Talk:Paper - Migration processes during ontogeny with reference to the venous development in the dorsal body wall (1946)
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MIGRATION PROCESSES DURING ONTOGENY WITH REFERENCE TO THE VENOUS DEVELOPMENT IN THE DORSAL BODY WALL
By J. AUER, Department of Anatomy, University of Utrecht
During the early months of development many organs migrate from their original positions. Such migration may have a considerable effect on the topographical relations of adjacent structures. The present paper gives an account of the migration of the permanent kidney, the Wolffian body and the suprarenal gland, and of the possible effects of this on the development of the veins in the dorsal body wall.
The migration of the permanent kidney has for many years constituted an interesting problem which is not yet completely solved. The question naturally arises whether the migration of the permanent kidney is an active process on the part of the kidney, or is merely passive and the result of outside dynamic factors.
Recently Brockmann (1936) has attempted to summarize all the dynamic factors, and has drawn attention in particular to (1) the straightening of the body of the embryo, (2) the changes in the ventro-dorsal and transverse diameters of the body, and (3) the displacement of the centre of gravity. He is of the opinion that, between the stages of 10 and 20 mm., the straightening of the body is the principal factor. He thinks of the body as consisting of two layers A and B (Fig. 1). He considers the two layers as being fixed at the cranial end, and thus when the straightening of the body occurs the organs in layer A must move cranially relative to layer B.
Objections to Brockmann’s theory have been made by Starkenstein (1938) on the grounds that (a) the embryos used by Brockmann were too far advanced for proper study of the migrating kidney, and (b) migration can be shown to be an active process, though he does not elaborate this. I believe that Brockmann’s theory is too mechanical, and it does not explain the caudal migration of such organs as the heart and the testis. Brockmann perhaps recognized this, for he states (p. 627): ‘Die Organe, die dem innerem Bande verglichen werden k6énnen, sind Blase, Enddarm, Ureter, Wolffscher Gang, Nieren, Urnieren und Aorta. Diese Organe werden bei der Entwicklung des Fétus in kranialer Richtung gegen die Wirbelsiule verschoben’, thus omitting the testis. With regard to the Wolffian bodies and aorta he is certainly incorrect, as will be shown. I agree with Starkenstein that Brockmann’s embryonic stages were too old. His youngest
embryo measured 10 mm. and showed the kidney primordia at the level of lumbar vertebrae 3-5. Starkenstein found the primordia in younger embryos (3-6 mm.) in the region of sacral vertebrae 2-5, a conclusion quite in accordance with my own observations.
Little attention has been paid to the relation between migration processes and venous development in the dorsal body wall. The permanent kidney, the suprarenal gland, the Wolffian body and the visceral aortic rami change their position relative to one another and to the vertebral column. As these organs are drained originally by paired veins, the postcardinal veins, changes in these veins associated with migration of these organs have an important bearing on the arrangement of the vessels responsible for the formation of the inferior vena cava.
MATERIAL AND METHODS
Human embryos ranging from 6 to 35 mm. in length were used for this investigation. They are listed in Table 1.
Table 1 C-R length C-R lengt Embryo mm. Embryo mm. I 6 VIII 18 I 6 Ix 20 II 8 x 20 Al ll A2 25-6 IV 12 L2 25 Ll 12 XII 26 V 14 XIII 30 VI 15-5 XIV 35 VII 18
I am greatly indebted to Prof. M. W. Woerdeman and Prof. J. A. J. Barge, Directors of the Departments of Anatomy in Amsterdam and Leiden Universities respectively, for the loan of the ‘A’ and ‘L’ series of embryos,
Two of the embryos were cut sagittally and the remainder transversely. They were stained with haematoxylin and eosin or by the Bodian silver impregnation method.
The embryos were studied in serial sections—still the best method for investigating the development of the veins in man (Gladstone, 1929). Wax-plate reconstructions were not made, but linear reconstructions were found to be useful in studying topographical relations. 62
12mm. Embryo, L1. At this stage the postcardinal vein is still the chief vein of the dorsal body wall, but it has lost part of its intersegmental drainage due, in my opinion, to two causes: the development of the suprarenal gland, and the separation or loosening of the mesonephros from the dorsal body wall, so that the postcardinal vein comes to be squeezed between the suprarenal gland and the mesonephros.
