Book - Embryology of the Pig 7

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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.

Patten 1951: 1 Foreword to the Student | 2 Reproductive Organs - Gametogenesis | 3 Sexual Cycle | 4 Cleavage and Germ Layers | 5 Body Form and Organs | 6 Extra-Embryonic Membranes | 7 Embryos 9-12 mm | 8 Nervous System | 9 Digestive - Respiratory and Body Cavities | 10 Urogenital | 11 Circulatory System | 12 Bone and Skeletal System | 13 Face and Jaws | Bibliography
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This historic 1951 embryology of the pig textbook by Patten was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.

By the same author: Patten BM. The Early Embryology of the Chick. (1920) Philadelphia: P. Blakiston's Son and Co.

Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381

Modern Notes


Historic Disclaimer - information about historic embryology pages 
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 7. The Structure of Embryos from Nine to Twelve Millimeters in Length

Pig embryos of about 10 mm. crown-rump length are especially valuable as material for laboratory study. They are neither so small and delicate that procuring and preparing them involves unusually skillful manipulation, nor so large that complete series of sections are tedious to make and difficult to study. Developmentally they are not so far advanced that their structure is hard to comprehend if the student is familiar with the early stages of development in any of the vertebrates. Yet if their morphology is thoroughly mastered it forms a starting point from which the later developmental processes in mammals may readily be traced. It seems advantageous, therefore, to deal with this, the most commonly utilized stage of development, somewhat less in narrative fashion than has been done heretofore. We shall, as it were, pause for a while and look over carefully the conditions which thus far have been established so that we may follow the story of the later phases of development more understandingly.

I. External Features


Like all forms which develop in the limited confines of an egg shell or within the uterine cavity, the body of a young pig embryo is curled up on itself. This flexion of the spinal axis is more marked in some regions than others. The points of conspicuous bending have received special designations from the region in which they appear. In a pig of 10 mm. (Fig. 58) the cranial flexure, cervical flexure, dorsal flexure, and lumbosacral flexure are all well developed.

The Head

In the cephalic region the thin skin leaves the contours of the brain clearly suggested (cf. Figs. 58 and 59). The nasal pits have appeared as definitely circumscribed depressions at the rostral end of the head and the bulging caused by the growing optic cups clearly marks the position of the eyes. Especially in fresh specimens the retinal pigment can be seen through the overlying skin.

Fig. 58. Drawing (X 7) showing external appearance of 10 min. pig embryo. (Modified from Minot.)

The Gill Arches

Flanking the oral cavity and caudal to it, in the region which will be under the chin, the gill (branchial) arches appear as strongly marked local elevations. This entire region is at this stage so compressed against the thorax that one gets a very incomplete view of its structure unless an embryo is decapitated and the head viewed in ventral aspect (Figs. 168 and 169). It is then seen that the maxillary processes form the lateral parts of the upper jaw, and that the two mandibular elevations meet each other in the mid-ventral line to form the arch (mandibular arch) of the lower jaw. Posterior to the mandibular are three similar arches, the hyoid and and the unnamed third and fourth post-oral arches. Later in development the arches posterior to the mandible become less conspicuous and are incorporated into the neck.

The Gill Clefts

Between the branchial arches are deep furrows which mark the position of ancestral gill clefts. Although in mammalian embryos these furrows do not ordinarily break through into the pharynx they are commonly called clefts because of their phylogenetic significance. Only the most cephalic of these clefts is named (hyomandibular cleft, Fig. 58); the others are designated by their post-oral numbers. The entire region about the third and fourth postoral clefts becomes especially deeply depressed and is known as the cervical sinus.

The Appendage Buds

Both anterior and posterior appendage buds in 10 mm, pigs are still paddle-shaped (Fig. 58). Not until embryos have grown to a length of between 15 and 20 mm. do the five digits show as terminal enlargements (Figs. 33 and 34). It is significant that the ancestral five-digit type of appendage appears transitorily in the pig embryo, before its characteristic highly specialized hoof is even suggested.

The Trunk

At this stage the superficial bulges due to the somites are evident all the way from the cervical to the caudal region of the body. Well-marked prominences in the lateral and ventral portions of the body-wall indicate the position of the heart, t^e liver, and the mesonephros (cf. Figs. 58 and 59).

The band of tissue (“milk ridge”) which gives rise to the mammary glands is not ordinarily developed in embryos of the 9 to 12 mm. range. It usually becomes clearly marked by 15 mm. (Fig. 33), and by 20 mm. the nipples can generallv be recognized (Fig. 34).

II. The Nervous System

The Brain

In younger embryos the brain consisted of three regions, fore-brain (prosencephalon), mid-brain (mesencephalon), and hind-brain (rhombencephalon) (Fig. 36, E). Now we find five regional divisions or vesicles, as they are called. The prosencephalon has divided to form the telencephalon and diencephalon; the mesencephalon has remained undivided; and the rhombencephalon has become differentiated into metencephalon and myelencephalon (Figs. 59, 60, 65, and 88).

The telencephalon consists of the most anterior median part of the brain and two lateral outgrowths from it called the lateral telencephalic vesicles (Figs. 60, 67, and 69). Its posterior boundary is conventionally established by drawing a line from a fold in the roof of the brain called the velum transversum (see Fig. 65 where the leader to Seessel's pocket crosses the dorsal wall of the neural tube) to the recessus opticus, a depression in the floor of the brain at the level of the optic stalks (Figs, 65 and 88).

