Book - Embryology of the Pig 9
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.
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Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381
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Chapter 9. The Development of the Digestive and Respiratory Systems and the Body Cavities
I. The Digestive System
In considering the structure of young embryos we traced the walling in of the primitive gut tract by entoderm, its regional division into fore-gut, mid-gut, and hind-gut, and the establishment of the oral and anal openings by the breaking through of the stomodaeal depression cephalically and the proctodaeal depression caudally (Figs. 16, 37, and 65). In embryos of from 9 to 12 mm., local differentiations in the gut tract clearly foreshadowed the development of certain organs and gave indications of the impending establishment of others. Starting with these now familiar conditions as a basis we shall trace briefly the more important steps by which the adult structure and relations of the various organs are established.
Oral Cavity
The oral cavity of the adult and its various special structures are derived from the stomodaeal region of the embryo. The entire face and jaw complex is formed from processes which arise about the margins of the stomodaeum. The progressive growth of these abutting structures results in a deepening of the originally shallow stomociaeal depression to form the oral cavity. An idea of the extent to which this growth progresses can be gained from the fact that the point of rupture of the stomodaeal membrane (oral plate) comes to lie, in the adult, at about the level of the tonsils. So many processes of special interest are involved in the changes which go on in this region that it has seemed wise to dismiss them for the present with this general statement and return to them later for special consideration (Chap. 13).
The Pharyngeal Region
In embryos of about 4 to 6 mm. the cephalic part of the fore-gut has become differentiated as the pharynx. Greatly compressed dorso-ventrally, the pharynx has a wide lateral extent with a series of pouch-Uke diverticula pushing out on either side between the branchial arches (Figs. 41, B, and 95). This stage of
Fig. 95. Lateral view of the pharynx of a young mammalian embryo with its relations to important adjacent structures indicated. (Patten: “Human Embryology,†The Blakiston Company.) The contours of the visceral arches are suggested and the broken lines between them indicate the location of the external gill furrows. The drawing is semischematic and equally applicable to conditions in a 4-week human embryo or a 5 mm. pig embryo.
the pharynx is a recapitulation of conditions which had an obvious functional significance in water-living ancestral forms. For the pharyngeal pouches of the mammalian embryo are homologous with the inner portion of the gill slits. The repetition of race history is here, as so frequently happens, slurred over. Although in the mammalian embryo the tissue closing the gill clefts becomes reduced to a thin membrane consisting of nothing but a layer of entoderm and ectoderm with no intervening mesoderm whatever (Fig. 62), this membrane rarely disappears altogether. Occasionally the more cephalic of the pharyngeal pouches break through to the outside, establishing op>en gill slits, but in such cases the opening is very short-lived and the clefts promptly close again.
Like many other vestigial structures which appear in the development of higher forms, the pharyngeal pouches give rise to organs having a totally different functional significance from the ancestral structures they represent. It is as if, to speak figuratively, nature was too economical to discard entirely structures rendered functionally obsolete by the progress of evolution, but rather conserved them in part at least and modified them to carry on new activities.
Discussion of the processes whereby various parts of the original pharyngeal apparatus become converted into other structures would involve too many details to permit of inclusion here. A bare statement of what these pharyngeal derivatives are and where they arise must suffice.
The main pharyngeal chamber of the embryo, that is, the central portion in distinction to its various diverticula, becomes converted directly into the pharynx of the adult. In this process its lumen is simplified in configuration and relatively reduced in extent. An important factor in these changes is the separation of various diverticula from the main part of the pharynx. The cell masses thus originating migrate into the surrounding tissues and there undergo divergent differentiation.
The first pair of pharyngeal pouches, extending between the mandibular and hyoid arches, come into close relation at their distal ends with the auditory vesicles (Fig. 60). They give rise, on either side, to the tympanic cavity of the middle ear and to the Eustachian tube.
The second pair of pouches become progressively shallower and less conspicuous. Late in fetal life the faucial tonsils are formed by the aggregation of lymphoid tissue in their walls, and vestiges of the pouches themselves persist as the supratonsillar fossae.
