total length measure. The correspondence, for example, between figs. 254 (series 143) and 259 (series 258) is remarkable.
 
In comparison with the series just considered figs. 259 and 260 show transverse sections of the lower thoracic region of series 258, slide xviii, sections 1 and 9, X 225. This embryo is rated as 17 mm. in crown-rump length, but the lymphatic development of the azygos region Ls relatively retarded and only slightly in advance of the conditions presented by the 15.5 mm, series 143. Direct comparison of figs. 252 to 258 with figs. 259 and 260 will show this correspondence, both at the lev^l of the intersegmental arteries {2 in fig. 260), and in the intervals between them (fig. 259) . In the older embrj'^o the right and left thoracic duct anlages (36) are somewhat larger and form longer channel segments, but otherwise the two series belong practically to the same developmental stage, in spite of their difference in total length measure. The correspondence, for example, between figs. 254 (series 143) and 259 (series 258) is remarkable.
Line 2,735: Line 2,560:
 
I. The two azygos veins (S and 6) have become reduced in size. At the level, at which the sections of figs. 261 and 262 are taken, both veins now lie dorsal to the intercostal arteries (2), whereas in the earlier stages (e.g. figs. 260, 258, 257) the azygos trunks are ventral to the arteries, and the interval between the latter, along the dorsal aspect of the aorta, is occupied by the supracardinal interazygos plexus.
 
I. The two azygos veins (S and 6) have become reduced in size. At the level, at which the sections of figs. 261 and 262 are taken, both veins now lie dorsal to the intercostal arteries (2), whereas in the earlier stages (e.g. figs. 260, 258, 257) the azygos trunks are ventral to the arteries, and the interval between the latter, along the dorsal aspect of the aorta, is occupied by the supracardinal interazygos plexus.
  
Reference to the reconstruction of series 96 (fig. 193) shows that this topographical readjustment has occurred in the caudal part of the azygos area, below the level A-B, by the longitudinal hydrostatic definition of the main supracardinal azygos
+
Reference to the reconstruction of series 96 (fig. 193) shows that this topographical readjustment has occurred in the caudal part of the azygos area, below the level A-B, by the longitudinal hydrostatic definition of the main supracardinal azygos trunks through confluence of elements of the preceding interazygos plexus. The original post-cardinal azygos veins, which in series 258 and 143 are seen to form the main channels ventral to the intercostal arteries, are, in the caudal region of series 96, represented by two slender longitudinal trunks, on the lateral aspect of the thoracic ducts between them and the sympathetic nerves (1). The main azygos veins have developed as distinct circumscribed axial chaimels out of the antecedent supracardinal interazygos plexus, dorsal to the intercostal arterial branches (S) . Correlated with this new azygos development is the reduction of the earlier postcardinal channels, ventral to the intersegmental arteries, and their topographical replacement in large part by the right and left thoracic ducts {36) (not labelled in fig. 193). Cephalad of the level A-B the reconstruction shows the earlier relation of the azygos trunks, ventro-lateral to the intercostal arteries. In ascending the left azygos vein gradually diminishes, while conversely the vessel of the right side increases in caliber.
 
 
 
 
 
 
142 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
trunks through confluence of elements of the preceding interazygos plexus. The original post-cardinal azygos veins, which in series 258 and 143 are seen to form the main channels ventral to the intercostal arteries, are, in the caudal region of series 96, represented by two slender longitudinal trunks, on the lateral aspect of the thoracic ducts between them and the sympathetic nerves (1). The main azygos veins have developed as distinct circumscribed axial chaimels out of the antecedent supracardinal interazygos plexus, dorsal to the intercostal arterial branches (S) . Correlated with this new azygos development is the reduction of the earlier postcardinal channels, ventral to the intersegmental arteries, and their topographical replacement in large part by the right and left thoracic ducts {36) (not labelled in fig. 193). Cephalad of the level A-B the reconstruction shows the earlier relation of the azygos trunks, ventro-lateral to the intercostal arteries. In ascending the left azygos vein gradually diminishes, while conversely the vessel of the right side increases in caliber.
 
  
 
II. The individual separate lymphatic anlages of the azygos segment of the thoracic duct of the preceding stages have united into a continuous and uninterrupted lymphatic channel.
 
II. The individual separate lymphatic anlages of the azygos segment of the thoracic duct of the preceding stages have united into a continuous and uninterrupted lymphatic channel.
Line 2,747: Line 2,566:
 
Caudad of the level A-B (fig. 193) the right and left thoracic ducts are fairly symmetrical bilateral channels, connected with each other by several broad and by a larger number of slender transverse anastomoses. They occupy the position of the earlier postcardinal azygos trunks, before these shifted to the supracardinal position, ventro-lateral to the intercostal arteries, along the medial border of the reduced longitudinal tributary channel representing on each side the remnant of the earlier post-cardinal azygos, and draining dorsad into the newly established supracardinal azygos line of each side.
 
Caudad of the level A-B (fig. 193) the right and left thoracic ducts are fairly symmetrical bilateral channels, connected with each other by several broad and by a larger number of slender transverse anastomoses. They occupy the position of the earlier postcardinal azygos trunks, before these shifted to the supracardinal position, ventro-lateral to the intercostal arteries, along the medial border of the reduced longitudinal tributary channel representing on each side the remnant of the earlier post-cardinal azygos, and draining dorsad into the newly established supracardinal azygos line of each side.
  
Figs. 261 and 262 give excellent views of the topographical relations of the vascular structures, arterial, venous and lymphatic. Both sections show the right and left thoracic ducts (36), ventral to the intercostal arteries (2), the lymphatic cross anastomosis between the two along the dorsal circumference of the aorta, and the main supracardinal azygos veins (5, 6) . Cephalad of the level A-B (fig. 193) this plexiform anastomosis between the
+
Figs. 261 and 262 give excellent views of the topographical relations of the vascular structures, arterial, venous and lymphatic. Both sections show the right and left thoracic ducts (36), ventral to the intercostal arteries (2), the lymphatic cross anastomosis between the two along the dorsal circumference of the aorta, and the main supracardinal azygos veins (5, 6) . Cephalad of the level A-B (fig. 193) this plexiform anastomosis between the ducts increases in extent, and the entire lymphatic channel inclines, in ascending, to the left. This, as previously explained (cf. supra, p. 129), is due to the greater development of the intermediate lymphatic line, along the ventral aspect of the interazygos anastomosis, and its union with the left thoracic duct to form the plexiform channel of the adult. The corresponding lymphatic of the right side, on the other hand, becomes reduced above the level A-B, and only its cephalic portion persists as the thoracic segment of the right lymphatic duct (47 in fig. 193).
 
 
 
 
 
 
DEVELOPMENT OP THE THORACIC DUCT 143
 
 
 
ducts increases in extent, and the entire lymphatic channel inclines, in ascending, to the left. This, as previously explained (cf. supra, p. 129), is due to the greater development of the intermediate lymphatic line, along the ventral aspect of the interazygos anastomosis, and its union with the left thoracic duct to form the plexiform channel of the adult. The corresponding lymphatic of the right side, on the other hand, becomes reduced above the level A-B, and only its cephalic portion persists as the thoracic segment of the right lymphatic duct (47 in fig. 193).
 
  
 
The extraintimal lymphatic anlages ventral to the thoracic ducts, which are eventually to establish the coimection between them and the mesenteric lymphatics, are in the process of development in series 96. The venous core (4) and the replacing perivenous lymphatic (5) are shown on the left side in figs. 261 and 262, and equivalent segments of this channel are seen in the reconstruction {61 in fig. 193). In other portions of the ventral and ventro-lateral periaortic mesoderm numerous independent intercellular spaces begonging to the same system are developing.
 
The extraintimal lymphatic anlages ventral to the thoracic ducts, which are eventually to establish the coimection between them and the mesenteric lymphatics, are in the process of development in series 96. The venous core (4) and the replacing perivenous lymphatic (5) are shown on the left side in figs. 261 and 262, and equivalent segments of this channel are seen in the reconstruction {61 in fig. 193). In other portions of the ventral and ventro-lateral periaortic mesoderm numerous independent intercellular spaces begonging to the same system are developing.
Line 2,759: Line 2,572:
 
While the thoracic ducts are in the majority of 16 mm. embryos complete continuous and definite channels, as in the present average example, the acquisition of this final condition is not infrequently delayed considerably beyond this period. I have a number of 16 and 16.5 nmi. embryos in which the thoracic duct development has not advanced beyond the stage typical of the average 14 mm. or 15 nmi. embryo. This is another example of the considerable range of chronological variation of lymphatic development in individual embryos encountered in examining a large number of series of about the same length measure.
 
While the thoracic ducts are in the majority of 16 mm. embryos complete continuous and definite channels, as in the present average example, the acquisition of this final condition is not infrequently delayed considerably beyond this period. I have a number of 16 and 16.5 nmi. embryos in which the thoracic duct development has not advanced beyond the stage typical of the average 14 mm. or 15 nmi. embryo. This is another example of the considerable range of chronological variation of lymphatic development in individual embryos encountered in examining a large number of series of about the same length measure.
  
The 16 mm. embryo again furnishes conclusive evidence as to the secondary character of the mutual relation between embryonic venous and lymphatic vessels, and shows, in typical individuals, the final stage of the process through which temporary venous, channels are replaced by the permanent components of the lymphatic system. This replacement, as demonstrated by the successive series of sections from the azygos region just considered, is a gradual process. The lymphatic channel has its inception in numerous small extraintimal mesenchymal spaces which form
+
The 16 mm. embryo again furnishes conclusive evidence as to the secondary character of the mutual relation between embryonic venous and lymphatic vessels, and shows, in typical individuals, the final stage of the process through which temporary venous, channels are replaced by the permanent components of the lymphatic system. This replacement, as demonstrated by the successive series of sections from the azygos region just considered, is a gradual process. The lymphatic channel has its inception in numerous small extraintimal mesenchymal spaces which form along and around the components of an atrophying embryonic venous plexus, or follow part of the circumference of a more definite early venous trunk. As these spaces increase, enlarge and become confluent to form longer segments of the lymphatic channel, the related venous element continues to recede, imtil, in certain areas, it is entirely replaced by the lymphatic vessel. In other districts, along embryonic veins which are carried into the adult organization, as permanent components of the venous system, the redundant vein of the earlier stages becomes more circumscribed and more clearly defined as the companion lymphatic channel develops in the perivenous area. In either case the complete or partial replacement of venous by lymphatic elements does not occur suddenly, but is accomplished by gradual and progressive ontogenetic stages. I can find no evidence at any point of the separation of portions of a venous plexus or of an already organized venous chaimel, such detached parts then uniting directly with each other to form systemic lymphatic vessels. In every region of the mammalian embryo the above described extraintimal development of the lymphatic elements, and the subsequent secondary topographical replacement of decadent veins by them, can clearly be demonstrated. This process, of course, leads to a stage in which the advancing development of the lymphatic and the correlated regression of the associated venous components are balanced, and in which both, proceeding in opposite genetic directions, are foimd side by side in the same field. The stages previously discussed in detail, between 13 mm. and 15 mm. offer abundant illustration of this intermediate stage in the azygos area.
  
 +
It is to be noted that in the earlier stages (13 to 14 mm.) the lymphatic extraintimal spaces first appear on the lateral aspect of the atrophying venules of the ventro-medial azygos plexus, and subsequently surround them. They are then situated at some distance ventral to the main azygos trunks. In the later stages (15 mm., 15.5 mm. and 16 mm. embrj'os) they have extended dorsad and lie in close apposition to the ventral and ventro-medial circumference of the larger veins, which position they maintain subsequently in their further development. Coincident with this relative change in position of the earlier lymphatic spaces is the appearance of additional thoracic duct anlages, which develop as extraintimal lymphatic elements along those lines of the embryonic azygos system which are destined to undergo, in the later stages, still further reduction and final elimination in the definite organization of the adult condition. [Compare on this point the sections of series 96 (figs. 261 and 262) with those of series 253 (figs. 265 to 270)].
  
 +
One other fact deserves careful consideration in analyzing the development of the azygos segment of the thoracic ducts in stages between 14 mm. and 15 mm. In a certain proportion of the embryos of this period the extraintimal replacement of the earlier ventro-medial azygos plexus in the cephalic part of the future azygos segment of the thoracic duct is delayed. In these cases peculiar histological pictures are obtained in this area. The components of the ventro-medial azygos plexus have become detached from the main veins, and have fused together into an elongated spindle shaped bag, completely filled with blood, beginning and ending blindly and not in commimication with any venous channel. This structure, when found, invariably occupies the site of the long lymphatic segment seen m the reconstruction of series 214 (fig. 190) on the right side, in the areas A and B (slides xiv and xv), ventral to the right intercostal arteries, in the interval between aorta and right azygos vein. The appearance presented by it on section is shown in figs. 263 and 264 in a 14 mm. cat embryo (series 211, slide xii, sections 2 and 8, X 225). It is indicated in these figures by the leader 5^, as a detached and confluent part of the original ventro-medial azygos plexus. Fig. 263 shows this structure in the interval between the intercostal arteries. Fig. 264 gives a typical view of it at the level of one of the right intercostal branches. It lies ventral to the same, between aorta (7) mesad, the right azygos trunk (5) laterad, dorsal to the oesophagus (5). The blood cells, which completely fill its lumen, diflferentiate from those circulating in the continuous vein channels by remarkably intense reaction to the Orange-G stain. As stated, this structure is completely detached from all adjacent veins, can be followed cephalad and caudad through many sections, and ends blindly at either end in a gradually diminishing pointed extremity. In a reconstruction of this embryo it would correspond absolutely in extent and relations to the long lymphatic channel above referred to on the right side of series 214 (fig. 190). A comparison of the reconstruction (fig. 190) and of the corresponding sections (figs. 214 to 240) of this latter embryo, clearly reveal the significance of this, at first sight, somewhat anomalous and puzzling structure. It will be noted that the reconstruction of series 214 (fig. 190) shows, at the level of the line C, the caudal end of a large ventro-medial tributary trunk {34) of the right azygos vein (S), which, in proceeding cephalad, becomes imbedded in the centre of the long extraintimal Isrmphatic channel. The continuation of this venous tributary line, detached from the functional veins, furnishes the central venous core (4) which is surrounded by the extraintimal lymphatic space (6) in all the sections of series 214 shown in figs. 214 to 240 inclusive. If this ventro-medial tributary trunk {34 in fig. 190) had organized in its entire length, by confluence of the radicles of the preceding plexus, and had then become detached from the fimctional right azygos vein, it would, before extraintimal development of the surrounding lymphatic spaces began, present exactly the conditions seen in series 211. In comparing, therefore, these two 14 mm. embryos, we find in series 214 the venous line, around which this important segment of the thoracic duct develops and which it subsequently replaces, present in the condition typical for the average embryo of this period, as a detached and nearly empty venous bag, surrounded by the extraintimal lymphatic channel. On the other hand, series 211 shows the same tributary trunk, also detached from the main vein, but still filled with blood, while the enveloping extraintimal lymphatic anlages have not yet made their appearance. Comparison with the reconstruction and with the sections of series 214 prove its identity with the latter's venous core (4) in the interior of the lymphatic channel (5).
  
144 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
+
A 15 mm. embryo of the Princeton embryological collection (series 53), which I had the opportunity of studying through the courtesy of Professor McClure, shows conditions identical with those just described in series 211. It is interesting and conclusive to further compare in this respect the reconstruction of series 214 (14 mm. fig. 190) with that of series 143 (15.5 mm. fig. 192). In the former embryo the secondary para-azygos channel of the right side (34), developed by confluence of components of the right ventro-medial azygos plexus, only persists in its caudal portion, and has in the rest of its original extent been detached and surrounded by the extraintimal lymphatic channel. On the other hand, in the 15.5 mm. series 143 (fig. 192) this normal process of detachment and replacement by extraintimal lymphatic anlages has been delayed. Fig. 192 shows the para-azygos ventro-medial channel (34) as an extensive tributary trunk of the right azygos vein, cephalad of the level C. Only a few areas of the extraintimal lymphatic development occur, as yet, along its course. Further development of these and their confluence will produce the condition already seen in the earlier embryo, series 214, which is typical for the average embryo of the 14 mm. stage. Thus unusual and atypical delay of lymphatic development occurs in some embryos of this period in the region above specified, and produces a definite and uniform aberrant picture. The correct interpretation of the same is given by the conditions obtaining in the average embryo of this period. I lay some stress on the facts just described, because isolated observation of an instance of this character might readily lead to the erroneous assumption that the detached venous bag (34) of figs. 263 and 264 became directly transformed into the lymphatic channel of the thoracic duct occupying in the later stages the identical topographical position and maintaining the same relations to surrounding structures. This error can only be avoided by determining, in numerous embryos of this period, the typical and average condition of lymphatic development.
 
 
along and around the components of an atrophying embryonic venous plexus, or follow part of the circumference of a more definite early venous trunk. As these spaces increase, enlarge and become confluent to form longer segments of the lymphatic channel, the related venous element continues to recede, imtil, in certain areas, it is entirely replaced by the lymphatic vessel. In other districts, along embryonic veins which are carried into the adult organization, as permanent components of the venous system, the redundant vein of the earlier stages becomes more circumscribed and more clearly defined as the companion lymphatic channel develops in the perivenous area. In either case the complete or partial replacement of venous by lymphatic elements does not occur suddenly, but is accomplished by gradual and progressive ontogenetic stages. I can find no evidence at any point of the separation of portions of a venous plexus or of an already organized venous chaimel, such detached parts then uniting directly with each other to form systemic lymphatic vessels. In every region of the mammalian embryo the above described extraintimal development of the lymphatic elements, and the subsequent secondary topographical replacement of decadent veins by them, can clearly be demonstrated. This process, of course, leads to a stage in which the advancing development of the lymphatic and the correlated regression of the associated venous components are balanced, and in which both, proceeding in opposite genetic directions, are foimd side by side in the same field. The stages previously discussed in detail, between 13 mm. and 15 mm. offer abundant illustration of this intermediate stage in the azygos area.
 
 
 
It is to be noted that in the earlier stages (13 to 14 mm.) the lymphatic extraintimal spaces first appear on the lateral aspect of the atrophying venules of the ventro-medial azygos plexus, and subsequently surround them. They are then situated at some distance ventral to the main azygos trunks. In the later stages (15 mm., 15.5 mm. and 16 mm. embrj'os) they have extended dorsad and lie in close apposition to the ventral and ventro-medial circumference of the larger veins, which position they maintain subsequently in their further development. Coincident with this
 
 
 
 
 
 
 
DEVELOPMENT OF THE THORACIC DUCT 145
 
 
 
relative change in position of the earlier lymphatic spaces is the appearance of additional thoracic duct anlages, which develop as extraintimal lymphatic elements along those lines of the embryonic azygos system which are destined to undergo, in the later stages, still further reduction and final elimination in the definite organization of the adult condition. [Compare on this point the sections of series 96 (figs. 261 and 262) with those of series 253 (figs. 265 to 270)].
 
 
 
One other fact deserves careful consideration in analyzing the development of the azygos segment of the thoracic ducts in stages between 14 mm. and 15 mm. In a certain proportion of the embryos of this period the extraintimal replacement of the earlier ventro-medial azygos plexus in the cephalic part of the future azygos segment of the thoracic duct is delayed. In these cases peculiar histological pictures are obtained in this area. The components of the ventro-medial azygos plexus have become detached from the main veins, and have fused together into an elongated spindle shaped bag, completely filled with blood, beginning and ending blindly and not in commimication with any venous channel. This structure, when found, invariably occupies the site of the long lymphatic segment seen m the reconstruction of series 214 (fig. 190) on the right side, in the areas A and B (slides xiv and xv), ventral to the right intercostal arteries, in the interval between aorta and right azygos vein. The appearance presented by it on section is shown in figs. 263 and 264 in a 14 mm. cat embryo (series 211, slide xii, sections 2 and 8, X 225). It is indicated in these figures by the leader 5^, as a detached and confluent part of the original ventro-medial azygos plexus. Fig. 263 shows this structure in the interval between the intercostal arteries. Fig. 264 gives a typical view of it at the level of one of the right intercostal branches. It lies ventral to the same, between aorta (7) mesad, the right azygos trunk (5) laterad, dorsal to the oesophagus (5). The blood cells, which completely fill its lumen, diflferentiate from those circulating in the continuous vein channels by remarkably intense reaction to the Orange-G stain. As stated, this structure is completely detached from all adjacent veins, can be followed cephalad and caudad through many sec
 
 
 
 
 
146 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
tions, and ends blindly at either end in a gradually diminishing pointed extremity. In a reconstruction of this embryo it would correspond absolutely in extent and relations to the long lymphatic channel above referred to on the right side of series 214 (fig. 190). A comparison of the reconstruction (fig. 190) and of the corresponding sections (figs. 214 to 240) of this latter embryo, clearly reveal the significance of this, at first sight, somewhat anomalous and puzzling structure. It will be noted that the reconstruction of series 214 (fig. 190) shows, at the level of the line C, the caudal end of a large ventro-medial tributary trunk {34) of the right azygos vein (S), which, in proceeding cephalad, becomes imbedded in the centre of the long extraintimal Isrmphatic channel. The continuation of this venous tributary line, detached from the functional veins, furnishes the central venous core (4) which is surrounded by the extraintimal lymphatic space (6) in all the sections of series 214 shown in figs. 214 to 240 inclusive. If this ventro-medial tributary trunk {34 in fig. 190) had organized in its entire length, by confluence of the radicles of the preceding plexus, and had then become detached from the fimctional right azygos vein, it would, before extraintimal development of the surrounding lymphatic spaces began, present exactly the conditions seen in series 211. In comparing, therefore, these two 14 mm. embryos, we find in series 214 the venous line, around which this important segment of the thoracic duct develops and which it subsequently replaces, present in the condition typical for the average embryo of this period, as a detached and nearly empty venous bag, surrounded by the extraintimal lymphatic channel. On the other hand, series 211 shows the same tributary trunk, also detached from the main vein, but still filled with blood, while the enveloping extraintimal lymphatic anlages have not yet made their appearance. Comparison with the reconstruction and with the sections of series 214 prove its identity with the latter's venous core (4) in the interior of the lymphatic channel (5).
 
 
 
A 15 mm. embryo of the Princeton embrj^ological collection (series 53), which I had the opportunity of studying through the courtesy of Professor McClure, shows conditions identical with those just described in series 211. It is interesting and conclusive
 
 
 
 
 
 
 
DEVELOPMENT OF THE THORACIC DUCT 147
 
 
 
to further compare in this respect the reconstruction of series 214 (14 mm. fig. 190) with that of series 143 (15.5 mm. fig. 192). In the former embryo the secondary para-azygos channel of the right side (34), developed by confluence of components of the right ventro-medial azygos plexus, only persists in its caudal portion, and has in the rest of its original extent been detached and surrounded by the extraintimal lymphatic channel. On the other hand, in the 15.5 mm. series 143 (fig. 192) this normal process of detachment and replacement by extraintimal lymphatic anlages has been delayed. Fig. 192 shows the para-azygos ventro-medial channel (34) as an extensive tributary trunk of the right azygos vein, cephalad of the level C. Only a few areas of the extraintimal lymphatic development occur, as yet, along its course. Further development of these and their confluence will produce the condition already seen in the earlier embryo, series 214, which is typical for the average embryo of the 14 mm. stage. Thus unusual and atypical delay of lymphatic development occurs in some embryos of this period in the region above specified, and produces a definite and uniform aberrant picture. The correct interpretation of the same is given by the conditions obtaining in the average embryo of this period. I lay some stress on the facts just described, because isolated observation of an instance of this character might readily lead to the erroneous assumption that the detached venous bag (34) of figs. 263 and 264 became directly transformed into the lymphatic channel of the thoracic duct occupying in the later stages the identical topographical position and maintaining the same relations to surrounding structures. This error can only be avoided by determining, in numerous embryos of this period, the typical and average condition of lymphatic development.
 
  
 
Thus in the average 16 mm. cat embryo the originally separate and independent extraintimal anlages of the two thoracic ducts have become confluent, and form from this stage onward usually a continuous and uninterrupted lymphatic channel, which is connected with the general venous system through the jugular lymph sac, by union with the thoracic duct approach of the latter.
 
Thus in the average 16 mm. cat embryo the originally separate and independent extraintimal anlages of the two thoracic ducts have become confluent, and form from this stage onward usually a continuous and uninterrupted lymphatic channel, which is connected with the general venous system through the jugular lymph sac, by union with the thoracic duct approach of the latter.
  
From this stage forward the subsequent growth and readjust
+
From this stage forward the subsequent growth and readjustment of the thoracic ducts is merely a question of continuous enlargement of the lymphatic channels at the eicpense of the azygos veins. The latter recede still further, while the former correspondingly increase until the adult relative condition of both systems is attained. In this readjustment the thoracic ducts of the adult cat acquire for themselves in large part the topographical position occupied in earlier embryonic stages by portions of the azygos complex. The method of this substitution has been sufficiently and repeatedly detailed in the preceding pages.
 
 
 
 
148 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS
 
 
 
ment of the thoracic ducts is merely a question of continuous enlargement of the lymphatic channels at the eicpense of the azygos veins. The latter recede still further, while the former correspondingly increase until the adult relative condition of both systems is attained. In this readjustment the thoracic ducts of the adult cat acquire for themselves in large part the topographical position occupied in earlier embryonic stages by portions of the azygos complex. The method of this substitution has been sufficiently and repeatedly detailed in the preceding pages.
 
  
 
Figs. 265 to 270 inclusive show the results in transverse sections of the middle thoracic region m a 19 mm. cat embryo (series 253, slide xxiv, sections 9, 11, 15, 16, 17 and 18, X 225).
 
Figs. 265 to 270 inclusive show the results in transverse sections of the middle thoracic region m a 19 mm. cat embryo (series 253, slide xxiv, sections 9, 11, 15, 16, 17 and 18, X 225).
Line 2,834: Line 2,619:
  
 
Reference to the dorsal view of the reconstruction of this embryo (fig. 171) shows that the azygos segment of the thoracic duct (36) has united with part of the caudal division of the preazygos chain of lymphatics (35), cephalad of the interazygos anastomosis, into a continuous lymphatic channel, although the con.tinuity with the main preazygos segment has not yet been completely established. In other words this condition has been very nearly, but not quite, attained. Some of the preazygos elements (36) have already fused with the azygos segment, thus carrying the latter cephalad beyond the interazygos anastomosis into the territory of the preazygos portion of the thoracic duct.
 