The diencephalon is the hinder portion of the old prosencephalon. Its caudal boundary is conventionally established by drawing a line from an elevation in the floor of the neural tube called the tuberculum posterius (Fig. 88) to a depression in the roof of the neural tube which is just appearing at this stage of development, and may (Fig. 60) or may not (Fig. 65) be evident in the particular embryo under observation. The most conspicuous special features of the diencephalon are the paired lateral outgrowths from it which form the optic vesicles (Figs. 60 and 63) and the median ventral diverticulum which constitutes the infundibulum (Fig. 65). The median dorsal outgrowth of the diencephalon known as the epiphysis, which is so conspicuous a feature in chick embryos of the third and fourth day, appears relatively late in the pig. No suggestion of an epiphyseal evagination has appeared in 9 to 12 mm. embryos.

The mesencephalon shows little change from its condition in younger embryos. Its demarcation from the metencephalon posteriorly is indicated by a conspicuous constriction in the neural tube (Figs. 59, 60, and 65).

The division of the primitive hind-brain (rhombencephalon) into metencephalon and myelencephalon is indicated at this stage, though not conspicuous or definite. The dorsal wall of the neural tube just caudal to the meso-rhombencephalic constriction is markedly thick, contrasting strikingly with the very thin roof of the more posterior part of the hind-brain (Fig. 65). The zone of the neural tube where the dorsal thickening exists is the metencephalon ; the posterior, thin-roofed portion of the hind-brain is the myelencephalon. Although all external indications of individual neuromeres have by this time disappeared, the internal face of the myelencephalic wall still shows definite neuromeric markings (Figs. 61 and 67).

The Cranial Nerves

The peripheral relations of the cranial nerves to cephalic structures and their central relations to the brain are strikingly constant throughout the vertebrate series. In fishes we recognize 10 pairs of cranial nerves. In the mammals we encounter these same 10 cranial nerves with essentially similar relations both centrally and peripherally. But the mammalian brain in its progressive specialization has incorporated a part of the neural tube which in primitive fishes was unmodified spinal cord. One of the clearest evidences of this process is the fact that we find in the mammals 12 pairs of cranial nerves, the first, 10 of which are homologous with the 10 cranial nerves of fishes and the last two of which represent a modification of nerves which in fishes were the most anterior of the spinal nerves.

The 12 cranial nerves of mammals are designated by numbers almost as commonly as they are referred to by name. Beginning with the most rostral they are: (I) olfactory, (II) optic, (III) oculomotor, (IV) trochlear, (V) trigeminal, (VI) abducens, (VII) facial, (VIII) acoustic, (IX) glossopharyngeal, (X) vagus, (XI) spinal accessory, and (XII) hypoglossal. In 9 to 12 mm. pig embryos all of the cranial nerves except the olfactory and optic are readily recognizable (Figs. 59-63, and 92). Those carrying sensory fibers show conspicuous ganglia near their point of connection with the brain (see nerves V, VII, VIII, IX, and X in Fig. 92). Except for the acoustic (VIII) all these ganglionated nerves carry also some motor fibers — that is, they are mixed nerves. The cranial nerves composed practically entirely of motor fibers have no external ganglia (nerves III, IV, VI, and XII, Fig. 92).

Fig. 60. Reconstruction (X 14) of the nervous, digestive, respiratory, and urinary systems of a 9.4 mm. pig embryo. Compare with Frontispiece and with figure 66. By laying a straight-edge across the numbered lines in the margin the locations of the accompanying cross-sections may be determined.


All. St., allantoic stalk, C.I., dorsal root ganglion of first

Br. ep., eparterial bronchus. cervical nerve.

Ch. t., chorda tympani branch of facial nerve.

Dien., diencephalon.

G.acc., accessory ganglion.

G.acou., acoustic ganglion of eighth nerve.

G.Fro., Froriep’s ganglion.

G.gas., Gasserian (semilunar) ganglion of fifth nerve.

(bgen., geniculate ganglion of seventh nerve.

G.jug., jugular ganglion of tenth nerve.

G.nod., nodose ganglion of tenth nerve., petrosal ganglion of ninth nerve.

G.pv., prevertebral sympathetic ganglion.

G.sup., superior ganglion of ninth nerve.

Gen. Em. genital eminence.

Gl. B., gall bladder.

Glom.j glomerulus.

L. , lung.

Lg. Int., large intestine.

M. Mes.j splanchnic mesoderm cut where reflected over mesonephros from mesentery.

Mand.V., mancfibular branch of fifth (trigeminal) nerve.

Max.V., maxillary branch of fifth (trigeminal) nerve.

Mes., mesencephalon.

Mes. D., mesonephric duct.

Met., metencephalon.

Met.D., metanephric diverticulum.

Myel.j myelencephalon.

N. IIL, third cranial (oculomotor) nerve.

N.IV., fourth cranial (trochlear) nerve.

N.VI., sixth cranial (abducens) nerve.

N.VI I., seventh cranial (facial) nerve.

N.X., tenth cranial (vagus) nerve.

N.XI., eleventh cranial (accessory) nerve.

N.XI I., twelfth cranial (hypoglossal) nerve.

N.cerv.d., descending cervical nerve.

N.Ph., phrenic nerve.

Na., location of nasal (olfactory) pit.

Nch., notochord.

Neur. Cr., neural crest.

Neph., nephrogenous tissue of metanephros.

Oes., esophagus.

Op.c., optic cup.

Oph.V., ophthalmic branch of fifth (trigeminal) nerve.

Ot., auditory vesicle (otocyst).

P.C.G., post-cloacal gut.

P.C.V., posterior cardinal vein.

Panc.d., dorsal pancreas.

Panc.v., ventral pancreas.

Pelvic Dil., pelvic dilation of metanephric diverticulum.

Ph.4., fourth pharyngeal pouch.

Rath, p., Rathke’s pocket.

Sm. Int., small intestine.

St., stomach.

Sub. Card. Sin., large venous sinus formed by the transverse anastomosis of the subcardinal veins (subcardinal sinus).

T.I., first thoracic spinal ganglion.

Tel., telencephalon (specifically the label is on the right lateral telencephalic vesicle).

Thym., thymus.

