Paper - On the developmental topography of the thoracic and abdominal viscera (1909)
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Jackson CM. On the developmental topography of the thoracic and abdominal viscera. (1909) Anat. Rec. 3(7): 361-396.
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- 1 On the Developmental Topography of the Thoracic and Abdominal Viscera
On the Developmental Topography of the Thoracic and Abdominal Viscera
C. M. Jackson.
From the Anatomical Laboratory, University of Missouri, Columbia.
With 23 Figures and 1 Table.
The purpose of the present paper is to describe the principal changes in position which the viscera undergo during the process of development, and to discuss some mechanical principles involved in these changes. The materials used include a considerable number of human embryos and fetuses in my collection. Several of these are cut in serial sections, and from four of them models were reconstructed by Bohn's wax-plate method. The embryos modeled are No. 60 (5th week, 11 mm. crown-rump length), No. 58 (6th week, 17 mm. crown-rump length), No. 57 (8th week, 31 mm. crown-rump length), and No. 55 (12th week, 65 mm. crown-rump length). I am greatly indebted to several of my students for assistance in reconstructing these models. That of No. 60 has already been described by Bonnot and Seevers. The model of No. 57 was in large part reconstructed by C. B. Rodes, Jr., and that of No. 55 by A. W. Kampschmidt and F. O. Kunz. The volumes of the various organs and parts of these embryos (excepting No. 55) have been given by nie in another paper. ^
- Vnatoini.seher Anzeiger, Bd. 21), 1000, S. 4r)2-4r)0.
- American Journal of Anatomy, Vol. VIII, No. 1, 1009.
The models arv especially designed to show the topographic relations of the thoracic and abdominal viscera to the skeleton, the projections of the sternum, ribs and vertebral centra being indicated upon the underlying organs in the models as shown in the various figures. In order to show the general jwsition and relations of the organs repnsented in the models, with reference to the body as a whole, graphic reconstructions were made (Figs. 1 to 4) showing from the left lateral view the outlines of the body, ribs, sternum, vertebral column, etc. The parts corresponding to the models are indicated in the figures by the shaded (stippled) areas.
In the following paper, the gc^neral relations of the viscera to tlie body wall, ])articularly to the vertebral column, will be discussed first, followed by a brief consideration individually of the principal viscera.
Relations to the Vertebral Axis
The most striking and important feature in the relations of the viscera to the vertebral column in the embryo is the so-called migration of the various organs along the body axis during the course of development. This phenomenon, which is of general occurrence among the higher vertebrates, has long been knowni, but its cause has never been fully and satisfactorily explained.
In general, we recognize that the migration of an embryonic organ may be either active or passive in character. An active migration is due to the (amoeboid?) movement of the component cells, e. g., of the early anlages of the sympathetic ganglia. It therefore does not require actual growth (increase in volume) of the parts concerned. Passive migration, on the other hand, is produced by pressure or tension due to unequal growth, relatively more or less rapid, either in the organ displaced or in neighboring organs or parts. It is evident that the migration of the viscera along the body axis belongs to the latter class. This principle was recognized by His,' who was the first to investigate in a systematic Avay the subject of developmental mechanics.
- Unsere KiJrperforni, Leipzig, 1874; also in Archiv f. Anat. u. Entw., 1894, S. Iflf.
His recognized that the heart and other organs which lie far forward on the ventral side of the embryo at a very early period are at first pushed backward (eaudad) by the ventral flexure of the body axis in the head region. Up to the third week in the human embryo, while the body axis remains comparatively straight, the heart lies chiefly ventral to the head region. During the third and fourth weeks, owing to the flexures in the head region, — the primary cephalic flexure opposite the midbrain, and the cervical flexure at the junction of the head and neck — the heart and adjacent organs are pushed backward farther and farther. In the fifth week (Fig. 1), at the close of the period of greatest flexure of the body axis, the heart lies entirely opposite the cervical region of the vertebral column. It is difiicult to determine with certainty the exact vertebral level of the various organs at this period, on account of the extreme flexure of the body.
The cause of this flexure ventralward of the body was originally explained by His as due to the restraint of the amnion upon the elongating embryo ; but it is now generally believed to be due to the early overgrowth of the central nervous system^ b'^^^S dorsal to the body axis. Later, when the structures lying ventral to the body axis begin to expand more rapidly, the head becomes more erect and the cervical flexure is. largely obliterated.
The influence of the brain and spinal cord in producing the flexure of the body is well sho^vn in embryos with anencephaly and spina bifida. In the absence of brain and spinal cord, the ventral flexure of the body axis is reduced or absent. An anencephalic embryo of about two months (27 mm. head-trunk length) in my collection shows no ventral flexure of the body ajcis. The axis is nearly straight, the face being directed ventralward. In older fetuses of this character, as is well known, the face looks upward as well as forward, giving the so-called "frog-face" expression. In a mid-sagittal section of such a fetus, I find the base of the cranium making an angle dorsalward at the junction with the vertebral column, instead of ventralward as in the normal condition. This is evidently due to the unopposed expansion in the facial region, not meeting the normal resistance of the cranial contents. A study of younger embryos (first and second months) of this class ought to throw more light upon the relation of the growth of the central nervous system to the production of the body flexure. This much is certain, however, that the absence or rudimentary condition of the brain and spinal cord (at least in the later stages) does not prevent the descent of the thoracic and abdominal viscera; for I find these organs at approximately their normal vertebral levels in fetuses with this deformity.
A study of the relations in the normal embrj'o leads to a similar conclusion. However important the ventral flexure of the body axis may be in displacing the heart and other organs in the early embryo, it is certain that this is not the only factor involved. At the end of the period of greatest flexure, as seen in the 11 mm. embryo (Fig. 1), the viscera still lie far above (i. e., cephalad to) their permanent levels. The continued descent of the viscera along the vertebral column is clearly apparent when Figs. 1, 2, 3, and 4 are compared. His* also recognized that the displacement of the viscera is only in part accounted for by the ventral body flexure. The final displacement was explained as an apparent migration of the viscera downward along the body axis. He states that in reality the cervical portion of the spinal cord (and column) undergoes a relative elongation about tlie sixth week, so that it actually pushes upward into the head region, thus moving apast the ventral viscera, which are left lying opposite a lower level on the vertebral column. The neck prominence (Nackenhocker) characteristic of this period he explained as due to this pushing upward of the cervical portion of the spinal cord.
On closer examination, however, there appears no valid evidence to support this view. In fact, His's own figures show no relative elongation of the cervical portion of the spinal cord or vertebral column at this time. On the contrary, there is during this period a constant decrease in the relative length of the cervical region, at least in the vertebral column. In order to show more exactly the growth in relative length in the various regions of the vertebral column during prenatal life, I have arranged Table I, based upon my own observations and those of Aeby,® His,® Merkel,^ Bardeen,® and Ingalls.®
- Anatoiiiie mensohl. Enibryonen, I, S. 78; III, S. 120 flf. Cf. also Archiv f. Anat., 1881. S. 319.