Fig. 96. Schematic diagrams indicating the origin and later interrelations of some of the derivatives of the embryonic pharynx. (Modified from SwaleVincent.) In the schematic cross section of one lobe of the adult thyroid (B) the numbers attached to parathyroids and thymus refer to their pouches of origin as indicated in A. Note that the thymic tissue which arises from the fourth pouch is drawn in lightly to indicate that it is not well developed in all mammals.
Fig. 97. Drawings (X 11) of transverse section through the pharyngeal region of a 15 mm. pig embryo. A, Upper laryngeal level. B, Level of third
Fig, 98. Pharynx of 15 mm. pig embryo schematically represented in relation to the outlines of other cephalic structures. (Adapted from several sources.) The heavy horizontal lines indicate the levels of the correspondingly lettered sections in the preceding figure.
From the floor of the pharynx, in the mid-line at about the level of the constriction between the first and the second pair of pharyngeal pouches, a divf^rticulum is formed which is destined to give rise to the thyroid gland (Figs. 95 and 96). The mass of epithelial cells making, up the walls of this evagination push into the underlying mesenchyme, break away from the parent pharyngeal epithelium, and migrate down into the neck (Figs. 97, D, 98, and 138). Only after arriving in its definitive location, relatively late in development, does the thyroid primordium undergo its final characteristic histogenetic changes.
Fig. 97 — {Continued)
pharyngeal pouch. C, Level of fourth pharyngeal pouch. D, Through the neck caudal to the level of the pharynx and larynx.
The level of each of the sections represented in this figure is indicated by the correspondingly lettered line in the next figure.
Abbreviations: Br. gr. Ill, third branchial groove; N. XII, hypoglossal nerve; Pharyng. Ill, third pharyngeal pouch; Pharyng. IV, fourth pharyngeal pouch; Preinusc., premuscular concentration of mesenchyme.
The third and fourth pairs of pharyngeal pouches give rise to outgrowths which are involved in the formation of the parathyroid glands, the thymus, and the post-branchial bodies. There are two pairs of parathyroid glands, usually spoken of as parathyroids III and parathyroids IV because they arise from the third and the fourth pharyngeal pouches (Figs. 96, A, and 98). As was the case with the thyroid, the parathyroid primordia soon break away from their points of origin and migrate into the neck. Here, as their name implies, they are positionally more or less closely associated with the thyroid. Parathyroids IV are particularly likely to become adherent to the thyroid capsule or even to become partially embedded in the substance of the gland (Fig. 96, B).
The thymus in the mammalian group is derived from outgrowths from the more ventral portions of the third and fourth pharyngeal pouches (Figs. 96, A, and 98). In different species there is considerable difference in the relative conspicuousness of the two pairs of primordia. In most of the higher mammals the primordia arising from the third pouches are much the more important thymic contributors. This is the situation for the pig as well as for man. In pig embryos of the 15-17 mm. range, however, it is usually possible to make out a rudimentary thymus IV (Fig. 98). The characteristic histogenetic changes in the thymus occur relatively late in development, and even in 15-17 mm. embryos thymus III is but a slender pair of cell cords growing into, the tissue at the base of the neck (Fig. 97, C, D).
The post-branchial bodies are structures of problematical significance. Arising as they do on the caudal face of the fourth pharyngeal pouches, many observers regard them as rudimentary fifth pharyngeal pouches. When the post-branchial bodies detach themselves from their site of origin they lie in the loose mesenchymal tissue (Fig. 97, D) close to the route followed by the thyroid gland in its descent. As the thyroid expands laterally the tissue of the post-branchial bodies becomes embedded in it on either side (Fig. 96, D). There is still difference of opinion as to whether this post-branchial tissue contributes to the formation of true thyroid glandular tissue or remains merely as an inconspicuous vestigial cell mass in the substance of the thyroid gland. Those who are convinced that these buds from the caudal face of the fourth pouches form true thyroid tissue generally designate them as lateral thyroid primordia. The non-committal
Fig. 100. Pig embryo of 35 mm. dissected to the mid-line to show the relations of the alimentary tract. (After Prentiss.)