The suprarenal gland extends at this stage from the 5th to 11th thoracic segment ; its topographical relations are shown in Fig. 2, in which the postcardinal vein is seen ventro-lateral to the sympathetic ganglia. In the figure another vein is seen ventro-medial to the sympathetic ganglia. It extends from the 11th to the 5th thoracic segment and there opens into the postcardinal vein. This vein, from its position between the dorsal aortic rami and the sympathetic trunk, corresponds to the later thoracic azygos vein and is regarded as the thoracic supracardinal of Huntington & McClure (1907), though this does not imply that it is continuous with the post-renal division of the inferior vena cava. A third, small, vein can be seen ventral to the suprarenal gland and medial to the Wolffian body—the subcardinal vein of Lewis (1902). The right and left subcardinal veins are joined at the level of the 12th thoracic segment by an extensive anastomosis. In later stages a migration of this anastomosis occurs.
While in the thoracic region it is the suprarenal gland that is of interest to us, in the lumbar region it is the permanent kidney that demands our attention. In the 12mm. stage the kidney lies ventral to the bifurcation of the aorta, and its cranial pole is dorsal to the caudal end of the mesonephros (Fig. 10a). The postcardinal vein, dorsal to the kidney, receives small branches from the Wolffian body, one of which is seen in Fig. 4 (c.m.v.). It seems as if the original single postcardinal vein had been split into two, and it is interesting to speculate whether the migrating kidney is at all responsible for this. Comparison of 6 and 12 mm. embryos suggests that such may be the case. Figs. 3 and 4 show such a comparison. The kidney, at first lying below the Wolffian body, shifts its position, as indicated by the arrows in Fig. 8, so as to lie dorsal to this organ and ventro-lateral to the vertebral column. At the same time there is a caudal movement and a reduction of the Wolffian body, accompanied by a detachment of it from the dorsal body wall, thus giving it a sort of mesentery. It seems as if the dorsal part of the dorsal body wall starts moving cranially ‘and the ventral part caudally, which, as will be seen, affects the venous system.
Fig. 4 shows the condition in a 12 mm. embryo, with one vein between the kidney and the Wolffian
body and another dorsal to the kidney and in the line of the original single postcardinal vein shown in Fig. 3. This line has been called the thoracolumbar line by some authors, e.g. Reagan (1920, 1927) and Gladstone (1929).
Hochstetter (1893), the pioneer in the investigation of this venous problem, lays great stress on the influence of the kidney, though he does not give details of the process of migration. But he speaks of the cardinal vein as having a ventral and a dorsal limb, as if it had been in his mind that it was divided into two at its caudal end.
Huntington, McClure and others have regarded this dorsal limb of the postcardinal vein as a new formation, the lumbar supracardinal system, but this does not seem correct, for the dorsal limb is not new, occupies the original position, and is directly continuous with the main postcardinal vein. Gruenwald (1938) also emphasized the fact that the supracardinal system does not take part in the formation of the inferior vena cava, but according to the above his conclusion that a so-called sacrocardinal vein forms the caudal division of the inferior vena cava cannot be accepted, for this would also mean that this vein is another formation. The data obtained after an examination of pig embryos (Auér, 1942) give the same evidence.
The dorsal limb is intimately related to the kidney, curving round its dorsal side as can be seen in Fig. 11a, and this curve straightens out as the kidney moves cranially.
Transverse sections at the level of the cranial pole of the kidney show a small vein, appearing first at the 12 mm. stage, which runs almost vertically from the postcardinal vein to the subcardinal. Huntington & McClure (1920) call this the supracardino-subcardinal anastomosis, but it should be more properly called a postcardino-subcardinal . anastomosis.
It is thus seen that in the lumbar region as in the thoracic, the original paired system of postcardinal veins, still present in the 11 mm. embryo (Fig. 5), is replaced by two veins. In the thoracic region these are the azygos and the postcardinal, and in the lumbar region the caudal mesonephric and the postcardinal.
18mm. Embryo, VIII. At this stage the cranial part of the postcardinal that was embedded in the meséntery of the mesonephros has almost disappeared. Similarly, the caudal mesonephric branch in front of the kidney is much reduced, owing to the
‘drainage from this part of the mesonephros being
taken over by the subcardinal vein. The main caudal postcardinal has rather increased in size, and has come to lie medial rather than dorsal to the kidney, due to the movement of the latter.