In Table I, the cervical region in embryos under 10 ram. (before the vertebra) are well differentiated) includes the eight cervical somites. There is evidently a constant decrease in the relative length of the cervical portion of the column, with a corresponding increase in the lumbar region, as was first shown by Aeby (Z. c). The change, as may be seen in the table, is most rapid in the first six weeks. During this time the cervical region decreases from about 35 per cent to about 30 per cent of the length of the corvicothoraco-lumbar division of the column. The lumbar region increases at the same time from about 17 per cent to about 22 per cent. During the remainder of the entire fetal period, the cervical region decreases from about 30 per cent to about 25 per cent; while the lumbar region increases from about 22 per cent to about 27 per cent. Aside from individual variations, the relative length of the thoracic region remains fairly constant (average about 48 per cent). The sacral region at the beginning increases in relative length, reaching approximately its permanent relation (individual variations excepted) about the end of the first month. The coccygeal region apparently reaches its maximum relative length in the second month, though it is exceedingly variable.
But even if there were, as His supposed, a relative elongation of the cervical region of the vertebral column in the second month, this would not account for the migration of the viscera downward along the thoracic as well as the cervical vertebne. So it is evident that some other cause must be sought. And that cause, it seems to me, is in gc^neral the relatively more rapid growth beginning at this time in the structures lying ventral to the body axis. This may be noted particularly in the cervico-facial (pharyngeal) region, as may be seen by comparing Figs. 1, 2, 3, and 4. The relatively rapid expansion in this region exerts pressure in all directions. The pressure upward against the massive head, on account of the great resistance, produces comparatively little displacement. The effect is merely to straighten out to a large extent the cephalic and cervical flexures. (The exception in Fig. 4 is only apparent, the cendco-cephalic angle being in reality less marked than in the preceding stages.)
- Archlv f. Anat. u. Entw., 1879.
- Anatomle menschl. Embryonen, I, S. 97.
- Menschliche Embryonen auf Medlanschnitteii uiitersueht. Gottiiigen, 1894.* American Journal of Anatomy, Vol. IV, p. 277, 1905.
- Archlv f. mikr. Anatomie, Bd. 70, 1907.
|Table I - to be formatted|
Relative Length of the Various Regions of the Vertebral Column in the Human Embryo and Fetus, expressed in Percentage of the Cervico-Thoraco-Lumbar Division. (Percentage of the Entire Column in Parentheses.)
Oown- I Total Rump I>enirth Lenfftn. ofCol'mn Mm. Mm.
7.5 10. 11. 11. 13. 14. 16.
Bardoen. . Jackson . .
Bardeon \ 3(».
Jackson I 35.
Merkel ; 43.
Jackson ' 65.
Morkol 1 73.
Aeby Aeby Aeby Aebv
Morkcl I 140.
His , 143.
Merkel | 170.
Jackson i 190.
Jackson I 205.
Jackson 1 210.
Morkcl I 310.
His Morkol 365.
Aeby (avorage of 8
Aeby (average of 13
35.1(28.5) 36. (31. ) 30.7
34.5(28.1) 30.8(22.9) 35. (26.5) 30.5C25.6) 31. (23.1) 31.9(23.8) 30.5(25. ) 29.9(22.7) 31.7(24. ) 30.6(23.9) 31.1(24.1) 29.3('23.4) 29.2(23. ) 27.9(23.2) 29.3(23.7) 20 8(24.3) 28.9(22.3) 26.1(21. ) I 27.7(22. ) 27.9(21.3) 29.2(23.1) 28.2(22.2) 25.4(19.7) 25.5(20.1) 28.1(22.5) 25.6(20.1) 26.1(20.6) 26.4(20.9) 25.5(20.9) 24.5(19.3) I 28.9(21.8) 25.9(20. ) 24.5(19.5) 23.9(19. ) 25. (20.4) 22.1(20.9) 24.
22.9(17. ) 24.5 23.9
Thoracic Reieion. Per Cent.
Lumbar Region. Per Cent.
Sacral Rejpon. Per Cent.
Coceyseal Refiion. Per Cent.
47.7(38.6) , 46. (40. ) , 50.8 '
45. (36.7) 1 49.2(36.6) I 44.4(33.6) I 46.7(30.2) '
48. (35.7) , 46.7(38.3) 1 47.1(36.5) 47.5(35.9) 46.9(37.1) 50. (38.7) 47.4(37.7) 47.2(37.2) 51.5(42.7)
49. (39.5) 47.9(37.8) 48.5(37.3) 49.1(31.5) 48.2(38.3) 47 5(36.1)
1 47.5(37.5) , 47.9(37.8) 48.3(37.4) I 50. (39.6) I 46.6(37.2) i 48.7(38.1) I 48.2(37.9) . 48.6(38.4) ■ 49. (40.1) , 50. (39.3) , 47.4(35.7) ^ 48.2(37.2) I 50. (39.9)
48.6(38.9) I 48.7(40.1) I 49.6(38.4)
50.5(37.6) I 48.5 , 48.9
17.2(13.9) 18. (15. ) 18.5
20.5(16.7) 20. (14.9) I 20.6(15.5) ! 22.8(19.2) 23. (17.1) , 20.1(15. ) ' 22.8(18.7) I 22.1(16.8) 20.8(15.7) 22.5(17.9) 18.9(14.6) 23.3(18.5) 23.6(18.6) ' 20.6(17.1) I 21.7(17.5) 21.3(16.9) 1 22.7(17.5) ' 24.8(20. ) 24.1(19.1) 24.6(18:7) 23.2(18.3)
24. (18.9) 26.3(20.4) I 24.5(19.4) I 25.3(20.1) 25.6(20.1)
25.6(20.1) 25. (19.8) 25.5(20.9) 25.5(20. ) 23.7(17.8) 25.9(20. ) 25.5C20.3) 27.5(21.9) 26.5(21.9) 28.3(19.8) 27.4
25.8(20.3) , 26.5(19.8)
12.5(10.1) 10.9(89) 12. (11. ) 4. (3. )
[22.8(18.6)] 19.2(14.3) 15.4(11.4) 20.6(15.5) ' 11.1(8.8)
[18.3(16.0)K?) 19.5(14.5) 14.9(11.1)
18.7(14. ) 13.7(11.3)
17.4(13.2) I 14.3(10.8) 17. (12.8) 15.4(11.6)
17.8(13.9) 17. (13.1) 17.3(13.8) 18.9(15. ) 14.0(11.6) 16.3(13.2) 18.8(14.9) 19.8(15.3) 18.2(14.6) 18.8(14.9) 18.6(14.2) 17. (13.4)
9.2( 7.2) 12. 2( 9.5) 8.3( 6.6) 8.1( 6.4) 6.6( 5.5) 7.5( 6.1) 7.8( 6.1) 9.9( 7.6) 6.1( 4.9) 7.1(5.7) 12.7(9.7) 9.7( 7.6)
17.9(14.1) 21.1(15.8) 19.6(15.2) 20.6(16.4) 18.3(14.6) 16.8(13.9)
10. 5( 8.2) 8.5( 6.7) 6.8( 5.4)
9.4( 7.4) 11. 8( 8.9) 9.8( 7.6) 5. ( 4) 7.3( 5.8) 4.4( 3.7)
21.9(16.3) 12. 4( 9.2)
7.4( 6.2) 11.8(8.8)
47.5(37.9) ' 26.8(21.4)
The effect of this expansion in the cervico-facial region is much more marked in the caudal direction, however, where there is relatively more room and the resistance is less. As I have showm in a previous paper (1. c), the alimentary tract, respiratory tract, and in fact most of the organs lying ventral to the body axis (excepting the heart) are also expanding with relatively great rapidity at this time. Thus in the profile views of His,^^ the oesophagus in the 5.5 mm., 7 mm., 10 mm., and 12.5 mm. embryos is found to equal in length about 25 per cent of the body length. In the 13.8 mm. embryo it has suddenly increased to about 40 per cent of the body length. At the same time the respiratory tract (larynx, trachea and lung) increases from about 20 per cent to about 33 per cent of the body length. The stomach similarly increases in relative size and length. The pyloric end becomes fixed at about the 12.5 mm. stage, hence further relative elongation of the oesophagus and stomach increases the obliquity of the stomach in later stages.