The region of narrowing where the trachea becomes confluent with the gut tract may be regarded as the posterior limit of the pharynx. From this point to the dilation which marks the beginning of the stomach the gut remains of relatively small and uniform diameter and becomes the Esophagus (Figs. 99 and 100). The original entodermal lining of the primitive gut gives rise only to the epithelial lining of the esophagus and to its glands. The connective tissue and muscle coats of the esophagus are derived from mesenchymal cells which gradually become concentrated about the original epithelial tube (Fig. 97, D).
Stomach
The region of the primitive gut which is destined to become the stomach is, in embryos of 10 mm., more or less clearly marked by a dilation (Figs. 60 and 64). Its shape, even at this early stage, is strikingly suggestive of that of the adult stomach. Its position is, however, quite different.
In young embryos the stomach is mcsially placed with its cardiac (esophageal) end somewhat more dorsal in position than its pyloric (intestinal) end. It is slightly curved in shape, with the convexity facing dorsally and somewhat caudally and the concavity facing vcntrally and somewhat cephalically (Fig. 99). The positional changes by which it reaches its adult relations involve two principal phases: (1) the stomach is bodily shifted in position so its long axis no longer lies in the sagittal plane of the embryo but diagonally across it, and (2) there is a concomitant rotation of the stomach about its own long axis so that its original dorso-ventral relations are altered as well. These changes in position are schematically indicated in figure 101. The shift in axis takes place in such a manner that the cardiac end of the stomach comes to lie to the left of the mid-line and the pyloric end to the right. Meanwhile rotation has been going on. In following the progress of rotation the best point of orientation is the line of attachment of the primary dorsal mesentery (Fig. 101). While the stomach occupies its original mesial position the mesentery is attached to it mid-dorsally, along its convex curvature (Fig. 111). As the stomach continues to grow in size and depart from the sagittal plane of the body it rotates about its own long axis. The convex surface to which the mesentery is attached and which was at first directed dorsally, now swings to the left. Since the long axis itself has in the meantime been acquiring an inclination, the greater curvature of the stomach comes to be directed somewhat caudally as well as to the left (Fig. 101, D).
The Omental Bursa. The change in position of the stomach necessarily involves changes in that part {dorsal mesogastrium) of the primary dorsal mesentery which suspends it in the body cavity (Figs. 101 and 111). The dorsal mesogastrium is pulled after the stomach and forms a pouch, known as the omental bursa. The opening from the general peritoneal cavity into the bursa is known as the epiploic foramen {foramen of Winslow). (See arrow in Fig. 101, D.)
The Intestines. The primitive gut is at first a fairly straight tube extending throughout the length of the body. Near its midpoint it opens ventrally into the yolk-sac (Figs. 37 and 40). The first conspicuous departure from this condition is the formation of a hairpin-shaped loop in the future intestinal region. The closed end of this loop extends into the belly-stalk (Figs. 60 and 64). The yolk-stalk connects with the gut at the bend of the loop and forms an excellent point of orientation in following the series of foldings and kinkings by which the definitive configuration of the intestinal tract is established. The attachment of the yolk-stalk is just cephalic to what will be the point of transition {ileo-cecal valve) from small to large intestine. Thus all the gut between the yolk-stalk and the stomach becomes small intestine, and, except for about 2 feet of the terminal part of the small intestine, the gut caudal to the yolk-stalk goes to form the large intestine.
Fig. 101. Diagrams illustrating the changes in position of the stomach, and the formation of the omental bursa. The broken line indicates the attachment of the mesogastrium along that surface of the stomach which is primarily mid-dorsal. The arrow passes through the epiploic foramen into the omental bursa.