The change that has occurred in the position of the kidney can be seen by comparing Figs 4 and 6, which show that at the 18 mm. stage, the kidney Migration processes and veins of dorsal body wall
has moved considerably dorsally and laterally. It lies at this stage at the level of lumbar vertebrae 1-3.
The posteardino-subcardinal anastomosis is much enlarged and now forms a wide channel, but it still lies caudal to the level of the intersubcardinal anastomosis (Figs. 10b and 110).
Stages older than 18 mm. of which we mention the 20, 25 and 30mm. stages, do not show any changes with regard to the level of the kidney relative to the vertebral column but as Figs. 6 and 7 demonstrate, a dorsal movement of the organs is still active.
385mm. Embryo, XIV. There is at this stage a considerable increase in the transverse diameter of the body, whereby the kidneys come to lie at the sides rather than in front of the vertebral column (Fig. 7). The mesonephros has become more detached from the dorsal wall, thus defining more clearly its mesentery.
The postcardino-subcardinal anastomosis has become wider and shorter and runs more horizontally than at the 18 mm. stage.
Thus in the lumbar region there is a movement of the metanephros and of the mesonephros in opposite directions, and this is associated with a lengthening of the postrenal segment of the postcardinal vein (or inferior vena cava), with a change in the direction of the postcardino-subcardinal anastomosis and with a change in position of the inter-subcardinal anastomosis and with a change of the postcardinal vein relative to the kidney.
Though it appears, from a comparison of Figs. 3 and 4, that the migration of the kidney is responsible for the splitting of the post-cardinal vein into a ventral and a dorsal portion, yet the two processes are in reality independent and both are part of a general developmental process.
Evidence for this statement is furnished by an abnormal embryo of 15-5 mm. that showed total absence of the metanephros and almost complete absence of the mesonephros, the latter being re cognizable only at its cranial end. Yet in this’
specimen development of the postcardinal and sub‘cardinal veins and of the anastomosis between the two has proceeded in the normal way, and the anastomosis has changed its direction from a vertical to a horizontal just as in a normal embryo (Figs. 8 and 9).
The azygos system. In embryos of more than 12 mm. length this system is represented.in the lumbar region by a longitudinal trunk lying between the sympathetic trunk and the dorsal rami of the aorta,
Gradually a plexiform system of veins appears between the two aortic dorsal rami (Strong, 1925; Reagan & Robinson, 1926). In Fig. 10 this plexus is drawn as a single vessel. It is convenient to refer to these azygos veins as lumbar supracardinals, for they lie in the same line as the thoracic supra 638
cardinals, i.e. between the sympathetic trunk and the dorsal aortic rami.
The anastomoses connecting the lumbar azygos, or supracardinal, veins are shown in Fig. 12. They anastomose with the veins of the dorsal aortic plexus and with the caudal postcardinal, or postrenal vena cava as it may now be called. This latter anastomosis, on the right side, is a very wide connexion more of the nature of a confluence than a simple anastomosis.
Reagan & Robinson (1926) have pointed out that the lumbar azygos veins disappear almost entirely as separate entities. Together with the dorsal aortic plexus (subcentral vein of Frazer) they form the terminal parts of the definitive left lumbar veins.
Continuity of thoracic and lumbar azygos systems. This difficult question is connected with the migration of the suprarenal gland, but though the development of the gland has been described by Wiesel (1902), Kohno (1925) and Politzer (1937), I have been unable to find any detailed account of its migration. The movement that this gland undergoes can be seen in Figs. 10 and 11. At the 12 mm. stage it extends from the 5th to the 11th thoracic vertebra; at the 18 mm. stage from the 9th thoracic to the 1st lumbar; at the 35 mm. stage from the 10th thoracic to the 1st lumbar vertebra. Associated with this caudal movement is a migration in a dorso-lateral direction (Figs. 15 and 16), and there is some caudal migration and atrophy of the mesonephros. The 20, 25, 26 and 30 mm. stages all show these changes to an extent relative to their age.
As atrophy of the cranial end of the mesonephros goes on, so there is a disappearance of the thoracic division of the postcardinal. We are thus left with a single pair of veins in the azygos line.
In the 12 mm. embryo this azygos vein extends from the 11th to the 5th thoracic vertebra, where it opens into the postcardinal; thus in the 18 mm. embryo the vein in the thoracic region is formed, as far as the 5th thoracic vertebra, by the original azygos (medial sympathetic vein of Robinson & Reagan) and, cranial to the 5th thoracic vertebra, by the thoracic postcardinal. But the thoracic postcardinal vein originally lay in the thoracolumbar line and not in the azygos line. It is, however, pressed in a medial direction, I believe, by the developing pleural cavities, and thus comes to lie in the azygos line (Fig. 13).