During the second month, this period of relatively rapid growth of the viscera is nearly ended. We find accordingly that in the 17 mm. embryo (Fig. 2), and especially in the 31 mm. embryo (Fig. 3), the viscera have reached nearly their permanent vertebral levels, there being relatively little descent after that time. We shall find similarly that the other topographic changes are also most rapid during the first two months. For purposes of developmental to])ography therefore the prenatal term may be divided into two periods, corresponding to those already generally recognized. The first or embryonic period includes the greater part of the first two months, during which the changes are most rapid. During the second or fetal period, from the third month onward, the changes in position are relatively slight.
- Auatomie menschlicher Embryonen, III, S. lG-19.
The visceral branches of the aorta, particularly the coeliac and mesenteric, follow their corresponding viscera downward by acquiring successively new roots of origin from the aorta. This was shown first by Mall,^^ later by Tandler^^ and Broman,*^ according to whom these arteries reach their permanent levels of origin during the second month. The nerves, on the other hand, are unable to shift their attachments to the spinal cord, hence their origin indicates approximately the level of the organ at the time it receives its innervation (e. g,, the diaphragm from the 4th cervical).
In addition to the migration caudad along the vertebral axis, the viscera undergo another change in their relation to the vertebral coluum. In cross sections of the trunk in earlier embryos, the body cavity ap|)ears somewhat elliptical in outline (long axis dorso-ventral), and is placed entirely ventral to the vertebral column. Later, as the trunk becomes flattened dorso-ventrally (cf. Rodes^^), the body cavity also appears more roundtd. At the same time, the vertebral column appears to move forward, forming a median dorsal projection into the body cavity. More accurately stated, the body cavity and viscera extend backward (dorsad) on each side of the vertebral axis. The groove thus formed in the viscera by the vertebral column may be called the vertebral groove.
This change is shown in lateral view in Figs. 1 to 4. In Fig. 1 (11 mm.) the vertebral column lies entirely behind the visa^ra. In Fig. 2 (17 mm.) the ribs have appeared, but the body cavity and thoracic viscera still lie entirely ventral to the vertebral column, and the dorsal aorta is visible in lateral view. In the abdominal region, on the other hand, the kidneys and suprarenal glands have begim to move backward on each side of the vertebral column, which projects forward into the shallow vertebral groove b(»tween them. In the 31 mm. embryo, this groove is much dee])er, involving also the lungs, and including about half of the width of the vertebral column as seen in lateral view (Fig. 3). The l)ody cavity and ribs extend still further backward, the angles of the latter now reaching nearly to the middle of the spinal cord. At 65 mm. (Fig. 4), the groove has become so deep as to include nearly the entire centra of the vertebral column. Corresponding to the width of the vertebral column (centra), the vertebral groove is narrower from side to side in the thoracic region and wider below in the lumbar region (cf. Fig. 20). The pleura at the angles of the ribs extends backward so that in lateral view it covers a large part of the spinal cord (Fig. 4). There is thus formed here (likewise in the 31 mm. and 17 mm. stages) behind the lungs in the pleural cavity a considerable space filled with fluid. In later stages, as the lungs expand the fluid is apparently absorbed, and this space largely disappears. So far as the body cavity is concerned, however, this groove for the vertebral column has nearly reached its I)ermanent relative (fetal) depth at the end of the third month.
- Journal of Morphology, Vol. V, 1801.
- Anatomlsche Ilefte, Bd.- 23, S. 180, 1903.
- Anatonilsohe Ilefte, Bd. 37, 1908.
- Zeitschrlft f. Morphol. u. AnthropoL, Bd. 9, 1905, S. 113 ft.
The cause of the formation of this vertebral groove in the viscera is in many resjiects difficult to understand. As previously stated, it cannot be explained as due to a pushing forward of the column, which appears to remain relatively fixed in size and position. The change is rather due to movement in the viscera lying ventral to the vertebral axis. The change in the general form of the l)ody wall (which is much elongated dorso-vent rally in the earlier stages, becoming nearly circular in cross-section about the third month) is readily explained as due to the pressure of the rapidly expanding viscera. Since more resistance is offered by the vertebral column, this pressure may be more effective laterally, thus producing grooves on either side of the vertebral column, so that it projects forward from the posterior body wall. Ilenke^^ has proposed an ingenious theory explaining the deepening of the vertebral groove in the thorax during childhood as due to a characteristic growth process in the ribs. This theory, however, will hardly apply to the origin of this groove in the embryo, and fails to account for a similar change in the form of the body wall in the lumbar region. Yet it is quite possible that the formation of the vertebral groove is due to growth processes in the body wall not explainable by simple mechanical principles.
- Anatomie des Kiiulesa Iters, 2. Aiifl., Tilbingeii, 1881, S. 101 ff.