Thyr., thyroid.

Tr., trachea.

V.T.M., superficial mesonephric veins connecting post- and subcardinals (named venae transversales mediales by Sabin).

Fig. 61. Transverse section of 9.4 mm. pig embryo passing through the myelencephalic region (X 15). This and the following drawings of crosssections were taken from the same series used in making the reconstructions which appear as figures 60 and 66. The serial number of this section is 96. Its location on the reconstructions may be determined by laying a straight-edge on the marginal lines numbered 96.

Fig. 62. Transverse section of 9.4 mm. pig embryo passing through the pharynx (X 15). (Its serial number on reconstructions appearing as figures 60 and 66 is 142.)

The Spinal Ckird and Spinal Nerves

The caudal part of the myelenccphalon merges without any definite line of demarcation into the spinal cord. The walls of the neural tube in the cord region have already begun to become differentiated. Dorsally and ventrally they remain thin, but laterally rapid proliferation of primordial nerve cells has caused them to increase greatly in thickness so that the originally oval lumen of the tube becomes slit-like (cf. Figs. 42, B-D, and 83).

The cells arising from the neural crests (Fig. 35) have become aggregated on either side of the cord into segmentally arranged clusters, the spinal ganglia (Fig. 59). From nerve cells in each of these ganglia, fibers grow both toward the cord and peripherally, establishing the dorsal root (afferent root) (sensory root) of a spinal nerve (Figs. 73, 84, and 85). The ventral root (efferent root) (motor root) of a spinal nerve is composed of fibers which grow out from cells lying in the wall of the neural tube (Fig. 84). Outside the cord the dorsal and the ventral root unite to form a spinal nerve trunk.

Immediately distal to the union of the dorsal and the ventral root of a spinal nerve, the nerve trunk breaks up into three main branches : a dorsal ramus carrying the fibers associated with the dorsal part of the body, a ventral ramus composed of fibers terminating in ventral parts of the body, and a ramus commtmicans containing the fibers which extend by way of the prevertebral sympathetic chain to the viscera (Fig. 85).

Fig. 63. Transverse section of 9.4 mm. pig at the level of the eyes (X 15). (Serial number on reconstructions, 159.)

Primarily the spinal nerves are strictly metameric in arrangement, each nerve carrying the sensory fibers from, and the motor fibers to, that segment of the body in which it arises. But the underlying metamerism of the body in the adult is greatly modified, almost obliterated in many regions, by such processes as the fusion of primordial tissues from several metameres to form new, highly specialized structures, or by the migration of entire organs from their place of origin to new positions in the body. Since the spinal nerves arise very early in development and are at first associated with structures at their own metameric level, their final arrangement constitutes a valuable record of evolutionary and developmental history. The appendages, for example, arise by the coalescence and organization of primordial tissue from several adjacent metameres. The corresponding spinal nerves innervate these tissues. Originally entirely separate, these nerves merge peripherally to form the nerves to the appendages. But the story of polymetameric development is permanently recorded by the series of nerve roots which retain their independent origin from the spinal cord in spite of their peripheral fusion in the brachial and sacral plexuses.

Furthermore, the caudal migration of the appendages during development is clearly evidenced by the fact that their nerves arise from the cord at a more cephalic level than that occupied by the appendages themselves. (Note the involvement of cervical nerves in the formation of the brachial plexus, Fig. 60.) Similarly the caudal migration of the diaphragm from its place of first appearance at what is destined to be the level of the neck, is indicated by the cervical origin from the cord of the phrenic nerve to the diaphragm. In a pig embryo of about 10 mm. the phrenic nerve (Fig. 60) can be seen extending directly from its level of origin toward the septum transversum (Fig. 59) which is the beginning of the diaphragm. Later in development, as the diaphragm moves caudad, the terminal portion of the phrenic nerve will be pulled caudad with it, constituting a permanent record of the migration of the diaphragm.

The Sense Organs

Although the cranial nerves associated with the nose and the eye are not as yet well developed in 10 mm. pig embryos, the primordia of the sense organs themselves are established. The olfactory organs are represented by a pair of depressions situated at the rostral end of the head (Figs. 58, 168, and 169). From specialized cells in the ectodermal lining of these nasal pits, nerve fibers will later arise and grow into the telencephalon establishing the olfactory nerves.

The single-walled spheroidal optic vesicle seen in younger embryos (Fig. 36, E) has been converted by invagination of its distal portion into a double-walled cup (Figs. 41, D, and 63), The inner layer of the optic cup is already much thickened, foreshadowing its development into the highly specialized sensory layer of the retina. The outer layer remains thinner and becomes the pigmented layer of the retina. The invagination of the primary optic vesicle to form the optic cup takers place eccentrically. As a result the lip of the cup is not, at first, complete. It shows a ventral gap, called the choroid fissure^ which does not ( lose until much later in development (Figs. 47 and 60, choroid fissure shown but not labeled; Fig. 41, D, section cut somewhat on a slant so that it passes through the choroid fissure in one eye but not in the other). The choroid fissure is continued as a groove on the ventral surface of the optic stalk. Wh(m the fibers which constitute the optic nerve grow from cells in the sensory layer of the retina to the brain, they pass along this groove in the optic stalk.

While these changes have been occurring in the optic vesicle, the lens has been established by invagination of the superficial ectoderm overlying the optic cup (Fig. 41, D). By the time the embryo has attained a length of 10 mm., the lens has been completely separated from the parent cctodcTm and appears as a spheroidal vesicle lying in the opening of the optic cup (Figs. 60 and 63).

The primordium of the internal ear mechanism is as yet very simple in form. It makes its first appearance as a loc:al thickening of the superficial ectoderm overlying the hind-brain. This thickened area, the olfactory placode, then sinks in to form a pit (Figs. 30 and 36, E) which soon becomes closed over to form the auditory (otic) vesicle. By 10 mm. the auditory vesicle has entirely lost all connection with the ectoderm from which it was derived. The only indication of its origin is a slender stalk, the endolymphatic duct, which extends dorsally toward the site of the original invagination.