The characteristic coiling of the small intestine is the first to become evident (Figs. 100 and 102, B). The only change of significance which has taken place meanwhile in the large intestine is the establishment of the cecum as a definite pouch-like diverticulum (Fig. 102, B). But the large intestine does not remain long uncoiled. In the pig it attains a greater length and a more complicated configuration than in man, its final condition being that of a loop closely spiraled on itself (Fig. 102^ E, F) and occupying a very conspicuous position among the abdominal viscera (Fig. 1).
Fig. 102. Diagrams illustrating the development of the intestinal tract in pig embryos. (After Lineback.) A, 12 mm.; B, 24 mm.; C, 35 mm.; D, 75 mm.; E, 110 mm.; F, 1 month after birth.
A to C show the entire gastro-intestinai tract viewed from the left side. D to F show the large intestine only. The relations of the last three figures will be made apparent by comparing C and D, taking for orientation the cecum and that part of the duodenum which loops across the large intestine.
Rectum and Anus. The attainment of adult conditions at the extreme caudal end of the digestive tract is so intimately associated with the development of the urogenital openings that changes in the cloacal region as a whole can more profitably be taken up later in connection with the reproductive organs.
The Liver. Wry early in development the diverticulum which gives rise to the liver is budded off from the entoderm of the primitive gut tract. In embryos as small as 4 mm. the hepatic diverticulum can be identified extending ventrad from the duodenal portion of the gut
Fig. 103. Reconstructions of the gut tract in the region of the hepatic and pancreatic diverticula.
A, From 4 mm. pig embryo in the Carnegie Collection.
B, From 5.5. mm. pig embryo. (After Thyng, modified.)
(Fig. 103, A). This original diverticulum, in embryos of 5 to 6 mm., has become clearly differentiated into several parts (Figs. 40 and 103, B). A maze of branching and anastomosing cell cords grows out from it ventrally and cephalically. The distal portions of these cords give rise to the secretory tubules of the liver and their proximal portions form the hepatic ducts. Originating where the hepatic ducts become confluent is a dilation which is the primordium of the gall-bladder. Closer to the gut tract is a separate outgrowth of cells which constitutes the ventral primordium of the pancreas.
The later changes in the biliary duct region are shown in figures 104 and 105. The gall-bladder elongates very rapidly and its terminal portion becomes distinctly saccular. The narrower proximal portion of this limb of the diverticulum becomes the cystic duct. That portion of the original diverticulum which lies toward the duodenum from the entrance of the hepatic ducts is called the common bile duct (ductus choledochus).
The mass of branching and anastomosing tubules which are distal continuations of the hepatic ducts constitute the actively secreting portion of the liver. Their position and extent in embryos of various ages are shown in figures 40, 65, 99, 100, and 106. The organization of these secreting units in the liver is quite characteristic. The hepatic tubules are not paeked so closely together in a framework of dense connective tissue as is usually the case in massive glands. Surprisingly little connective tissue is formed between them and the intertubular spaces become pervaded by a maze of dilated and irregular capillaries known as sinusoids. This tremendously extensive meshwork of small blood vessels among the cords of liver cells is a condition which we shall find of great importance in the development of the circulatory system in this region.
Fig. 104. Reconstruction of gut tract of a 9.4 mm. pig embryo showing pancreatic and hepatic diverticula (X 33). C'ompare with figure 60.
The Pancreas, The pancreas makes its appearance in the same region and at about the same time as the liver. It is derived from two separate primordia which later become fused. One primordium arises dorsally, directly from the duodenal entoderm; the other arises ventrally, from the entoderm of the hepatic diverticulum (Fig. 103). As they increase in size, these two buds approach each other and eventually fuse (Figs, 104 and 105). The glandular tissue of the pancreas is formed by the budding and rebudding of cords of cells derived from this primordial mass. The terminal parts of the cords gradually take on the characteristic configuration of pancreatic acini while their more proximal portions form the duct system draining the acini.
- The ventral pancreatic diverticulum in a certain number of cases may be paired instead of single. It is probable that the usual unpaired diverticulum seen in mammalian embryos represents originally paired ventro-lateral diverticula.