Ribs and Body Wall
As is apparent in lateral views, the ribs, which extend nearly horizontally forward when first formed (17 mm., Fig. 2, and 31 mm., Fig. 3), soon extend obliquely downward and forward. This obliquity is unusually marked in the 65 mm. specimen (Fig. 4). The obliquity of the ribs is due to the fact that they elongate relatively more rapidly than the corresponding body wall. This may also account for the formation of the angles of the ribs and costal cartilages. There is also a depression of the anterior body wall as a whole, however, as is shown by the oblique course of the abdominal nerves, myotomes, etc., at G5 mm. and later stages. The umbilicus, which from the G5 mm. stage onward is foimd opposite the 4th lumbar vertebra, is located at a higher level in the earlier stages. This is in part, at least, due to a more rapid growth in the anterior abdominal wall, resulting in its relative elongation.
As already mentioned, the heart migrates downward (caudad) from its primitive position in the ventral part of the head region. In the 11 mm. embryo (Fig, 1) it lies opposite the 2d to the 7tli cervical segment; in the 17 mm. (Fig. 2), opposite the 7th cervical to the 5th thoracic; in the 31 mm. (Fig. 3), opposite the 1st to the 6th thoracic; and in the 65 mm. (Fig. 4), opposite the 4th to the 9th thoracic. In later fetal stages, it usually e^xtends approximately from the 3d to the 8th thoracic vertebra.
At the beginning of the second month, the plane of the diaphragm, between the heart and the liver, extends from above and behind in a direction downward and only slightly forward, nearly parallel with the body axis. The downward displacement of the thoracic viscera takes place more rapidly near the dorsal wall than near the ventral wall, however, so that the heart and diaphragm are apparently rotated on a transverse axis. At 11 mm. (Fig. 1), the diaphragm is rapidly approaching the horizontal position, and at 17 rain. (Fig. 2) has pavssed it, now extending from behind upward and forward. The direction is similar at 31 mm. (Fig. 3) ; but at 65 mm. (Fig. 4) the anterior portion of the heart and diaphragm have descended (due to the relative decrease in the size of the liver?) so that the diaphragm has reached a position approximately horizontal.
The long axis of the heart is at first nearly in the mid-sagittal plane, the apex being nearly in the midline at 11 mm. (Fig. 13). It is slightly inclined to the left at 17 mm. (Fig. 14), more so at 31 mm. (Fig. 15), and very decidedly at 65 mm. (Fig. 16). The primitive right side of the heart is thus turned to the front, and the right auriculo-ventricular groove comes to lie behind the sternum near the midline. The asymmetrical position of the heart (which in turn increases the asymmetry of the lungs?) seems to be correlated with the progressive dorso-ventral flattening of the thorax, to which reference has already been made.
The hings in the 11 mm. embryo lie entirely behind and below the heart, being scarcely in contact with it. (Figs. 1, 5, 9, 1). They gradually extend upward and forward upon the auricles at 17 mm. and 31 mm., and upon the left ventricle at 65 mm. (Figs. 1 to 16).
Although the thymus appears in the 11 mm. embrv^o, it does not extend down in front of the heart until in the 65 mm. stage (Figs. 4j 16, th), where it is in contact with the upper part of the right auricle and ventricle. In later fetal stages it continues to expand in the anterior mediastinal space, so that it covers to a large but variable extent the anterior surface of the heart in the new-born.
The lungs arise during the first month from the oesophagus in the mid-cervical region, in the angle behind and between the heart and liver (as seen in reconstructions by His, Mall and others). Until the middle of the second month, the lungs remain comparatively small and retain this primitive position. In the model of the 11 mm. embryo (Figs. 1, 5, 9, 17, 1), the lungs are separated by the cpsophagus. They lie on each side in a pocket between the liver and upper end of the Wolffian body below, and arched over by the posterior cardinal vein and ductus Cuvieri above. This pocket is deeper on the right side, which perhaps accounts for the occurrence of an accessory lobe of the lung in this position in the adult more often on the right side. So far as the vertebral level is concerned, the lungs of the 11 mm. embryo correspond approximately to the position later of the lung apex, lying opposite the last oen'ical and the first two thoracic segments, and under cover of the anlage of the upiK»r limb. The migration continues downward along the vertebral column, however, so that the lung in the 17 mm. embryo (Fig. 2) lies opposite the 4th-9th thoracic vertebrae, entirely below the upper limb. In the 31 mm. embryo (Fig. 3), the lung has expanded so as to extend from the 1st to the 10th and in the 65 mm. (Fig. 4) lies opposite the 2d to the 11th thoracic vertebra.
Similarly, the bifurcation of the trachea, which arose in the midcervical region, has in the 11 mm. embryo descended to the level of the 1st thoracic segment; in the 17 mm. to the 3d thoracic; in the 31 mm. to the disk between the 3d and 4th ; and in the 65 mm. to the disk between the* 4th and 5th.
The pulmonary arteries in the 11 mm. embryo arise from the ductus arteriosus and descend along the trachea for a considerable distance Ix^fore reaching the lungs. The distance is relatively less in the 17 mm. embryo, but it is not until the 31 mm. stage that approximately the permanent relation is reached. The change is apparently due, chiefly not to the ascent of the hilum of the lung, but rather to the descent of the heart with the accompanying vessels.
The lungs are relatively small in the 11 mm. embryo, but expand rapidly upward and forward over the auricles in the 17 mm. and 31 mm. stages, reaching the ventricle on the left side at 65 mm. There is little expansion (relatively) in the fetal limg after this time, the anterior borders of the lungs being separated by a wide anterior mediastinal space occupied largely by the thymus. The fetal lungs are relatively largest about the 4th month, when they average 3.3 per cent of the total body volume.
The lobes of the limgs are not distinct in the 11 mm. embryo, but are well marked at 17 mm. The fissures are carried upward and forward by the expansion of the lungs, as may be seen by comparing the relations to the ribs in the various stages (Figs. 1 to 12, I, Is, Im, li). Owang to the large size of the thymus, the cardiac notch is usually not well marked in the fetal lung.
The pleural cavity is from the beginning 'more extensive than the lung, which at no time fills it completely. In the embryonic stages the pleural cavity always contains a relatively large amount of fluid, which surrounds the lung and distends the cavity. The pressure of this fluid may assist in extending the boundaries of the pleural cavity during the process of development.
In the 11 mm. embryo, the pleural cavities are not yet separated oS from the general body cavity. At 17 mm. (Fig. 2, pi) the pleura extends from the 2d to the 9th ribs, and from the plane of the anterior surface of the vertebral column posteriorly nearly to the extremities of the ribs anteriorly. It still communicates with the peritoneal cavity (Fig. 2, pp). In a 24 mm. embryo, and in all later stages, I find the pleura extending from the 1st to the 12th ribs. Anteriorly, however, the pleurse of the twq sides are widely separated, extending forward only to the neighborhood of the internal mammary vessels. They do not approach each other in the anterior mediastinal space until after the atrophy of the thymus in postnatal life. Posteriorly, the pleural cavity in the 31 mm. embryo reaches the plane of the anterior surface of the spinal cord (Fig. 3), and at 65 mm. almost covers the spinal cord in lateral view (Fig. 4).