Although the nerve connections of the auditory apparatus with the brain have not yet been definitely established, they are clearly indicated by the nerve fibers growing from cells in the acoustic ganglion to the brain on the one hand, and toward the otic vesicle on the other. Of significance, also, is the close proximity of the first pharyngeal (hyomandibular) pouch to the otic vesicle (Fig. 60).

This pouch is destined to give rise to the middle ear chamber and the Eustachian tube.

III. The Digestive and Respiratory Organs

The Oral Region

The digestive system of 10 mm. pigs still shows most of the primary landmarks seen in younger embryos. The stomodaeal depression has broken through into the fore-gut establishing the oral opening, but a small diverticulum called Seessel’s pocket persists as a vestige of the pre-oral gut (cf. Figs. 37, 40, and 65). SeessePs pocket is of no especial interest in itself for it gives rise to no adult structure. In embryos of this age, however, it is a valuable landmark indicating as it does precisely the point at which stomodaeal ectoderm and fore-gut entoderm became continuous when the oral plate ruptured.

The part played by the growth of nasal, maxillary, and mandibular processes in deepening the original stomodaeal depression and in the formation of the face and jaws is discussed elsewhere (Chap. 13). But mention should be made here of another stomodaeal structure called Rathke’s pocket. Rathke’s pocket arises in the mid-line as a slender diverticulum of stomodaeal ectoderm growing toward the infundibulum (Fig. 65). Later in development it separates from the ectoderm and its deep portion becomes fused with the infundibulum to form an endocrine gland known as the hypophysis.

The Pharynx

Caudal to the oral opening, the fore-gut becomes very broad and considerably flattened dorso-ventrally to form the pharynx. A series of four pairs of pocket-like diverticula, the pharyngeal pouches, arise from it laterally (Fig. 60). Each pharyngeal pouch is situated opposite one of the external gill furrows, representing, as it were, an abortive attempt at establishing an open gill cleft. In mammalian embryos this process ordinarily stops just short of completion, the gill clefts remaining closed by a thin membrane (Figs. 41, B, and 62).

The Trachea and Lung Buds

In the floor of the pharynx at the level of the most posterior pair of pharyngeal pouches, a median ventral groove appears which is rapidly converted into a tubular outgrowth parallel to the digestive tract. This groove is the tracheal (Iciryngo-tracheal) groove, and the tubular outgrowth which is formed by its prolongation caudad is the trachea. In 10 mm. embryos the caudal end of the trachea has become enlarged and bifurcated to form the lung buds. Thus the original evagination from the pharynx is the primordium of larynx, trachea, bronchi, and lungs, but for the sake of brevity it is often referred to as “the lung bud” (Figs. 40, 60, 65, and 69-74),

Fig. 64. Model of 10 mm. pig embryo dissected to the sagittal plane. (After Prentiss.)

The Esophagus and Stomach

Posterior to the pharynx the digestive tube is distinctly narrowed to form the esophagus (Figs. 65 and 69—74). A marked local dilation, already of suggestive shape, indicates the beginning of the stomach (Figs, 59, 60, 64, and 67).

The Liver and Pancreas

Immediately caudal to the stomach are the outgrowths of the gut which constitute the primordia of the pancreas and of the liver and gall-bladder. The pancreas at this stage consists of two independent parts, a large dorsal bud and a small ventral bud (Figs. 60, 76, and 104). The original hepatic diverticulum (Figs. 40 and 103, A) has given rise to a very extensive mass of glandular tissue which is crowded ventrally and cephalically from its point of origin to constitute the liver (Figs. 59, 64, and 65). The narrowed proximal portion of the original evagination from the gut persists as the duct draining the liver, and a diverticulum of it becomes enlarged to form the gall-bladder (Fig. 104).

The Intestines

The elongation of the intestines which later results in their characteristic coiling has just commenced in 10 mm. embryos. The gut has become relatively thinner than in earlier stages and protrudes into the belly-stalk in the form of a slender U-shaped loop (cf. Figs. 37, D, 40, 64, and 65). Communicating with the gut at the apex of the loop is the yolk-stalk. The yolk-stalk by this time has become greatly attenuated and the yolk-sac with which it communicates is reduced to a shriveled vesicle embedded in the belly-stalk (Figs. 64 and 65).

In some of the older embryos in the 9 to 12 mm. range the Ushaped bend of the gut has been twisted to form a loop and a slight enlargement of the gut just caudal to the yolk-stalk will be found indicating the beginning of the cecum (Fig. 64). Cephalic to this enlargement the gut will become small intestine, and caudal to it, large intestine.

The Cloaca

The dilated caudal end of the gut where the allantoic stalk and the mesonephric ducts enter is called the cloaca (Fig. 60). It is in this region that the proctodaeal depression ruptures into the gut establishing its posterior opening to the outside. In 10 mm. embryor the tissue intervening between the gut and the proctodaeum is nevet thick and usually shows definite signs of impending disintegration if not an actual rupture (Fig. 65). Caudal to the level of the proctodaeum a variable portion of the hind-gut persists for a time as the so-called post-cloacal (post-anal) gut (Fig. 60).

IV. The Coelomic Cavity

When first established the coelom consisted of paired cavities bounded by the splanchnic and somatic layers of the lateral mesoderm (Fig. 108, A). With the folding off of the body from the extraembryonic membranes and the closure of the embryonic body ventrally, the primary right and left coelomic chambers are carried toward each other in the mid-line (Fig. 108, B~D). In this process the gut tract is caught between the layers of splanchnic mesoderm which form the mesial boundaries of the coelomic chambers. The double layer of splanchnic mesoderm thus formed serves as a supporting membrane for the gut and is known as the mesentery. Shortly after its formation the part of the mesentery ventral to the gut breaks through, bringing the right and left coelomic chambers into confluence and thus establishing a single body cavity within the embryo (Fig. 108, F).