Reference to the dorsal view of the reconstruction of this embryo (fig. 171) shows that the azygos segment of the thoracic duct (36) has united with part of the caudal division of the preazygos chain of lymphatics (35), cephalad of the interazygos anastomosis, into a continuous lymphatic channel, although the con.tinuity with the main preazygos segment has not yet been completely established. In other words this condition has been very nearly, but not quite, attained. Some of the preazygos elements (36) have already fused with the azygos segment, thus carrying the latter cephalad beyond the interazygos anastomosis into the territory of the preazygos portion of the thoracic duct.
 
 
  
 
==Part II. Plates==
 
==Part II. Plates==
Line 3,879: Line 3,662:
  
 
The eventual reduction of the left azygos vein, and of the caudal part of the right trunk, by transferance of the blood current to the vertebral chain, accounts for the topographical relations of the replacing thoracic duct complex of the adult.
 
The eventual reduction of the left azygos vein, and of the caudal part of the right trunk, by transferance of the blood current to the vertebral chain, accounts for the topographical relations of the replacing thoracic duct complex of the adult.
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[[Paper - The anatomy and development of the systemic lymphatic vessels in the domestic cat|Huntington1911]]: [[Huntington1911 - 1|Part I. The development of the systemic lymphatic vessels in their relation to the blood-vascular system]] | [[Huntington1911 - 2|Part II. The development of the preazygos and azygos segments of the thoracic ducts]] | [[Huntington1911 - Bibliography|Bibliography]]
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{{Ref-Huntington1911}}
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Huntington GS. The anatomy and development of the systemic lymphatic vessels in the domestic cat. (1911) Memoirs of the Wistar Institute of Anatomy and Biology No. 1.

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Huntington 1911: Part I. The development of the systemic lymphatic vessels in their relation to the blood-vascular system | Part II. The development of the preazygos and azygos segments of the thoracic ducts | Bibliography

Part II. The development of the preazygos and azygos segments of the thoracic ducts

The details of the adult organization of the thoracic ducts, and the genesis of the observed adult variations on the basis of the development of the embryonic channels, will be considered subsequently (Part V). The present conmiunication is confined to the consideration of the development of the preazygos and azygos segments of the ducts. The thoracic ducts develop in cat embryos between 11 and 16 mm. crown rump measure. In the 16 mm. embryo, McClure ^'• states that they ^4or the first time make their appearance as definite, continuous lymph channels, which are disconnected from the venous system, except through the mediation of the jugular lymph sacs.'^ This is the case in the majority of 16 mm. embryos, but in a number of 15, 15 5 and 16 mm. series in my collection the final union of the individual segments has not yet taken place. It is possible, by combining a large number of observations on numerous embryos of the same length, to establish an average developmental condition of the thoracic ducts for each stage in embryos between 12 mm. and 17 mm. crown-rump measure. Individual embryos will, however, show marked diflfereoces in development, and many departures from the standard plan are encountered. The same conditions obtain here which McClure and I found in our study of the development of the jugular lymph sacs. Some embryos show, for their length, unusually advanced thoracic duct development in certain areas, while in other parts this development is relatively retarded and more primitive. In other series the entire region of thoracic duct development appears more uniformly ahead of the average condition, normally encountered in embryos of this period. Finally, in a third group, the appearance of the typical lymphatic development seems to be delayed until a later stage than is normally the case. But in spite of these individual differences, a uniform and consistent ground plan of thoracic duct development can be demonstrated, to which the majority of embryos conform and into which even the aberrant types fit. As a matter of fact, the regularity with which the thoracic duct anlages first appear in the majority of the individuals of the proper stage, and the uniform and constant character of the resulting histological pictures, is one of the most striking and significant features of the entire process. Prior to the 11 mm. stage no definite anlage of the anterior segments of the future ducts is observable.

I believe that the adult thoracic ducts of the cat are developed by fusion of three distinct and separate regional segments. Each of these segments is in turn formed by confluence of a number of originally discrete and independent anlages, which develop independently of the venous system as extra-intimal or perivenous mesenchymal spaces in the sense previously defined. ^*' '» -'•

^' These spaces are applied to, or surround, the walls of the embryonic veins of the lower cervical and of the dorsal mediastinal region. The three main divisions, thus developed independently of the venous system, unite with each other to form the channels of the left and right thoracic ducts, and these usually gain their point of entrance into the systemic veins by uniting with a process of the jugular lymph sacs ("thoracic duct approach") derived from their dorsal aspect, just cephalad to the common jugular approach. Exceptionally the main (left) thoracic duct, by combining with the ventral mediastinal trunk presently to be described, unites (fig. 98) with the ventral process of the "subclavian approach, prolonged from the jugular lymph sac over the ventral surface of the jugulo-subclavian confluence, and thus establishes its connection with the venous system (cf. pp. 84 89, figs. 89 to 99).

The ontogenetic history of the ducts may therefore (fig. 29) be considered under four headings, viz. :

1. The THORACIC duct approach of the jugular lymph sac, forming the terminal of the adult duct on each side (fig. 29, 1, blue) .

2. The preaztgos segment (fig. 29, II). This includes two distinct and separate channels:

A. The bronchO'Tnediastinal or ventral mediastinal lymphatic trunk (fig. 29, 37, green) , which drains the ventral mediastinum cephalad of the pericardium, viz., the pericardial, tracheal, bronchial, lateral oesophageal, and thymic areas.

This lymphatic channel, associated with the embryonic venous plexuses of the pretracheal region, the pulmonary vessels and the vagi, develops through confluence of a large number of separate and independent extra-intimal lymphatic spaces following and surrounding the embryonic venous plexuses of the ventral mediastinum. The chain formed by these spaces eventually unites with the similar chain forming the anlages of the preazygos segment of the thoracic duct.

B. The preazygos segment of the main thoracic duct (fig. 29, 55, brown). This portion of the main duct extends from the point of its entrance into the jugular lymph sac, through the thoracic duct approach of the latter, caudad to its intersection with the dorsal surface of the aortic arch.

In the adult animal this segment forms the relatively long portion which ascends cephalo-sinistrad from the point where the duct parts company with the right azygos vein, to its junction with the jugular lymph sac, under cover of the aortic arch, and the vertical portion of the left subclavian artery, dorsal to the vertebral vein and to the left innominate confluence. In this part of its course the thoracic duct frequently receives the lymphatic return from the ventral mediastinum through channels which unite it to the ventral mediastinal trunk as just defined. The preazygos segment of the main duct is again formed in the embryo by confluence of independent mesenchymal spaces around and along the prevertebral and dorsal mediastinal venous plexuses of the embryo.

3. The azygos segment (fig. 29, III, 36, yellow) comprises the portions of the thoracic ducts caudal to the level of the aortic arch. It develops, again independently, as the result of fusion of a number of extraintimal mesenchymal spaces closely applied to the ventral surface of the azygos veins, or of their ventromedial tributaries, or surrounding the latter.

4. The post- azygos segment (fig. 29, IV, 52, green), through which the thoracic ducts establish their connection with the Receptaculum and the system of the abdominal lymphatics.


Inasmuch as the development of this segment of both ducts is intimately connected with that of the principal abdominal lymphatic channels, its consideration will be included in that of the latter system (Part III). The present communication deals only with the first three segments above enumerated.

Fig. 29 shows in a color schema the genetic composition of the thoracic ducts according to the segments just outlined, and their relation to the embryonic venous system.

In proceeding caudo-cephalad the post-azygos portion (IV) develops in the larger right segment (52) by replacement of the caudal part of the ventro-medial tributary plexus of the right azygos vein.

The smaller channel of the left side replaces the caudal part of the left azygos vein. In the azygos area (III) the left channel follows, as a much reduced left thoracic duct, the prevertebral portion of the left azygos vein. The larger right lymphatic channel {36), gradually inclining cephalo-sinistrad, develops along the ventral aspect of the inter-azygos cross-anastomosis and unites with the left duct eventually in a close plexiform interlacement perforated by the intersegmental arteries, which continues cephalad to near the level of the aortic arch. Here the plexus condenses into two more or less well defined channels which turn up to the right and left into the preazygos segment (II). The smaller right channel follows, cephalad of the right azygos terminal, the dorso-medial aspect of the right precaval, innominate and common jugular veins, developing by confluence of extraintimal spaces along the course of these vessels (ef . reconstruction of series 88, figs. 27 and 28), and forms the cephalic end of the reduced right thoracic duct of the adult. It terminates by joining the thoracic duct approach (12) of the right jugular lymph sac. The left and larger trunk replaces the extra-pericardial portion of the left precava, and ascends dorsal to the left innominate confluence to its junction with the thoracic duct approach {12) of the left jugular lymph sac. 37 in fig. 29 indicates* the broncho-mediastinal trunk in one of its commonly encountered types, joining caudally the preazygos portion of the thoracic duct (35), and terminating cephalad in the ventral

process of the subclavian approach (14) of the jugular lymph sac, m company with the mam lymphatic channels retummg from the anterior extremity.

1. The Thoracic Duct Approach Of The Jugular Lymph Sacs

(FIG. 29, 12)

This Structure is an integral part of the jugular lymph sacs and forms in the adult animal the portal of entry of the thoracic ducts into the lymph sacs, and through them into the systemic veins. McClure, in the paper quoted ^^^^' has described its general formation and relation to the jugular lymph sac and has given an excellent illustration of its appearance in the dorsal view of a reconstruction of a 14 mm. cat embryo. I have here defined it as the "thoracic duct approach" of the sacs, in conformity with the nomenclature which McClure and I adopted in describing ^^s, 27; n^^ development of those portions of the jugular sac which, forming the "jugular" and "subclavian" "approaches," project as blind processes from the caudal end of the sac, and are destined to effect subsequently the permanent lymphatico-venous connections at either the common jugular, or the jugulo-subclavian confluence, or at both of these venous angles. The term, as here used in describing the similar process of the sac which is to establish definite connections with the thoracic duct, is intended to emphasize the fact that the thoracic approach" of the lymph sac is, like the rest of the structure from which it proceeds, verums in origin, while the thoracic ducts proper, with which it secondarily unites, are not derived from the veins, but are, from their beginning, independent of the bloodrvascular channels, and develop by confluence of independent extra- or perivenoiLs mesenchymal spaces.

This fundamental difference in origin is shared, m my ophiion, by all the other main systemic lymphatics which, in addition to the thoracic ducts, enter the jugular sacs as the subclavian, external jugular, cephalic and internal jugular lymphatics through the processes which the sac sends to meet them.

The thoracic duct approach of the earlier embryos arises as a short, blunt, curved, conical process from the dorso-medial aspect of the sac, a short distance cephalad of the jugular approach and of the common jugular venous confluence.

It extends meso-caudad across the dorso-lateral and dorsal aspects of the mtemal and common jugular veins between them and the thyro-cervical artery, towards the vagus-sympathetic strand, dipping ventro-mesad between the main vein and the nerves and ending blindly in this situation.

The process makes its first distinct appearance, after the jugular sac has been fully established, in embryos of between 11 mm. and 14 mm. crown-rump measure.

It is noteworthy that individual embryos within these limits show considerable diflference in the degree of development of the process. In some it appears earlier, in others* a little later, so that in individual instances an embryo of slightly shorter measure may have the thoracic duct approach of the jugular lymph sac further developed than one which exceeds the first in total length by the fraction of a millimeter.

Thus one of the earUest distinct stages of the process is seen in a 14 nam. embryo (series 210, slide viii, sections 39-42, figs.

30 to 33 inclusive).

In this embryo, the thoracic duct approach appears as a short blunt process which projects dorso-mesad from the caudal end of the left jugular lymph sac, a short distance cephalad of the common jugular confluence. Sections 39, 40 and 41 (figs. 30,

31 and 32, 12) show the earliest stage in the development of this process. In them the area dorsal to the main vein, between the sympathetic nerve and the jugular lymph sac, is occupied, in the direction meso-laterad, by the sympathetic nerve (/), the termination of the large dorso-meJial venous tributary (left sup. intercostal vein) (/6), the thyro-cervical artery {S4), and, laterad to this, by the blind terminal portion of the thoracic duct approach (/^), derived from the dorso-medial aspect of the jugular lymph sac (//). In this embryo the process of the sac ends blindly laterad to the thyro-cervical arter>^ (figs. 33, 34, ^4). It has not yet extended meso-caudad across the dorsal surface of the main vein toward the interval between the thyro-cervical artery and the sympathetic nerve. The thyro-cervical artery (24) lies at a little distance directly dorsad of the internal jugular vein. The dorso-medial angle of the main vein is related to a large compound medial venous tributary, draining the dorsomedial (prevertebral), (17) and the dorsal somatic areas (16). This compound vessel is the anlage of the future superior intercostal vein. In fig. 33 (section 42) the common trunk opens into the main vein just dorsal to the sympathetic nerve (1). In this section, the blunt medial extremity of the thoracic duct approach (12) is separated from the portion still connected with the jugular lymph sac, thus expressing the caudo-mesal curve of the entire process. It ends blindly in the following section, and does not reappear in the succeeding sections. The common jugular confluence occurs in section 44 of this slide.

We are dealing, therefore, with a short, blunt conical process of the left jugular lymph sac, which curves meso-caudad for two sections, and terminates blindly dorsal to the main vein.

In another 14 nun. embryo (series 212) the thoracic duct approach is much more extensively developed and occupies sections 11 to 26 of slide ix. Selected sections of this series are shown hi figs. 34 to 42 inclusive.

Fig. 34 (series 212, slide ix, sec. 14) shows part of the caudal end of the left jugular lymph sac (11) with the thoracic duct approach (12) arising from its medial aspect and extending mesad across the dorsal surface of the internal jugular vein (25). On the mesal aspect of the vein are seen the sympathetic (1) and the vagus (22).

The large common trunk formed by the union of the dorsal somatic and prevertebral venous tributaries {16, 17) lies dorsomedial to the main jugular channel. It joins the latter, as left superior intercostal vein, {16, 46f) in section 26 of this slide (fig. 41). On the lateral aspect of this vessel is the ascending dorsal branch of the thyro-cervical artery {24).

The six succeeding figures (35 to 40 inclusive) are cut down so as to only show the dorsal circumference of the internal jugular vein and the superincumbent thoracic duct approach of the jugular lymph sac.

In fig. 35 (section 15) the approach has extended further mesad across the main vein. In fig. 36 (section 16) a blind terminal protrusion (12) separates from the main lumen of the sac (H), indicating the frequent bifid character of the thoracic duct approach. The succeeding sections (18 and 20, figs. 37 and 38) hence show an apparent recession of the entire process. In section 22 (fig. 39) the approach has again extended mesad toward the thyro-cervical artery and the superior intercostal vein. In the succeeding section (fig. 40) the fundus of the process terminates for the second time in a blind pouch-like protrusion (12), In the following section (24, fig. 41) -the approach (12) again appears fully developed and in open connection with the main jugular lymph sac (//). It maintains the character in section 25, and in section 26 (fig. 42) the terminal of this second protrusion (12) ends blindly. It does not reappear in the succeeding sections.

We are dealing here with an instance of very common occurrence, in which the thoracic duct approach of these earlier stages not only gives off a number of blind terminal diverticula, but in which the entire approach arises by two or three distinct and separate origins from the main jugular lymph sac. This character will again be discussed below (cf. p. 66). in connection with the interpretation of certain adult conditions, but the instance now under discussion shows the early type of this variation very clearly, so that in a graphic reconstruction the thoracic duct approach of this embryo would appear as in figure on page 64.

This embryo therefore presents for its age an unusually long and well developed thoracic duct approach, with triple terminal pouches, extending through sixteen sections, while at the same time it remains throughout this extensive course in the early position, viz.: lateral to the thyro-cervical artery (24) and the conmion trunk of the dorsal and dorso-medial tributaries of the internal jugular vein (16, 17, 46). The approach has not as yet arched mesad into the neuro-venous recess, and is hence in marked contrast to the succeeding stages presently to be described.

In a 13.5 mm. embryo, series 189, the development of the thoracic duct approach has gone much further, although the embryo, a litter mate of series 210, measures half a millimeter less.

The thoracic duct approach of the left jugular lymph sac occupies sections 10 to 23 of slide vii in this embryo, and is shown in the dorsal view of the reconstruction, fig. 89, to which the individual sections following are referred for orientation.

The process arises in the usual situation from the dorso-medial aspect of the left jugular lymph sac and trends dorso-mesad across the internal jugular vein (series 189, sUde vii, section 10, fig. 43). In reference to the level of the common jugular confluence, which occurs in slide vii, section 26, the thoracic duct approach in series 189 arises more anteriorly from the jugular lymph sac than in the preceding series 210. Its extent and course can be followed accurately in the successive sections shown in figs. 43 to 55 inclusive (series 189, slide vii, sections 10 to 22 inclusive).


In section 11 (fig. 44) the thoracic duct approach is separated by a thin endothelial partition from the lumen of the main jugular lymph sac, which it rejoins in the following section (section 12, fig. 45), only to again separate in the next section (section 13, fig. 46). In the meanwhile, the thoracic duct approach has arched from the lateral to the medial side of the internal jugular vein {25), closely applied to its dorsal surface, between the same and the thyro-cervical artery (24) j and its blind terminal is beginning to bend ventro-mesad into the neuro-venous recess between the vagus-sympathetic strand {1 , 22) and the internal jugular vein {26). In the next three sections (sections 14, 15, 16, figs. 47, 48 and 49) the thoracic duct approach becomes slightly dilated and diverticular (sections 15 and 16, figs. 48 and 49), but remains detached from the jugular sac. In section 15 (fig. 48) it appears subdivided, by endothelial partition, into two, and in section 16 (fig. 49) into three components. The protrusion which the common jugular confluence sends towards the jugular approach of the lymph sac and which forms the point of the subsequent invagination of the venous angle by the lymphatic tap, begins to appear in section 16 (fig. 49), and can be traced through the succeeding sections as a derivative of the lateral aspect of the main internal jugular vein, between it and the median surface of the jugular lymphatic sac.


In the six following sections, 17 to 22 inclusive, (figs. 50 to 55 inclusive) the proximal end of the process, connected with the sac, gradually recedes, while the blind distal end {12') conti. ues to develop in the ventro-medial direction between the main vein {25, in fig. 55) and the dorsal and dorso-medial venous tributaries {16 and 17, fig. 55), dipping ventrad and approaching the interval between the internal jugular vein {25) and the sympathetic nerve (i), the site of its subsequent junction with the independently developed perivenous lymphatics of the ventral mediastinal and dorsal prevertebral regions, which together constitute the anlages of this cephalic portion of the future thoracic duct, (cf. series 143, slides x and xi, figs. 172 to 183). The thoracic duct approach ends blindly in section 26 of slide vii. The course and the relations to surrounding structures of the thoracic duct approach of this embryo are shov^rn in the dorsal view of the reconstruction (fig. 89), containing the 50 sections of slide vii in a magnification of 225 diameters.


This view of the reconstruction presents from the lateral to the mesal border the following structures.

  1. Caudal end of left jugular lymph sac, with the jugular (13) and subclavian {14) approaches, between which passes the ventral branch of the thyro-cervical artery (24) The fifth spinal nerve (54) penetrates the lymph sac, while the sixth nerve lies on its lateral aspect with the primitive ulnar veno-lymphatic arching over it to enter the lymph sac.
  2. The dorsal aspect of the internal jugular vein (25).
  3. The main trunk and branches of the thryo-cervical artery (24) .
  4. The left superior intercostal vein (46), with the dorsal {16) and dorso-medial or prevertebral {17) tributaries, enters thedorsomedial angle of the internal jugular vein between the thyrocervical artery {24) and
  5. the double strand of the vagus {22) and sympathetic (/) nerves.
  6. The thymus {21).
  7. The aortic arch (7), with the left carotid artery {23) and the origin of the innominate artery {45).


The thoracic duct approach {12) arises by two branches from the dorso-medial aspect of the sac, turns caudo-mesad between the internal jugular vein and the ascending branch of the thyrocervical artery, in front of the left superior intercostal vein, and approaches the recess formed between the main vein and the bundle of the nerves.

The reduplication of the origin of the thoracic duct approach from the jugular lymph sac present in this embryo is not unusual, and leads to the type of divided termination of the thoracic duct in the adult shown in fig. 56. The left jugular lymph sac (S) connects with the venous system at the common jugular angle {2) by a larger ventral and smaller dorsal jugular approach {18). It also taps at the jugulo-subclavian angle {19). The thoracic duct enters the dorsal aspect of the sac by a divided terminal, the lower branch connecting with the subclavian approach.


Finally an intermediate condition is seen in another 14 mm. embryo, series 122, of which selected sections are appended, from slides ix and x (figs. 57 to 62). Section 21 of slide ix (fig. 57) is taken at the level where the jugular lymph sac has divided for the passage of the thyro-cervical artery (^34) into its two main terminal components, viz., the dorsally placed subclavian approach {14)j and the ventrally situated jugular approach (13), The latter appears in the interval between the internal jugular vein {25) and the jugulo-cephalic trunk (29), These veins are approaching each other to form the common jugular confluence (fig. 39, 26), at which point the jugular approach usually makes the secondar^-^ permanent lymphatico-venous tap. The thoracic duet approach {12) arises from the dorsal aspect of the jugular approach l.y a relatively narrow pedicle and then hooks, as an elongated and expanded curved pouch, ventro-mesad over the dorsal aspect of the internal jugular vein towards the neurovenous recess.

In section 28 of the same slide (fig. 58) the internal jugular and jugulo-cephalic veins have met in the coimnon jugular confluence {26} which includes the jugular approach. Consequently the latter structure only appears in its ventral prolongation which is seen occupying the ventral angle of the conamon jugular junction (not labelled in fig. 58).

The subclavian approach {14) has moved relatively nearer to the dorsal surface of the main vein channel.

The blind terminal end of the thoracic duct approach {12) is seen dorsal to the common jugular vein, between a large dorsomedial tributary of the latter {16) mesally, and the thyro-cervical artery {24) laterally. Its pointed extremity is directed further ventro-mesad toward the interval between the sympathetic nerve (/) and the medial surface of the common jugular vein {26).

In section 7 of slide x (fig. 59) the terminal of the thoracic duct approach {12) has relatively receded and lies in the narrow interval between thyro-cervical artery (24) and common jugular vein {26),

In fig. 60 (section 9 of slide x) a large dorso-medial venous tributary {16, left superior intercostal vein) empties into the dorso-medial angle of the common jugular trunk. The thoracic duct approach {12), occupying the same relative position as in the preceding figure, lies between this tributary and the thyro-cervical artery, dorsal to the main vein. Section 12 of slide x (fig. 61) shows the distal reduced end of the approach in this same situation. The subclavian approach of the jugular lymph sac (14) is reduced to a narrow longitudinal cleft on the dorso-lateral aspect of the main vein. This reduction is coincident with the derivation from the dorsal aspect of the subclavian approach of the primitive ulnar veno-lymphatic (31). This channel arches dorsad over the Vlth spinal nerve (30), and appears in this section in its typical relation to this nerve. Finally in the succeeding section, 13 of slide ix, (fig. 62), the thoracic duct approach ends blindly. The structures shown in the section dorsal to the common jugular trunk are now encountered in the following order: mesally the terminal of one of the branches of the dorsomedial venous plexus (16) ; next the thyro-cervical artery (24) ; then the Vlth segmental nerve (30), and, dorsal of this, the primitive ulnar veno-lymphatic (31),


The series of sections of this embryo shown here are interesting not only in respect to the behavior of the thoracix; duct approach, but also in reference to the mutual relations of this structure and the terminals of the dorso-medial tributary venous plexus. These relations are to be discussed subsequently in considering, as whole, the evidence upon which some investigators have based their conception of direct venous origin of the lymphatic system by fusion of a number of separate elements detached from the main venous trunks (cf. infra p. 153.)


The variable appearance of the thoracic duct approach in the younger stages is perhaps best illustrated by the exaggerated development presented in a 13 mm. embryo (series 92), in which the process occupies the last thirteen sections of slide x, and the first fourteen sections of slide ix. Selected microphotographs of this series are shown in figs. 63 to 78. The embryo presents the largest and most dilated thoracic duct approach of any in my collection. Fig. 63 gives a topographical view of the region involved. The succeeding sections are cut down and confined to the immediate neighborhood of the thoracic duct approach. Fig. 63 shows the sections of the internal jugular vein {25) and jugulo-cephalic trunk {29), approaching their point of union in the common jugular confluence. A large process of the jugular lymph sac {IS) extends, as the jugular approach, into the interval between the main veins. Along the mesal aspect of the internal jugular vein {25) are placed the sympathetic (i), the vagus {22), the left common carotid artery {23), and, further ventrad, the thymus {21 ) . The dorsal portion of the sac is about to form the subclavian approach {11, 14) In fig. 64, the jugular approach {13) has entered more deeply into the narrowing interval between the internal jugular {26) and jugulo-cephalic trunks {29), and the sharply marked constriction between it and the remainder of the sac (subclavian approach, 14) foreshadows its coming separation from the latter. A protrusion of the dorso-medial angle of the jugular approach, adjacent to the internal jugular vein, indicates the beginning derivation of the thoracic duct approach {12).


In the succeeding section (fig. 65) the jugular approach {IS) has separated from the subclavian approach {14) of the jugular lymph sac and thus cleared the path for the passage of the thyrocervical artery {24) between the two cardinal divisions of the lymph sac, whose caudal terminations are represented by the jugular and subclavian approaches. The origin of the thoracic duct approach {12) from the dorso-medial angle of the jugular approach -is now defined.

In the succeeding section (fig. 66) the triple termination of the jugular lymph sac, in the subclavian (i^), jugular {IS) and thoracic duct approaches {12), is accomplished, and the latter is beginning to extend mesad across the dorsal surface of the internal jugular vein. The main thyro-cervical artery is approaching the junction with its secondary dorsal branches.

In fig. 67 the jugular approach {IS) has divided into the intermediate part between the internal {25) and jugulo-cephalic {29) veins, which is to establish the tap at the common jugular angle, and a ventral process which extends from the tap caudad over the ventral surface of the common jugular angle and vein.

The thoracic duct approach has enlarged, is partially subdivided (cf. p. 65, figs. 48 and 49, series 189) and its blind terminal extremity is reaching ventro-mesad into the angle between internal jugular vein and sympathetic (i). The thyro-cervical artery (£4) lies between the subclavian and thoracic duct approaches, dorsal to the jugulo-cephalic trunk (29).