Fig. 105. Reconstruction of pancreas and hepatic duct system of 20 mm. pip^ embryo. (After Thyng, modified.)
There is, in different forms, considerable variation in the relations of the main pancreatic ducts which persist in the adult. In the horse and dog, for example, there are two ducts, a dorsal one (duct of Santorini) which opens directly into the duodenum, and a ventral one (duct of Wirsung) which opens into the duodenum by way of the common bile duct. These two ducts represent the two original pancreatic buds which appear in mammalian embryos generally. In other forms the two original ducts become confluent within the pancreas and the terminal portion of one duct only is retained. Thus in the sheep and in man the ventral duct persists communicating with the duodenum by way of the common bile duct, while the terminal portion of the dorsal duct usually atrophies. In the pig and the ox the ventral duct ordinarily disappears and the dorsal one persists as the definitive pancreatic duct. /
Ro. 106 . Sagittal section of 24 mm pig embryo. (After Minot.)
II. The Respiratory System
The Trachea. The first indication of the diflferentiation of the respiratory system is the formation of the laryngo-tracheal groove. This mid-ventral furrow in the primitive gut tract appears in embryos of about 4 mm. at the posterior limit of the pharyngeal region. As it becomes deepened it is constricted off from the gut except for a narrowed communication cephalically. Once established as a separate diverticulum, it grows caudad as the trachea, ventral to, and roughly parallel with, the esophagus (Figs. 40, 65, and 103).
The anatomical relations of the trachea in the embryo, even in early stages, are quite similar to adult conditions. We can recognize its communication with the posterior part of the pharynx as the future glottis, and the slightly dilated portion of the embryonic trachea just caudal to the glottis as foreshadowing the larynx (Fig. 106).
Only the epithelial lining of the adult trachea is derived from fore-gut entoderm. The cartilage, connective tissue, and muscle of its wall arc formed by mesenchymal cells which become massed about the growing entodermal tube (Figs. 72, 97, D, and 161).
Fig. 107. Stages in the development of the trachea, bronchi, and lungs in the pig. (After Flint.) The pulmonary arteries are shown in black; the veins arc cross hatched. Ep, bud of eparterial bronchus.
The Bronchi and Lungs. As the tracheal outgrowth lengthens, it bifurcates at its caudal end to form the two lung buds (Fig. 107, A). These in turn continue to grow and rebranch, giving rise to the bronchial trees of the lungs (Fig. 107). The terminal portions of the branches where cell proliferation is exceedingly active tend to remain somewhat bulbous. Later in development these terminal portions of the bronchial buds become still more dilated, their epithelium thins markedly, and they give rise to the characteristic air sacs of the lungs. As was the ease with the trachea, th(‘ connective-tissue framework of the lung is derived from mesenchyme, which collects about the entodcrmal buds during their growth. The entoderm gives rise only to the lining epithelium of the bronchi and the air sacs. The pleural covering of the lungs is derived from splanchnic mesoderm pushed ahead of the lung buds in their growth (Fig. 113).
The lungs do not at first occupy the position characteristic of adult anatomy. In very young embryos they lie dorsal to the heart (Figs. 40 and 110). A little later when they have extended caudad, they are situated dorsal to the heart and liver (Figs. 99 and 138). The changes by which they eventually come to occupy their definitive position in the thorax can best be taken up in connection with the partitioning of the primitive coelom to form the body cavities of the adult.
Ill. The Body Cavities and Mesenteries
The body cavities of adult mammals are the pericardial cavity containing the heart, the paired pleural cavities containing the lungs, and the peritoneal cavity containing the viscera lying caudal to the diaphragm. All three of these regional divisions of the body cavity ^re derived from the coelom of the embryo. The general location and extent of the coelom are already familiar from the study of young embryos, but it may be well, nevertheless, to restate some of the more important relations here.
The Primitive Coelom. The coelom arises by the splitting of the lateral mesoderm on either side of the body into splanchnic and somatic layers (Fig. 108, A).