There is considerable individual variation in the development of the lungs and pleurae, as well as of other organs. For purposes of comparison, the reconstructions by MalP^ wnll be found of especial value.
The descent of the stomach along the vertebral column has been mentioned previously. In the 11 mm. embryo the cardia lies opposite the 3d or 4th thoracic segment, and the pylorus opposite the 7th or 8th. In the 17 mm. embryo the two ends of the stomach seem to have reached approximately their permanent positions, the cardia opposite the 10th thoracic and the pylorus opposite the 1st or 2d lumbar vertebra. The greater curvature may later extend much lower, however, especially in cases where the stomach is distended or the liver unusually enlarged.
- Archiv f. Anatomie u. Entw., 1897, Suppl. Bd. S. 403-434; also in .Tolins Hopkins Hospital Bulletin, Vol. XII, 1901, Nos. 121, 122, 123.
Superiorly the fundus region is related to the base of the left lung in the 11 mm. embryo (Fig. 1). The liver extends between them in the 17 mm. and 31 mm. stages (Figs. 2, 3, 21, 22), but has retracted at 05 mm. (Fig. 23) leaving this relation constant in all later stages.
Externally the stomach is separated from the hody wall in the 11 mm. embryo (Figs. 1, 5, 17, x) by the thick-wallcd great omentum, in which, in the 17 mm. stage, the spleen appears (Figs. 2, 6, 21, sp). Both are overlapped by the enormous expansion of the liver at 31 mm. (Figs. 3, 22, sp). The stomach is not yet again visible at 65 mm. (Fig. 8), though it is often visible to the left of and below the liver in the later fetal stages.
Anteriorly and to the right, the stomach is at all stages in contact with the posterior surface of the liver (Figs. 17, 21, 22, 23). Posteriorly it is related in the 11 mm. embryo to the left suprarenal gland, sexual anlage, and Wolffian body (Fig. 17). At 17 ram., the pancreas extends across behind the stomach (Fig. 21). Beginning with the 31 nmi. stage, the stomach becomes separated from the sex gland and Wolffian body, but comes into more or less intimate relation with the kidneys and intestines (Figs. 22 and 23).
The developmental topography of the intestines has been thoroughly worked out by Mall, so that it is unnecessary to describe in detail the relations shown in my models, and indicated in the various figures.
- Arohlv f. Anatoinie ii. Entw., 1897, Suppl. Bd, "Anatomiseher Anzelger, Bd. 27. 1905, S. 488-510.
The developmental topography of the pancreas has been described by me in a previous paper, to which the reader is referred. Figs. 20, 21, 22, and 24 of the present paper exhibit well many of the relations of the pancreas.
From the beginning, the liver is in intimate relation with the diaphragm, and through this with the lieart and lungs. In connection with these organs, it undergoes the migration previously described. The upper surface of the liver apparently reaches its permanent level in the fetus at some time between the 31 mm. and 65 mm. stages.
In size, the liver is at first relatively small, but it rapidly enlarges. In the 11 mm. embryo it measured 4.85 per cent of the total body volume (or approximately the same as at birth) ; in the 17 mm. embryo it has increased to 6.9 per cent; while in the 31 mm. embryo it has reached 10.56 per cent, which represents its maximum relative size. In the 65 mm. specimen the volume of the liver was not measured, but it has apparently decreased to about 5 or 6 per cent, which is the average for the remainder of the fetal period. The importance of the relatively enormous expansion of the liver in producing the characteris^tic embryonal umbilical hernia has been emphasized by Mall.
The relation of the liver to the body wall and ribs anteriorly and laterally is evident in the various figures, and calls for no special discussion.
The posterior or visceral surface of the liver in the 11 mm. embryo (Figs. 5, 9, 17) is related to the suprarenal gland, Wolffian body and sexual anlage on the right side ; to the stomach on the left side ; and to the duodenum and head of the pancreas below. At 17 mm., the relations are similar (Figs. 18, 21). At 31 mm. (Figs. 19, 22), the kidneys and spleen come into contact with the liver. The visceral surface at this stage forms a relatively small depression on the posterior surface of the liver. At 65 mm. (Figs. 20, 23) the relations approach those found throughout the remainder of the fetal period. The lower part of the visceral surface is related to the transverse colon and coils of the small intestine. The relations of the liver to the pancreas are described in detail in my paper on the topography of the pancreas (1. c).
The spleen is not well differentiated in the 11 mm. embryo, but the indistinct anlage lies in the thick-walled great omentum in the region of the window shown in the model (Figs. 1, 5, 17, x).
In the 17 mm. stage, the spleen is relatively small but distinct (Figs. 2, 6, 18, 21, sp). It is an elongated prismatic structure, extending from above downward, slightly outward and forward. It has three distinct surfaces (best seen in Fig. 21) corresponding to those of the adult organ. These are: (1) an external surface^ corresponding to the (later) diaphragmatic surface, but here in contact with the lateral abdominal wall below the ribs; (2) an antero-internal gastric surface, in contact with the stomach; and (3) a posterior (later renal) surface, here in contact wath the left suprarenal, sex gland, and Wolffian duct.
In the 31 mm. stage, the liver has expanded so as to separate the external surface of the spleen from the body wall (Figs. 3, 7, 22). The spleen is here nearly vertical, and in contact antero-internally with the tail of the pancreas, as well as the stomach (Fig. 22, sp). Postero-internally, it is in contact with the left suprarenal.
In the 05 mm. stage, the spleen is relatively larger, but still relatively smaller than in the later fetal stages. It here lies entirely under cover of the ribs. The liver has partly retracted, so that it here covers only the anterior portion of tlie external splenic surface ; the posterior portion being in contact with the diaphragm between the 0th and 10th ribs (Figs. 4, 8, 20, 23, sp). The upper extremity of the diaphragmatic surface is now related to the base of the left lung. Antero-internally and postero-internally the relations are similar to those at 31 mm. The lower extremity of the spleen comes in contact with the splenic flexure of the colon (Figs. 20, 23), a relation which is constant throughout all later stages.
In the later fetal stages, the spleen becomes relatively larger, expanding farther upward, inward and downward. The liver usually retracts somewhat, but is often slightly in contact Avith the spleen, even at birth. As the suprarenal gland becomes relatively smaller, the posterior surface of the spleen comes into relation more and more with the left kidney. The fetal spleen is constantly related to the hase of the lung above, and is usually entirely pre-pleural (as in Fig. 4). The spleen is occasionally enlarged, however, in which case it may extend downward below the lower margin of the pleura.