In fig. 68 the thoracic duct approach has further enlarged and is still subdivided into two compartments the terminal of which abuts directly against the sympathetic nerve. The jugular approach (13) communicates in this section with the internal jugular vein, while a narrower slit-like division continues further caudad toward the common jugular angle.

In fig. 69 the internal (25) and jugulo-cephalic (29) trunks are only separated from each other by a narrow septum containing the lumeYi of the slit-like prolongation of the jugular approach. Both in this and in the preceding section the ventral process of the jugular approach lies in the angle between the two veins, closely applied to their ventral surface.

The thoracic duct approach is a large sau.sage-shaped structure, curved over the dorsal surface of the internal jugular vein. The septum in its interior has disappeared and the process presents a clear lumen.

In fig. 70 (slide ix, section 2), the confluence of internal jugular and jugulo-cephaUc trunks has taken place (26). The thoracic duct approach (12) has extended both ventro-mesad into an area of indifferently defined mesodermal spaces, mesad to the sympathetic nerve (1), and laterad- toward the subclavian approach of the jugular sac (14)j passing between common jugular vein (26) and thyro-cervical artery (24) In section 5 of slide ix (fig. 71) the thoracic duct approach (12) appears as a long curved multilocular channel, which laterad reaches the subclavian approach of the jugular lymph sac, between the thryo-cervical arterj^ (24) and common jugular vein (26)j while its blind mesal extremity terminates just dorsal to the sympathetic-vagus strand. In the following section (fig. 72) the thoracic duct approach (12) connectswith the subclavian approach (14) of the jugular lymph sac affording an instance of the genesis of the reduplicated or multiple thoracic duct terminations occasionallj' encountered in the adult as above described (cf.p. GH, fig. 56, series 212, slide ix, figs. 34 to 42).

In fig. 73 (series 92, slide ix, section 9) the subclavian approach [14) is much reduced, and lies along the lateral aspect of the common jugular vein (26). Its connection with the thoracic duct approach (12) is plainlj' visible, passing between the main vein (26) and the thyro-cervical arterj' (24)- Dorsal somatic and prevertebral venous tributaries (16), approaching the dorsomedial angle of the common jugular vein (26), intersect the course of the thoracic duct approach, and begin to divide the same into a medial and a lateral element. This division is completed in the following section (fig. 74) in which the united dorsal and prevertebral tributaries form a venous trunk of considerable size (16) which is approaching its confluence with the comimon jugular vein by passing between the lateral and medial components of the thoracic duct approach. In other words, the appearance of the thoracic duct approach in this and in the following sections is the expression of the ventro-mesal arched course of this structure across the fork or angle between the common jugular vein (26) and the terminal of its ventro-medial tributary plexus (16). The reconstructions shown in figs. 89, 90 and 91 indicate this topographical relationship clearly. The blind mesal end of the arched process (12) now lies mesal to the tributary vein (16) and dorsal to the sympathetic nerve (7). In the following section (section 11 of slide ix, fig. 75), the dorso-medial venous tributary (16) is elongated and close to the main vessel, while the reduced sections of the thoracic duct approach lie on either side of the same, as indicated by the forked leader (12), The subclavian approach (14)y still prominent and connected with the medial section of the thoracic duct approach in the preceding section 10 (fig. 74), appears now in section 11 reduced to a detached small blind end (not labelled in the figure), close to the lateral wall of the main vein and ventral to the thyro-cervical artery (24)- In the succeeding section 12 (fig. 76) the dorso-medial venous tributary (superior intercostal vein) enters the common jugular trunk. The lateral portion of the thoracic duct approach (not labelled in the figure) is seen in the deep recess between this entrance, the main vein {S6) and the thyro-cervical artery (24) . The mesal end of the thoracic duct approach terminates blindly in this section just dorsal to the sympathetic nerve (1). In section 13 (fig. 77) the lateral terminal of the thoracic duct approach (IS) can still be clearly seen in the interval between the wide dorsal opening of the combined somatic and prevertebral (17) tributary stem into the main vein (26) and the thyro-cervical artery {S4) . It appears very much reduced in the same situation in section 14 (fig. 78), and ends blindly in the following section. The last three sections of this series (figs, 76, 77 and 78) show the beginning of the jugulo-subclavian confluence.


In the later stages the thoracic duct approach has extended further ventro-mesad into the recess between the common jugular vein and the sympathetic nerve, and in this situation it eventually makes its connection with the two independently developed lymphatic channels of the anterior mediastinal region, viz., the preazygos segment of the thoracic duct dorsally, and the bronchomediastinal trunk ventrally. The mode of this union is discussed below under a separate heading (cf. infra, p. 111). Figs. 79 to 82 show transverse sections of the left side of the lower cervical region in a 15 mm. cat embryo (series 245, slide xi, sections 24 to 27, X 225), and fig. 90 gives the dorsal view of the reconstruction on the same scale of the thoracic duct approach and the related structures in the same embryo (slide xi, sections 1-44). In fig. 79 (sUde xi, section 24) the terminal extremity of the thoracic duct approach (12) is seen between the common jugular vein {S6) and the sympathetic nerve (1), having reached this situation by extending ventro-mesad across the dorsal tributary 16 (left superior intercostal vein), between this vessel and the main venous trunk.

The section is taken below the common jugular confluence, consequently the jugular approach is no longer seen, and the blind end of the thoracic duct approach appears isolated. The only other portion of the jugular lymph sac carried caudad to this level is the subclavian approach (14), seen on the lateral aspect of the main vein. The sequence of structures now successively encountered along the mesal and dorsal aspects of the main common jugular vein (26) are the following, proceeding meso-laterad :

  1. Thymus (21 ) , with sub- and suprathymic venous tributaries entering the main vein.
  2. Left common carotid artery (23),
  3. Vagus nerve (22).
  4. Sympathetic nerve (i).
  5. Thoracic duct approach (12),
  6. Dorsal somatic tributary of main vein (16). (Anlage of left superior intercostal vein).
  7. Thyro-cervical artery (24) 8. Subclavian approach of jugular lymph sac (14), on the dorso-lateral aspect of the common jugular vein.


This order and relationship is maintained uniformly and clearly in the three succeeding sections shown in f gs. 80, 81 and 82.

In the last of these the dorsal venous tributary {16) enters the main common jugular channel.

The sections can be easily followed and oriented by reference to the dorsal view of the reconstruction shown in fig. 90. The same is composed of the plates of all 44 sections of slideix of series 245. In this embryo the thoracic duct approach has already established its definite and permanent connection with the independently developed channel of the preazygos segment of the thoracic duct {36, in fig. 90), as described in detail below (cf. p. 111). The point of this junction is indicated in the reconstruction by the narrowing of the two spindles 12 and 35 (thoracic duct approach and preazygos segment of thoracic duct) to form an isthmus situated just cephalad of the leader marked 16-46,

The reconstruction includes the terminal of the jugular approach {13), with the origin from the same of the thoracic duct approach {12), and the entire subclavian approach {14) of the jugular lymph sac. The ventral division of the thyro cervical artery {24) passes forward between the jugular and subclavian approaches of the lymph sac, accompanied by a vein (not labelled in the figure) which is constantly found in this situation in the adult, draining from the prevertebral area into the common jugular trunk.


The dorsal and dorso-medial venous tributary plexus has united into the stem of the left superior intercostal vein (16-46). The reconstruction shows well the caudo-mesal turn of the thoracic duct approach (12) in the fork between the main vein (26) and the superior intercostal (16-46), accompanied by the mesal branch of the thyro-cervical artery. The terminal of the thoracic duct approach (IZ) meets and unites with the preazygos segment of the thoracic duct (35) on the mesal shoulder of the superior intercostal vein terminal, between it and the sympathetic nerve (1). This is one of the two types of topographical relationship encountered (cf. infra pp. 75, 76). Further laterad follow the vagus (22) and the sympathetic strand (1), intersecting at an acute angle. Then the left common carotid artery (23) and the thymus (21) with the perithymic venous plexuses.

Figs. 83 to 88 show the caudal end of the left jugular lymph sac and the thoracic duct approach in a selected series of sections from a 17 mm. cat embryo (series 142), all magnified 225 diameters, and fig. 91 gives the dorsal view of the reconstruction of this region from the same embryo.

These preparations are introduced here for the purpose of illustrating, in comparison with the preceding series (245) the second type of relationship which the thoracic duct approach may bear to the combined vagus-sympathetic strand.

Fig, 83 (series 142, sUde x, section 13) shows the transection of the caudal end of the left jugular lymph sac in the interval between the internal jugular (25) and jugulo-cephalic (29) veins which are approaching their confluence to form the common jugular trunk.

The form of the jugular lymph sac already indicates its coming division into the three terminal approaches," the jugular approach (14), the subclavian approach (IS) and the thoracic duct approach (12). The latter covers the entire dorsal circumference of the internal jugular vein (25) and its termination points to the angular recess between the sympathetic (1) and vagus (22). Ventrad to the latter the field includes the thymus (21). The thyro-cervical artery is cut in several of its divisions, two of which are indicated by the forked leader 24

In fig. 84 (series 142, slide x, section 16), the subclavian approach (14) has separated from the jugular approach (13) and the thyro-cervical artery (24) is passing meso-laterad between these two divisions. The origin of the thoracic duct approach

(12) from the dorso-medial circumference of the jugular approach

(13) is well shown in the section. The concavity of the jugulocephalic arch is expressed by the two cross-sections labelled 29.

In fig. 85 (series 142, slide x, section 8) the thoracic duct approach (12) has separated from the jugular approach (IS). The curve of the process, with the concavity directed mesocaudad, gives the two cross sections of its lumen included in the forked leader 12. The termination of the thoracic duct approach has still further invaded the recess between sympathetic (1) and vagus (22) and a slight interval has developed between the two nerves.

The lateral circumference of the vagus (22) covers practically the entire mesal aspect of the internal jugular vein (25).

In the following figure (fig. 86, series 142, slide x, section 20) the termination of the thoracic duct process (12) is seen slipping through the interval between sympathetic (1 ) dorsally and vagus (22) ventrally. Instead of lying therefore dorso-lateral to the sympathetic — as in the preceding series 245 — the end of the thoracic duct approach in the present instance passes mesad on the ventral aspect of this nerve, between the same and the vagus. This relationship is also seen in the two succeeding figures, 87 and 88 (series 142, slide x, sections 22 and 24.)

We are dealing therefore apparently with two potential paths which the blind terminal of the thoracic duct approach may take in its passage from the jugular lymph sac mesad beyond the vagussympathetic line to the upper mediastinal region, where it establishes eventually its secondary connections with the preazygos segment of the thoracic duct and with the broncho-mediastinal trunk. In one type (illustrated by the sections and reconstructions of series 245 (figs. 79 to 82, fig. 90) the process dips, after passing between the left superior intercostal and main jugular vein at an acute angle, caudad on the lateral side of the sympathetic strand (fig. 90).


In the other type, illustrated by series 142 (figs. 83 to 88; fig. 91) the thoracic duct approach, maintaining the identical relation to left superior intercostal and common jugular veins, gains the ventro-medial aspect of the sympathetic cord, by passing obliquely between the same and the vagus.

In schematic cross section these two conditions may be illustrated by the accompanying text figures:


In regard to the remaining structures fig. 87 (series 142, slide X, section 22) shows, by comparison with the preceding figure 86, the tap of the jugular approach into the internal jugular vein (25) just above the confluence with the jugulo-cephalic trunk (29), and the prolongation of its ventral process {IS) over the ventral aspect of the common jugular angle. Fig. 88 (series 142, slide X, section 24) shows the confluence of the internal jugular (S5) and jugulo-cephalic (^9) veins fully established, and gives a very clear picture of the relation which the ventral prolongation (IS) from the jugular approach of the lymph sac bears to the common jugular angle. In both figs. 87 and 88 the dorsal venous tributary (sup. intercostal vein anlage) is indicated by the leader 16. It enters the main vein in section 3 of the following slide (xi). Fig. 91 gives the dorsal view of the reconstruction of the vascular and nervous structures of the left side in slides X and xi of this embryo, carried caudad to the level of the vertebral vessels Wl, 4^). Comparison with fig. 90 will show the correspondence of the two preparations in all essential points and will accentuate the above described difference in the course of the thoracic duct approach (12) and tjie preazygos segment of the thoracic duct (35) in relation to the vagus-sympathetic strand.


It is readily seen, in comparing the reconstructions of the two older stages just described (figs. 90 and 91) with that of the earlier embryo shown in fig. 89, that the latter, in the construction of the terminal of its thoracic duct approach, combines potentially the ability of developing into either one of the two more advanced conditions just described. The process which the thoracic duct approach turns nearly horizontally mesad into the interval between vagus and sympathetic is present in all three reconstructions. In many embryos, as will appear presently in describing the critical stages in detail, the thoracic duct approach appears to make its connection with the independently developed anlages of the preazygos portion of the thoracic duct in this situation, and continued development will lead to the condition seen in series 142 (fig. 91). If, on the other hand, the caudal extremity of the thoracic duct approach of the 13.5 mm. embryo (fig. 89) joins the independently developed lymphatic anlages at the root of the left superior intercostal vein and along the common jugular lateral to the sympathetic line, the resulting relation will be as seen in series 245 (fig. 90).


These observations are further supported by comparison of the preparations just described with the corresponding portions of the two reconstructions shown in figs. 170 and 171.


Fig. 170 (series 218) shows the stage in a 15 mm. embryo just prior to the junction of the thoracic duct approach with the independently formed preazygos segment of the thoracic duct (35). The latter is in its greater part already a distinct and considerable segment of the future continuous duct channel. Between its blind cephalic extremity and the blind caudal end of the thoracic dUct approach (12) are a number of scattered and still separate lymphatic anlages along the common jugular and innominate veins on each side of the sympathetic line. By continued extension of these as yet isolated lymphatic anlages the still lacking intermediate bondpiece will be produced, which, by union of its cephaUc extremity with the thoracic duct approach of the jugular lymph sac and junction of its caudal end with the cephali? termnal of the preazygos segment of the thoracic duct {35), will produce the continuous and uninterrupted lymph channel of the succeeding stage (se.ies 143, 15.5 mm, embryo, fig. 171). A study of the reconstruction of series 218 makes it evident that the opportunity still exists of developing this final junction either on the dorsolateral or ventro-medial side of the sympathetic cord, for the lymphatic anlages exist in both situations. The end of the thoracic duct approach of this embryo rides with a well marked lateral and medial prolongation in the fork formed by the entrance of the left superior intercostal vein (16-46) into the common jugular. The lateral process, carrying the leader 12 in the figure, is in position to join the line of separate lymphatic anlages extending caudad on the lateral side of the sympathetic, and a prominent cephalic process of the already established segment of the preazygos lymph channel extends cephalo-laterad across the dorsal surface of the nerve, as if to meet the lymphatic line just described. If development had proceeded along this line in this embr^^o, the condition seen in series 245 might have resulted, with the modification of swinging the thoracic duct approach to the lateral instead of to the medial side of the left superior intercostal vein, but carrying the thoracic duct along the dorso-lateral aspect of the sympathetic nerve. In this case the large separate lymphatic anlage labelled 35' in fig. 170 would take an important share in building up the resulting lymph channel. On the other hand, the definite connection with the preazygos segment of the thoracic duct might have been established through the ventromedial terminal prolongation of the thoracic duct approach. A piece of the sympathetic nerve has been removed in the reconstruction in order to show the position of this structure, in the interval between sympathetic nerve (1) and vagus (22), The long slender prolongation extending forward between vagus (22) and common carotid arter\^ (23) is the path of junction for the ventral or broncho-mediastinal lymphatic trunk. I am under the strong impression that the observed variations in the relation of the embryonic thoracic duct approach to the sympathetic nerve hinge largely for their production upon the share which the broncho-mediastinal duct is to take in individual cases in the establishment of the preazygos segment of the thoracic duct channel. (Compare relations of 35 and ^7 in the schematic cross sections shown above in the text figures.)


If now this ventro-medial end piece of the thoracic duct approach had effected its junction with the preazygos segment of the thoracic duct along the line of the lymphatic anlages on the medial side of the sympathetic and in the interval between this strand and the vagus, then the individual condition already described in series 142 (fig. 91) would have resulted.


This same topographical relationship of sympathetic nerve and lymph channel is also seen in the 15.5 mm. embryo, series 143, shown in reconstruction from the right side in Fig. 171. The thoracic duct approach (12) intersects the sympathetic nerve {1) on its ventro-medial aspect at an acute angle to establish its connection with the cephalic end of the preazygos segment of the thoracic duct (36). This t>T)e appears the prevalent one in the embryos in my collection.

I am strongly inclined, on the evidence of the embrj^os of my collection, to regard the development of the thoracic duct approach of the jugular lymph sac as occupying three distinct chronological stages :

1. Early stage. Embryos of 13-13.5 mm.

Series 92 — 13. nam.

Series 189 — 13.5 mm. In this period the thoracic duct approach appears large, at times multilocular, and shares the redundancy and capacious lumen which characterizes the entire jugular h^mph sac in the earlier genetic period.

2. Intermediate stage. Embryos of 14 mm.

Series 210 — 14 mm. Series 212 — 14 mm. Series 122 — 14 mm. The sac, as a whole, and the thoracic duct approach in })articular, condenses and becomes more clearly defined against the surrounding mesoderm. The multilocular character due to remnants of the capillary walls in the interior, disappears and the approach obtains a clear and clean-cut lumen. The entire structure appears to contract somewhat on itself and develops more distinct walls. The recession, during which the thoracic duct approach occupies a relatively smaller territory is succeeded by 3. Final stage. Embryos of 15-17 mm.

Series 245 — 15 mm.

Series 218 — 15 mm.

Series 143 — 15.5 mm.

Series 142—17 mm. In this period the sac, through its digitate processes, establishes the definite connections with the independently formed systemic extraintimal lymphatic channels. Specifically the thoracic d uct approach of the sac unites secondarily with the preazygos segment of the thoracic duct and with the broncho-mediastinal lymphatic trunk.

While I do not insist on the above outlined details in the chronological order of development of the thoracic duct approach, the embryos of my collection certainly tend to support the view expressed. Thus the 13 mm. embryo (series 92) offers the highest degree of expansion of the thoracic duct approach. The structure recedes and diminishes somewhat in extent in the 13.5 mm. stage (series 189), appears uniformly further reduced and shortened in the 14 mm. embryos (series 210, 212, 122), and finally, from the 15 and 15.5 mm. stages on, makes its definite connections with the preformed independently developed channels of the thoracic and broncho-mediastinal ducts.


In this entire genetic process there is not the slightest indication of an extension of lymphatic trunks from the jugular sac or from its thoracic duct approach toward the periphery. There is absolutely no suggestion of so-called ^'budding" or ^* sprouting, or of any other form of centrifugal extension of lymphatic vessels. On the contrary, the sac as a whole, and the processes therewith connected, recede actually for a time, and only then establish a secondary junction with the independently developed general systemic lymphatic channels. These have in the meanwhile formed along and around the temporarj"^ embryonic veins, as the result of the confluence of large numbers of extraintimal perivenous spaces in the sense previously defined in detail. Then- line of further growth, extension and union with each other is altogether centripetal^ from the outlying regions toward the jugular lymph sacs, and theh- union with the latter 's processes is the last and final step in the definite organization of the lymphatic sj^stem.


PART II, PLATES

FIGURES 29 TO 91


FIGTRi: 2?


TSf Cc4or B^ita^aL. Otfviz^ ilie 5 cmr enwiir seexDesic oi ^ht ikorMor tens 1. IL IIL n~ IK lirt- ca^L &ad l2«Lr rrdkiiocis :<:< ihs ^s^hrpomal tcebs. ike


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11 iufolkr hnz-p^ fiii^ 12 infnliLr }tz4^ «{fc.ei. iboni^i? -ii;^ &3?pnMch.

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33 ScbfliTitiJ »rt«rT

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35 TbfWirif^ d-r:- prekxygos

37 Br*:-c*cii«>-c>'»'iikftsrjJ c-n.

£ Pon^jj^o* M«cL«i-. of :LorA^-> i-n.


FIGUHKS 3<) AND 31

30 'VriiiiHXfrHO. Hortion throiif|;h left hu\c of lower cervical region in a 14 iiitn. rat embryo («ericH 210, sliile VI 11. section 39>, X 225.

31 Same, flection 40.

1 Sympathetic nerve.

1 1 Jugular lymph sac.

12 JuKular lymph sac, thoracic duct approach.

16 DorHo-medial somatic venous tributaries.

17 Pn? vertebral venous plexus.

18 Peritracheal venous plexus.

19 Suprathymic venous plexus.

20 Subthymic venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein.


FIGURES 32, 33 AND 34

32 Same, section 41.

33 Same, section 42.

34 Transverse section through left side of lower cervical region in a 14 nun. rat embryo (series 212. slide IX. section 24), X 200.

1 Sympathetic nerve.

8 Oesophagus.

11 Jugular lymph sac.

12 Jugular lymph sac, thoracic duct approach.

16 Dorso-medial somatic venous tributaries.

17 Prevertebral venous plexus.

18 Peritracheal venous plexus. 22 Vagus.

24 Th3rrocervical artery. 2o Internal jugular vein.


FIGURES 35 TO 41

35 Sainc, section 15.

36 Same, section 16.

37 Same, section 17.

38 Same, section 20.

39 Same, section 22.

40 Same, section 23.

41 Same. secti(m 24.

1 Sympathetic nerve.

11 Jugular lymph sac.

12 Jugular lymph sac, thoracic duct approach. 16 Dorso-metlial somatic venous tributaries. 22 Vagus.

24 Thyrocervical artery.

25 Internal jugular vein.

46 Left superior intercostal vein.


FIGUKKS 42 AND 4.S

42 Same, section 26.

43 Transverse section through left side of lower cervical region of a 13.5 mm cat embryo (series 189, slide VII, section 10), X 150.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

11 Jugular lymph sac.

12 Jugular lymph sac, thoracic duct approach.

16 Dorso-medial somatic venous tributaries.

17 Prevertebral venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein.

46 Left superior intercostal vein.


FIGURES 44 TO 49

44 Same, section 11.

45 Same, section 12.

46 Same, section 13

47 Same, section l4.

48 Same, section 15.

49 Same, section 16.

1 Sympathetic nerve.

8 Oesophagus

9 Trachea.

11 Jugular lymph sac.

12 Jugular lymph sac, thoracic duct approach.

16 Dorso-medial somatic venous tributaries.

17 Prevertebral venous plexus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein. 29 Jugulo-subclavian tnmk


FICJUJiKS 50 TO 5o

50 8ainc, section 17.

51 Same, section 18. 62 Same, section 19.

53 Same, section 20.

54 Same, section 21.

55 Same, section 22.

1 Sympathetic ner\e.

S Oesophagus.

11 Jugular lymph sac.

12 Jugular lymph sac, thoracic duct approacli. IG Dorso-medial somatic venous tributaries. 17 Prevertebral venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugidar vein.

21» Jugulo-subclavian trunk.


FIGURE 56

56 Dissection of left cervical region of adult cat, showing reduplication (»f thoracic duct approach of left jugular lymph sac (Adult No. H3).

1 External jugular vein and lymphatics.

2 Common jugular vein.

3 Jugular lymph sac. 5 Subclavian vein.

9 Thyrocervical artery.

1 1 Vertebral vein.

13 Left subclavian artery.

18 Jugular approach and tap.

10 Subclavian approach and tap.

20 Innom inate artery .

24 Vena cava superior.

25 Left carotid artery.

26 Aorta.


FIGURES 57 AND 58

57 Transverse section through left lower cervical region of a 14 mm. cat embryo (series 122, slide IX, section 21) , X 150.

58 Same, section 28.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

12 Jugular lymph sac, thoracic duct approach.

13 Jugular lymph sac, jugular approach.

14 Jugular l3nmph sac, subclavian approach. 16 Dorso-medial somatic venous tributaries.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein.

26 Common jugular vein. 29 Jugulo-subclavian trunk« 



FIGURES 59, 60 AND 61

59 Same, slide X, section 7.

60 Same, section 9.

61 Same, section 12.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

12 Jugular lymph sac, thoracic duct approach.

14 Jugular lymph sac, subclavian approach.

16 Dorso-medial somatic venous tributaries.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery. 26 Common jugular vein.

30 Sixth spinal nerve.

31 Primitive ulnar veno-ljrmphatic.


FIGURES 62 AND 63

62 Same, section 13.

63 Transverse section of left lower cervical region in a 13 mm. cat embryo (series 92, slide X, section 27), X 225.

1 Sjrmpathetic nerve.

8 Oesophagus.

11 Jugular lymph sac.

13 Jugular lymph sac, jugular approach.

14 Jugular Ijrmph sac, subclavian approach. 16 Dorso-medial somatic venous tributaries.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein.

26 Common jugular vein.

29 Jugulo-subclavian trunk.

30 Sixth spinal nerve.

31 Primitive ulnar veno-lymphatic.


FIGURES 64, 65 AND 66

64 Same, section 31.

65 Same, section 32.

66 Same, section 33.

1 Sympathetic nerve.

12 Jugular lymph sac, thoracic duct approach.

13 Jugular lymph sac, jugular approach.

14 Jugular lymph sac, subclavian approach. 22 Vagus.

24 Thyrocervical artery.

25 Internal jugular vein.

29 Jugulo-subclavian trunk.


FIGURES 67, 68 AND 69

67 Same, section 35.

68 Same, section 36.

69 Same, section 38.

1 Sympathetic nerve.

12 Jugular lymph sac, thoracic duct approach.

13 Jugular lymph sac, jugular approach.

14 Jugular lymph sac, subclavian approach.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein.

29 Jugulo-subclavian trunk.


FIGURKS 70, 71 AND 72

70 Same, slide IX, section 2.

71 Same, slide IX, section 5.

72 Same, slide IX, section 6.

1 Sympathetic nerve.

12 Jugular lymph sac, thoracic duct approach.

14 Jugular lymph sac, subclavian approach.

22 Vagus.

24 Thyrocervical artery.

26 Common jugular vein.


FIGURES 73, 74 AND 75

73 Same, slide IX, section 9.

74 Same, slide IX, section 10.

75 Same, slide IX, section 11.

1 Sympathetic ner\'e.

8 Oesophagus.

9 Trachea.

12 Jugular lymph sac, thoracic duct approach.

14 Jugular lymph sac, subclavian approach.

16 Dorso«medial somatic venoua tributaries,

22 Vagus.

24 Thyrocervical artery.

26 Common jugular vein.


FIGURES 76, 77 AND 78

76 Same, .slide IX, section 12.

77 Same, slide IX, section 13.

78 Same, slide IX, section 14.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

12 Jugular lymph sac, thoracic duct approach.

16 Dorso-medial somatic venous tributaries.

17 Prevertebral venous plexus. 22 Vagus.

24 Thyrocervical artery. 26 Common jugular vein.


EXPLANATION OF FIGURE 79

79 Transverse section of left lower cervical region in a 15 mm. cat embryo (series 246, slide XI, section 24), X 225.

1 Sympathetic nerve.

8 Oesophagus.

12 Jugular lymph sac, thoracic duct approach.

14 Jugular lymph sac, subclavian approach.

16 Dorso-medial somatic venous tributaries.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery. 26 Common jugular vein.