Fig. 65. Sagittal section of 10 mm. pig embryo (X 16)

Only in the mid-body region do these changes take place exactly as described above. While they are essentially similar elsewhere there are certain local modifications of the process which are of special interest. At the level of the pancreas and liver the ventral mesentery persists, supporting the liver (Figs. 108, E, and 111). Where the extraembryonic membranes are continuous with the embryo at the bellystalk, the body cavity remains for a long time continuous with the extra-embryonic coelom (Figs. 108, C, and 111). In the cardiac region the digestive tract develops in the dorsal body-wall so that no dorsal mesentery is formed. Here the heart is formed ventral to the gut and suspended in the coelom by a double layer of splanchnic mesoderm in a manner quite suggestive of that in which the liver is suspended in the ventral mesentery farther caudally in the body (cf. Figs. 43, D, and 108, E).

In 10 mm. pig embryos the body cavity is not yet divided into separate pericardial, pleural, and peritoneal chambers. Between the liver and the heart, however, there has appeared a shelf-like structure which partially separates the thoracic region of the coelom from the abdominal. This incomplete partition is called the septum transversum. At this stage it is an ingrowth of ventral body-wall tissue fused to the cephalic face of the liver (Fig. 64). The septum transversum itself never extends all the way across the coelom. Later in development we shall see it supplemented by the pleuroperitoneal folds which arise from the dorsal body-wall, and complete the diaphragmatic partition across the coelom.

V. The Urinary System

The Pronephros

In mammalian embryos the pronephros is a vestigial organ appearing only transitorily in the form of a few rudimentary tubules. In 10 mm. pig embryos even these tubules have almost completely disappeared. The paired ducts which originally appeared in connection with the pronephric tubules persist, however, and are appropriated by the developing mesonephric tubules. After forming this new association they are called the mesonephric (Wolffian) ducts.

The Mesonephros

In younger embryos we saw the development of mesonephric tubules from the intermediate mesoderm and the manner in which they attained connection with the mesonephric duct (Figs. 38, E, F, 40, and 41, G, H). In pigs of 10 mm. mesonephric tubules have been formed in great numbers, and each tubule has become much elongated and exceedingly tortuous. As a result the mesonephros becomes an organ of great bulk — in fact the most conspicuous organ in the body of an embryo of this age (Figs. 59 and 60).

Sections passing through the mesonephros (Figs. 74-79) convey a very vivid impression of the interwoven mass of tubules of which it is composed. Details as to the shape of the tubules and the relations of the blood vessels to them can best be considered in connection with the development of the urogenital system (Chap. 10).

In the more cephalic part of the mesonephros it is not always easy to distinguish the duct from the tubules. Farther caudally the duct can easily be identified lying along the ventral border of the mesonephros. After leaving the substance of the mesonephroi, the mesonephric ducts curve ventro-mesially to enter the cloaca together with the allantoic stalk (Figs. 60 and 122).

The Metanephros

Just cephalic to the cloacal end of the mesonephric duct an outgrowth arises from it and extends antero-dorsad. This is the metanephric diverticulum (Figs. 60 and 122). The terminal portion of this diverticulum becomes dilated, presaging its ultimate fate as the pelvic cavity of the kidney. Its proximal portion remains slender as the ureter. About the enlarged pelvic end of the metanephric diverticulum a mass of mesoderm accumulates. Because it is destined to form the excretory tubules of the permanent kidney, this mass of mesoderm is known as the nephrogenous tissue of the metanephros (Figs. 60 and 79). Like the cell clusters which form the mesonephric tubules it arises from intermediate mesoderm.

VI. The Circulatory System

A functionally competent circulatory system is laid down before the developing pig has reached a length of 10 mm. Paired primordia fuse to establish the heart as a median tubular organ receiving the blood at its posterior end and pumping it out from its cephalic end (Figs. 43 and 44). The aortae arise as paired vessels which lead away from the heart, swing around to the dorsal side of the pharynx, and then extend caudad throughout the length of the embryo as the main distributing channels (Fig. 45). Large collecting vessels develop in the shape of the paired cardinal veins which receive the blood from the anterior and posterior regions of the body and return it over the ducts of Cuvier (common cardinal veins) to the sino-atrial part of the heart (Fig. 45). Pigs in the 9 to 12 mm. range still retain to a large extent this primary bilaterally symmetrical arrangement of the main vessels which is so characteristic of young erribryos. There have been, however, many local elaborations and modifications which are of special interest because they initiate the series of changes which lead toward adult conditions.

The Heart

The chief factor in changing the configuration of the heart from its primitive straight tubular shape to the condition seen in 10 mm, embryos is its own rapid elongation and consequent bending. In this process the cephalic end of the heart remains anchored by the aortic roots and its caudal end by the omphalomesenteric veins. Thus the receiving and discharging ends of the heart suffer no radical change in position. The intervening portion of the heart is, however, bent into a loop which is carried first ventrally and then caudally to form the pumping part of the heart or ventricle (Figs. 142 and 143). The details involved in the bending of the heart tube and the special features of its internal structure can best be considered in connection with later stages of development when their significancp will be more apparent (see Chap. 11). At present we are chiefly interested in becoming acquainted with the more outstanding structural features of the embryonic heart.

Fig. 66 — {Continued) Abbreviations

A.C.V., anterior cardinal vein.

Ao., aorta. ^

Ao.A., aortic arch.

A.O.M., omphalomesenteric artery. Bas.A., basilar artery.

Cerv. Seg. A., intersegmeiital

branches of aorta in cervical region. Coel. A., celiac artery.