It is, therefore, primarily a paired cavity bounded proximally by splanchnic mesoderm and distally by somatic mesoderm. In forms such as birds and mammals which have highly developed extraembryonic membranes, the coelom extends between the mesodermal layers of the extra-embryonic membranes beyond the confines of the
Fig. 108. Diagrams illustrating the development of the coelom and
mesenteries.
developing body. In mammalia where the nutrition of the embryo depends on the uterine relations established by the extra-embryonic membranes, they develop exceedingly precociously. It is not surprising, in view of this fact, that the splitting of the mesoderm in mammals occurs first extra-embryonically and progresses thence toward the embryo (Fig. 108, A, B). When the body of the embryo is folded off from the extra-embryonic membranes the extra- and intraembryonic portions of the coelom are thereby separated from each other, the last place of confluence to be closed off being in the region of the belly-stalk (Fig. 108, C). It is the intra-embryonic portion of the primitive coelom, thus delimited, which gives rise to the body cavities.
It will be recalled that the typical configuration of the mesoderm indicated diagrammatically in figure 108 does not pertain in the cephalic part of the embryo. The mesoderm in the head region consists of mesenchymal cells which wander in from the more definitely organized mesoderm located farther caudally in the body. Thus the intra-embryonic coelom established by the splitting of the lateral mesoderm extends headwards only to the level of the pharynx, and the heart is developed in its most cephalic portion (Figs. 43, 44, 109, and 110).
The Mesenteries. The same folding process that separates the embryo from the extra-embryonic membranes completes the floor of the gut (Figs. 37 and 108). Coincidently the splanchnic mesoderm of either side is swept toward the mid-line enveloping the now tubular
Fig. 109. Schematic plan of lateral dissection of young mammalian embryo to show the relations of the pericardial region of the coelom to the primary paired coelomic chambers caudal to the level of the heart. (Patten; “Human Embryology,†The Blakiston Company.) The proportions of the illustration were based in part on Heuser’s study of human embryos about 3 weeks old, but all the essential relationships shown are equally applicable to pig embryos of 3 to 4 mm.
Fig. 110. Diagrams showing the relations of the pericardial, pleural, and peritoneal regions of the coelom while they are still confluent. (Patten: “Human Embryology,†The Blakiston Company.)
A, Semischematic frontal plan with body of embryo represented as if it had been pulled out straight. The position of the lungs is indicated by broken lines, and arrows indicate the location of the pleural canals, on either side, dorsal to the liver. (Cf. part C of this figure.)
B, Lateral dissection to show left pleural canal opened with lung bud bulging into it. (Modified from Kollmann.)
C, Schematized section diagonally through body at level of line in B.
digestive tract. The two layers of splanchnic mesoderm which thus become apposed to the gut and support it in the body cavity are known as the primary or common mesentery. The part of the mesentery dorsal to the gut, suspending it from the dorsal body-wall, is the dorsal mesentery. The part of the mesentery ventral to the gut, attaching it to the ventral body- wall, is the ventral mesentery (Fig. 108, D). The primary mesentery, while intact, keeps the original right and left halves of the coelom separate. But the part of the mesentery ventral to the gut breaks through very early, bringing the right and left coelom into confluence and establishing the unpaired condition of the body cavity characteristic of the adult (Fig. 108, F).
Fig. 111. Semidiagrammatic drawing showing the arrangement of the viscera, body cavities, and mesenteries in young mammalian embryos. (Patten: ‘‘Human Embryology,’’ The Blakiston Company.) In all essentials the conditions here represented will be found in pig embryos of 12-15 mm.
In the region of the developing lungs the body is cut parasagittally, well to the left of the mid-line, in order to show the relations of pleuropericardial and pleuroperitoneal folds. Below the developing diaphragm, dissection has been carried to the mid-line.
Abbreviations: G, gall-bladder; Y, yolk-sac.