FIGURES 80, 81 AND 82

80 Same, section 25.

81 Same, section 26.

82 Same, section 27.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

12 Jugular lymph sac, thoracic duct approach.

14 Jugular lymph sac, subclavian approach.

16 Dorso-medial somatic venous tributarieh.

22 Vagus.

23 Left carotid arterv.

24 Thyrocervical artery. 26 Common jugular vein.


FIGURES So AND 80

85 iSame, section 18.

86 Same, section 20.

1 Sympathetic nerve.

12 Jup:ular lymph sac. thoracic duct approach.

18 Jupular lymph sac, jujijular approach.

14 Juguhar lymph sac. subclavian approach.

21 Thymus.

22 VagUis.

24 Thvroccrvical artcrv. 2o Internal jugular vein. 29 Jugulo-subclavian trunk.


FIGUIIES 87 AND HS

87 Same, section 22. SS Same, section 24.

1 Sympathetic nerve.

12 Jugular lymph sac, thoracic duct approach.

13 Jugular lymph sac, jugular approach.

14 Jugular lymph sac, subclavian approach. 16 Dorso-medial somatic venous tributaries.

21 Th>Tnus.

22 Vagus.

24 Thyrocervical artery.

25 Internal jugular h-mphatics. 29 Jugulo-subclavian trunk.


FIGURES 89, 90 AND 91

89 Reconstruction of caudal end of left jugular lymph sac. with thoracic duct approach, in a 13.5 mm. cat embryo (series 189, slide VII. sections 1 to 50), X 225. Dorsal view.

90 Reconstruccion of caudal end of left jugular lymph sac, with thoracic duct approach, in a 15 mm. cat embryo (series 245, slide XI, sections 1 to 44), X 225. Dorsal view.

91 Reconstruction of caudal end of left jugular lymph sac with thoracic duct approach, in a 17 mm. cat embryo (series 142. slide X. sections 1 to 28. slide XI, ^ sections 1 to 29), X 225. Dorsal view.

1 Sympathetic nerve. 7 Aorta.

12 Jugular lymph sac, thoracic duct approach.

13 Jugular lymph sac, jugular approach.

14 Jugular lymph sac, subclavian approach. 16-46 Dorso-medial somatic venous tributaries.

(Sup. intercostal v.)

17 Prevertebral venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein.

26 Common jugular vein. 29 Jugulo-subclavian trunk.

31 Primitive ulnar veno-lymphatic.

33 Left subclavian artery.

33' Left subclavian vein.

35 Thoracic duct, preazygos segment.

41 Vertebral arterv.

42 Vertebral vein.

46 Innominate artery.

46 Left superior intercostal vein.

54 Fifth spinal nerve.

55 Sixth, seventh and eighth spinal nerve.


1 Ua^KS 92 AND 93

92 Dissection of the veins and lymphatics at the root of t lie neck in an aduM cat. (Xo. 67).

93 Same, No. 13.

(Figs. 92, 93, and 94 arc republished from Uuniingtou and McClure's ** Anatomy and Development of the Jugular Lymph Sacs in the Domestic Cat," Am. Jour. Anat., Vol. X, No. 2.)


FIGUHK 94 94 Same. No. .56.


FIGURE 95

05 Dissection of main lymphatic vcssols of neck and thorax in an a<luU cat

(No. 22).

1 External jugular vein and lymphatics.

5 Subclavian vein.

7 Internal mammary artery and lymphatics.

S Broncho-mediastinal or ventral mediastinal lymphatic trunk.

10 Cephalic vein and lymphatics.

11 Vertebral vein.

12 Thoracic duct, preazygos segment. 15 Oesophagus.

17 Internal jugular vein and lymphatics.

18 Jugular approach and tap.

19 Subclavian approach and tap.

20 Innominate artery.

21 Azygos vein.


FIGURE 96

96 Dissection of main lymphatic vessels of neck and thorax in an adult cat (No. 29).

1 External jugular vein and lymphatics.

2 Common jugular vein.

3 Jugular lymph sac.

4 Inferior thyroid lymphatics.

5 Subclavian vein.

6 Internal mammary lymphatics.

7 Internal mammary artery.

S Hroncho-mediastinal or ventral mediastinal lymphatic trunk

9 Thyrocervical artery:

10 Cephalic vein and l\^nphatic8.

11 Vertebral vein.

12 Thoracic duct, preazygos segment.

13 Left subclavian artery.

14 Junction of preazygos and azygos segments of thoracic duct, lo Oesophagus.

16 Azygos segment of thoracic duct.


FIGURE 97

97 Dissection of main lymphatic vessels of neck and thorax in an adult cat (No. 131).

1 External jugular vein and lymphatics.

2 Common jugular vein. 5 Subclavian vein.

7 Internal mammary artery.

8 Broncho-mediastinal or ventral mediastinal lymphatic trunk.

9 Thyrocervical artery.

12 Thoracic duct, preazygos segment.

13 Left subclavian artery.

15 Oesophagus.

16 Azygos segment of thoracic duct.

17 Internal jugular vein and lymphatics. 20 Innominate artery.

23 Innominate vein.

26 Aorta.

27 Vagus nerve.

28 Inferior thyroid vein.

29 Jugulo-cephalic venous trunk


FIGURE »S

98 Dissection of left jugular lymph sac and thoracic duct in an adult cat (\o. 168).

2 Common jugular vein.

5 Subclavian vein.

8 Broncho-mediastinal or ventral mediastinal lymphatic trunk.

9 Thyro-cervical artery. 13 Left subclavian arterv. 15 Oesophagus.

17 Internal jugular vein and lymphatics.

18 Jugular approach and tap.

21 Azygos vein.

22 Trachea.

23 Innominate vein*

24 Vena cava superior

25 lieft carotid artery.


FKU'RK no

M9 Schema showing composite picture of left juguhvr lymph sac, broncho iiu (liastinal trunk and preazygos segment of thoracic duct.

1 External jugular vein and lymphatics.

3 Jugular lymph sac.

5 Subclavian vein.

6 Internal mammary lymphatics.

8 Broncho-mediastinal or ventral mediastinal lymphatic trunk.

9 Thyro-cervical artery.

10 Cephalic vein and lymphatics.

11 Vertebral vein.

12 Thoracic duct, preazygos segment.

13 Left subclavian artery.

10 Azygos segment of thoracic duct.

17 Internal jugular vein and lymphatics.

IS Jugular ai)i)roach and tap.

19 Subclavian approach and tap.

20 Innominate arterv.

23 Innominate vein.

24 Vena cava superior. 2') Left carotid arterv. 2() .\orta.


The Development of the Broncho-Mediastinal Trunk and of the Preazygos Segment of the Thoracic Duct

The area in which the development of the broncho-mediastinal and preazygos segments of the thoracic duct occurs is shown in fig. 100, which gives a topographical view of a transection of the upper thoracic region in a 12 mm. embryo (series 78, slide IX, section 5, X 50).

The section includes the entire coelom ventrally, and the structures between it and the notochord dorsally. The right (S) and left {6) precaval veins occupy the lateral limits of the dorsal region. In the interval between them are seen the oesophagus (5), the trachea (9), the vagi {22) and the dorsal aortic arches (7). Dorsal to the main veins on each side are the strands of the brachial plexus (not labelled in the figure), and dorsal to the nerves is the primitive ulnar veno-lymphatic (Si). Further mesad follow the subclavian arteries {33), the entrance of the dorso-medial venous tributaries {16) into the main veins, and the sympathetic strands (i). The letter Y in the figure, between the notochord and the dorsal mediastinal structures named, indicates the center of the area in which the development of the preazygos segment of the thoracic duct will proceed in the succeeding stages.

The ventral portion of the field contains the coelom, with right {IfS) and left {J^O) auricles, right ventricle {50), ascending aorta (7) and ascending trunk of main pulmonarj'^ arter>' {10), Dorsal to this are seen the cross-sections of the right and left pulmonary arteries {10) descending to the lun^s. Between them ventrally, and trachea (9), aortae (7), vagi {22) and praecaval veins (S, 6) dorsally, is the area marked by the letter X, in which the development of the broncho-mediastinal trunk will proceed.


A. The development of the broncho-mediastinal or ventral mediastinal lymphatic channel, which drain subsequently caudo-sinistrad into (lie left thoracic duct, or cephalo-sinistrad into the subclavian approach of the jugular lymph sac, or into both.

In the earlier stages (embrj^os of 11-14 mm.), an extensive ventral venous capillary network develops between the main venous trunks of the right and left sides, involving the distal part of the internal jugular, the common jugular and the innominate veins.

Out of this general antecedent sinistro-dextral ci^illary reticulum the definite retro- or suprathymic innominate anastomosis develops subsequently in normal individuals, but in the earlier stages this plexus extends both ventral and dorsal to the thymus anlage.

The venous network develops, in addition to the prevertebral and dorsal somatic components, chiefly on the lateral aspect of the oesophagus, in the peri-tracheal area, and around the thymus and the common carotid artery. It trends cephalo-laterad towards the main venous line (jugulo-innominate), into whose mesal surface the terminals of the plexus discharge.

In so doing they encounter, and partially surround, the sympathetic and vagus nerves, and the thymus body. Consequently the terminal branches of this ventral mediastinal plexus enter the main vein both ventral and dorsal to these three structures and through the intervals between them. Thus the composite plan of this venous area appears as shown in the schematic fig. 101, which represents the combination of a number of transverse sections of the anterior thoracic region.

The terminal connections of the ventral mediastmal plexus may be grouped in reference to their derivation and their relation to adjacent fields of venous drainage into the following five sets, (fig.fOl):

1. Dorso-medial somatic tributaries of internal and common jugular and of innominate veins (fig. 101, 16) entering the main venous channel dorsal to the sympathetic nerve, and frequently combined with

2. Terminal of prevertebral venous plexus anterior to azygos veins (fig. 101, 17).

3. Terminal of peritracheal plexus passing laterad between sympathetic {1) and vagus {22) nerves (fig. 101, 18).

4. Ventral mediastinal branch, draining, as "suprathymic plexus," the area between the thymus {21), carotid {23) and vagus {22) . This is the line of the future typical permanent left innominate anastomosis dorsal to the thymus (fig. 101, 19).

5. Subthymic plexus, passing ventral to the thymus anlage (fig. 101, SO). This forms, if retained and further developed in the adult, the occasionally observed '*prethymic" left innominate vein ^'*\

Any number of good examples of all these conditions are afforded by every 13 or 14 mm. embryo. Thus in a 14 nun. embryo (series 210, slide viii, section 41, X 225), fig. 102, direct comparison of the schema shown in fig. 101 with the section will, without further description, make the arrangement of the elements composing this plexus (16, 17, 18, 19, 20) clear. (Compare also fig. 43 [series 189, slide vh, section 10, 13.5 mm.] and fig. 30 [series 210, slide viii, section 39, 14 mm.])

The ventral divisions of this plexus extend in the earlier embryonic stages (11-12 nun.) as a well developed capillary reticulum into the upper thoracic region around and between oesophargus, trachea, precardinal veins, vagi and pulmonary arteries.

Beginning in some 12 nun. embryos, but more constantly and characteristically in the 13 nun. stage, certain components of this ventral mediastinal venous plexus appear partly surrounded by independently developed extraintimal lymphatic spaces, which form the anlages of the future broncho-mediastinal lymphatic trunk.

Thus, if the ventral portion of this venous plexus is followed caudad into the upper thoracic region, into the area of the developing broncho-mediastinal trunk marked X in fig. 100, the following observations can be made in stages of the proper length, and adequately fixed and stained:

1. In embryos between 11 and 12 nmi. only venous capillaries are found. Thus figures 103 and 104 show transverse sections of this part of the upper thoracic region in a 12 nmi. embryo (series 217, slide viii, sections 21 and 29, X 200). The field, caudal to oesophagus and aorta (7), contains in the middle hne the trachea (S), the vagi {22), and further ventrad, the pulmonary arteries (10). The branches of the peritracheal venous plexus (18) are well developed. There is as yet no indication of lymphatic development.


  • Geo. S. Huntington : Contribution to the topographical anatomy of the thorax in the foetus at term and the new-bom child." Rep. Soc. N. Y. Lying-inHospital, 1897, p. 343, fig. xxxi.


2. In 13 mm. embryos certain of the venous radicles entering into this plexus, are partly surrounded and enveloped by extraintimal lymphatic spaces, which arise independently of the venous system, as the first anlages of the future ventral mediastinal lymphatic channel. Figures 105 and 106 show transverse sections of this region in a 13 mm. cat embryo (series 92, slide vii, sections 30 and 38) in a magnification of 225 diameters. Ventral to trachea (9), aorta (7) and left vagus {2£) are seen sections of a number of venous radicles (4) of the ventral mediastinal plexus, some of which are partially surrounded by very smiall extraintimal perivenous lymphatic anlages (5). Only a few of the most prominent of these are indicated in the figures by leaders, but they exist in many other parts of the field.

A similar section from another 13 nmi. embryo (series 107, slide ix, section 40, X 225) is shown in figure 107. Dorsal to the pulmonary arteries (10), between them and the trachea (9) and aorta (7), the section has cut a prominent extraintimal lymphatic space (5) in typical relation to one of the radicles (4) of the ventral mediastinal venous network.

3. In the 13.5 mm. embryo the full and convincing proof of the extraintimal derivation of this channel is given. In some embryos of the period the process is still seen in its earlier stages. Thus figs. 108 and 109 (13.5 mm. embryo, series 223, slide x, sections 11 and 12, X 225) show on each side, ventral to the trachea (9) and the vagi (22) y a number of radicles (4) of the ventral mediastinal capillar}^ plexus. One of these small vessels, symmetrically disposed on each side in front of the respective vagus, is in process of being surrounded and replaced by an extraintimal lymphatic space (5).

Other 13.5 mm. embryos show a rapid advance in the perivenous Ijinphatic development.

Fig. 110 shows a transverse section of the upper thoracic region of another 13.5 mm. embryo (series 189, slide viii, section 35, X 225). Just ventro-mesad of the left vagus nerve and its encircling vein is a venous radicle (4) almost completely surrounded by an extraintimal lymphatic space (5) in the process of replacing the atrophying vein with which it is so closely associated. The same structures (4, 5) are seen still more clearly defined in the corresponding position on the right side of the embryo.

Comparison with tl\e corresponding sections just given in figures 103 to 109 show that the extraintimal anlage of the earlier stages, developed along the identical venous radicle, has increased in the 13.5 nrni. stage, so as to nearly envelop the vein, and that the latter, if followed cephalad and caudad, is now separated from the functional venous channels of this region and is in the process of further recession and degeneration as the lymphatic perivenous space enlarges and more and more replaces the antecedent venous channel. The vein in question in the older embryo (series 189) appears collapsed and shrunken, and contains only a few degenerating red blood cells. In the same situation on the right side of fig. 110 (series 189, slide viii, section 35), the section has cut the corresponding vein and the enveloping extraintinr al space at right angles, so that the central kernel of the shrinking vein (4), still containing a few degenerating red blood cells, is nearly completely surrounded by the replacing extraintimal lymphatic (5).

Fig. Ill shows the following section of the same embryo (series 189, slide viii, section 36), magnified 225 diameters, in a larger field which gives the topographical relations and may serve in in the orientation of the succeeding plates. The ventral circumference of the oesophagus iS) and the aorta (7) are seen dorsally. The trachea (9), with the vagi {22) on either side, occupies the central area. Ventrally are the two pulmonary arteries {10). The section is a most important one in interpreting the early histogenetic stages of the broncho-mediastinal lymphatic trunk and its relation to the embryonic mediastinal venous plexus. The same venous radicle (4), ventral to the vagus {22), already noted in the preceeding section (fig. 110), is seen on each side. On the left side it has been cut obliquely by the plane of the section, and hence shows a segment of considerable length, as a practically empty and partially collapsed endothelial bag (4), surrounded by the extraintimal lymphatic space (6). On the right side of the embryo the corresponding venous radicle (4) and the perivenous lymphatic anlage (6) have been again cut more at right angles, so that the envelopment of the former by the latter can be clearly traced nearly throughout the entire circimiference.

Fig. 1 11 A shows the decadent vein and the replacing extraintimal lymphatic on the left side of this section in a higher magnification, ( X 300). The structure of the collapsed venous endothelial tube, and the beginning development of endothelial character in the mesodermal cells limiting the perivenous extraintimal lymphatic anlage, can be here clearly made out.

Figs. 112 to 116 show the five succeeding sections of the same embryo (series 189, slide viii, sections 37 to 41 inclusive) in a magnification of 225 diameters.

I publish this series of plates in order to prove on the one hand that the extraintimal lymphatic anlages can be followed with certainty and accuracy in their relation to the contained decadent venule for longer or shorter distances, and that, on the other hand, they are, in these earlier stages, still isolated and not yet continuous with similar anlages in adjacent areas. They form, in other words, in these earlier periods, detached links of a lymphatic chain, not yet united to each other to form the continuous channel of the later stages.

In figs. 112, 113 and 114 (sections 37, 38 and 39) beautiful and clear pictures of the atrophying central venule (4) and the perivenous extra-intimal lymphatic space (5) are found on both sides in the typical situation noted in the preceding sections, with which they of course can easily be coordinated by simple comparison.

Sections 38 and 39 (figs. 113 and 114) are particularly instructive inasmuch as the detached central vein (4) on the left side still contains a few degenerating red blood cells, while its former connection with the vahd and functional venous plexus around the left vagus nerve can still be faintly traced.

In section 40 (fig. 115) the atrophying vein (4) and the surrounding lymphatic (5) of the left side appear divided into two components. On the right side the corresponding structures end blindly.

In the following section (41, fig. 116) the left venule and lymphatic also terminate almost entirely in the indifferent mesodermal tissue, only a small renmant (4, 5) persisting between trachea and left vagus. This disappears entirely in the following section.

The succeeding stage (14 mm.) shows the same lymphatic anlages surrounding and replacing the atrophying venules of the ventral mediastinal plexus in still higher development. It appears from the study of numerous embryos of this length, that the average 14 mm. stage represents the point in the lymphatic ontogenesis of this region at which the pictures are most striking. The central kernel of the decadent vein is still large and, in places, as yet incompletely separated from the definite venous channels. At the same time the perivenous lymphatic space has acquired an increased lumen and hence the two combined structures occupy a relatively large field.

Thus figs. 117 to 122 show transverse sections of the ventral thoracic region in a 14 mm. cat embryo (series 212, slide x, sections 4, 5, 6, 7, 10 and 11, X 225). In figs. 117 and 118 the developing lymphatic anlages (5) of the broncho-mediastinal trunk can ke readily recognized and present the same relation to the atrophying venules, which they surround-, as in the sections of the preceding series just described and figured. In contradistinction to these structures the components of the permanent functional venous plexus of this region {32) form well-differentiated vascular channels.

In fig. 119 (series 212, slide x, section 6) the scattered lymphatic anlages occupying in the two preceding sections the space between the left pulmonary artery {10) and the left vagus {22) j with its accompanying ventral mediastinal vein {32) j are gathered together into a more extensive elongated space (5). In the interior of this is seen the endothelial bag of the empty and atrophying venule (4) which the lymphatic is about to replace. The vein renmant is almost completely surrounded by the lymphatic space and only connected to the latter's endothelial wall by a bridge at one point. Other areas of similar lymphatic development are seen on the opposite side {4-5) and (not labelled) in the center of the field, but the structures on the left side are, as in the earlier stafres (cf. series 189) better developed and larger.

The succeeding figures, 120, 121 and 122 (sections 7, 10 and 11) show the same structures, which can be distinctly followed for some distance caudad of the last section figured. In fig. 122 the original connection of the large upper venous kernel of the left side (4) with one of the components of the peritracheal venous plexus (18) is still suggested By the arrangement of the adjacent cells, although the degenerating vein has been cut off from all actual communication with the permanently valid channels of the mediastinal venous plexus.

The 14 mm. stage is so important, and the results gained by the examination of a laige number of embryos so conclusive in respect to the true ontogenesis of the systemic lymphatic channels, that I have selected a well marked example and pubUsh here eighteen consecutive, or nearly consecutive, sections from the same individual. I do this in order to show the extent to which extraintimal perivenous lymphatic development has proceeded in this stage, in respect to the length of the resulting segments of the future lymphatic channel, still separated from each other, and to accentuate, by a large number of successive illustrations from the same embryo, the histogenesis of the lymphatic anlages in their relation to the decadent embryonic venules which they eventually entirely replace. The conception involved by the theory of extraintimal systemic lymphatic development has proved apparently, to judge from published comments, difiScult to understand in some laboratories from the histo-mechanical standpoint. The following eighteen reproductions of sections of a 14 mm. embryo (series 214, slide xiii) surely explain, without detailed description, the principles underlying lymphatic ontogenesis in the mammalian embryo. I very gladly welcome the opportunity, which this series of Wistar publications offers, for presenting them.

The plates comprised in this group are shown in order cephalocaudad in figures 123 to 141 inclusive. In all of them the extraintimal lymphatic anlages of the broncho-mediastinal trunk are indicated by the leader 5, while the included remnant of the atrophying embryonic vein is labelled 4, as in the preceding and following figures.

Figs. 123, 124, 125 and 126 (sections 4, 5, 6 and 7) show well the relation of the venule (4), undergoing atrophy and replacement by the extraintimal lymphatic space (5), to the permanent venous channels of the mediastinal plexus. Thus in fig. 123 the dorsal large lighter area marked 4 is the blind cephalic end of a venule which is nearly detached from the permanent peritracheal (18) and ventral mediastinal (32) venous channels, and which in the succeeding sections is seen to be partially surrounded by the extraintimal lymphatic space (5) eventually designed to replace it entirely.

In figure 124 (section 5) this perivenous space (5) has developed. The included vein (4) is still partially connected with the functional peritracheal plexus (18). Further ventrad is another portion of this same plexus. The venous kernel here still contains a number of red blood-cells. These characteristic relations of developing lymphatic and receding vein can be followed clearly throughout the succeeding sections. The contrast between the well filled permanent veins {e.g.j 18, 32 in figs. 123, 124, 125 and 126), with walls fitted closely into the surroimding mesenchyme, and the atrophying venule surrounded by the extraintimal lymphatic anlage, is striking throughout the series. Some of the sections show in certain regions good pictures of nearly complete reduction of the decadent vein to an insignificant central kernel of connective tissue surrounded by the extraintimal lymphatic space; thus, e.g., figs. 137 to 141 in the region ventro-mesad to the left vagus {22).

The 15 nma. stage shows the developing lymphatic plexus of the broncho-mediastinal trunk further advanced. Figs. 142 to 145 show four successive transverse sections through the ventral part of the upper thoracic region in a 15 mm. cat embryo (series 216, slide x, sections 32, 33, 34 and 35, X 225). The central decadent venule in the interior of the developing perivenous lymphatic space has in many places disappeared entirely, leaving only the clear lymphatic lumen (5). In other places remnants of the replaced vein (4) can still be distinctly determined. This clearance has resulted in a relative diminution of the actual area of a cross section of the meshes of the lymphatic plexus, since now the entire lumen is given over to the newly formed lymphatic channel, whereas in the earlier stages its central area was still largely filled by the degenerating veil. The lymphatic spaces in these sections have increased considerably in number. (Only a few of the more prominent vessels are indicated m the figures by leaders.)

A transverse section of this region in a 16.^ mm. cat embryo (series 215, slide xiv, section 31, X 226) is shown in a fig.146. The preceding more diffuse lymphatic plexus of the earlier stages is condensed to a much smaller mesenchymal network. In a few of the larger meshes a remnant of the former central vein (4) can still be traced as a solid plug of mesoderm surrounded by the lymphatic channel (5).

In general the 15 and 15.5 mm. embryos of the cat represent the stages in the development of the broncho-mediastinal trunk in which the histological pictures of the lymph channels are least distinct aod striking. This is due to the fact that on the one hand they have lost the central venous kernel around which they were originally developed through confluence of extramthnal spaces, and which gave to the combined structures their area on section and their distinctive appearance in the earlier (13.5 and 14 mm.) stages, while on the other hand the independent growth and enlargement of the lymph channels proper begins in the 16 mm. stage. In some embryos, measuring 16 mm., the appearances characteristic of the 15 mm. and 15.5 mm. stages are still encountered. Thus fig. 147 shows a transverse section through the ventral area of the upper thoracic region in a 16 mm. embryo (series 230, slide xii, section 25, X 225). The correspondence of the histological picture to that presented by the 15 mm. or 15.5 mm. embryo is seen by comparison with series 215 and 216 (figs. 142 to 146).

The majority of 16 mm. embryos show, however, a marked advance in the independent growth of the lymphatic channels, which have become fully established. Their lumen now appears for the most part entirely cleared of the remnants of the decadent venules originally contained therein. In only a few instances does the original vein kernel appear reduced to an insignificant rudiment. The resulting plexus of the broncho-mediastinal lymphatic duct fills the typical position occupied in the earlier stages by the extensive preceding perivenous lymphatic reticulum. Thus figs. 148-153 give transverse sections through the ventral part of the upper thoracic region in another 16 mm. cat embryo (series 222, sUde X, sections 1, 2, 3, 10, 15 and 16, X 225). This embryo affords remarkably clear pictures of the fully developed lymphatic plexus of the ventral or broncho-mediastinal trunk.