No other vein than this was found in the thoracic region, and I was unable to identify the transient vein of the thoraco-lumbar line to which Gladstone refers. I may have missed the stage at which it is most developed, but in view of my closely graded series of embryos this seems unlikely.
It is thus seen that veins of the azygos line are present in the thoracic and lumbar regions, and I have nothing to add to the account by Gladstone (1929) and by Reagan and his co-workers of the 64
manner in which the two chains are linked together. Though there was a direct continuation of lumbar and thoracic azygos systems through a small channel in the 18 mm. embryo, I was unable to recognize it at the 35 mm. stage.
The circumaortic venous ring. This was first described by Huntington & McClure (1920). Fig. 12 shows a composition of the ring that differs from the classical description of the above authors in some respects, especially with regard to the postcardino-subcardinal anastomosis.
It has been shown that this anastomosis changes from a vertical to a horizontal direction and that this is associated with the migration of the kidney, the suprarenal gland and the mesonephros. This migration is accompanied by a shifting of the postcardinal vein in a cranial direction and a shifting of the subcardinal in a caudal direction as is indicated by the arrows in Fig. lla. The result is that the inter-subcardinal anastomosis also moves caudally. It is at the level of the upper border of the 12th thoracic vertebra at the 12 mm. stage, at the level of the 1st lumbar vertebra at the 18 mm. stage, and at a little lower down the Ist lumbar vertebra at the 35 mm. stage. This is the level of the left renal vein in the adult and thus it seems that by the 35 mm. stage migration has ceased.
Another factor involved in the migration of the kidney and affecting the topographical relations of the veins is the ventro-dorsal flattening and the associated increase in transverse diameter of the body. This ventro-dorsal compression is a symptom of the general change in diameter of the embryo as described by Brockmann. It is similar to the well-known phylogenetic change seen in the mammalian as compared with the human thorax (see Weidenreich, 1923). As a result of these changes of body form (Figs. 14-16) the kidney moves laterally ; the distance between the subcardinal and postrenal division of the inferior vena cava is reduced; the subcardinal vein (prerenal division of the inferior vena cava) is brought into line with the postrenal
. division ; the horizontal renal division (postcardinosubcardinal anastomosis), which originally has a vertical course (12 mm. stage), is thus brought into line, with the other divisions. Here we have a typical form of the development in a roundabout way, a principle in ontogeny, which has been introduced by Nauck (1931), who has already given us many striking examples of this interesting mechanism. To appreciate this change we refer to Fig. lla, b and c.
Brockmann, as well as referring to a ‘sliding plane’ in the movement of the abdominal viscera, mentions a ‘compression’ as a result of the changes in the ventro-dorsal and transverse diameters of the body. _I would quite agree with this ‘compression’ being
an agent in causing visceral movement, but cannot support his ‘sliding plane’ theory. Only a part of the ‘inner layer’ mentioned by Brockmann shifts cranially, i.e. the kidney and the persistent caudal part of the postcardinal vein; the suprarenal gland, the mesonephros, the testis and the subcardinal vein migrate caudally.
It seems that the forces, shown in Fig. lla, tend to telescope the structures towards the plane of lumbar vertebrae 1 and 2, thus, perhaps, leaving room for the developing thoracic and pelvic contents.
It is difficult to assess the causes of these movements, but there does seem to be an inherent tendency for movement in the tissues of the dorsal wall. For example, absence of the kidneys, as seen in our 15-5 mm. embryo, does not mean absence of movement in the tissues of the dorsal wall in which the kidney normally lies, and absence of mesonephros and metanephros does not seem to disturb the arrangement of the veins such as postcardinal, subcardinal and circumaortic venous ring.
Considerable difficulty in understanding the development of the inferior vena cava has been caused by the various names that have been applied to the veins involved. McClure & Butler (1925), Reagan (1927) in particular, Gladstone (1929) and Gruenwald (1988) have all called attention to the confusing terminology.
It may, therefore, be of value to attempt to establish a clear terminology for the veins concerned.