In position, the longitudinal axis of the fetal spleen is always oblique, but it usually approaches the vertical more nearly than the horizontal. When the colon is distended, however, the lower extremity of the spleen is flattened and often pushed upward so as to throw the spleen into a position more nearly horizontal.
Wolffian Bodies (Mesonephros)
In the 11 mm. embryo, the volume of the Wolffian bodies is .000734 ec. (.6 per cent of the total body volume) ; at 17 mm., the volume is .00055 cc. (.124 per cent of total body) ; and at 31 mm., .00045 cc. (.0212 per cent of total body). It is therefore evident that the Wolffian bodies decrease in size, not only relatively but absolutely, from the beginning of the second month.
At the end of the first month, the Wolffian bodies extend from the lower cervical to the lumbar region (according to the observations of His and Mall). In the 11 mm. embryo (Figs. 1, 5, 9, 17, w), they extend along the posterior body wall from the 1st or 2d thoracic segment down to the lower lumbar region, lying just anteroexternal to the posterior cardinal veins. From the 3d to the 6th thoracic, they are also related internally to the suprarenal anlages. Anteriorly, they are related to the lungs, stomach, liver and sexual anlages.
In the 17 mm. embryo (Figs. 2, 6, 10, 18, w), the Wolffian bodies have apparently been drawn downward (being more firmly attached at the lower end), so that they extend from the 10th thoracic to the sacral region. They are separated relatively more widely by the expansion of the kidneys and suprarenals (cf. Figs. 17 and 18, w), and they no longer touch the lungs.
At 31 mm. the Wolffian bodies lie opposite the lower two lumbar and the upper sacral vertebrae. Internally they are related to the ovaries and lower part of the kidneys (Fig. 22a). At 65 mm,, the Wolffian bodies appear as rudimentary appendages of the testis, lying opposite the first sacral vertebra (Fig. 20).
The early developmental topography of the sex glands is very similar to that of the Wolffian bodies, with which they are intimately connected. They are likewise firmly connected with the body wall below, so that they are also dragged downward during the relative elongation of the lower portion of the trunk in development. In the 11 mm. embryo, the sexual anlages (sex not yet determinable) form narrow strips extending along the antero-intemal aspect of the Wolffian bodies, from the 9th thoracic to the 3d lumbar segment (Figs. 1, 5, sx). The relations are similar to those of the Wolffian l)odics in this region. At 17 mm, (Fig. 21a, ov), 31 mm. (Fig. 22a, ov), and C5 mm. (Fig. 20, t), the general position and relations are very similar to those already stated for the Wolffian bodies.
In the 11 mm. embryo, the suprarenal glands (Figs. 1, 5, 17, sr, si) are ratlier ill-defined elongated bodies, extending on each side at tlie level of the 3d to the 6th thoracic segments, between the aorta and the posterior cardinal vein. Anteriorly, they are related to the lungs above, and to the liver (right side) and stomach (left side) hi low.
In the 17 mm. stage, the suprarenals have descended so as to extend from the 10th thoracic to the 1st lumbar vertebra (Figs. 2, r>, 10, 18, 21a, sr, si). They are shorter, thicker, and flattened from within outward. Internally they are closely related to each other, being separated posteriorly by the aorta. Posteriorly they are related to the 10th, 11th and 12th ribs below the pleural cavity. Superiorly they are related to the bases of the lungs (Fig. 18). Anteriorly the right suprarenal is related to the liver, the left to the stomach and spleen, Inftriorly they are in contact with the kidneys, and externally with the sex glands and Wolffian bodies.
At 31 mm., the suprarenals (Figs. 3, 7, 11, 19, 22a, sr, si) extt^nd from the 11th thoracic to the 1st lumbar vertebra. Owing to the change in position of the vertebral column (previously discussed), the suprarenals are separated more widely from each other by the lx)dies of the vertebne, and their former internal surfaces are rotated so as to face postero-internally. The aorta is displaced forward, so that it here lies between the anterior margins of the suprarenals. The relations to the body wall, etc., posteriorly are nearly as in the 17 mm. embryo. They do not now extend up to the lungs or 10th ribs, however, although the pleural cavity has extended down behind the upper half of their posterior surfaces. Anteriorly the right suprarenal is in contact with the liver ; the left with the stomach and body of the pancreas, and externally with the spleen and left lobe of the liver. Inferiorly and posteriorly, the concave base of each suprarenal is in close contact wnth the upper portion of the corresponding kidney.
At 65 mm., the suprarcnals (Figs. 4, 8, 12, 20, 23a, sr, si) extend from the 10th thoracic to the 1st lumbar vertebra. They are still more widely separated by the vertebral column, and have rotated so as to lie almost in the frontal plane. They are decidedly flattened antero-posteriorly, and show a distinct groove on the anterior surface (Fig. 23a). The relations are much as at 31 mm., except that the kidneys have pushed up behind them to a greater extent, and the basrs of the lungs are again related to them above. The left lobe of the liver has retracted, so that it is no longer in contact with the left suprarenal. The relations of the suprarenal glands found here persist nearly unchanged throughout the remainder of the fetal period.
The kidneys form a notable exception to the general rule in developmental topography, since they migrate upward (cephalad) along the vertebral column, instead of downward like the other viscera. Their relations therefore deserve especial attention.
As is well known, the kidneys (kidney-ureter anlages) arise as a dorsal outgrowth of the Wolffian duct on each side in the sacral region. By active growth, the anlage elongates, extending forward on each side along the line of least resistance in the loose mesenchyme of the space bounded by the aorta dorsally, the rectum ventrally, and the umbilical arteries laterally.
In the 11 mm. embryo (Fig. 9, u) the T-shaped upper extremity of the anlage (representing the renal pelvis) lies in the upper sacral region, under cover of the wide origin of the umbilical artery. It is extending upward, and is approaching the lower extremity of the Wolffian body. In succeeding stages, as the anlage elongates, the kidney is pushed upward on either side of the aorta in the loose mesenchyme dorsal and internal to the Wolffian body and sexual anlage.
In the 17 mm. embryo, the kidneys are well developed, with a distinct capsule, and extend from the 1st to the 5th lumbar vertebra (Figs. 2, 0, 10, 18, 21a, kr, kl). They have come in contact with the lower ends of the suprarenal glands, and further extension upward (except to a very slight extent) is possible only later when the suprarenals diminish in relative size. Antero-lateral to the kidney are the Wolffian body and ovary.
At 31 mm., the kidneys (Figs. 3, 7, 11, 19, 22a, kr, kl) still lie below the 12th rilw, extending from the 1st to the 5th lumbar vertebra. Their upper ends reach almost to tlie lower pleural margin (Fig. 3). The separation and rotation of the kidneys in connection with the changed position of the vertebral column is very similar to that descrilx?d for the suprarenal glands. Between the kidneys at the floor of the vertebral groove are the aorta and vena cava inferior (Fig. 19, 22a, a, vc). Anteriorly the right kidney is related to the corresponding suprarenal and the liver and ovary below; the left kidney is related to the suprarenal above, and to the liver, pancreas, stomach and ovary below.