Figs. 148 and 153 (sections 1 and 16) give the topographical pictures of the entire region involved. The broncho-media^^tinal lymphatic plexus {37) is fully developed in the typical area between the trachea (5) and vagi (32) dorsally and the pulmonary arteries (10) ventrally. The lymphatic spaces possess a clear lumen and distinct endothelial walls. The remnants of the degenerating ventral mediastinal venous plexus which they have replaced have almost entirely disappeared. Only slight traces of them are still to be seen here and there. In the main the lymphatic channels are clear of venous kernels and form a rich interlacing lymphatic plexus. The central field contains the oesophagus (5), aorta (7) and part of the medial circumference of the left praecava (6). Dorsally on the right side the right azygos vein (S) and sympathetic nerve (1) appear, while on the left side the section includes the cephalic part of the azygos segment of the left thoracic duct (36).

Sections 2, 3, 10 and 15 of the same slide are shown in their ventral portions in figs. 149 to 152. They all exhibit the extent and continuity of the broncho-mediastinal lymphatic channel complex in this stage. The figures are not leadered, but the individual structures can be readily identified by reference to the topographical figures 148 and 153.

The succeeding stages are characterized by a condensation of the extensive lymphatic plexus of the typical 16 nmi. and 17 mm. embryos into the more definite channel of the broncho-mediastinal trunk with larger caliber.

Figs. 154 to 158 show the region of the broncho-mediastinal channel in four successive sections in a 20 mm. cat embryo (series 241, slide xix, sections 19, 20, 21 and 22, X 225). Fig. 154 again gives a topographical view of the entire field, while the same is cut down to the ventral portion in the remaining plates. In all the multi-meshed lymphatic plexus of the preceding stage is replaced by the more definite and larger channel of the bronchomediastinal trunk proper (37) y in the classical situation between the pulmonary arteries (10) ventrally and the trachea (9), vagi (SS) and permanent ventral mediastinal veins (18) dorsally.

This stage marks the practical attainment of adult conditions for the lymphatic channel of the broncho-mediastinal trunk, and from this period on its further development is merely a question of continued growth and adaptation to the environment in establishing secondary connections with the tributary channels, developed independently, the preazygos segment of the thoracic duct, and (usually) the ventral prolongation of the subclavian approach of the jugular lymph sac.

As will be seen from the foregoing analysis of the development of the broncho-mediastinal lymphatic trunk in embryos of the cat, the principles of mammalian systemic lymphatic ontogenesis defined in Part I of this memoir are consistently sustained by the actual evidence afforded. The development of the perivenous extraintimal anlages which form the basis for the subsequent evolution of the systemic lymphatic channels, is a uniform and remarkably consistent process.

The existence of these spaces in this and other regions of the embryo has been so often denied by recent contributors to the subject, or, if admitted, explained in every possible way except through the correct- interpretation, that I pubUsh in this paper a series of micro-photographs of a considerable number of sections taken from embryos of all the important stages. Of course these observations could be indefinitely multiplied. The larger the embryological collection, and the better the individual preparations are fixed and stained, the more conclusive will be the results of a careful examination of lymphatic development along the lines here indicated.

No competent and impartial observer can mistake the significance of the conditions here shown. Every stage of the process can be followed in detail. The behavior of the decadent embrvonic vein, and its relation to the enveloping extraintimal lymphatic channel, are absolutely demonstrated. The endothelium of the shrinking, vein has no share in furnishing the independent lymphatic endothelimn of the replacing mesenchymal space, and nowhere, in the entire process, is there the faintest suggestion of an "out-bud" or of a "splitting ofif" from the circumference of an otherwise vaUd embryonic vein of "lymphatic or "venolymphatic anlages.

It is evident in comparing a number of embryos between 13 and 16 mm., crown-rump measure, that the extensive area occupied by the lymphatic anlages in the earlier stages (13 mm., 13.5 mm. and 14 mm.) is due to the relatively large size of the central decadent vein. In the earlier stages {e.g., figs. Ill, 112, 113, 114, 119, 121, 125, 126, 127, series 189, 212 and 214) the evacuated and abandoned vein appears as a wide loose and partially collapsed endothelial bag, surrounded by a narrow extraintimal lymphatic space, with but as yet indifferently defined endothelial lining, continuous at one or more points with the external surface of the endothelium of the degenerating vein. As development proceeds the actual perivenous lymphatic spaces increase but they concentrate and narrow down on the contained venous kernel, and, as the latter continues to decrease in size and finallj^ become completely eliminated, the resulting clear lymphatic channel occupies relatively less space than that filled in the earlier stages by the decadent venule and the early surrounding lymphatic anlage combined.

Thus in the 15, 15.5 and in some 16 nmi. embryos (figs. 142, 143, 144, 145, 146, and 147, series 216, 215 and 230), while the actual lumen of the lymphatic channel has increased that of the enclosed and abandoned vein has corresponding!}^ receded to an insignificant remnant, which projects into the lymphatic space from a narrow pedicle attached to the latter's endothelial lining.


Consequently, in the pictures of the earlier stages (13, 13.5 and 14 mm. embryos) the area occupied by the still capacious abandoned venous channel, combined with that of the relatively narrow enveloping lymphatic space, appears larger and more prominent than in the later periods (15, 15.5 mm.) in which the permanent lymphatic channel and its reduced venous kernel occupies less histological territory.

It should also be noted that in the earlier stages (13, 13.5, 14 mm.) the perivenous extraintimal lymphatic network is more diffuse and redundant than in the later stages (15, 15.5, 16 mm.). This early plexiform lymphatic reticulum is subsequently condensed into more limited and better defined lymph channels, in exact conformation to the processes observed in the haemal vascular development, in which the definite blood channels crystallize along main lines out of an antecedent indefinite and more diffuse plexus. The result of this genetic process is strik; I ing in tracing developing lymphatics through successive stages.

Thus in the region here imder discussion the pictures offered by the 13, 13.5 and 14 mm. embryos are more conclusive than those furnished bj' the 15 and 15.5 mm. stages in reference to the genesis of the Ijonphatic channels. In the former, the lymphatic anlages and the decadent venules around which they develop are taken together, relatively larger and more numerous, and hence easier to recognize. In the latter, this early redimdant perivenous lymphatic plexus has been replaced by a relatively smaller, but much more definite and distinct channel, occupjdng, however, always a part of the area filled in the earlier stages by the less circumscribed antecedent lymphatic plexus with its contained remnants of abandoned embryonic veins. I lay special stress on this fact, because it is absolutely neoessarj' to take it into account in comparing stages between 13.5 and 15.5 nmi. In the latter stage the final IjTiiphatic anlages have definitely formed and have reached the relation to adjacent main venous lines which they henceforth are to occupy. The further growth and enlargement of these chamiels begins in the 16 mm. embrj'o, and proceeds from this point on, in many regions at the expense of the adjacent embryonic vein undergoing atrophy.


The only conclusion which seems to me to be warranted by actual observation is that certain embryonic veins form, during the process of their atrophy and final elimination from the definite venous organization, the supporting lines along which certain of the perivenous extraintimal lymphatic anlages first develop.

The fact that numerous early embryonic venous channels, large and small, atrophy and disappear during the normal course of subsequent development, appears to afiford a more favorable field for the greater development of the adjacent mesenchymal inintercellular spaces, so that these enlarge more rapidly, as the correlated vein recedes. This relationship appears, however, to be based exclusively on the physical and mechanical advantages which the abandoned and shrinking primary venous line affords to the adjacent mesenchymal spaces for more rapid enlargement, in the sense of replacing the disappearing vein and occupying secondarily the space formerly filled by the haemal channel. This is evidently an important factor in determining the size and extent of the final lymphatic channel resulting from the confluence of the originally separate and independent perivenous anlages. Consequently, in the adult, the largest and best defined systemic lymphatic vessels either accompany reduced adult remnants of a relatively larger embryonic venous channel, or, in case of the latter's entire default, topographically replace the same. Now, while this relation manifests itself strikingly in many parts of the body, it is quite evident that the development of lymphatic chaimels occurs in other parts independently of preceding veins, by the confluence of independent intercellular mesenchymal spaces.

The cat may, and does, differ in its details of lymphatic development, and in its adult lymphatic organization, from the conditions obtaining in many other mammalian forms. In fact, even in the present very incomplete state of our knowledge of the comparative anatomy of the mammalian lymphatic system, we realize that what we regard as the mammaUan ground plan of lymphatic organization may in certain genera undergo great and far-reaching modifications, as for example the conditions determined by Silvester'^ for the whole group of the Cebidae, and those found by myself in Macropua rufus. And yet these diflferences, established and maintained within the natural limits of the mammalian class, cannot, in whatever degree they actually exist, in individual forms be basic. In any given mammalian type, the systemic lymphatic vessels, whatever their adult relation to and connection with the venous system may be, must develop in accordance with a genetic groimd plan common to all mammalia.


•' C. F. Silvester. On the Presence of Permanent Lymphatico-Venous Communications at the Level of the Renal Veins in Adult South American Monkeys."

AnaU Am,, Erganzungsh. z. 37. Bd., Vevh. Anat. Ges., 24 Vevs., Brttssels, 1910, S. 111-114.


Part II Plates

FIGURES 100 TO 158


FIGURE 100

100 Transverse section of upper thoriicic region in a 12 mm. cat embryo (series 78f slide IX, section 5), X 50.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Precardinal or precava, resp. azygos of left side.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonary artery.

16 Dorso-medial somatic venous tributaries.

22 Vagus.

31 Primitive ulnar veno-lymphatic.

33 Subclavian artery.

48 Right auricle.

49 Left auricle.

50 Right ventricle


FIGURE 101

101 Schema of venous plexuses of left side in lower cerviciil region of cat embryo between 13 mm. and 14 mm. crown-rump measure in transverse section.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

16 Dorso medial somatic venous tributaries.

17 Prevertebral venous plexus.

18 Feri tracheal venous plexus.

19 Suprathymic venous plexus.

20 Subthymic venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery. 20 Coiiunon jugular vein.


FIGURE 102

102 Transverse section of lower cervical region in a 14 mm. cat embryo (series 210, slide VIII, section 41) , X 225.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

16 Dorso-medial somatic venous tributaries.

17 Prevertebral venous plexus.

18 Peritracheal venous plexus.

19 Suprathymic venous plexus.

20 Subthymic venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery.

24 Thyrocervical artery.

25 Internal jugular vein.


FIGURES 103 AND 104

103 Transverse section of ventral part of upper thoracic region in a 12 mm. cat embryo (series 217, slide VIII. section 21), X 200.

104 Same, section 29.

7 Aorta.

9 Trachea.

10 Pulmonary artery.

IS Peritracheal venous plexus.

22 Vagus.


FIGUKES 10.-) AND 106

105 Transverse section of ventral part of upper thoracic region in a 13 mm. cat embryo (series 92, slide VII, section 30), X 225.

106 Same, section 33.

3 Right precaval vein.

4 Atrophying embryonal vein, fonning kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerating

embryonal vein.

7 Aorta.

8 Oesophagus.

9 Trachea. 22 Vagus.


FIGURE 107

107 Transverse section of ventral part of upper thoracic region in a 13 mm. cat embryo (series 107, slide IX, section 40), X 225.

3 Right precaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing Ijinphatic space.

5 Extraintimal or perivenous hmiphatic space surrounding degenerating

embryonal vein. 7 Aorta. 9 Trachea. 10 Puhnonarv arterv


FIGURES 108 AXD 109

108 Transverse section of ventral part of upper thoracic region in a 13.5 mm. cat embryo (^series 223, slide X, section 11), X 225.

109 Same, section 12.

• 4 Atrophying embryonal vein, fonning kernel in interior of developing

and replacing lymphatic space. 5 Kxtraintimal or perivenous lymphatic space surrounding degenerating

embryonal vein. 7 Aorta. 9 Trachea. 22 Vagus.


FIGURES 110 AND lllA

110 Transverse section through ventral part of upper thoracic region in a 13.5 mm. cat embryo (series 189, slide VIII, section 35), X 225.

111 A Left part of section 36 of same embryo, shown in the following figure 111, X300.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerating

embrvonal vein.


FIGURE 111

111 Transverse section through ventral part of upper thoracic region in a 13.5 mm. cat embryo (series 189, slide VIII, section 36), X 225.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerating

embryonal vein.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonarv artorv. 22 Vagus.


FIGURES 112 AND 113

112 Same, section 37, X 225.

113 Same, section 38, X 225.

4 Atrophying embryonal vein, fonning kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degener ating embryonal vein.


FIGURKS 114 AND 115

114 Same, section 39. X 225.

115 Sjime, section 40, X 225.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraiutimal or perivenous lymphatic space surrounding degenerat ing embryonal vein. 10 Pulmonarv arterv.


FIOUHKS IH) AND 117

116 Same, section 41, X 225.

117 Transverse section through ventral part of upper thoracic region in a 14 mm. cat embryo (series 212, slide X, section 4h X 225.

3 Kight precaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Kxtraintimal or perivenous h'mphatic space surrounding degenerat ing embryonal vein.

7 Aorta.

9 Trachea. 10 Pulmonar)' artery. 22 Vagus. 32 Ventral nr'diastinal venous plexus.


FIGURES 118 AND 119

118 Same, section 5.

119 Same, section 6.

3 Right precaval vein.

4 Atrophying embryonal vein, fonning kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

7 Aorta.

9 Trachea. 10 Pulmonary artery. 22 Vagus. 32 Ventral mediastinal venous plexus.


FIGURES 120 AND 121

120 Same, section 7.

121 Same, section 10.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

7 Aorta

9 Trachea. 10 Pulmonary artery. 22 Vagus. 32 Ventral mediastinal venous plexus.


FIGURES 122 AND 123

122 Saiiio. section 11.

123 Transverse section through ventral part of upper thoracic region in a 14 mm. cat embryo (series 214, slide XIII, section 4) X 225.

4 Atropliying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Kxtraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

7 Aorta.

9 Trachea.

10 Pulmonary artery.

18 Peritracheal venous ])lexus.

22 Vagus.

32 Ventral mediastinal venous plexus.


FIGURKS 124 AND 120

124 Same, section o.

125 Same, section 0.

3 Right precaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

7 Aorta.

9 Trachea.

10 Pulmonary artery.

18 Peritracheal venous plexus.

22 Vagus.

32 Ventral mediastinal venous plexus.


FKU'RKS llV) AND 127

120 J:*:Linc, section 7. 127 Same, section S.

3 Kipht j)reciiviil vein.

4 Atrophying enibryoniil vein, forming kernel in interior of <leveloping

and replacing lymphatic space, o Kxtraintimal or perivenous lymphatic space surroinuling degenerating embrj'onal vein. 7 Aorla. Trachea. 10 Pulmonary artery. 22 Vagus. 32 Ventral mediastinal venous plexus.


FKiUHES Vis AM) \2^

12S Same, sor-tioii 10. 129 Sanio. section 11.

3 Kiglit prcK'aval vein.

4 Atrophying embryonal vein, fonuing kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous hinphatic space surrounding degenerat ing embryonal vein.

7 Aorta.

9 Trachea. 10 Pulmonary arterv. 22 Vagus.


FIGURES 130 AND 131

130 Same, section 14.

131 Same, section 15.

3 Right preraval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous l>^nphatie space surrounding degenerat ing embryonal vein.

6 Precardinal or precava, resp. azygos of left side.

7 Aorta.

9 Trachea. 10 Pulmonary artery. 22 Vagus.


FIGURES 132 AND 133

132 Same, section 16.

133 Same, section 17.

3 Right precaval vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing Ijrmphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein. 9 Trachea. 10 Pulmonary artery. 22 Vagus.


FIGURES 134 AND 135

134 Same, section IS.

135 Same, section 19.

3 Right precaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal* vein. 9 Trachea. 10 Pulmonary artery. 22 Vagus.


FIGURES 136 AND 137

136 Same, section 20.

137 Same, section 21.

3 Right i)recaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Kxtraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

6 Precardinal or precava of left side.

7 Aorta.

9 Trachea. 10 l*ulmonary artery. 22 Vagus.


FIGURES 138 AND 139

13S Same, section 22. 139 Same, section 23.

3 Right pre^aval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 P^xtraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

6 Precardinal or precava of left side.

7 .\orta.

9 Trjichea. 10 Pulmonarv arterv. 22 Vagus.


FIGURES 140 AND Ul

140 Same, section 24.

141 Same, section 25.

3 Right precaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing l3rmphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

6 Precardinal or precava of left side.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonary artery.

22 Vagus.


FIGURES 142 AND 113

142 Transverse section through ventral part of upper thoracic region in a 15 mm. cat embr>'o (series 216, slide X. section 32), X 225.

143 Same, section 33.

3 Right precaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space. 5» Extrainthnal or perivenous lymphatic space surrounding degenerating embryonal vein.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonary artery. 22 Vagus.


FIGURES 144 and 145

144 Same, section 34.

145 Same, section 35.

3 Right prccaval vein.

4 Atrophying embryonal vein, forming kernel in interior of devel oping and replacing lymphatic space.

5 Kxtraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

6 Precardinal or precava of left side.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonarv arterv.

18 Peritracheal venous plexus.

22 Vagus.


FIGURES 146 AND 147

146 Transverse section through ventral part of upper thoracic region in a 15.6 mm. cat embryo (series 215, slide XIV, section 31), X 225.

147 Transverse section through ventral portion of upper thoracic region in a 16 mm. cat embryo (series 230, slide XII, section 25), X 225.

3 Right precaval vein.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

6 Precardinal or precava of left side.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonary artery.

22 Vagus.

32 Ventral mediastinal venous plexus.


FIGURE US

148 Tninsvorso section through ventral portion of upper thoriicic region in a 16 mm. eat embryo (series 222, slide X, section 2), X 225.

1 Sympathetic nerve.

3 Right Azygos vein, thoracic porticm.

() Precardinal or precava of left side.

7 Aorta.

8 Oesophagus. Trachea.

10 Puhnonarv arterv.

22 Vagus.

3fi Thoracic duct, azygos segment.

37 Hrnncho-mediastinal duct.


FIGURES 149 AND 150


149 Same, section 2.

150 Same, section 3.


FIGURES 151 AND 1.52


151 Same, section 10.

152 Same, section 15.


FIGURE 153

153 Same, section 16.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Precardinal or prccava, resp. azygos of left side.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonary artery.

22 Vagus.

36 Thoracic duct, azygos segment.

37 Broncho-mediastinal duct.


FIGURES 151 AND 152


151 Same, section 10.

152 Same, section 15.


FIGURE 153

153 Same, section 16.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Precardinal or precava, reap, azygos of left side.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Puhnonary artery.

22 Vagus.

36 Thoracic duct, azygos segment.

37 Broncho-mediastinal duct.


FIGURE 154

154 Transverse section through ventral portion of upper thoracic region in a 20 mm. cat embryo (series 241, slide, XIX, section 18). X 225.

3 Right precaval vein.

7 Aorta.

8 Oesophagus.

9 Trachea.

10 Pulmonary artery.

IS Peritracheal venous plexus.

22 Vagus.

37 Broncho-mediastinal duct.


FIGURES 155 AND 156


155 Same, section 19.

156 Same, section 20.

3 Right precaval vein.

7 Aorta.

9 Trachea.

10 Pulmonary artery.

22 Vagus.

37 Broncho-mediastinal duct.


FIGURES 157 AND 158


157 Same, section 21.

158 Same, section 22.

9 Trachea.

10 Pulmonary artery.

22 Vagus.

37 Broncho-mediastinal duct.


The development of the preazygos segment of the thoracic duct

B. The development of the proximal portion of the thoracic duct proper between the termination of thoracic duct approach of the jugular lymph sac and the beginning of the azygos segment of the thoracic ducts, caudal to the level of the aortic arch, viz, the preazygos segment of the thoracic duct.

In the earlier purely venous stages a venous plexus between oesophagus and vertebral column (i7, fig. 101), drains caudolaterad into the mesal surface of the main jugular and innominate trunks. This plexus continues the supracardinal venous line cephalad beyond the level of the azygos-Cuvierian junction. The terminals of this plexus are frequently joined by dorsal somatic venous tributaries {16, fig. 101), near their entrance into the main vein. These vessels have been previously discussed in their relation to the general tributary system of thejuguloinnominate lin-e (cf. supra p. (92), and their disposition is schematically shown in fig. 101.

In a 14 mm. embryo (series 210, slide viii, section 41, X 225) the dorso-medial or prevertebral tributary {17) and the dorsal somatic branch {16) are shown in fig. 102, just prior to their union into the common large trunk emptying into the dorsomedial angle of the internal jugular vein {25), dorsal to the sjinpathetic nerve (i).

The same plexus elements {16 and 17) are seen in another 14 mm. embryo (series 212, slide vii, section 23, X 150) in fig. 159. In this section their point of confluence and the entrance of the resulting combined trunk into the internal jugular vein takes place ventral to the sympathetic nerve (i), between the same and the left subclavian artery (55), by utilizing the pathway afforded by the peritracheal plexus {18, in fig. 102).

Figs. 160 and 161 show these tributaries {16 and i7) in a 13.5 mm. cat embryo (series 189, slide vii, sections 20 and 21) in a magnification of 225 diameters, as they are approaching their confluence and point of entrance into the internal jugular vein {25), In addition these sections afford admirable examples of two other conditions, previously noted (vide supra pp. 49) :

a. The development of independent mesenchymal intercellular lymphatic spaces, with well defined endothelial limiting walls.


Such spaces {77) are seen in the two sections lateral to the oesophagus. They apparently develop from the beginning as intercellular clefts, without relation to embryonic veins. The preazygos region presents these spaces in almost all 13 and 14 mm. embryos. Their development in the mammalian embryo is of the utmost phylogenetic significance and affords important evidence in the interpretation of systemic lymphatic ontogenesis in the sauropsida, and especially in reptiles. The investigation of avian and reptilian lymphatic development has been carried on for the past year in the anatomical laboratory of Columbia University. These researches are now nearly completed, and some of the main results were presented at the 27th session of the American Association of Anatomists at Cornell University in December of last year. They establish a common genetic ground plan for the development and adult organization of the amniote lymphatic system. The avian and reptilian type only differs from that encountered in the mammal in respect to the higher degree of development of the jugular lymph sac, as an anterior or cervical veno-lymphatic heart, in the ontogenetic appearance of other areas of equal phylogenetic significance, as remnants of the multiple ancestral series of segmental veno-lymphatic hearts, and in the preponderance of peripheral lymphatic development by confluence of independent intercellular mesodermal spaces, not associated topographically, as in the mammal, with temporary embryonic venous channels. In other words, the extraintimal perivenous development of the majority of peripheral lymphatic channels described in this communication appears as a caenogenetic process in the mammalian embryo, whereas in the sauropsid amniotes most of the systemic lymphatic channels develop along more primitive phylogenetic lines, by the direct confluence of numerous intercellular mesodermal spaces.

These relative conditions have been outlined in a previous publication on the phylogenetic relations of the lymphatic and blood-vascular systems in vertebrates.*^^ The results of the more recent detailed investigations of Ijonphatic development in birds and reptiles have fully sustained the interpretation given in the publication quoted.

Geo. S. Huntington, "The phylogenetic relations of the lymphatic and blood -vascular systems in vertebrates.*' Anat. Record, vol. IV, no. 1, January 1910.


b. The darkly stained mass of cells seen in the interval between 16 and i7 in both sections is the result of obliteration of early embrj^onic vascular channels, which have lost their connection with the vessels, and whose endotheUal walls are apparently in the process of reverting to indifferent mesodermal cells. Many of these degenerated vascular aggregations are formed in embryos between 13 and 15 mm. crown-rump measure.

Some of the elements of this early embryonic prevertebral venous plexus are secondarily replaced by perivenous or extraintimal IjTnphatic spaces in exactly the same way as is observed in the development of the ventral mediastinal duct. The resulting, originally separate, extraintimal lymphatic anlages, having replaced the venule along and around which they primarily developed, unite with each other and form the preazygos segment of the thoracic duct, between the thoracic duct approach of the jugular lymph sac and the level of the aortic arch, at which the azygos portion of the thoracic duct begins.

Fig. 162 shows a transverse section of the left side of the upper thoracic region in a 14 nron. cat embr^^o (series 210, slide IX, section 26, X 200). The prevertebral tributarj^ plexus (17) turns mesad of the sympathetic nerve (1) to enter the dorso-medial angle of the left innominate vein (40), Dorsad of the main vein is the ascending portion of the left subclavian artery (S3). Between aorta (7) and innominate vein (40) are other elements of the mediastinal venous plexus, in the interval between the sympathetic nerve (1) and the vagus {22) j with ventral branches (32) in front of the latter.

Between the oesophagus and the prevertebral plexus (17) is an area in which venous elements (4), formerly connected with the same, are undergoing replacement by the extraintimal lymphatic anlages (5) of the preazygos segment of the thoracic duct. These structures cannot be distinctly made out in fig. 162, but they are shown in a higher magnification in detail in fig. 163, which includes the region of the same section dorsal to the oesophagus magnified 225 diameters. The prevertebral venous plexus (17) extends in an arch concentric with the dorsal circumference of the oesophagus (5), in front of the vertebral centers. Between it and the oesophagus, mesal to the sympathetics (i), are seen on each side former elements of the venous plexus (4) surrounded by the extraintimal lymphatic anlages (5) of the preazygos segment of the thoracic duct.

Figs. 164, 165 aiid 166 show the same structures in the next three sections (25, 24 and 23), proceeding cephalad. Fig. 166 is especially clear. In it the original line of connection of the atrophying venous kernel (4) with the prevertebral plexus (17) can still be made out as a band of condensed mesoderm, and the endothelium of the perivenous lymphatic space (5) is clearly marked.

Figs. 167 and 168 show this area in two successive sections of the same embryo further caudad (series 210, slide ix, sections 43 and 44, X 150), and fig. 169 gives the last section magnified 200 diameters. The empty central venous remnant (4), with the enveloping extraintimal lymphatic anlage (5), and the still functional elements of the prevertebral plexus (17), can be seen in all of the sections. They also show, nearer to the dorsolateral wall of the oesophagus, one of the collections of darkly stained mesodermal cells above referred to as representing remnants of formerly patent blood-vessels.