Cuv.d., common cardinal vein (duct of Cuvier).

D.V., ductus venosus.

Ex. Car. A., external carotid artery. Il.A., iliac artery.

Int. Car. A., internal carotid artery. P.C.V., posterior cardinal vein. Pul.A., pulmonary artery.

P.V.C., posterior vena cava.

S.C.V., subcardinal vein.

Subcl.A., subclavian artery.

Subcl.V., subclavian vein.

U.A., umbilical (allantoic) artery.

U. V., umbilical (allantoic) vein.

V. Cap., vena capitis (tributary of anterior cardinal vein).

V.O.M., omphalomesenteric (portal) vein.

V.T.L., lateral transverse veins of mesonephros.

V.T.M., medial transverse veins of mesonephros.

V. Vent., ventral vein of mesonephros.

Vert. A., vertebral artery.

For cardiac structures refer to figure 145.

The great veins converging to enter the heart become confluent in a thin-walled chamber called the sinus venosus (Fig. 144). The sinus venosus opens into the atrial portion of the heart by a slit-like orifice guarded against return flow by well-developed flaps known as the valvulae venosae (Figs. 46, 145, 146, and 147).

The atrial region has undergone extensive transverse enlargement so that it bulges out into pouch-like right and left chambers (Figs. 71, 72, 142, 144, and 147). Although the beginning of the separation of these chambers from each other is clearly indicated by the presence of an interatrial septum, this septum is not complete, and the atrial chambers remain in communication through a secondary perforation in the septum called the interatrial foramen secundum {ostium II) (Figs. 71, 145, and 148).

Leaving the atrium, the blood passes through a constricted region known as the atrio-ventricular canaL Previously a single channel (Fig. 147, A), in 10 mm. embryos this canal has, as was the case with the atrium, become more or less completely divided into right and left channels. The division is effected by a pair of plastic masses of mesenchymal tissue, the so-called endocardial cushions of the atrio-ventricular canal. Located one dorsally and one ventrally (Fig. 39), these cushions grow together and fuse to divide the atrio-ventricular canal (Figs. 147 and 148). Except for the least advanced specimens their fusion will ordinarily have occurred in embryos of the 9-12 mm. stage.

In the ventricle, also, there are indications of the impending separation of the heart into right and left sides. The interventricular septum has appeared as a well-marked median ridge extending from the apex of the ventricular loop toward the atrio-ventricular canal (Figs. 71, 72, 147, and 148). Above this septum the two parts of the ventricle are still in open communication.

Correlated with its activity in pumping, the ventricular wall has become greatly thickened. Irregular branching bands of developing muscle tissue protrude from the main part of the wall into the lumen. These trabeculae carneae already suggest the muscular bands which project so characteristically into the cavities of the adult ventricles.

From the ventricle the blood passes into the truncus arteriosus and thence out to the body by way of the ventral aortic roots. Aside from the marked thickening of its walls, the truncus arteriosus shows little change from its original condition as the anterior part of the heart tube. Its diameter remains small and the longitudinal division it is destined to undergo later in development is barely suggested by the irregular shape of its lumen seen in cross-sections (Fig. 70).

Fig. 68. Reconstruction of 12 mm. pig embryo showing the relation of the main venous channels to the viscera. (From Minot after Lewis.)

It should not be inferred from the modifications which have occurred in the different regions of the heart that it has as yet altered its primitive method of functioning. The heart tube has become bent and shows local dilations and constrictions which we name because of their future fate. Many internal conditions point toward its division into right and left sides. But the blood enters the heart posteriorly by way of the sinus venosus, is collected in the atria, and passes into the ventricle whence it is pumped out by way of the truncus arteriosus as an undivided stream, just as was the case in younger embryos where the heart was still a straight tube.

The Aortic Arches

In vertebrate embryos six pairs of aortic arches are formed extending around the pharynx from the ventral to the dorsal aorta. Of these the most cephalic are the first to appear and the other pairs are formed in sequence caudal to the first (Figs. 134 and 136). In pig embryos of 10 mm. the two most cephalic arches have already degenerated. The functional arches at this stage are the third, fourth, and vsixth (Figs. 66, 67, 134, E, and Frontispiece). The fifth arch is always poorly developed in mammals and usually appears only as a small collateral channel appended sometimes to the fourth, but more commonly to the sixth arch (Fig. 66). From the sixth arches small branches (the pulmonary arteries) extend caudad to the developing lungs (Frontispiece).

Fig. 69. Transverse section of 9,4 mm. pig embryo at level of posterior part of pharynx (X 15). (The serial number of this section on the reconstructions appearing as figures 60 and 66 is 180.)

Fig. 70. Transverse section of 9.4 mm. pig embryo through telencephalon and cephalic part of pericardial chamber (X 15). (Serial No. 212.)

Arteries of the Cephalic Region

The portions of the ventral aortic roots which led to the two anterior aortic arches do not disappear when the arches themselves degenerate but persist as the external carotid arteries (Figs. 66 and 67). The dorsal aortae are prolonged cephalad as the internal carotid arteries (Figs. 45, 66, and 67).

Throughout the length of the aorta small branches appear at regular intervals and extend dorsad on either side of the neural tube. Since these vessels are formed between adjacent somites they are known as the intersegmental arteries (Figs. 45, 67, and 159). In the cervical region the intersegmental vessels form a series of connections with each other which eventually result in the establishment of longitudinal vessels dorsal to, and parall^ with, the aortae. These are the vertebral arteries (Fig, 67). This longitudinal anastomosing between the cervical intersegmental arteries appears cephalically first (Fig. 66) and then progresses caudad. As the vertebral arteries become established, the more cephalic intersegmental branches feeding them disappear. Only the most caudal of the intersegmentals concerned in the formation of the vertebral arteries persists. Since this intersegmental artery (seventh cervical) is at the same time the one situated at the level of the anterior appendage bud and consequently the vessel which is enlarged with the growth of the appendage to form the subclavian artery, the vertebral artery eventually appears as a branch of the subclavian (Figs. 67 and 133, B, C).