At 65 mm. (Figs. 4, 8, 12, 20, 23a, kr, kl), the kidneys have reached approximately their permanent fetal position, extending from the 12th thoracic to the 3d lumbar vertebra. Both kidneys are nearly at the same level, the higher level of the left kidney later being correlated with the atrophy of the left lobe of the liver. Posteriorly the kidneys reach the 12th rib above (and, on the left side, the 11th rib). The pleural cavity covers the upper portion of the posterior surface of each kidney (Fig. 4). Anteriorly the right kidney is in contact with the suprarenal gland above, and with the liver, and small intestines (including the duodenum) below; the left kidney, with the suprarenal above, and the intestines (l)eginning of jejunum and descending colon) below. The relations of the kidneys found here are but little changed throughout the remainder of the fetal period.
Fig. 1. — Graphic reconstruction of an 11 mm. human embryo (No. GO) from the left side, showing the body outline, extremities, central nervous system, vertebral centra, viscera, etc. The parts con-esponding to the viscera in the model (Fig. 5) are indicated by stippling. The various regions of the vertebral column are indicated (ceph.-cerv., cerv.-thor., thor.-lumb., lumb.sacr., sacr.-cocc.).
A, ascending aorta ; a, descending aorta ; a3, a4, Sd and 4th aortic arches ; ac, anterior cardinal (jugular) vein ; c, anlage of the cecum ; cd, caudal aorta ; cl, cloaca ; co, colon ; dC, ductus Cuvieri ; 1, lung ; L, liver ; la, left auricle ; Iv, left ventricle ; pc, posterior cardinal vein ; ph, pharynx ; R, rectum ; sx, sexual anlage ; si, left suprarenal anlage ; sa, origin of subclavian artery ; th, anlage of thymus; tl, tm, lateral and median anlages of the thyroid; U, umbilical cord; ua, umbilical artery; uv, umbilical vein; w, Wolffian body; wd, Wolffian duct ; x, window cut into great omentum ; ys, attachment of yolk-stalk to intestinal loop.
Fig. 2. — Graphic reconstruction of a 17 mm. human embryo (No. 58) from the left side, showing the body outline, extremities, central nervous system, vertebral centra, ribs, etc. The parts corresponding to the wax model (Fig. 6) are indicated by stippling. The various regions of the vertebral column are indicated (ceph.-cerv., cerv.-thor., thor.-lumb., lumb.-acar., sacr.-cocc. ) .
a, aorta ; ac, anterior cardinal (jugular) vein ; c, anlage of cecum and appendix ; co, beginning of colon ; da, ductus arteriosus ; ic, ileo-cecal junction ; kl, left kidney; Is, li, sui)erior and inferior lobes of left lung; L, liver; la, left auricle; Iv, left ventricle; o. oesophagus; pi, pleural boundary; pp, pleuro-peritoneal foramen ; R, rwtum, s, stomach ; si, left suprarenal ; si. small intestine; sp, si)leen ; tr, trachea; U, umbilical cord; w, Wolffian body; 1 to 12, 1st to 12th ribs.
Fig. 3. — Graphic reconstruction of a 31 mm. human embrj-o (No. 57) from the left side, showing the body outline, extremities, central nervous system, vertebral centra, ribs, sternum, viscera, etc. The parts corresponding to the wax model (Fig. 7) are indicated by stippling. The various regions of the vertebral column are indicated (ceph.-cerv., cerv. thor., thor.-lumb., lumb.-sacr., sacr.-cocc. ) .
kl, left kidney; Is, 11, superior and inferior lobes of left lung; L, liver; la, left auricle ; Iv, left ventricle ; o, cesoiihagus ; pi, pleural boundary ; R, rectum ; si, left suprarenal ; si, small intestine ; st, sternum ; tr, trachea ; U, umbilical cord; ua, umbilical artery; uv, umbilical vein; 1 to 12, 1st to 12th ribs.
Fig. 4. — Graphic reconstruction of a (35 mm. human embryo (No. 55) from the left side, showing the body outline, extremities (cut off short), central nervous system, vertebral centra, ribs, sternum, viscera, etc. The imrts corresponding to the wax model (Fig. 8) are Indicated by stippling. The various regions of the vertebral columnn are indicated (ceph.-cerv., cerv.-thor., thor.-lumb., lumb. sacr., sacr.-cocc.).
CO, colon ; kl, left kidney ; Is, li, superior and inferior lobes of left lung : L, liver ; Iv, left ventricle ; o, cesophagus ; p, penis ; pi. pleural boundary ; R, rectum; si, small intestine: si, left suprarenal ; sp, spleen; st. sternum; T, lower extremity, cut off just below the hip joint; t, testis; th^ thymus; tr, trachea; U, umbilical cord; 1-12, 1st to 12th ribs.
Figs. 5 to 8. — From photographs of wax models reconstructed by Bom's method from human embryos, showing the thoracic and abdominal viscera, with projections of the ribs, viewed from the left side.
( For Figs. 5 to 8. ) A, ascending aorta ; a, descending aorta ; a3, a4, 3d and 4th aortic arches; ac, anterior cardinal (jugular) vein; bw, body wall; c, anlage of cecum and appendix; ca, left common carotid artery; co, bt»ginning of colon ; da, ductus arteriosus ; dC, ductus Cuvierl ; hi, hind limb ; ia, innominate artery: ic, ileo-cecal junction; kl, left kidney; L, liver; 1, left lung; Is, 11, superior and inferior lobes of left lung; la, left auricle; Iv, left ventricle ; o, oesophagus ; pc, posterior cardinal vein ; ph, pharynx ; R, rectum ; r2-rl2, projections of 2d to 12th ribs ; ra, right auricle ; rv, right ventricle; s, stomach; sa, left subclavian artery; si, small intestine: si, left suprarenal : sp, spleen ; t, testis ; th, thymus, tl, tm, lateral and median thyroid anlages ; tr, trachea : ua, uv, umbilical artery and vein ; w. Wolffian body; x, window in great omentum; ys, attachment of yolkstalk to intestinal loop.
Fio. 5. — From an embryo of 11 mm. (No. 60), showing also the lower extremity, and the lower portion of the body wall.
Fig. 6. — From an embryo of 17 mm. (No. 58).
Fig. 7. — From an embryo of 31 mm. (No. 57).
Fig. 8. — From an embryo of G5 mm. (No. 55).
Figs. 9 to 12. — From photographs of wax models reconstructed by Born's method from human embryos, showing the thoracic and abdominal viscera with projections of the ribs, viewed from the right side.