The series of microphotographs shown in figs. 160 to 169 inclusive again conclusively demonstrates the origin of this portion of the lymphatic channel by confluence of independent extraintimal mesenchymal spaces surrounding and replacing degenerating embryonic venules. The reproductions only give relatively faint and indistinct views of the actual conditions observed under the microscope. The general mesenchymal tissue in this region is extremely loose and the area is very difficult to photograph sharply and distinctly. The actual sections, however, offer histological pictures identical with those observed as above described in the development of the ventral mediastinal lymphatic duct. The extraintimal lymphatic spaces are either closely applied to part of the circumference of the wall of the shrinking venule, or they occasionally completely surround the same. In the former case the contained venous remnant is connected at part of its periphery with the wall of the enveloping lymphatic space. In the latter and more exceptional case, the free ventral nucleus of the lymphatic space is formed by the atrophied vein. In either case the lumen of the' latter may contain a few degenerated red blood cells. These pictures are again constant in embryos of the appropriate stages. The lymphatic anlage can be accurately traced from its indefinite beginning among the perivenous mesenchymal intercellular clefts through a number of successive sections to its similar distal terminations in the same intercellular plexus. Following the sections from this point caudad through a varying intervening area in which no distinct lymphatic channel appears, the same line will sooner or later reveal the repetition of the same process, and the formation of another link in the still disjointed chain of primitive lymphatic aniages. It is again to be noted here that in the earlier stages (13, 13.5,

14 mm.), the mdividual spaces, plus thek kernel of atrophied vein, are relatively larger than in the later (15-15.5 mm.) embryos. In the earlier stages (13 nrni., 13.5 mm. and 14 mm. embryos) these separate and still isolated segments of the future continuous lymphatic channel begin and end blindly. In the

15 and 15.5 mm. stages they have increased in number and adjacent links have united to form longer segments. Finally, in the 16 mm. cat embryo, the preazygos portion of the thoracic duct is usually established as a nearly continuous channel through the further confluence of the originally separate and distinct components. Every stage of the extraintimal lymphatic development can be observed with absolute certainty and accuracy in every series within the proper length limits.


FIGURE 159

159 Transection of upper thoracic region in a 14 mm. cat embryo (series 212, slide VII, section 23), X 150.


1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

11 Jugular lymph sac.

16 Dorso-medial somatic venous tributaries.

17 Prevertebral venous plexus.

20 Subthymic venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery. 25 Internal jugular vein. 33 Left subclavian artery.


PART II, PLATES

FIC;URES 159 TO 169


FIGURE 159

159 Transection of upper thoracic region in a 14 mm. cat embryo (series 212, slide VII, section 23), X 150.


1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

11 Jugular lymph sac.

16 Dorso-medial somatic venous tributaries.

17 Prevertebral venous plexus.

20 Subthymic venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery. 25 Internal jugular vein. 33 Left subclavian artery.


FIGURES 162 AND 163

162 Transverse section of upper thoracic region in a 14 mm. cat embryo (series 210, slide IX, section 26), X 200.

163 Dorsal portion of same section, X 226.

1 Sympathetic nerve.

4 Atrophying embryonal vein, forming kernel in interior of develop ing and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

7 Aorta.

17 Prevertebral venous plexus.

22 Vagus.

32 Ventral mediastinal venous plexus^

33 Left subclavian artery. 40 Innominate vein.


FIGURES 162 AND 163

162 Transverse section of upper thoracic region in a 14 ram. cat embryo (series 210. slide IX, section 26), X 200.

163 Dorsal portion of same section, X 225.

1 Sympathetic nerve.

4 Atrophying embryonal vein, forming kernel in interior of developing and replacing lymphatic space, o Kxtraintimal or perivenous lymphatic space surrounding degenerating embryonal vein. 7 Aorta.

17 Prevertebral venous plexus. 22 Vagus.

32 Ventral mediastinal venous plexus*

33 Left subclavian arterv. 40 Innominate vein.


FKIUHES im, 165 AND \m

164 Same, section 25.

165 Same, section 24.

166 Same, section 2ii.

1 Sympathetic nerve.

4 Atrophying embryonal vein, fonniug kernel in interior of develop ing and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic apace surrounding <iegen crating embryonal vein. 8 Oesophagus. 17 Prevertebral venous plexus.


FIGURES 167 AND 168

167 Same, section 43 X 150.

168 Same, section 44 X 150.

1 Sympathetic nerve.

4 Atrophying embryonal vein, forming kernel in interior of develop ing and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degen erating embryonal vein, 17 Prevertebral venous plexus.


FIGURE m) 169 Same, section 44. X 200.

1 Sympathetic nerve.

4 Atrophying embryonal vein, forming kernel in interior of develop ing and replacing lymphatic space.

5 Kxtraintimal or perivenous lymphatic space surroumiing degener ating embryonal vein. 8 Oesophagus. 40 Innominate vein.


FIGURE 170

170 Reconstruction of left vascular complex of lower cervical and upper thoracic regions of a 15 mm. cat embryo (series 218, slide VIII, sections 25 to 44; slide IX, sections 1 to 48; slide X, sections 1 to 48; slide XI, sections 1 to 48; slide XII, sections 1 to 48, X 225), dorsal view.

1 Sympathetic nerve.

3' Right azygos vein.

3 Right precava.

6 Left precava.

6' Left azygos vein, thoracic portion.

7 Aorta.

10 Pulmonary artery.

12 Jugular lymph sac, thoracic duct approach.

16 Dorso-medial somatic venous tributaries.

22 Vagus.

24 Thyrocervical artery.

33 Left subclavian artery.

35 Thoracic duct, preazygos segment.

35' Detached lymphatic space in path of preazygos channel.

37 Broncho-mediastinal duct.

41 Vertebral artery.

42 Vertebral vein.

45 Innominate artery.

46 Left superior intercostal vein.

53 Precaval lymphatics.

54 Fifth spinal nerve.

55 Sixth, seventh and eighth spinal nervt

56 Ductus arteriosus Hotalli.

57 Right subclavian artery.


FIGURE 171

171 Reconstruction of left vascular complex of lower cervical and upper thoracic regions in a 15.5 mm. cat embryo, (series 143, slides VIII, IX, X, and XI), X 225 View from behind and from right side.

1 Sympathetic nerve. 3' Right azygoB arch. 3 Right precava.

6 Left precava.

6' Left azygos vein, thoracic portion.

7 Aorta.

12 Jugular lymph sac, thoracic duct approach.

13 Jugular lymph sac, jugular approach. 17 Prevertebral venous plexus.

19 Suprathymic venous plexus.

20 Subthymic venous plexus.

21 Thymus.

22 Vagus.

23 Left carotid artery. 23' Right carotid artery.

24 Thyrocervical artery.

29 Jugulo-subclavian trunk.

35 Thoracic duct, preazygos segment.

36 Thoracic duct, azygos segment.

40 Innominate vein.

41 Vertebral artery.

42 Vertebral vein.

45 Innominate artery.

54 Fifth spinal nerve.

55 Sixth, seventh and eighth spinal nerve 57 Right subclavian artery.


FIGURES 172 AND 173

172 Traiiaverso section of loft side of upper thoracic rej^iori of a 15 mm. cat embryo, (series 143, slide XI, section 10), X 2(K).

173 Same, section 14.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

17 Prevertebral venous plexus.

33 Left subclavian arterv.

35 Thoracic duct, ])reazygos segment.

37 Broncho-mediastinal duct.

38 Confluence of broncho-mediastinal duct and prcazygos segment of

thoracic duct. 40 Innominate vein.


KIGUKKS 174. 175 AND 176

174 Same, section 12.

175 Same, section 11.

176 Same, section 6.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

33 Left subclavian artery.

35 Thoracic duct, preazygos segment.

37 Broncho-mediastinal duct.

38 Confluence of broncho-mediastinal duct and preazygos segment of

thoracic duct. 40 Innominate vein.


FIGURES 177, 178 AND 179

177 Same, section 5.

178 Same, section 2.

179 Same, slide IX, seetion 32.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

35 Thoracic duct, prcazygos sej^ineni.

37 Bron(!ho-mediaatinal duct.

38 Confluence of broncho-mcdiastinal (hict and preazvRos sei^eni <»f

thoracic (hict.

40 Innominate vein.

41 Vertebral arterv. 44 (\)sto-rervical vein.


FICiURKS ISO, 181 A\D 182

ISO Same, slide X, section 28. 181 Same, slide X, section 20. 1S2 Same, slide X, section 23.

1 Sympathetic nc^-ve.

8 Oesophagus.

9 Trachea.

33 Left subclavian artery.

38 Confluence of l)roncho-inediastinal duct and preazygos segment of thoracic duct.

40 Innominate vein.

41 \'ertel)ral artery.

43 ('o<to-cervical arterv.

44 (\)sto-c(M*vical vein.


FIGURE 183 183 Same, slide X, section 22.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

12 Jugular lymph sac, thoracic duct approach.

22 Vagus.

23 Left carotid artery.

33 Left subclavian artery.

35 Thoracic duct, preazygos segment.

40 Innominate vein.

41 Vertebral artery.

43 Costo-cervical artery.

45 Innominate artery.


FIGURES 184 AND 185

184 Transverse section through upper part of left thoraric region in a 15.5 mm. cat embryo, (series 141. slide X, section IS), X 200.

185 Same, section 21.

1 Sympathetic nerve.

8 Oesophagus.

9 Trachea.

16 Dor.so-medial somatic venous tributaries.

18 Peritracheal venous plexus.

22 Vagus.

23 Left carotid artery.

33 Left subclavian artery.

38 Confluence of broncho-niediastinal duct and preazygos segment of

thoracic duct. 40 Innominate vein.


FIGURE 18S

188 Reconstruction of cephalic portions of azygos veins and anlages of azygos segment of thoracic ducts in a 13.5 mm. cat embryo, (series 76, slide X, sections 1 to 27; slide XI, sections 1 to 38; slide XII, sections 1 to 40; slide XIII. sections 1 to 33; slide XIV, sections 1 to 7) , X 225. Ventral view.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.


FKJURK 189

189 Reconstruction of cephalic portions of azygos veins and anlages of azygos segment of thoracic ducts in a 14 mm. cat embryo; series 34 (Princeton Embryological (Collection), slides XXIV to XXXTI, 222 sections, X 225. Ventral view.

1 Sympathetic nerve. 3 Right precava.

3' Right azygos vein, tlioracio portion. 6 Left precava.

6' Left azygos vein, thoracic portion. 34 Ventro-medial azygos plexus.


FIGURE 190

190 Reconstruction of cephalic portion of azygos veins and anlages of azygos segment of thoracic ducts in a 14 mm. cat embryo; (series 214, slides XIII, XIV, XV, XVI and XVII i, X 225. Ventral view.

1 Sympathetic nerve. 3 Precardinal or precava of right side. 3' Right azygos vein, thoracic portion. 6 Precardinal or precava of left side. 6' Left azygos vein thoracic portion. 34 Ventro medial azygos plexus.


FIGURE 19()

100 Roconstriiction of (•e[)hiilic portion of azygos veins and milages of azygos segment of thoracic duets in a 14 mm. cat embryo; (series 214. slides XIII, XIV, XV, XVI and XVII ), X 22.'). Ventral view.

1 Sympathetic nerve.

3 Precardinal or precava of right side.

3' Right azygos vein, tlioracic portion.

6 Precardinal or i)recava of left side,

f)' Left azygos vein thoracic portion.

34 Ventro medial azygos ple.xus.


FIGURE 191

191 Ucoonstnirtion of cephalic portions of azygon veins and aniages of azygoH segment of thoracic ducts in a 15 mm. cat embryo; (series 219, slides XTV, XV, XVI and XVII), X 225. Ventral view.

1 Sympathetic nerve.


FIGURE 192

192 Reconstruction of cephalic portions of azygos veins and anlages of azygos segment of thoracic ducts in a 15.5 mm. cat embryo; (series 143, slides XIV, XV, XVI, XVII and XVTII) X 225. Ventral view.

34 Ventro-medial azygos plexus.


FIGURK 193

193 Reconstruction of azygos veins and of thoracic and right lymphatic ducts In a 16 mm cat ombrvo: ('serios96, slides VII to XVI) X 225. Ventral view.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.

47 Right lymphatic duct.

51 Mesenteric lymphatics.


(f)) is larger and still contains a number of red blood cells. In the four succeeding sections, 19, 20, 21 and 22 (figs. 248, 249, 250 and 251) the former line of communication of the decadent venule (4) with the functional azygos channel can still be distinctly traced as a strand of connective tissue cells joining it to the beginning of the right interazygos plexus. The separation of the venous radicle undergoing extraintimal lymphatic replacement has just been accomplished in this area.

The six sections of this embryo shown in figs. 246 to 251 should he compared with the slightly more advanced 14 mm. embryo 214; where, e.g., in figs. 230, 231, 232, 214, 215, 216, 217, 218 and 219 exactly equivalent sections are given, showing the next step in extraintimal lymphatic development in this region. The atrophying venule (4) in the earlier embryo (series 34) still contains red blood cells and still betrays its original connection with the functional venous channels. In the genetically slightly older embryo (series 214) the same structure appears now as an empty endothelial bag (4), completely detached from the azygos veins. In both embryos this decadent venous radicle is surrounded by the developing extraintimal lymphatic space (5). The comparison of the sections shows every step in the process of development in these two closely connected stages. The reduced photographic reproductions do not give the clear and unmistakable pictures presented by the actual sections, owing to the loss of the diflFerential stain and of the focal adjustment. Still the contour of the venous core, undergoing atrophy and replacement by the extraintimal lymphatic anlage, its content of red blood cells and its recently interrupted connection with the main azygos vein in the younger embryo (series 34), can be followed with a sufficient degree of clearness and can be contrasted with the conditions presented by the succeeding stage (series 214).

Figs. 244 and 245 give two successive sections from the caudal part of the thoracic region in series 34 (slide xxxi, sections 18 and 19, X 225) in which the earlier conditions, preceding the first appearance of the lymphatic anlages of the thoracic duct in the azygos segment, are shown.

Between the right azygos vein {3) and the aorta (7) are seen the components of the ventro-medial azygos plexus {4)f partially separated from the main venous trunks, but not, as yet, surrounded by extraintimal lymphatic anlages, as, further cephalad, '(slide xxviii) in the sections just described, (figs. 246 to 251).

The 15 mm. and 15.5 mm. embryos show, in the majority of instances, an advance over the preceding stage in two directions :

I. The central venous core of the 14 mm. embryo is further reduced or entirely eliminated, leaving the lymphatic anlage, which formerly surrounded it, with a clear lumen.

II. The individual scattered lymphatic areas have united to form longer segments of the future lymphatic channel system. This condition is well shown in the ventral view of the reconstruction of an average 15 mm. embryo (series 219) in fig. .191 (cf. supra, p. 126).

Figs. 252 to 258 inclusive show transverse sections of the lower thoracic region in a 15.5 mm. cat embryo, (series 143, slide xviii section 7, 11, 12, 13, 14; slide xvii, sections 31 and 32, X 225), and fig. 192 gives the ventral view of the reconstruction of the azygos region in this embryo, comprising slides xiv, xv, xvi, xvii, and xviii.

The sections (figs. 252 to 258) show the following changes in the vascular structures, compared with the preceding stages

I. The right (S) and left (6) azygos veins are concentrated, more sharply defined and relatively smaller, although still equally developed on both sides.

II. The interazygos anastomosis is still well developed and uniform (cf. also fig. 192).

III. The entire azygos complex is much more closely apphed to the dorsal and dorso-lateral circumference of the aorta (7), resulting in the practical obliteration of the interval between the main veins and the artery seen in the preceding stages.

IV. This topographical rearrangement has forced the ventromedial azygos tributaries, where they still persist {34 in figs. 252, 253, 257), and the thoracic duct imlage (36) to pass from the ventro-medial to the ventral aspect of the main azygos trunks.

V. Both azygos veins lie in the region ventro-lcUeral to the intersegmental arteries. The supra-aortic area between them is still occupied by the interazygos (supra-cardinal) anastomosis (cf. infra, figs. 261 and 262).

VI. The sections taken from the caudal portion of the thoracic region (slides xvii and xviii) show the right and left thoracic duct anlages (36) following the ventral aspect of the respective azygos veins (cf. fig. 192). They are not yet united into a continuous channel, but their lumen is clear, without trace of the venous kernel around which they developed.

Fig. 252 (slide xviii, section 7) shows renmants of the ventromedial azygos plexus (34) emptying on both sides into the azygos trunks. Lateral to these venous radicles lies the thoracic duct anlage of each side {36) .

In section 11 of the same slide (fig. 253) the right thoracic duct anlage has enlarged and presents a beautifully clear picture of the relation of this structure to the rightazygos vein at this stage.

On the left side the conditions are as in the preceding figure, both persistent ventral venous tributary (34) and left thoracic duct anlage (36) lying side by side.

In the two succeeding sections, 12 and 13, (figs. 254 and 255) the right thoracic duct continues well developed and presents a pointed dorso-mesal prolongation indicative of its situation in the earlier stages, on the mesal aspect of the vein, between it and the aorta, and ventral to the interazygos anastomosis, (cf. series 34 and 214). The left duct anlage continues as a small channel.

In section 18, (fig. 256) both right and left lymphatic anlages cease as distinct chaimels, marking one of the intervals between the still disconnected segments of the future continuous duct.

Figs. 257 and 258 (sections 31 and 32 of slide xvii) show on alternating sides small thoracic duct anlages (36), and in the root of the dorsal mesogastrium sections of the well-developed anterior mesenteric lymphatics (61). Some of the later stages (series 253, figs. 265 to 270) will show the full development of these mesenteric channels and their confluence with the thoracic duct. Tliis area, is, however, responsible for the development of what I have above defined as the post-azygos segment of the thoracic duct and will hence be considered in detail in the second volume of this series.


In comparison with the series just considered figs. 259 and 260 show transverse sections of the lower thoracic region of series 258, slide xviii, sections 1 and 9, X 225. This embryo is rated as 17 mm. in crown-rump length, but the lymphatic development of the azygos region Ls relatively retarded and only slightly in advance of the conditions presented by the 15.5 mm, series 143. Direct comparison of figs. 252 to 258 with figs. 259 and 260 will show this correspondence, both at the lev^l of the intersegmental arteries {2 in fig. 260), and in the intervals between them (fig. 259) . In the older embrj'^o the right and left thoracic duct anlages (36) are somewhat larger and form longer channel segments, but otherwise the two series belong practically to the same developmental stage, in spite of their difference in total length measure. The correspondence, for example, between figs. 254 (series 143) and 259 (series 258) is remarkable.

On the other hand the majority of 16 mm., 16.5 mm. and 17 mm. embryos offer decided advances in the lymphatic development of this region beyond that normally found in the 15 mm. and 15.5 mm. stages.

Two successive sections of a typical 16 mm. embryo (series 96) in the azygos area are shown in figs. 261 and 262, and fig. 193 gives the ventral view of the reconstruction of the same embryo, in which the level of figs. 261 and 262 is indicated by the line A-B.

Comparison with the immediately preceding series (143 and 258) will show that the following fundamental changes have occurred in the azygos venous and lymphatic systems :

I. The two azygos veins (S and 6) have become reduced in size. At the level, at which the sections of figs. 261 and 262 are taken, both veins now lie dorsal to the intercostal arteries (2), whereas in the earlier stages (e.g. figs. 260, 258, 257) the azygos trunks are ventral to the arteries, and the interval between the latter, along the dorsal aspect of the aorta, is occupied by the supracardinal interazygos plexus.

Reference to the reconstruction of series 96 (fig. 193) shows that this topographical readjustment has occurred in the caudal part of the azygos area, below the level A-B, by the longitudinal hydrostatic definition of the main supracardinal azygos trunks through confluence of elements of the preceding interazygos plexus. The original post-cardinal azygos veins, which in series 258 and 143 are seen to form the main channels ventral to the intercostal arteries, are, in the caudal region of series 96, represented by two slender longitudinal trunks, on the lateral aspect of the thoracic ducts between them and the sympathetic nerves (1). The main azygos veins have developed as distinct circumscribed axial chaimels out of the antecedent supracardinal interazygos plexus, dorsal to the intercostal arterial branches (S) . Correlated with this new azygos development is the reduction of the earlier postcardinal channels, ventral to the intersegmental arteries, and their topographical replacement in large part by the right and left thoracic ducts {36) (not labelled in fig. 193). Cephalad of the level A-B the reconstruction shows the earlier relation of the azygos trunks, ventro-lateral to the intercostal arteries. In ascending the left azygos vein gradually diminishes, while conversely the vessel of the right side increases in caliber.

II. The individual separate lymphatic anlages of the azygos segment of the thoracic duct of the preceding stages have united into a continuous and uninterrupted lymphatic channel.

Caudad of the level A-B (fig. 193) the right and left thoracic ducts are fairly symmetrical bilateral channels, connected with each other by several broad and by a larger number of slender transverse anastomoses. They occupy the position of the earlier postcardinal azygos trunks, before these shifted to the supracardinal position, ventro-lateral to the intercostal arteries, along the medial border of the reduced longitudinal tributary channel representing on each side the remnant of the earlier post-cardinal azygos, and draining dorsad into the newly established supracardinal azygos line of each side.

Figs. 261 and 262 give excellent views of the topographical relations of the vascular structures, arterial, venous and lymphatic. Both sections show the right and left thoracic ducts (36), ventral to the intercostal arteries (2), the lymphatic cross anastomosis between the two along the dorsal circumference of the aorta, and the main supracardinal azygos veins (5, 6) . Cephalad of the level A-B (fig. 193) this plexiform anastomosis between the ducts increases in extent, and the entire lymphatic channel inclines, in ascending, to the left. This, as previously explained (cf. supra, p. 129), is due to the greater development of the intermediate lymphatic line, along the ventral aspect of the interazygos anastomosis, and its union with the left thoracic duct to form the plexiform channel of the adult. The corresponding lymphatic of the right side, on the other hand, becomes reduced above the level A-B, and only its cephalic portion persists as the thoracic segment of the right lymphatic duct (47 in fig. 193).

The extraintimal lymphatic anlages ventral to the thoracic ducts, which are eventually to establish the coimection between them and the mesenteric lymphatics, are in the process of development in series 96. The venous core (4) and the replacing perivenous lymphatic (5) are shown on the left side in figs. 261 and 262, and equivalent segments of this channel are seen in the reconstruction {61 in fig. 193). In other portions of the ventral and ventro-lateral periaortic mesoderm numerous independent intercellular spaces begonging to the same system are developing.

While the thoracic ducts are in the majority of 16 mm. embryos complete continuous and definite channels, as in the present average example, the acquisition of this final condition is not infrequently delayed considerably beyond this period. I have a number of 16 and 16.5 nmi. embryos in which the thoracic duct development has not advanced beyond the stage typical of the average 14 mm. or 15 nmi. embryo. This is another example of the considerable range of chronological variation of lymphatic development in individual embryos encountered in examining a large number of series of about the same length measure.

The 16 mm. embryo again furnishes conclusive evidence as to the secondary character of the mutual relation between embryonic venous and lymphatic vessels, and shows, in typical individuals, the final stage of the process through which temporary venous, channels are replaced by the permanent components of the lymphatic system. This replacement, as demonstrated by the successive series of sections from the azygos region just considered, is a gradual process. The lymphatic channel has its inception in numerous small extraintimal mesenchymal spaces which form along and around the components of an atrophying embryonic venous plexus, or follow part of the circumference of a more definite early venous trunk. As these spaces increase, enlarge and become confluent to form longer segments of the lymphatic channel, the related venous element continues to recede, imtil, in certain areas, it is entirely replaced by the lymphatic vessel. In other districts, along embryonic veins which are carried into the adult organization, as permanent components of the venous system, the redundant vein of the earlier stages becomes more circumscribed and more clearly defined as the companion lymphatic channel develops in the perivenous area. In either case the complete or partial replacement of venous by lymphatic elements does not occur suddenly, but is accomplished by gradual and progressive ontogenetic stages. I can find no evidence at any point of the separation of portions of a venous plexus or of an already organized venous chaimel, such detached parts then uniting directly with each other to form systemic lymphatic vessels. In every region of the mammalian embryo the above described extraintimal development of the lymphatic elements, and the subsequent secondary topographical replacement of decadent veins by them, can clearly be demonstrated. This process, of course, leads to a stage in which the advancing development of the lymphatic and the correlated regression of the associated venous components are balanced, and in which both, proceeding in opposite genetic directions, are foimd side by side in the same field. The stages previously discussed in detail, between 13 mm. and 15 mm. offer abundant illustration of this intermediate stage in the azygos area.

It is to be noted that in the earlier stages (13 to 14 mm.) the lymphatic extraintimal spaces first appear on the lateral aspect of the atrophying venules of the ventro-medial azygos plexus, and subsequently surround them. They are then situated at some distance ventral to the main azygos trunks. In the later stages (15 mm., 15.5 mm. and 16 mm. embrj'os) they have extended dorsad and lie in close apposition to the ventral and ventro-medial circumference of the larger veins, which position they maintain subsequently in their further development. Coincident with this relative change in position of the earlier lymphatic spaces is the appearance of additional thoracic duct anlages, which develop as extraintimal lymphatic elements along those lines of the embryonic azygos system which are destined to undergo, in the later stages, still further reduction and final elimination in the definite organization of the adult condition. [Compare on this point the sections of series 96 (figs. 261 and 262) with those of series 253 (figs. 265 to 270)].