Fig. 71. Transverse section of 9.4 mm. pij^ embryo through heart at level of interatrial foramen secundum (X 15). (Serial No. 251.)

Cephalic to the cervical level the vertebral arteries bend toward the mid-line and unite with each other to form a median vessel lying ventral to the myelencephalon. This is the basilar artery (Figs. 65, 66, and 67). Ventral to the cephalic flexure in the neural tube, the internal carotid arteries send branches mesiad to unite with the basilar (Figs. 67 and 133). This anastomosis between the internal carotid and the basilar is the first step in the formation of the arterial circle (circle of Willis) which is such a conspicuous landmark in the adult anatomy of the hypophyseal region.

The Dorsal Aorta and Its Branches

When first formed the dorsal aorta is a paired vessel. This paired condition is retained in the branchial region, but posteriorly the two primitive aortae soon fuse with each other to form a median vessel. The fusion first occurs in the mid-body region (Fig. 51) and extends thence cephalad to about the level of the anterior appendage buds and caudad throughout the length of the aorta (Fig. 67).

In young embryos the most conspicuous vessels arising from the dorsal aorta are the omphalomesenteric trunks which are prolonged as the vitelline arteries to the yolk-sac, and the allantoic or umbilical arteries to the vascular plexus of the allantois (Figs. 45 and 51). Both these vessels arise from the aorta before its fusion and are themselves paired. The umbilical arteries retain their paired condition (Fig. 79). When the body is closed ventrally, the right and left omphalomesenteric roots are brought together in the mid-line and fuse with each other to form a median vessel running in the mesentery. With the early degeneration of the yolk-sac, this vessel becomes relatively less conspicuous and is known as the anterior (superior) mesenteric artery (Figs. 66 and 67). Its original relations are, nevertheless, apparent from its course along the intestinal loop into the belly-stalk to the place where the small yolk-sac still retains its attachment to the gut.

Fig. 72. Transverse section of 9.4 mm. pig embryo through heart at level of atrio-ventricular canals (X 15). (Serial No. 258.)

Somewhat cephalic to the anterior mesenteric artery, the celiac artery arises from the aorta and extends in the mesentery toward the gastric region of the gut tract (Fig. 67). In the adult, the celiac, anterior mesenteric, and posterior mesenteric arteries constitute a group of vessels which one naturally thinks of together because of their similar ventral origin from the aorta, their course through the mesenteries, and their termination in the gastro-intestinal tract. The third member of this enteric group, the posterior (inferior) mesenteric cannot ordinarily be found in pigs of 9 to 12 mm. It arises from the aorta, caudal to the other two vessels, at a slightly more advanced stage of development.

Fig. 73. Transverse section of 9.4 mm. pig embryo at level of tracheal bifurcation (X 15). (Serial No. 293.)

At the level of the mesonephros the aorta gives off, in addition to the series of dorsal intersegmental branches, many small branches which extend ventrally. These vessels feed the capillary plexuses (glomeruli) in the dilated ends of the mesonephric tubules and the network of capillaries which surround the tubules themselves (Fig. 117). Individually these branches are very small, but the volume of blood they handle collectively is surprisingly large as evidenced by the size of the veins (post- and subcardinals. Fig. 68) which drain the mesonephroi.

The Anterior Cardinal Veins

In 9 to 12 mm. embryos little alteration from primitive conditions (Fig. 47) has occurred in the veins of the cephalo-thoracic part of the body. Numerous large tributary vessels have appeared, especially in the cephalic region where they converge on either side of the head as the so-called venae capitis (Fig. 66). It is already possible to recognize in the larger of these branches the primordial vessels from which the main venous sinuses pf the adult cranial region are derived (Fig. 68). Fundamentally, however, these veins are but an elaboration of the original anterior cardinal system. From them the blood passes caudad along the less modified portion of the anterior cardinals to enter the heart by way of the common cardinal veins (Fig. 66). Just before the anterior cardinal vein enters the common cardinal vein a series of small tributaries (dorsal segmental veins, Fig. 68) return to it the blood distributed by the intersegmental arteries to the cervical region. Near this point, also, a welldeveloped branch brings back the blood from the mandibular region. The vessel which thus returns the blood distributed by the external carotid artery is the beginning of the external jugular vein (Fig. 68). The anterior cardinal vein itself is later known as the internal jugular.

Fig. 74. Transverse section of 9.4 min. pig embryo through cephalic part of liver (X 15). (Serial No. 309.)

The Posterior Cardinal Veins

In very young embryos (Fig. 45) the posterior cardinal veins are the only conspicuous venous channels in the caudal half of the body. In 9 to 12 mm. pigs these veins have already begun to degenerate. Their relative position as vessels lying dorsal to the mesonephroi remains unchanged, but much of the blood formerly returned by them now reaches the heart over new channels (cf. Figs. 45, 66, and 139, A~F). As a result the posterior cardinal veins in the mid-mesonephric region have been interrupted. Toward the heart from this point the old channels persist, although they are much reduced in size. Caudal to their interruption the posterior cardinals rapidly degenerate as main channels (Figs. 66 and 139, D, F).

Fig. 75. Transverse section of 9.4 min. pig embryo just cephalic to attachment of belly-stalk (X 15). The belly-stalk and the tip of the tail have not been included in the drawing. (Serial No. 388.)