The datum lines are the sympathetic trunk and the dorsal rami of the aorta. Lateral to the sympathetic trunk Reagan and others distinguish a ‘thoraco-lumbar’ drainage, which is not mentioned by Huntington & McClure. In the thoracic region this drainage line is only temporary, as Gladstone has pointed out, and I was unable to find it in my material. In the lumbar region. this drainage line is identical with the line of the postrenal vena cava.
Huntington states that the supracardinal vein forms in the lumbar region the postrenal vena cava, and in the thoracic region it forms the azygos vein. Reagan and others have shown that the azygos line in the lumbar region is medial to the sympathetic trunk and the postrenal vena cava lateral. I suggest that the term ‘supracardinal’ be employed to designate the vein lying medial to the sympathetic trunk and the primordium of the azygos system. We should thus have a ‘thoracic supracardinal’ and a ‘lumbar supracardinal’ both in the same line and both medial to the sympathetic trunk.
With regard to the postcardinal vein, we have seen that this lies in the ‘thoraco-lumbar line’ lateral to the sympathetic trunk. From my investigations I am of opinion that the postcardinal vein divides at its caudal end into a caudal mesonephric vein and a dorsal postcardinal. Whether Migration processes and veins of dorsal body wall
this latter vein is regarded as the dorsal remains of the original postcardinal or whether we call it thoraco-lumbar does not make much difference in principle. What I wish to emphasize is that the ventral limb atrophies with the reduction of the mesonephros but the dorsal limb, the original vessel, persists as the postrenal vena cava, and thus I cannot agree with Gruenwald in his suggestion to introduce the name ‘sacrocardinal’ for what is not at all a new vein but only the dorsal remnant of the original postcardinal in the lumbar region.
1. An account is given of the course taken by the permanent kidney in its migration, and reference is made to the migration of the suprarenal gland and mesonephros.
2. These movements are part of an inherent tendency in the structures of the dorsal body wall,
and movement of one tissue is not necessarily dependent on that of another.
8. There are forces causing a general movement from above and below towards the first two lumbar vertebrae, so giving space for the development of the contents of thorax and pelvis.
4. These movements affect the disposition of the Veins.
5. It is suggested that the name ‘supracardinal’ be applied to veins lying in the ‘azygos line’ medial to the sympathetic trunk, thus avoiding ambiguity.
6. The postrenal division of the inferior vena cava is derived from the dorsal remnant of the original postcardinal vein. There is no need to introduce a new name for this vessel.
I wish to express my sincere gratitude to Prof. C. M. West for his kindness in helping me with the reduction in length of the original manuscript, necessary because of the paper situation.
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KEY TO LETTERING
aa, azygos anastomosis
a.b. aortic bifurcation
a.c. auricular canal
al, azygos line
amg. abnormal mesonephric glomerulus
a.8.v. confluence of azygos and subcentral veins av. azygos vein
a.v.C. aorta, with projected vertebral column at. atrium
cd. crus of diaphragm
C.m.v. caudal mesonephric vein
ca.p.m. caudal pole of mesonephros
cr.p.m. cranial pole of mesonephros
cr. pe. cranial remains of postcardinal vein. cr.sc. cranial end of subcardinal vein d.a.p. dorsal aortic plexus 66
g-gh.-8e.p. td. 1.m.a. 4.80.0. av.
la. La.v. lv,
p.m. pp. p.8. pur. pe.a.v. pe.c.
dorsal aortic plexus line
dorsal aortic ramus
hepato-subcardinal junction of postcava
inferior mesenteric artery
line of azygos vein
line of vertebral column
line of postcardinal vein
lung © ,
plane of section
combined postcardinal and azygos veins
postcardinal line and postrenal division of postcava
220.127.116.11. rv. rb.
8.0. (c.8¢.) 8.0.0. 8C.p.
8p.g. 8p.m. sp.n. sp.v.
vcd. V.8.9~ v.87.
postrenal division of postcava peritoneal cavity
region of compressed azygos vein renal vein
sympathetic ganglion suprarenal gland
future sex vein (caudal subcardinal) superior vena cava
subcardinal division of postcava subcardinal vein
suprarenal vein, cranial remnant of subcardinal thoracic azygos vein
vein compressed by diaphragm visceral sympathetic ganglion visceral sympathetic ramus ventricle
Wolffian body Wolffian duct
Fig. 1. Schematic illustration of Brockmann’s hypothesis on the migration of the kidney. On the left, a and b represent two metal laminae fixed at the top end: in }, a straightening process has changed the position of the inner lamina relative to the outer. On the right, this mechanism has been transferred to the embryo.