One other fact deserves careful consideration in analyzing the development of the azygos segment of the thoracic ducts in stages between 14 mm. and 15 mm. In a certain proportion of the embryos of this period the extraintimal replacement of the earlier ventro-medial azygos plexus in the cephalic part of the future azygos segment of the thoracic duct is delayed. In these cases peculiar histological pictures are obtained in this area. The components of the ventro-medial azygos plexus have become detached from the main veins, and have fused together into an elongated spindle shaped bag, completely filled with blood, beginning and ending blindly and not in commimication with any venous channel. This structure, when found, invariably occupies the site of the long lymphatic segment seen m the reconstruction of series 214 (fig. 190) on the right side, in the areas A and B (slides xiv and xv), ventral to the right intercostal arteries, in the interval between aorta and right azygos vein. The appearance presented by it on section is shown in figs. 263 and 264 in a 14 mm. cat embryo (series 211, slide xii, sections 2 and 8, X 225). It is indicated in these figures by the leader 5^, as a detached and confluent part of the original ventro-medial azygos plexus. Fig. 263 shows this structure in the interval between the intercostal arteries. Fig. 264 gives a typical view of it at the level of one of the right intercostal branches. It lies ventral to the same, between aorta (7) mesad, the right azygos trunk (5) laterad, dorsal to the oesophagus (5). The blood cells, which completely fill its lumen, diflferentiate from those circulating in the continuous vein channels by remarkably intense reaction to the Orange-G stain. As stated, this structure is completely detached from all adjacent veins, can be followed cephalad and caudad through many sections, and ends blindly at either end in a gradually diminishing pointed extremity. In a reconstruction of this embryo it would correspond absolutely in extent and relations to the long lymphatic channel above referred to on the right side of series 214 (fig. 190). A comparison of the reconstruction (fig. 190) and of the corresponding sections (figs. 214 to 240) of this latter embryo, clearly reveal the significance of this, at first sight, somewhat anomalous and puzzling structure. It will be noted that the reconstruction of series 214 (fig. 190) shows, at the level of the line C, the caudal end of a large ventro-medial tributary trunk {34) of the right azygos vein (S), which, in proceeding cephalad, becomes imbedded in the centre of the long extraintimal Isrmphatic channel. The continuation of this venous tributary line, detached from the functional veins, furnishes the central venous core (4) which is surrounded by the extraintimal lymphatic space (6) in all the sections of series 214 shown in figs. 214 to 240 inclusive. If this ventro-medial tributary trunk {34 in fig. 190) had organized in its entire length, by confluence of the radicles of the preceding plexus, and had then become detached from the fimctional right azygos vein, it would, before extraintimal development of the surrounding lymphatic spaces began, present exactly the conditions seen in series 211. In comparing, therefore, these two 14 mm. embryos, we find in series 214 the venous line, around which this important segment of the thoracic duct develops and which it subsequently replaces, present in the condition typical for the average embryo of this period, as a detached and nearly empty venous bag, surrounded by the extraintimal lymphatic channel. On the other hand, series 211 shows the same tributary trunk, also detached from the main vein, but still filled with blood, while the enveloping extraintimal lymphatic anlages have not yet made their appearance. Comparison with the reconstruction and with the sections of series 214 prove its identity with the latter's venous core (4) in the interior of the lymphatic channel (5).

A 15 mm. embryo of the Princeton embryological collection (series 53), which I had the opportunity of studying through the courtesy of Professor McClure, shows conditions identical with those just described in series 211. It is interesting and conclusive to further compare in this respect the reconstruction of series 214 (14 mm. fig. 190) with that of series 143 (15.5 mm. fig. 192). In the former embryo the secondary para-azygos channel of the right side (34), developed by confluence of components of the right ventro-medial azygos plexus, only persists in its caudal portion, and has in the rest of its original extent been detached and surrounded by the extraintimal lymphatic channel. On the other hand, in the 15.5 mm. series 143 (fig. 192) this normal process of detachment and replacement by extraintimal lymphatic anlages has been delayed. Fig. 192 shows the para-azygos ventro-medial channel (34) as an extensive tributary trunk of the right azygos vein, cephalad of the level C. Only a few areas of the extraintimal lymphatic development occur, as yet, along its course. Further development of these and their confluence will produce the condition already seen in the earlier embryo, series 214, which is typical for the average embryo of the 14 mm. stage. Thus unusual and atypical delay of lymphatic development occurs in some embryos of this period in the region above specified, and produces a definite and uniform aberrant picture. The correct interpretation of the same is given by the conditions obtaining in the average embryo of this period. I lay some stress on the facts just described, because isolated observation of an instance of this character might readily lead to the erroneous assumption that the detached venous bag (34) of figs. 263 and 264 became directly transformed into the lymphatic channel of the thoracic duct occupying in the later stages the identical topographical position and maintaining the same relations to surrounding structures. This error can only be avoided by determining, in numerous embryos of this period, the typical and average condition of lymphatic development.

Thus in the average 16 mm. cat embryo the originally separate and independent extraintimal anlages of the two thoracic ducts have become confluent, and form from this stage onward usually a continuous and uninterrupted lymphatic channel, which is connected with the general venous system through the jugular lymph sac, by union with the thoracic duct approach of the latter.

From this stage forward the subsequent growth and readjustment of the thoracic ducts is merely a question of continuous enlargement of the lymphatic channels at the eicpense of the azygos veins. The latter recede still further, while the former correspondingly increase until the adult relative condition of both systems is attained. In this readjustment the thoracic ducts of the adult cat acquire for themselves in large part the topographical position occupied in earlier embryonic stages by portions of the azygos complex. The method of this substitution has been sufficiently and repeatedly detailed in the preceding pages.

Figs. 265 to 270 inclusive show the results in transverse sections of the middle thoracic region m a 19 mm. cat embryo (series 253, slide xxiv, sections 9, 11, 15, 16, 17 and 18, X 225).

In fig. 265 (section 9) both azygos veins are reduced, but the right trunk (S) has mamtamed itself as the principal vessel, stiU connected by the interazygos anastomosis (16) with the very much smaller left azygos (6). Ventral to the veins, on each side of the aorta (7), are the right and left thoracic ducts (36). The right duct, as is mvariably the case in this region, is the larger, and shows a pointed dorso-medial extension which passes heLd the aorta towards the ventral aspect of the interazygos anastomosis. This is a further development of the same conditions seen in the 15.5 nmi. and 17 mm. embryos, in figs. 254, 255 and 259, and is responsible for the formation of the intermediate dorsal lymphatic element seen in the 16 mm. embryo, (figs. 261 and 262) behind the aorta, between the azygos veins and the intercostal arteries. (cf. especially left side of fig. 262).

On the left side of fig. 265, ventral to the left thoracic duct, is seen a venous radicle (4) in relation to an extraintimal anlage (5). In section 1 1 (fig. 266) this space has enlarged. In the succeeding sections 15, 16, and 17 (figs. 267, 268 and 269) the same structure appears as a well developed lymphatic channel (61), in the root of the dorsal mesentery, and in section 18, (fig. 270) it opens into the left thoracic duct. It marks one of the points of connection of the mesenteric lymphatic plexus with the thoracic duct, which leads to the ultimate dififerentiation of the postazygos segment of this channel. The anlage is the same previously noted in the 16 mm. embryo, (4 and 5, in figs. 261 and 262, series 96), and in the 15.5. mm. embryo, series 143 {61 in figs. 257 and 258).


DEVELOPMENT OF THE THORACIC DUCT 149

Fig. 266 (section 11) is taken at the level of a pair of intercostal arteries (j?), which perforate the interazygos anastomotic plexus {16) and separate the same from the azygos trunks. The dorsomedial extension of the right thoracic duct towards the interval between the interazygos channel (16) and the dorsal circumference of the aorta (7) has increased. In section 15 (fig. 267) it has crossed to &ie mesal side of the right intercostal arterj'- and occupies part of the area filled in the preceding sections by the interazygos anastomosis. This latter is reduced to insignificant remnants. The left azygos vein (6) appears as a minute channel between the left intercostal artery (;?) and the left thoracic duct {S6). It can be traced in the same condition in the succeeding sections.

In sections 16, 17 and 18, (figs. 268, 269 and 270) the intermediate l3rmphatic channel appears along the dorsal circumference of the aorta, between the intercostal arteries, which now pass through meshes separating the three chief channels of the lymphatic plexus, viz. the left and right thoracic ducts and the intermediate supra-aortic lymphatic.

The three last figures of this series show very clearly, by comparison with figs. 261 and 262, the advances in lymphatic development, and the recession of the azygos system, especially of the left azygos vein, in passing from the 16 mm. (series 96) to the 19 mm. stage (series 253). The latter presents practically adult conditions in both the venous and the lymphatic systems.

It only remains to be noted that, in the adult cat, the right azygos vein shifts secondarily dorsad to a submuscular prevertebral anastomotic venous chain at the 9th or 10th costo-vertebral level, and thus affords the opportunity for the greater development of the caudal part of the right thoracic duct which replaces the original azygos channel and establishes connection with the abdominal lymphatics, forming the above defined Post-azygos segment of the thoracic duct."

It is not the purpose of the present paper to enter into the development of the abdominal connections of the thoracic ducts. This subject requires a minute consideration of the conditions encountered in the adult manrntial, and is hence properly reserved for future detailed consideration.


150 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS

Junction of azygos and preazygos segments of the thoracic ditct

These segments of the main adult lymphatic channel usually unite by end-to-end fusion in embryos between 15 mm. and 16 mm. crownrump measure, although, as previously stated, the establishment of a continuous lymphatic channel may be delayed in individual instances beyond this period.

Figs. 271 to 275 inclusive show five successive transverse sections of this junctional area of the two segments in the cephalic portion of the dorsal mediastinal region in a 15.5 mm. cat embryo (series 143, slide xii, sections 22, 23, 24, 25 and 26, X 200).

Fig. 271 (section 22) contains the right precava (S) and the left azygOvS-C^uvierian arch (ff), with the cephalic termination of the interazygos plexus (15,) dorsal to the oesophagus (8). The caudal extremity of the preazygos segment of the thoracic duct (36) is seen on the loft side, and continues in the next section (fig. 272) as the link between the preazygos and azygos segments of the duct {35, 36 in fig. 272). In the following section 24 of this slide (fig. 273) the darker area dorsal to the lymphatic channel marks the blind cephalic end of the left azygos arch, and hence characterizes the lymphatic channel (36) as the beginning of the azygos portion of the duct. In fig. 274 (section 25) the left azygos arch (dorsal 3) is opened by the section and receives the terminal of the cephalic end of the interazygos plexus (16) seen in the three immediately preceding sections (sections 24, 23, and 22, figs. 273, 272 and 271). We are therefore now at the cephalic end of the azygos segment of the thoracic duct (36), ventral to the azygos vein (3) and the interazygos anastomosis, and can trace the same unintorniptedly fn)m this point cephalad into continuity with the beginning of the preazygos channel (55), as seen in sections 23 and 22, figs. 272 and 273.

In the following section (26, fig. 275) the left azygos arch (S) is fully opened in its turn ventrad to join the left precaval vein, and the azygos segment of the thoracic duct (36) occupies the typical position of this period, between the ventro-medial aspect of the right azygos trunk (5) and the dorso-lateral circumference of the oesophagus (S), trending sinistro-cephalad into the Ime of the preazygos segment {36, m figs. 271 and 272).

Reference to the dorsal view of the reconstruction of this embryo (fig. 171) shows that the azygos segment of the thoracic duct (36) has united with part of the caudal division of the preazygos chain of lymphatics (35), cephalad of the interazygos anastomosis, into a continuous lymphatic channel, although the con.tinuity with the main preazygos segment has not yet been completely established. In other words this condition has been very nearly, but not quite, attained. Some of the preazygos elements (36) have already fused with the azygos segment, thus carrying the latter cephalad beyond the interazygos anastomosis into the territory of the preazygos portion of the thoracic duct.

Part II. Plates

FIGl RK8 194 TO 275


191 Transvorso scM-tion of iiiiddlo thoracic region of 11 nini. cat embryo; (series 213. slide XI, section 9) X 225. 195 Same, section 10.

1 Syini)athetic nervt*.

'i Right azypos vein, thoracic portion.

6 Left azyjjos vein, thoracic portion.

7 .\orta.

S Oesojihagiis. 34 Ventro-medial azygos plexus.


FI(;URKS llHi AM) 197

196 Siune, section 27.

197 Same, section 29.

1 Sympathetic nerve.

2 Intercostal arterv.

3 Right azygos vein, thoracic portion. Left azygos vein, thoracic portion.

.'U Ventro-medial azygos plexus.


FIGURES 198 AND 199

198 Transverse section of middle thoracic region of 12 mm. cat embryo, (Reries 217, slide XI, section 27( X 225.

199 Same, section 28.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.

7 Aorta.

34 Ventro-medial azygos plexus.


FIGUKES 2(K) AND 201

200 Same, s(Mtion 2).

201 Samo. slide X. section 18.

1 Sympathetic nerve.

2 InterscKniental arteries.

3 Right azygos vein. thoraci<* portion.

6 Left azygos vein, thoracic portion.

7 Aorta.

34 Ventro-nuMlial azygos plexus.


FIGURES 202 AND 203

202 Same, section 30.

203 Same, section 31.

1 Sympathetic nerve.

3 Right azvKos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of develop ing and replacing lymphatic space, o Extraintimal or perivenous lymphatic space surrounding degenerating embryonal vein.

6 Left azygos vein, thoracic portion.

7 Aorta.

8 Oesophagus.

34 Ventro-medial azygos plexus.


FIGURES 204 AND 205

204 Transverse soction through iniddlo thoni^'ic region of a 13.5 mm. cat embryo; (.series 189. slide X. seeti(m 15) X 225.

205 Same, section 10.

1 Sympathetic nerve.

3 Right azygos vein, thoracic ])ortion.

6 Left azygos ve.n, thoracic portion.

7 Aorta.

S Oesophagus. 34 Ventro-medial a/ygos jilexus.


FKUJKKS 2(Mi AND 2()7

206 TraiisvcTsc Miction through middle thoracic rogion in a 13.5 mm. cat embryo, ' scries 7(). slide XI, section 15) X H(K). 2()7 Sjune. section Hi.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of develop ing and replacing lymphatic space.

5 Kxtraintimal or perivenous lymphatic space surrounding degener ating embryonal vein

7 Aorta.

8 Oesophagus.

15 Interazygos venous plexus.


FI(;i KKS 2(IS AND 2()9

'J{)s Sjiiiir. «»i*rn«u 17. NH» S.iiiu-. Motion !*).

■« hitri'smtiuMiiul artorio>.

.1 Kmht :uvKi»s \oin. thorarir pi>rli<)n.

I VtrophviiiK oinbryoual vein, forming kernol in interior of develop iiiK hihI ivplHi'ing lymphatic vein. » KvtraiiiiiiUHl i>r jHTivenous lymphatic space surrounding deKenor atiii^ cml>rvoiial voin. (I Lcl't a/.yi5o> vein, thoracic {xisition. / Voria. \ lVMipha\;UN.

|.'» lhtoniJi>nt»^ venous plexus.


P^IGURES 210 AND 211


210 Same, section 21.

211 Same, section 22.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of develop ing and replacing lymphatic space, o Extraintimal or perivenous lymphatic space surrounding degenerating embryonal vein.

7 Aorta.

8 Oesophagus.

15 Interazygos venous plexus.


FIGURES 212 AND 213

212 Same, section 23.

213 Same, section 28.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

6 Left azygos vein, thoracic portion.

7 Aorta.

8 Oesophagus.

15 Interazygos venous plexus.


FIGURES 214 TO 217

214 Transverse section through middle thoracic region in a 14 mm. cat 'I embryo, (series 214, slide XIV, section 15) X 225.

215 Same, section 16.

216 Same, section 17.

217 Same, section 19.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space sourrounding degenerat ing embryonal vein.


FIGURES 218 TO 221

218 Same, section 20.

219 Same, section 22.

220 Same, section 26.

221 Same, section 27.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.


FIGURES 222 TO 225

222 Same, section 28.

223 Same, section 32.

224 Same, section 34.

225 Same, section 1.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerat ing embryonal vein.

6 Left aeygos vein, thoracic portion.

7 Aorta.

8 Oesophagus.

16 Interazygos venous plexus.


FIGURES 226 TO 229

226 Same, section 2.

227 Same, section 3.

228 Same, section 4.

229 Same, section 5.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerating

embryonal vein.


FIGURES 230 TO 233

230 Same, slide XV, section 10.

231 Same, section 11.

232 Same, section 14.

233 Same, section 16.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerating

embryonal vein.

6 Left azygos vein, thoracic postion.

7 Aorta.

8 Oesophagus.


FIGURES 234 TO 237

234 Same, section 19.

235 Same, section 21.

236 Same, section 27.

237 Same, section 28.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerating

embrvonal vein.


EXPLANATION OF FIGURES 238, 239 AND 240

238 Same, section 29.

239 Same, section 30.

240 Same, section 31.

4 Atrophying embryonal vein, forming kernel in interior of developing and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degenerating embryonal vein.


FIGURE 241

241 Transverse section of lower thoracic region of 14 mm. cat embryo (series 127, slide X, section 3) X 225.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.

7 Aorta.

8 Oesophagus.

34 Ventro-medial azygos plexus.


FIGURES 242 AND 243


'il^ 242 Same, section 4.

243 Same, section 10.


1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

6 Left azygos vein, thoracic portion.

7 Aorta.

8 Oesophagus.


FIGURES 248 TO 251

« 

248 Same, section 19.

249 Same, section 20.

250 Same, section 21.

251 Same, section 22.

4 Atrophying embryonal vein, forming kernel in interior of develop ing and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degen- ^i

crating embryonal vein. -^


»


FIGURES 252 AND 253

252 Transverse section though lower thoracic region of a 15.5 mm. cat embryo, (series 143, slide XVIII, section 7) X 225.

253 Same, section 11.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.

7 Aorta.

34 Ventro-medial azygos plexus. 36 Thoracic duct, azygos segment.


FIGURES 254 AND 255


254 Same, section 12.

255 Same, section 13.


1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.

7 Aorta.

36 Thoracic duct, azygos segment.


FIGURES 256 AND 257

256 Same, section 14.

257 Same, slide XVII. section 31.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.

7 Aorta.

34 Ventro-medial azygos plexus. 36 Thoracic duct, azygos segment. 51 Mosentoric lymphatics.


FIGURES 258 AND 259

258 Same, section 32.

259 Transverse section or lower thoracic region in a 17 mm* cat embryo, (series 258, slide XVII I, section 1) X 225.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

6 Left azygos vein, thoracic portion.

7 Aorta.

36 Thoracic duct, azygos segment. 51 Mesenteric Ijonphatics.


FIGURES 260 AND 261

260 Same, section 9.

261 Transverse section of lower thoracic region in a 16 mm. cat embryo, (series 96, slide XII, section 32) X 200.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

6 Left azygos vein, thoracic portion.

7 Aorta.

36 Thoracic duct, azygos segment.


FIGURES 262 AND 263

262 Same, section 33.

263 Transverse section of middle thoracic region in a 14 mm. cat embryo, (series 211, slide XII, section 2) X 225.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Extraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

6 Left azygos vein, thoracic portion.

7 Aorta

8 Oesophagus.

15 luterazygos venous plexus. 34 V'entro-medial azygos plexus. 36 Thoracic duct, azygos segment.


V


FIGIRES 2CA AND 2r>.5

264 Same, sect ion 8.

265 Transverse section of middle thoracic regjion in a 19 mm. cat embryo, (series 253, slide XXIV, section <)) X 225.

1 Sympathetic nerve.

3 Right azygos vein, thoracic portion.

4 Atrophying embryonal vein, forming kernel in interior of developing

and replacing lymphatic space.

5 Kxtraintimal or perivenous lymphatic space surrounding degen erating embryonal vein. () Left azygos vein, thoracic porticm.

7 Aorta.

8 Oesophagus.

15 Interazygos venous plexus. 34 Ventro-medial azygos plexus. 36 Thoracic duct, azygos segment.


FIGURES 260 AND 267

266 Same, section 11.

267 Same, section 15.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

5 Extraintimal or perivenous lymphatic space surrounding degen erating embryonal vein.

6 Left azygos vein, thoracic portion.

7 Aorta.

15 Interazygos venous plexus. 36 Thoracic duct, azygos segment. 51 Mesenteric lymphatics.


FIGURES 268 AND 269

268 Same, section 16.

269 Same, section 17.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion. • ('} Left azygos vein, thoracic portion.

7 Aorta. 36 Thoracic duct, azygos segment. 51 Mesenteric lymphatics.


FIGURES 270 AND 271

270 Same, section 18.

271 Transverse section of anterior thoracic region in a 15.5 mm. cat embrj'o (series 143, slide XII, section 22), X 200.

1 Sympathetic nerve.

2 Intersegmental arteries.

3 Right azygos vein, thoracic portion.

6 I^eft azygos vein, thoracic portion.

7 Aorta.

8 Oesophagus.

15 Interazygos venous plexus.

35 Thoracic duct, preazygos segment.

36 Thoracic duct, azygos segment. 51 Mesenteric lymphatics.


FIGURES 272 AND 273

272 Same, section 23.

273 Same, section 24.

1 Sympathetic nerve.

3 Right precaval vein.

6 Left azygos-precaval junction.

8 Oesophagus.

9 Trachea.

15 Interazygos venou.s piexu.s.

35 Thoracic duct, preazygos .segment.

36 Thoracic duct, nzygos segment.


FRIURKS 274 AND 275

274 Same, section 25.

275 Same, section 26.

1 Sympathetic nerve.

3 Right aygos vein, thoracic portion.

3 Right precava.

6 Left azygos-precaval junction.

8 Oesophagus.

36 Thoracic duct, azygos segment.


154 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT

It is quite true that it is possible to inject the jugular lymph sacs in certain manimalian embryonic stages and thus, by grading the preparations, to obtain pictures which might be interpreted in the sense of this theory, provided all questions as to the ontogenetic history of lymphatic organization are designedly left out of consideration. There may be a royal road to the solution of the vertebrate lymphatic problem, but surely the injection method has not found it. It will furnish abundant illustrations of a presumably progressive extension of lymphatic development from centre to periphery, but no proof of the actual occurrence of the process. The tissues of the developing vascular system in mammalian embryos afford at no period and in no region the slightest evidence of such an occurrence. On the other hand, the mammalian embryo does offer conclusive evidence of the development of mesenchymal spaces, arising independently of the veins, and lined by a lymphatic endothelium which is not derived from a pre-existing haemal endothelium. These spaces form the anlages of the future systemic lymphatic channels, and they are continuous with intercellular tissue spaces limited by the surrounding indifferent mesodermal cells which have not yet assumed endothelial character. This relation of lymphatic anlages, lined by lymphatic endothelium, to the intercellular mesenchymal tissue spaces, while clearly determined in mammalian embryos, is moreover strikingly and diagrammatically demonstrated in reptilian embryos.

I have given in the preceding pages my reasons for assigning an absolutely negative value to injection experiments in determining lymphatic ontogeny. These preparations yield interesting topographical pictures in the different stages, showing the extent of continuous lymphatic organization at different periods, but they do not touch the problem of the genetic processes responsible for the establishment of a set of vascular channels capable of injection. Much of the general confusion of ideas evident in discussions of the question arises, in my opinion, from the failure to correctly appreciate the value and significance of the lymphheart remnants of venoiLS origin which in the mammal are interpolated as junctional links between the independently developed systemic lymphatic vessels and the permanent functional veins.


SUMMARY AND CONCLUSIONS 155

It seems difficult, in spite of carefully chosen expressions, to make it clear that in my interpretation of mammalian lymphatic ontogeny I am not trying to straddle the question of venous or non-venous origin of the lymphatic vessels. What we ordinarily describe as 'lymphatic channels' both in the embryo and in the adult animal I am forced, by the results of my investigations, to regard in anmiotes as vessels developed absolutely independently of the hsemal vascular system. The association of these channels, in the mammalian embrj^o, with certain embryonal venous lines is purely a secondary, mechanical and topographical relationship, expressed by the condensed term of * extraintimal' development of mammalian systemic lymphatic vessels, and absolutely devoid of genetic significance. This is, without reference to other vertebrate classes, proved by the development within restricted areas in the mammaUan embryo of systemic lymphatic channels through the direct confluence of intercellular mesenchymal clefts, not related topographically or in any other sense to the embryonal veins. It is true that in the mammal this independent lymphatic genesis is extremely limited, and that the majority of the lymphatic vessels develop in close association with embryonal veins, as products of the confluence of perivenous extraintimal spaces. But this is merely, as shown by comparison with other anmiote embryos, the expression of the peculiar relations obtaining in the mammal between the venous and lymphatic circuits of the vascular system, developed independently of each other.

On the other hand, I am trying to give its full value to a hitherto unrecognized component of the finished lymphatico- venous organization of the mammal. I am trying to establish the extremely reduced and rudimentary mammalian lymphatico-venous heart in the position which its phylogeny entitles it to occupy, as the link or bond-piece between vertebrate lymphatic and venous channels. I desire to make it as perfectly clear as it is possible to do in the English language that this does not mean the acceptance of a mixed, partly venous and partly independent genesis of the lymphatic system as a whole. It merely calls for the morphological recognition of a distinct and valid structure, interposed, as the connecting link, between the definite venous channels and the


166 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT

lymphatic vessels, genetically a derivative from the former, but in no sense responsible for the development of the latter. It is perhaps possible to illustrate my meanmg by a comparison drawn from an entirely foreign field. The pronephric, and the subsequent Wolffian duct, in approaching the cloaca, is met by a responsive outgrowth of the latter and eventually gains its permanent opening into the genito-urinary cavity by union with the same. Nobody would hold this cloacal horn responsible for the Wolffian duct development by a process of 'outgrowth' of 'budding.' The embryonal veins respond in exactly the same way to the approach of the independently developed systemic lymphatics. This response in the typical and prevalent mammal takes the form of a rudimentary and foreshortened lymphatico-venous heart, and continues to function as such throughout the life of the individual, as the jugular lymph sac. There is here certainly a distinct genetic principle involved. We, to a certain extent, disregard the cloacal participation in the final establishment of the Wolffian duct connection with the genito-urinary sinus, because the ontogeny of the duct in itself presents such marked and striking stages. In exactly the same general way I believe that we are prone to misinterpret the vertebrate lymphatico-venous junctions, unless we recognize lymphatico-venous hearts and their remnants in their true morphological significance, as links uniting structures genetically distinct and of different origin. In the matter here under discussion this does not mean a double genesis for the lymphatic vessels, part derived from the veins, part by mdependent mesenchymal confluence of intercellular spaces. This would be no more in accordance with the actual facts than a genetic description ascribing a portion of the Wolffian duct to cloacal ' outgrowth,' while another portion is credited to independent development. I would refrain from laying stress on these facts were they based on guess-work, but the entire genetic history of lymphatic and venous organization in all amniote types heretofore examined is so strikingly consistent and so uniformly constant that my personal conviction of the truth of the above statements is very firmly rooted.