In the liver region the ventral mesentery does not entirely disappear. The liver arises, as we have seen, from an outgrowth of the gut and in its development pushes into the ventral mesentery (Fig. 108, E). The portion of the ventral mesentery between the liver and the stomach persists as the gastro-hepatic omentum (ventral mesogastrium), and the portion between the liver and the ventral body- wall, although reduced, persists in part as the falciform ligament of the liver (Fig. Ill),
While the ventral mesentery, except in the region of the liver, eventually disappears, almost the entire original dorsal mesentery persists. It serves at once as a membrane supporting the gut in the body cavity and a path over which nerves and vessels reach the gut from main trunks situated in the dorsal body-wall. Its different regions are named according to the part of the digestive tube with which they are associated, as, for example, mesogastrium^ that part of the dorsal mesentery which supports the stomach; mesocolon^ that part of the dorsal mesentery supporting the colon, etc. (Fig. 111).
The Partitioning of the Coelom. The structure which initiates the division of the coelom into separate chambers is the septum tramversum. The septum transversum appears very early in development (Figs. 40 and 110) and is already a conspicuous structure in embryos of 9 to 12 mm. (Figs. 64 and 111). Extending from the ventral bodywall dorsad, it forms a sort of semicircular shelf. Fused to the caudal face of the shelf is the liver and on its cephalic face rests the ventricular part of the heart.
The septum transversum is the beginning of the diaphragm. It should be clearly borne in mind, however, that the diaphragm is a composite structure embryologically, and that the septum transversum gives rise only to its ventral portion. The septum transversum itself never grows all the way to the dorsal body-wall. Dorsal to the septum transversum, the region of the coelom occupied by the heart and lungs is confluent with that occupied by the developing gastrointestinal tract and liver (Fig. 110). Thus, although the division of the coelom into thoracic and abdominal regions is clearly indicated even at this early stage, it is not as yet complete.
The complete isolation from one another of the pericardial, pleural, “and peritoneal portions of the coelom is brought about by the growth of the paired pleuroperitoneal and pleuropericardial folds. These folds arise from the dorso-lateral body-walls where the ducts of Cuvier bulge into the coelom as they swing around to enter the sinus venosus of the heart (Figs. 110 and 113, B). The folds thus established rapidly acquire a roughly triangular shape with their bases diagonally along the body-wall and their apices extending toward, and eventually fusing with, the dorsal part of the septum transversum. Because of their different fate and relations the cephalic parts of these primary triangular folds have been called the pleuropericardial folds and their caudal parts pleuroperitoneal folds (Fig. 111).
In the growth processes which lead toward the separation of the thoracic from the abdominal region, the dorsal mesentery is caught
Fig. 112. Diagram indicating the embryological derivation of the various regions of the diaphragm. (Modified from Broman.)
between the coiiverging septum transversum and the pleuroperitoneal folds. Fusions along the lines of contact complete the diaphragm (Fig. 112). The last place to close is near the dorsal body- wall on either side of the mid-line where the pleuroperitoneal folds are bent caudad by the growing lungs (Fig. 111). The manner in which the margins of the pleuroperitoneal folds are forced caudad by the growing lungs is one of the chief factors in establishing the characteristic dome-shaped configuration of the adult diaphragm.
Later in development, the margins of the diaphragm, especially dorso-laterally, are invaded by body-wall tissue which contributes the main part of the diaphragmatic musculature (Fig. 112).
In the thoracic region of the coelom, changes have in the meantime been going on which lead toward its subdivision into a pericardial
Fig. 113. Schematic diagrams showing the manner in which the pleural and pericardial regions of the coelom become separated.
and paired pleural chambers. The cephalic portions of the primary triangular folds arising about the ducts of Cuvier, constitute, it will be recalled, the pleuropericardial folds (Fig. 111). The convergent growth and ultimate fusion of the pleuropericardial folds isolate the heart from the lungs (Fig. 113, B, C). The pleural cavities thus established lie very far dorsally and are greatly restricted in extent as compared with the pleural cavities of the adult. The schematic diagrams of figure 113 indicate the manner in which, with the growth in mass of the lungs, the pleural cavities are expanded ventralwards on either side of the heart.
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