The Subcardinal Veins

The diversion of the blood from the posterior cardinals is brought about by the development of a collateral system of veins in the mesonephroi. When it first appears, this system of vessels is but an irregular plexus tributary to the postcardinals (Fig, 139, A). The organization of longitudinal channels in these plexuses establishes the main subcardinal veins as vessels extending cephalad in the ventro-mesial border of the mesonephroi, parallel with and ventral to the posterior cardinal veins. In the cephalic part of the mesonephros the newly established subcardinal blood stream enlarges some of the small channels already entering the posterior cardinal and discharges through them into the posterior cardinal vein/ (Figs. 47 and 139, C). Other vessels of the primitive plexus persist as a meshwork of small veins lying superficially in the mesonephros. These veins afford free intercommunication between the postcardinals dorsally and the subcardinal vessels in the ventromesial portion of the mesonephros (Figs. 117 and 139).

From the same primitive subcardinal plexus a minor longitudinal channel develops along the ventral border of each mesonephros. These vessels are called the ventral veins of the mesonephroi (Figs. 66 and 139). The fact that they are rather conspicuous in pig embryos at this stage sometimes leads to their confusion with the main subcardinal channels. Their characteristic superficial position on the ventral border of the mesonephros (Fig. 139, E) should preclude such a possibility. They are to be regarded as an incidental modification of the plexus of small vessels connecting the sub- and postcardinals rather than as main channels of any special importance.

Fig. 76. Transverse section of 9.4 inm. pijEj embryo at level of pancreatic outgrowth from gut (X 15). Tail and part of belly-stalk omitted. (Serial No. 406.)

Fig. 77. Transverse section of 9.4 mm. pig embryo at level of inter-subcardinal anastomosis. (Cf. Fig. 139, D, F.) (Serial No. 426.)

Fig. 79. Transverse section of 9.4 mm. pig embryo passing through caudal end of mesonephros (X 15). (Serial No. 529.)

With the growth of the mesonephroi the rapidly enlarging subcardinal veins are brought very close to each other (cf. Fig. 139, B, E). Where they are approximated, cross-communication is established, first by small vessels and then by a broad anastomosis (Figs. 60, 68, 77, and 139, D-F). The large median venous sinus thus formed probably offers less resistance to the flow of blood than surrounding channels; in any case all the vessels connecting with it tend to drain toward it. The diversion of blood toward this sinus by way of the small vessels which connect the sub- and postcardinals is responsible for the breaking down of the postcardinal veins at this level (Figs. 66 and 139, D, F).

The Posterior Vena Cava

One might expect that the great volume of blood entering the subcardinal sinus would cause a corresponding enlargement of the cephalic portion of one or both subcardinal veins. Instead, a new and more direct channel toward the heart appears. In its growth the liver is crowded very close to the mesonephroi. The developing liver contains a maze of vascular channels, as does the mesonephros. Capillaries ramifying in the base of the mesentery between the liver and the right mesonephros form the connecting link between the vessels of these two organs (Fig. 140). Once the blood begins to find its way by this route, the small irregular channels are rapidly enlarged and straightened. The new and more direct channel thus established leads from the subcardinal sinus through the right subcardinal vein for a short distance and thence, by the newly excavated channels in the mesentery, through the liver to the heart (Fig. 139, D). This is the start of that embryologically composite^ vessel which we know in the adult as the posterior, or inferior, vena cava (Figs. 66, 68, and 74-77).

The Omphalomesenteric Veins

Primarily the omphalomesenteric veins are the main channels into which the vitelline veins from the yolk-sac converge (Fig. 45). Two factors radically change their original relations. The early degeneration of the yolk-sac reduces their peripheral drainage area, with a resultant decrease in their relative size; and the growing liver envelops and breaks up their proximal portions (Fig. 141, A-C). Thus we find them in 10 mm. pig embryos reduced to small vessels which collect the blood delivered by the enteric arteries to the gut. Distally the omphalomesenteric (vitelline) veins are still paired but where they discharge into the liver they have been reduced to a single vessel, which we can now quite properly call by its adult name, Xht portal vein (Figs. 66, 76, and 77).

  • The developmental complexity of the posterior vena cava is recognized by designating the part of it which arises by the straightening of small channels in the liver as its intra-hepatic portion; the part which arises from the capillaries in the caval fold as its mesenteric portion; the part formed by the inter-subcardinal anastomoses as the inter-renal portion; and that part which at a later stage is added by the appropriation of the right supracardinal as the post-renal portion.

The Umbilical Veins

Except for a striking increase in size, the distal portions of the umbilical veins in 9 to 12 mm. pig embryos have undergone little change (cf. Figs. 45 and 66). Proximally, however, they have been rerouted through the liver. The underlying factor in this change is the extensive growth of the liver which brings it into contact with the lateral body-walls in which the umbilical veins are embedded in their course from the belly-stalk to the sinus venosus. Fusion follows the contact, and small vessels develop between the umbilicals and the network of channels in the liver (Fig. 141, B). As these new vessels develop the portions of the umbilical veins cephalic to them gradually drop out altogether and all the placental blood passes through the liver (Figs. 66, 68, 74, 75, and 76).

With the completion of this change in the umbilical circulation, the liver has become the common returning path for both of the original extra-embryonic circulatory arcs and most of the intraembryonic circulation of the posterior half of the body. Only the dwindling current of the postcardinals and the unchanged anterior cardinal circulation now enter the sinus venosus without first passing through the liver. When we consider all this volume of blood passing through one organ, it leaves little room for surprise at the relatively enormous bulk attained by the liver in mammalian embryos.

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Patten 1951: 1 Foreword to the Student | 2 Reproductive Organs - Gametogenesis | 3 Sexual Cycle | 4 Cleavage and Germ Layers | 5 Body Form and Organs | 6 Extra-Embryonic Membranes | 7 Embryos 9-12 mm | 8 Nervous System | 9 Digestive - Respiratory and Body Cavities | 10 Urogenital | 11 Circulatory System | 12 Bone and Skeletal System | 13 Face and Jaws | Bibliography

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