SUMMARY AND CONCLUSIONS 157

Observations made on vascular structure and growth in the transparent tails of living anure amphibia have of late been offered in support of the theory of contmuous centrifugal lymphatic development from pre-existing venous endothelium. These observations recall vividly the corresponding historical period in the developmental analysis of the embryonic blood-vascular channels, prior to Thoma's classical researches on this subject. Sigmund Mayer nearly thirty years ago employed the tails of living anure larvae for the purpose of studying this phase of the circulatory system. It seems strange that to-day it should be equally, or even more, difficult to induce observers to differentiate between development and growth of vascular channels. Development means the genesis of the first embryonic anlages of a vascular system. When this has once been established its further extension by growth is an entirely different problem which demands equally careful and competent observation and presents many phases difficult of mterpretation, involving, among others, the entire question of the 'specificity of vascular endothelium.' Much futile and controversial discussion could be avoided if the growing generation of contributors to this subject will restrain their enthusiasm long enough to bear this vital difference in mind.

On the other hand, the theory which regards the lymphatic channels of the mammalian embryo as developing by the coalescence of multiple venous 'outgrowths' or portions of embryonic venous plexuses, detached and separated secondarily from the parent veins, deserves detailed consideration.

According to the postulates of this view, the lymphatic endothelium is the direct descendant of the pre-existing hsemal vascular endothelium lining the detached venous elements which enter into the formation of the lymphatic channels. The latter, after being thus established, make secondary and permanent connections with the venous system.

It is of course a very easy matter to prove in siiccessive stages the topographical correspondence of venous and lymphatic channels, to prove, in other words, that the area occupied by a vein in the earlier stages is filled in the following periods by a lymphatic


158 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT

channel. The genetic succession of mammalian lymphatic vessels, developing along and around atrophying antecedent embryonal veins and eventually replacing them absolutely in the topographical sense, has been sufficiently elaborated in the preceding pages. There is no question whatever concerning the anatomical fact of such replacement. My quarrel lies with the deductions which the sponsors of the theory just quoted draw from the evidence of their slides.

If in an earlier stage they find a vein or venous radicle occupying a definite and clearly circumscribed topographical position in relation to surrounding structures, and if they encounter in a later stage a lymphatic channel in the identical position of the earlier vein, they reason that this topographical coincidence proves the direct transformation of the preceding vein into the succeeding lymphatic. They speak of the earlier vein as an 'outgrowth' from the main venous channel and describe the same as being ' detached' or 'split off' the latter to form a Veno-lymphatic anlage,' which then, by fusion with multiple equivalent detached venous elements, forms finally a valid and permanent systemic lymphatic channel. It appears to me that in this process of reasoning topography is forced to substitute for histogenesis, and that the vital intermediate genetic stages, which finally produce the result under discussion, have been disregarded. I believe that this error in deduction is based on perfectly correct but insufficient premises and is due to the lack of sufficient material capable of demonstrating the intermediate histogenetic stages through which the replacement of a venous by a lymphatic channel is accomplished. There are many individual embryos of certain stages, which, interpreted by themselves, tend to support strongly the hypothesis of direct conversion of venous into lymphatic anlages. It requires a very large number of embryos of approximately the same age in order to demonstrate the true genetic processes governing the early relations of the venous and lymphatic channels in the mammalian embryo. Hence the futility of reasoning, from a few human embryos, concerning primate lymphatico-venous relations, which we know, from the already ascertained conditions in adult platyrrhine and catarrhine forms, present one of the most complicated


X '■ '--



SUMMARY AND CONCLUSIONS 169

and involved problems in the entire genetic history of mammalian vascular organization.

I am convinced that the theory of direct derivation of the mammalian systemic lymphatic vessels from the embryonal veins is utterly wrong and that it is based primarily on insufficient observation. It lends itself to an easy interpretation of a relatively small number of mammalian embryos of each appropriate stage, but fails utterly in reckoning with the conditions presented by a large consecutive series of the critical periods.

I will analyze this view on the basis of the material discussed m the preceding pages, first m the region of the thoracic duct approach of the jugular lymph sac and in the preazygos segment of the thoracic duct, and subsequently in the azygos district of the latter.

I. THORACIC DUCT APPROACH AND PREAZYGOS SEGMENT OF THfe

THORACIC DUCTS

The terminal portion of th^ thoracic duct has been described as developed by confluence of venous outgrowths, arising along the dorsal surface of the left common jugular and innominate veins in the earlier (14 mm.) stages, which occupy the same relative position with respect to the main venous channel, the thyrocervical artery and the sympathetic nerve as does the thoracic duct in the later (16 mm.) stages.

In my opinion the so-called ' outgrowths' arising from the main venous trunks are the terminals of dorso-medial tributary branches entering the same, and have nothing to do with the thoracic duct. They are part of the prevertebral capillary plexus in relation in this stage to the dorso-medial surface of the main venous channels, as described m detail in the second part of this paper dealing with the development of the preazygos segment of the thoracic duct (supra, p. 84). This opinion is based on the fact that in many 14 mm. embryos sections through this region show the socalled ' venous outgrowth' and the thoracic duct approach of the jugular lymph sac coexisting side by side.

As previously described in detail (supra, pp. 56 to 77) the thor


160 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT

acic duct approach of the jugular lymph sac, in certam typical 14 mm. embryos, extends as a curved blunt conical process mesocaudad for some distance between the main vein and the thyrocervical artery, towards the interval between the former and the sympathetic nerve, where it ends blindly. Mesad of the thoracic duct approach the internal and common jugular veins receive a number of dorso-medial tributary radicles of the above mentioned prevertebral venous plexus. Now, it will depend altogether upon the degree of development of any particular embryo, and the selection of the sections, as to whether both the thoracic duct approach and the dorso-medial tributary terminal appear side by side in the same field, or whether one or the other, or both are absent at the particular level examined. Thus in a 14 mm. embryo (series 122, slide ix, section 28, fig. 58), the dorso-medial tributary (16) of the main vein (left internal jugular, 25) together with the thoracic duct approach of the jugular lymph sac {12), are both present in the typical relation to the thyro-cervical artery (24) and the sympathetic nerve (1). Traced back in series 122 to section 21 of slide ix (fig. 57) the thoracic duct approach (12) is seen to arise from the dorso-mesal aspect of the caudal end of that portion of the jugular lymph sac, which, as the terminal of the ventral di\'ision of the entire structure, forms the jugular approach (IS).

But since in section 28 of slide ix of series 122, this portion of the future thoracic duct (lymph sac segment) is present in the classical position between thyro-cervical artery and sympathetic nerve, dorsal to the common jugular vein, together with the well developed dorso-medial venous tributary {16), it must become evident that the latter cannot be regarded as a 'venous outgrowth' representing the anlage of the thoracic duct, since it is manifestly impossible to include the ' anlage' of a structure and the actual structure itself side by side in a single section.

As a matter of fact, a selection of sections in this neighborhood in series 122 (14 mm.) will furnish all possible combinations of the venous dorso-medial tributary of the internal and common jugular vein and of the thoracic duct approach of the jugular sac.

The two structures, as can be clearly seen, both occupy in sections at the proper level the topographical position along the dor


SUMMARY AND CONCLUSIONS


161


sal aspect of the main vein and between the thyro-cervical artery and sympathetic nerve, which has been alone employed as the basis of the conclusion that, in comparing the 14 mm. and 16 mm. embryo, the thoracic duct develops from a portion of the dorsomedial plexus, secondarily split off from the main vein.

Thus, for example, the series 122 imder discussion, if employed for purposes of topographical comparison with a 16 mm. embryo, should be represented by the following sections :


1 2

3 4 5 6


BXRIB8


122


IX


122


ix


122


X


122


X


122


X


122


X


8LIOK


sscnoN


['


nouRS


21


57


28


58


7


59


9


60


12


61


13


62


1. It will be seen that in section 21 of slide ix (fig. 57), the interval between thyro-cervical artery (£4) ^uad sympathetic nerve (i) is occupied by the thoracic duct approach (IB) directed meso-caudad from the jugular lymph sac.

2. In section 28 of the same slide (fig. 58), the thoracic duct approach (12) has separated from the sac, and appears, as the free terminal of the future adult duct, mesad to the thyro-cervical artery (24)- Further dorso-mesad is a venous tributary (16) approaching the main vein. Elements of the mesal peri- and interneural venous plexus are also seen.

3. Section 7 of slide x (fig. 59) shows, in the interval between the thyro-cervical artery (24) and the sympathetic nerve (1) the section of the thoracic duct approach (12) close to the dorsal circumference of the common jugular vein (26),

4. In section 9 of the same slide (x) (fig. 60), the blind end of the thoracic duct approach (12) is still seen on the mesal side of the thyro-cervical artery (24) f while further mesad the common jugular vein (26) receives the terminals of the dorso-medial venous plexus. Here, in other words, the Venous outgrowths' or 'anlages of the thoracic duct,' and the thoracic duct approach of the


162 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT

jugular lymph sac appear side by side, dorsal to the left common jugular vein, between the thyro-cervical artery and the sympathetic nerve.

5. Section 12 of the same slide (fig. 61) is caudal to the blind terminal of the thoracic duct approach. A radicle of the dorso-medial venous plexus {16) lies close to the dorsal ^ircimiference of the common jugular vein (26) between the thyro-cervical artery (24) and the sympathetic nerve (i), approximately in the situation occupied in the sections further cephalad by the terminal of the thoracic duct approach.

6. In the following section (fig. 62) this venous tributary opens into the main vein.

A comparison of the structures labelled respectively 16 and 12 in figs. 61 and 59 will show their close topographical correspondence and will suffice to indicate how easily the latter, a valid portion of the future thoracic duct, might be confused with the former, a dorso-medial venous terminal. Hence, in comparing successive stages, the greatest care is necessary in order to avoid the mistake of considering a lymphatic channel of the later period to be the direct derivative of a ^venous outgrowth' as seen in an earlier embryo. Other embryos of this stage will give equally conclusive pictures of the juxtaposition of thoracic duct approach (12) and dorso-medial venous tributary (16) in their various mutual relations (figs. 41, 42, 49, 50, 55).

Of course, caudal to the level at which in this stage the thoracic duct approach ends blindly, and before the independent lymphatic channels, which are subsequently (15 mm. and 15.5 mm.) to join it, are developed, the sections will only show the dorso-medial venous branches entering the main channel

But these venous terminals of a well defined plexus are in no sense ' outgrowths' from the veins, nor should they be considered as anlages of a lymphatic structure which does not appear in the particular section figured, because it has not as yet developed at that particular level, although, as in the case in point, it may be perfectly evident and coexisting with the 'venous outgrowths' few sections further cephalad in the same embrj^o.

The venous plexuses, both in the prevertebral and in the ventral


SUMMARY AND CONCLUSIONS 163

mediastinal region, which form the basis for the extraintimal lymphatic channel development in the correlated segments of the thoracic ducts, are sufficiently described above (p. 85).

There is no question that the mtemal and common jugular veins and the innominate vein receive, along their mesal and dorsomesal aspects duringihe 13 and 14 mm. stage, numerous tributaries of these two plexuses, as indicated above schematically in fig. 101. These terminals cannot, however, be interpreted as ' outgrowths' of the main venous line, constituting the venous anlages of the thoracic duct in this region and subsequently uniting to form the continuous channel of this duct in its preaortic segment. They are a direct caudal continuation of the extensive inter- and perineural venous plexus connected, cephalad of the thoracic duct level, with the mesal aspect of the internal jugular vein throughout nearly its entire extent.

The elements of these early venous plexuses serve as lines of extraintimal perivenous lymphatic development.

Both in the prevertebral region cephalad of the aortic arch and in the ventral mediastinal area, as above described (pp. 84 to 110) the extraintimal anlages of both channels, the preazygos segment of the thoracic duct doraally, and the mediastinal trunk ventraUyy can be followed in the proper stages from their earliest inception to their full completion and union with each other.

The lymphatic channel system in both areas, as in all other regions of the body, is at first very extensive and plexiform, composed of a larger number of intercommunicating spaces. Later the definite channel, smaller in cross-cut area and more direct in course, crystallizes along definite static lines out of the preexisting lymphatic network, just as we now know that hsemal channels do. In other words, the systemic lymphatic channels, as would be expected, repeat in their development the ontogenetic stages of the haemal channels. Both sets of vessels, lymphatic as well as hsemal, result from the condensation of an antecedent plexus along definite and determined lines and thus give rise to the characteristic vessels of the adult.

In this entire process there is never a question of the direct implication of a vein in the formation of a 'ymphatic space. The


164 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT

majority of the spaces develop, as above described, extraintimally along or around a venule, but at the same tune a considerable number of these spaces develop as independent mesenchymal clefts not directly related to a degenerating embryonic vein. The embryonic history of this portion of the mammalian lymphatic system, up to the jimction of the completed preazygos segments of the thoracic ducts, includmg their ventral mediastinal tributary, with the thoracic duct approaches of the left and right jugular lymph sacs, is, as above stated, clearly shown in cat embryos of between 13 and 15.5 mm. and need not be repeated here.

Comparison of the 13 and 14 mm. stages with slightly older embryos (15 mm.) definitely proves the correctness of the view just expressed. In the course of further development the thoracic duct approach, turning ventro-mesad into the neuro-venous interval between sympathetic nerve and common jugular vein in proceeding latero-mesad across the dorsal aspect of the latter vessel, necessarily intersects the line of the dorso-medial venous tributaries. Its blind terminal must exchange the position which it occupies in the earlier (13-14 mm.) stages, lateral to the dorsal somatic venous branch, for one situated on its mesal aspect, if the vessels are fimctional venous tributaries and not, as has been held, venous anlages of the future thoracic duct in this region. In other words, they should continue in the later stages side by side with the thoracic duct more fully developed, if they have not become detached from the main vein to form by fusion the lymphatic channel. This is seen to be the case, for example, in the sections of the 15 mm. cat embryo (series 245) shown in figs. 79, 80, 81 and 82. The thoracic duct approach (12) here has extended further meso-ventrad into the recess between sympathetic nerve (1) and common jugular vein (26) and now lies on the mesal aspect of the ventro-medial venous tributary (16) which opens into the main vein in fig. 82. Fig. 90 shows clearly in the dorsal view of the reconstruction of this embryo the lymphatic channel {12-S5) intersecting at an acute angle the coexisting dorsomedial tributary


Sl'MMAUY AND CONCLUSIONS 165

TI. AZVCIOS SEGMENT OF THORACIC Dl'C 'T

The genesis of the azygos segment of the thoracic ducts is in my estimation the region in which the vahdity of divergent views as to systemic lymphatic development can l)est b(* tested.

Certain mechanical and genetic conditions combine to make the investigation of the development of the thoracic ducts in this region perhaps difficult in one sense, while in another the results of thorough investigation are most conclusive and convincing.

Part of the difficultv refen*ed to lies hi the relativelv small amount of the retro- and peri-aortic space available for the development both of the azygos veins and of the subsequent lymphatic ducts replacing the same to a large extent. Again, the process of systemic lymphatic development hi this n^gion appears to proceed with great rapidity. The first anlages of the thoracic ducts in a given stage are succeeded in stages only slightly older by well marked longer segments of the future lymphatic channel. On the other hand, the observer lias the advantage^ of dealing with a circumscribed region and with axial venous trunks, which, both in their main line and in their tributaries, are remarkably constant, regular and easily distinguished.

As above stated, in the earlier stages a ventral tributary plexus of the azygos veins drains the periaortic region. In later stages this ventral venous plexus in large part disaj)pears, a bilateral lymphatic duct occupying its former plac(\ The two conditions cover (^ach other absolutely.

No one, })()ss(*ssing a knowledge of the embryonic venous development of the cat, can examine a number of injections of the thoracic ducts and azygos veins of the adult animal without IxH'oming convinced that in course of development the thoracic duct complex, caudal to th(» aortic arch-level, rej)laces a large part of th(^ embryonic azygos venous system, and comes to occupy absolutely its topographical j)osition and relation to surrounding structures. The relation of the dorsal intersegmental aortic branch(\^ to the* interaz3"gos venous anastomosis of the (*ml)ryo, compared with the intercostal arteries of the adult perforating through the meshes of the plexiform lymphatic channel of the thoracic ducts, will alone absolutely demcmstrate this mutual topographical corresp(mdeiic(».


166 SYSTEMIC LYMPHATIC VESSELS IN DOBfESTIC CAT

The only question at issue is the determination of the genetic processes through which the embryonic azygos venous plexus is replaced by the lymphatic network of the thoracic ducts in the adult.

Some observers have derived this portion of both ducts directly from the ventral aspect of the azygos veins by comparing transections of 14 mm. and 16 mm. embryos, and finding in the former ventral tributaries of the main channels, whose place in the latter is occupied by the thoracic ducts. They hence conclude that the ventral azygos plexus, forming Hhe veno-lymphatic anlages' of the thoracic ducts, separates at numerous points from the parent vein, and that the resulting detached portions of the originally continuous venous plexus, secondarily unite to form the channel of the thoracic ducts. The latter structures are therefore, in their estimation, direct derivatives of the azygos veins and their tributaries.

The preceding pages give my reasons for regarding this replacement as having occurred in course of the development of the thoracic ducts by confluence of numerous isolated mesenchymal spaces, formed along and around the azygos veins and their ventral tributaries, but genetically independent of the latter and lined by an endothelium not derived from the pre-existing haemal endothelium. •

In the region under discussion these mesenchymal clefts develop, owing to local conditions, very largely as extraintimal or perivenous spaces accompanying and surrounding the branches of the retrograding ventromedial azygos plexus. The resulting thoracic ducts thus come to occupy the place formerly filled by the ventral azygos venous network, and later are led into direct ventral contact with the main azygos veins, which they in their further growth to a large extent secondarily replace.

In the early (11-12 mm. stage) the azygos veins receive on their ventral and ventro-medial aspect the terminals of a venous plexus originating in the retro- and peri-aortic mesenchyme. While the individual embryos offer many striking pictures at various levels, I believe that my purpose of demonstrating the genesis of the thoracic ducts in this area will best be accomplished by taking


SUMMARY AND CONCLUSIONS 167

sections in successive stages which will present approximately the same topographical picture, allowing for the necessary changes consequent on increasing development. The area to which I thus desire to call detailed attention is the mesenchymal tissue, situated on the right side of the thoracic aorta, between this vessel and the mesal aspect of the right azygos vein, ventral to one of the right dorsal somatic (intercostal) branches of the aorta, and under cover of the dorsal surface of the oesophagus.

These limitations, as is well known, define the position of this portion of the main thoracic duct of the adult, and consequently the development of this structure, however produced, must run its course in this situation. The successive stages, which in my opinion definitely and conclusively prove the independent extraintimal anlage of this portion of the right thoracic duct, may be grouped as follows, and referred to selected illustrations above described in the body of the text in detail :

A. Vencyus stage. (Embryos from 10 mm. to 13 mm.). Illustrations in body of text: figs. 194 to 203.

The sections show an extensive ventral and ventromedial azygos tributary plexus.

B Stage of replacement of ventral azygos venov^ plexiLS by independently developed perivenous or extraintimal lymphatic anlages which accompany and surround the branches of the antecedent venous network, and form the first anlages of the azygos segment of the thoracic ducts. (Embryos from 13 to 14 mm.) Illustrations m body of text: figs. 204 to 213; 247 to 251.

1. The earliest anlages of the future thoracic ducts are seen in certain sections of the thoracic region in embryos between 12 and 13 mm. crown-rump measure. I have found these early traces almost exclusively on the right side, the sinistral position of the aorta appearing to delay their development on the left side. They make their first appearance as delicate endqtheliaUined extremely sparse and widely separated independent mesenchymal clefts, at some distance ventral to the main right azygos trunk and near the right lateral wall of the aorta.

At first they are not clearly related to the co-existing ventromedial tributaries of the azygos. But they soon extend to involve these in the characteristic manner above detailed, and continue to develop as extraintimal or perivenous lymphatic spaces gradually replacing the shrinking ventral azygos plexus.

2. In the succeeding stage (13+ mm.) the aorta sinks more deeply into the interazygos interval and is spanned on its dorsal aspect by the horse shoe of the abimdant supra-aortic cross anastomosis between the azygos trunks of the two sides. The perivenous extraintimal lymphatic spaces surrounding and accompanying the ventral azygos tributaries have increased in size, and a few spaces of the same character begin to make their appearance along the ventro-lateral aspect of the main azygos veins.

3. In the 13.5 mm. embryo the extraintimal anlages replacing the ventro-medial azygos tributaries have increased in number, united with each other to form a series of longer continuous segments, and have, following the lead of the venules which they are replacing, approached nearer to the ventral aspect of the main azygos veins, and appear more closely interlocked with the ventromedial tributaries of the latter.

C 14 w^. stage. Illustrations in body of text : figs. 214 to 240.

The most instructive and conclusive pictures are obtained in embryos of approximately this crown-rump measure.

The sections are clearer and most immistakable in their meaning and the individual spaces have become confluent for longer stretches, represe^jting longer segments of the thoracic duct channel which is subsequently to be formed by their continued elongation and fusion with each other. This becomes clearly apparent in contrasting the ventral view of the reconstruction of series 76 (13.5 mm.) with that of series 214 (14 mm.) shown in figs. 188 and 190.

The diffuse and scattered elements of the parazygos lymphatic plexus anlages in the 13.5 mm. stage have, in the 14 mm. embryo, become condensed and united into longer segments representing the future line of right axial prevertebral lymphatic drainage. It is noteworthy, as an important fact, that notwithstanding the clear development of unmistakable longer segments of the future right thoracic duct, these anlages are still placed at some distance ventro-mesal to the right azygos vein and ventral to the intersegmental arteries.


Summaby And Conclusions

The 14 mm. embryo represents thus in general the full development of the histological picture of secondary replacement of a decadent embryonic venous plexus by the secondarily developed extraintimal perivenous lymphatic spaces and channels.

The following conditions have been noted in individual sections:

a. Ventro-medial tributary of right azygos connected with main vein, detached on left side.

Anlage of right thoracic duct lateral to right ventral azygos tributary.

b. Ventro-medial tributaries of both azygos veins surrounded by extraintimal anlages of both ducts.

c. Ventro-medial tributaries detached and surrounded by extraintimal anlages of thoracic ducts on both sides.

d. Ventro-medial tributaries in connection with both azygos veins and accompanied by ventro-lateral lymphatic anlages on both sides.

e. Azygos veins without ventro-medial branches or their remnants and accompanied on both sides by the thoracic duct anlages

/. Azygos veins with ventro-medial tributaries without duct anlages.

g. Azygos veins without either ventro-medial tributaries or thoracic duct anlages.

D. Later transitional lymphatic stages. Illustrations in body of text: figs. 252 to 258.

In the 15 and 15.5 mm. embryo three further developmental characters appear:

a. The earlier more redundant and plexiform lymphatic anlages condense into a definite channel of relatively smaller caliber.

6. The central remnants of the atrophied embryonic vein, around which the lymphatic anlages developed, have to a large extent disappeared, leaving the lymphatic lumen clear.

c. The individual and originally separate segments of the lymphatic channels have united into segments of greater length, and have approached more closely to the ventral surface of the main azygos trunks. The definition of a right and left lymphatic trunk with supra-aortic cross anastomoses is beginning to appear, although the former is still the larger and better defined. It occupies the original position between aorta and ventromesal surface of the right azygos vein, ventral to the intercostal artery, dorsal to the oesophagus.

E. Definite lymphatic stage. From the 16 mm. embryo on. Illustrations in body of text : figs. 259 to 262 ; figs. 265 to 270.

Here all the thoracic duct anlages have united into the continuous and connected plexiform channel of the thoracic ducts, ventral to the azygos trunks, dorsal to the aorta.

The early independent genetic history of the spaces, which I have above described as the first anlages of the thoracic duct channels in the embryos of the cat, and the fact that in subsequent stages they appear consistently and in every possible combination as extraintimal or perivenous mesenchymal spaces, following and surroimding the branches of the ventral azygos plexus, exclude to my mind the possibility of regarding them as derived directly from ventral azygos venous plexus, or from socalled ' venous outgrowi^hs' of the main azygos trunks, subsequently detached from the parent trunks.

In the foregoing genetic history of part of the thoracic duct lymphatic plexus, by fusion of extraintimal and perivenous mesenchymal spaces, accompanying and surrounding certain of the ventral venous tributaries of the right azygos trunk, the following points should be noted :

1. The portion of the entire adult thoracic duct complex here considered develops in its earliest stages at some distance ventromesad to the right azygos vein. It only subsequently, by extending proximad along the venule which it replaces, reaches the position in close relation to the ventral surface of the azygos which it occupies in the 16 mm. stage.

2. In the 13.5 and 14 mm. embryos the radicles of the ventromedial tributary plexus are in the process of being surrounded by the growing perivenous extraintimal lymphatic spaces which are approaching along this line the ventral surface of the azygos, but in the 14 mm. stage have not yet acquired this position. The 16 mm. embr>o shows the lymphatic anlages in their definite relation to the azygos trunk, and the primitive ventral venous tributary along which the lymphatic spaces expanded to reach this position, has disappeared. The successive stages of this process can be followed in such detail and in such conclusive preparations that all doubt as to the accuracy of the statements here made seems to me to disappear. It is merely a question of suitable material, accurately fixed, sectioned and stained, and sufficiently abundant to offer complete corroboration in a number of series of the same age. There can then be no question as to the genetic principles involved.

3. The portion of the thoracic duct, whose genetic history has just been outlined, forms only a component of the entire lymphatic complex entering into the formation of the adult duct. Two other equivalent components combine with it to produce the adult conditions, viz., a corresponding segment on the left side, developed in the same way in association with the ventral tributary plexus of the left azygos vein, and an intermediate plexiform lymphatic reticulum which arises as the result of fusion of extraintimal lymphatic spaces formed along the ventral aspect of the supraaortic interazygos venous plexus.

In addition to these three cardinal elements entering into the organization of the adult thoracic ducts, these channels, when once they have gained their position ventro-mesad to the azygos trunks, grow partly by their own expansion, partly by addition of numerous new extraintimal lymphatic spaces which form along the azygos veins, expecially on the left side.

The eventual reduction of the left azygos vein, and of the caudal part of the right trunk, by transferance of the blood current to the vertebral chain, accounts for the topographical relations of the replacing thoracic duct complex of the adult.


Huntington1911: Part I. The development of the systemic lymphatic vessels in their relation to the blood-vascular system | Part II. The development of the preazygos and azygos segments of the thoracic ducts | Bibliography


Huntington GS. The anatomy and development of the systemic lymphatic vessels in the domestic cat. (1911) Memoirs of the Wistar Institute of Anatomy and Biology No. 1.


Cite this page: Hill, M.A. (2020, February 17) Embryology Huntington1911 - 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Huntington1911_-_2

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