(After Brockmann.) Migration processes and veins of dorsal body wall _ 67
Fig. 2. Transverse section of the 12mm. embryo showing the relations § Fig. 3. Scheme of a transverse section of of the suprarenal gland. the 6 mm. embryo. The arrow indicates the direction in which the kidney migrates.
Fig. 4. Schemeof atransversesectionof the 12mm. Fig. 5. Longitudinal section of the 11 mm. embryo. embryo (L1). Note the position of the kidney. . 68 J. AUER
w.d. plc. SC.U.
Fig. 6. Transverse section of the 18 mm. embryo at the level of the 2nd lumbar vertebra. Note the position of the kidneys relative to the vertebra] column.
Fig. 7. Transverse section of the 35 mm. embryo. Note the position of the kidneys relative to the vertebral column. Migration processes and veins of dorsal body wall 69
see) E = pe.-sc.a.
Fig. 8. Transverse section of the 15-5 mm. anomalous embryo in the region of the circumaortic venous ring Note the abnormal mesonephros and the absence of the kidney.
m. o.b. d.t. 1.M.a. ppe.-SC.a. ay, 2.8. Ss. , V.8.8. w.b. pe.v. PeUe S.C. la. v. p.m. La. Sp.v. d.a.p.l. a.
Fig. 9. Transverse section of the 15-5 mm. anomalous embryo showing normal relations of the veins below the level of the intersubcardinal anastomosis. Note the abnormal mesonephros and the absence of the metanephros (X). a.v.c.
C.-SC.A. e.v. P ‘
Fig. 10a. Scheme of the dorsal body wall seen from behind in the 12 mm. embryo (L1). A transverse section has been interposed to show the topographical relations.
Fig. 12. Scheme of a transverse section of the 35 mm. stage in the region
1. Subcardinal vein. 2. Intersubc. 4. Postcardino-subcardinal
of the circumaortic venous ring. anastomosis. 3. Caudal part of poste. vein. anast. 5. Azygos vein. (lumb. supracard. v.) 6. Supracardino-postc. anast. 7. Symp. chain of ganglia. 8. Visceral symp. ganglia. 9. Dorsal aortic rami. 10. Intersupracard. anast. with dors. aortic plexus. 1l. Permanent (ant.) renal vein. 12. Temporary (post.) renal vein. 13. Renal artery. 14. Permanent kidney. 15. Corpus vertebrae.
J. AUER 8-8.
Fig. 106. Scheme of the dorsal body wall seen from behind in the 18 mm. embryo. A transverse section has been interposed to show the topographic relations of the organs and veins.
SC.p. i.sc.a. d.a.p.
Fig. 10c. Scheme of the dorsal body wall seen from behind in the 35 mm. embryo.
An interposed section has been omitted for reasons of clearness. 72
3 3 cS a _ . . ~~ ~ as 3 Q 3 e ~ S ® Ef ¢ 5 % = e ~ a 2
a zi 3 S = 3 : 3 3 - sO 5 = S S S * a, ~ x a Qo 8 i wn ° ~ 0 la 2 = nN * le 1 EEE aT Ses Ree ARAITY” | Pn SS 2 YRS \ a itm SS % & gH 5s 9 3 3 & & gs ¢ > es YS = na g . : ae S 3 = S S = QQ Ss = Q. gy = os | wi QoN
2 S a”
median plane of the 35 mm. embryo.
Fig. llc. Scheme of the right half of the dorsal body wall seen from the
dorsal body wall seen from the m plane of the 18 mm. embryo.
Fig. 116. Scheme of the right half of the
ig. 1la. Scheme of the right half of tho dorsal body wall seen from the median plane of the 12 mm. embryo (L1). S.U.C.
Fig. 13. Transverse section of the 15-5 mm. embryo showing the azygos venous line on a level with the third thoracic vertebra. This part of the azygos may be derived from the poste. vein (note position of the symp. chain).
5.8: id. sp.n.
Fig. 14. Transverse section of the 12 mm. embryo (L1), for comparison of the sagittal and transverse diameters of the dorsal body wall. 74
Fig. 15. Transverse section of the 18 mm. embryo. Note the sagittal and transverse diameter of the dorsal body wall.
Fig. 16. Transverse section of the 35 mm. embryo in order to compare sagittal and transverse diameters of the dorsal body wall.