(For Figs. 9 to 12.) A, ascending aorta (or arch) ; a, descending aorta; ac, anterior cardinal (Jugular) vein; al, allantois; a2, a3, a4, 2d, 3d, and 4th aortic arches ; c, anlage of cecum and appendix ; cl, cloaca ; co, beginning of colon ; du, duodenum ; gb, gall bladder ; kr, right kidney ; 1, lung ; Is, Im, 11, superior, middle and inferior lobes of the right lung; L, liver; n, notochord ; o, oesophagus ; po, posterior cardinal vein ; ph. pharynx ; r2-rl2, projections of 2d to 12th ribs; ra, right auricle: rv, right ventricle; sa, sulK*lavlan artery ; sc, spinal cord ; si, small intestine ; sp, spleen ; sr, right suprarenal gland ; th, thymus ; tl, tm, lateral and medium thyroid anlages ; tr, trachea ; u, ureter ; ua, umbilicirl artery ; uv, umbilical vein ; vv, vitelline vein; w. Wolffian body; ys, attachment of yolk-stalk to intestinal loop.
Fig. 9. — From an embryo of 11 mm. (No. (X)). In the lower part of the model, the body wall is represented as having been dissected away nearly to the mid-sagittal plane, so as to show a portion of the spinal cord, notochord, aorta, pelvic viscera, etc.
Fig. 10. — From an embryo of 17 mm. (No. 58).
Fig. 11. — From an embryo of 31 mm. (No. 57).
Fig. 12. — From an embryo of G5 mm. (No. 55).
Figs. 13 to 10. — From photographs of wax models reconstructed by Bom's method from human embryos, showing the thoracic and abdominal viscera, with projections of the sternum and ribs, anterior view.
(B'or Figs. 13 to 1(5.) A, ascending aorta; ac, anterior cardinal (jugular) vein; al, allantois; a2, 2d aortic arch; bl, bladder; c, anlage of cecum and appendix ; ca, left common carotid artery ; co, colon ; da, ductus arterioRUR ; gb, gall bladder ; hi. lower extremity ; ia, innominate artery ; ic, ileo-cecal junction ; L. liver ; is, Im. li, superior, middle and inferior lobes of the lung ; la, left auricle ; Iv, left ventricle ; Ix, larynx ; o, oesophagus : ph, pharynx ; r2-r8, projections of 2d to 8th ribs, with corresponding costal cartilages joining the sternum; ra, right auricle; rv, right ventricle; sa, left subclavian artery; sc, spinal cord; si, small intestine; th, thymus anlage: tl, tm, lateral and median thyroid anlages; tr, trachea; ua, umbilical artery; uv, umbilical vein; va, vitelline artery (with accompanying velnj.
Fio. 13. — From an embryo of 11 mm. (No. 00). Lower extremity and a I>ortion of the lower body wall showni on the left side of the model.
Fig. 14. — From an embryo of 17 mm. (No. 58).
Fig. 15. — From an embryo of 31 mm. (No. 57).
Fig. 10. — From an embryo of 05 mm. (No. 55).
Figs. 17 to 20. — From photographs of wax models reconstructed by Born's method from human embryos, showing the thoracic and abdominal viscera, with projections of the ribs and intervertebral disks, posterior view.
(For Figs. 17 to 2().) A, arch of aorta; a» descending aorta; ac, anterior cardinal (jugular) vein; ba. bifurcation of aorta; bw, body wall; co, descending colon; dcO-7. level of disk between 0th and 7th cervical vertebne; dt3-4, disk between 3d and 4th thoracic vertebra*; dtl, disk between 12th thoracic and Ist lumbar; dl3-4. disk between 3d and 4th lumbar; du, duodenum ; hi, lower extremity ; hp, head of pancreas ; kr, kl. right and left kidneys; L, liver; 1, lung. Is, Im, II, superior, middle and Inferior lobes of lung; la, left auricle; o, (esophagus; pb, body of pancreas; pc, posterior cardinal vein ; pco, pelvic colon ; ph, pharynx ; r2-rl2, projections of 2d to 12th ribs ; ra, right auricle ; sa, subclavian artery ; sc, spinal cord ; si, small Intestine; si, sr, left and right suprarenals; sp, spleen; t, testis; th, thymus anlage ; tr, trachea ; u, ureter ; vc, vena cava inferior ; w. Wolffian body ; X, window in great omentum.
Fio. 17. — From an embryo of 11 mm. (No. 00). rx)wer extremity and a portion of the lower body wall shown on the left side of the model.
Fm. 18. — From an embryo of 17 mm. (No. 58).
Fio. 19. — From an embryo of 31 mm. (No. 57).
Fig. 20. — From au embryo of 05 mm. (No. 55).
Figs. 21 to 23. — Posterior view of the models as shown in Figs. 18 to 20, excepting that the kidneys, suprarenal glands, Wolffian bodies and sex glands have been removed.
(For Figs. 21 to 23, in addition to the explanations given for Figs. 18 to 20.) cr, cardia; dj, duodeno-jejunal flexure; dls, disk between last lumbar and first sacral ver^ebrje ; gc, groove for inferior vena cava ; gk, groove for kidney ; go, groove for ovary ; gs, groove for suprarenal gland ; Ic, caudate lobe of liver ; s, stomach ; tp, tail of pancreas.
Fig. 21. — From an embryo of 17 mm. (No. 58).
Fig. 22. — From an embryo of 31 mm. (No. 57).
Fig. 23. — From an embryo of 05 mm. (No. 55).
Fig. 21a. — Anterior view of detached model showing the kidneys, suprarenal glands and sex glands of the 17 mm. embryo. A posterior view of this model Is shown In Fig. 18, but It has been detached in Fig. 21. a, aorta, showing origins of the coellac, superior and inferior mesenteric branches; ba, bifurcation of the aorta, with beginning of the caudal (middle sacral) artery ; ov, ovary ; od. oviduct ; sr, si, right and left suprarenal glands, below which a small portion of the kidneys is visible.
Fig. 22a. — Anterior view of detached model showing the suprarenal glands, kidneys, etc., of the 31 mm. embryo. A posterior view of this model is shown in Fig. 10, but It has been detached in Fig. 22. a, aorta ; ba, bifurcation of aorta; kr, kl, kidneys; ov, ovary; od, oviduct; sr, si, suprarenals; u, ureter; vc, vena cava inferior; w. Wolffian bodies.
Fig. 23a. — Anterior view of detached model showing the suprarenal glands and kidneys of the (>5 mm. embryo. A posterior view of this model Is shown in Fig. 20, but it has been detached in Fig. 23. a, aorta; kr, kl, kidneys; sr, si, suprarenals ; u, ureter.
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