Book - A Laboratory Manual of Vertebrate Embryology (1947) Pig

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Adamstone FB. and Shumway W. A Laboratory Manual of Vertebrate Embryology. (1947) John Wiley & Sons, London.

   A Laboratory Manual of Vertebrate Embryology 1947: Frog | Chicken | Pig
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A Laboratory Manual of Vertebrate Embryology

Part III Anatomy of the 10 mm Pig

10 mm Pig Embryo

External Form

The 10-mm. pig is a much larger and more complicated embryo than the 72-hour chick last studied, and it has not only begun to take on its final external form but it has also developed the primordia of nearly all the organs and organ systems. The body is clearly divisible into head, trunk, and tail regions and the limb buds are well developed.

The head is bent sharply almost at right angles to the longitudinal axis of the body owing to the cervical and cranial flexures. The head is marked by prominent lateral bulges corresponding to the location of the five vesicles of the brain. At the ventrolateral angles of the head the olfactory pits are clearly evident, bounded by the median and lateral nasal processes. The eye, lateral to the diencephalon, bulges prominently. The depression surrounding it is joined to the nasal pits by the nasolacrymal groove which marks the anterior boundary of the maxillary process of the first visceral arch. The mandibular process, or primordium of the lower jaw, which is the main body of the first arch, lies posterior to the maxillary process, the primordium of the upper jaw. The mandibular process is separated from the second or hyoid arch by the hyomandibular groove which gives rise to the external auditory meatus. The tuberculated edges fif the hyomandibular groove furnish small tissue masses which form the pinna of the ear. The second arch and groove are followed by the third arch and groove, but the fourth and succeeding arches are squeezed in beneath the head in the region of the future neck by the cervical flexure and their location is indicated only by the cervical sinus, a pitlike depression on the side of the head.

In the region of the trunk, the heart is visible through the thin epidermis just below the head. The liver lobes and mesonephros lie posterior to the heart in the order given. The liver is approximately the same size as the heart, but the mesonephros is very large, occupying most of the space between the anterior and posterior limb buds. Along the dorsolateral sides of the body adjacent to the spinal cord a series of small bulges marks the position of the spinal nerves and indicates the number of somites present. The anterior limb buds project from the sides of the body at the level of the heart and the posterior limb buds at the level of the posterior end of the mesonephros. The body stalk is attached to the ventral abdominal body wall.

On the ventral side of the body at the base of the tail is a small rounded prominence, the genital tubercle, which is the primordium of the external genitalia.

Laboratory Directions

Study a 10-mm. pig embryo, and make a drawing showing the external features. Label fully.

Internal Anatomy

General Directions

In studying the 10-mm. pig embryo it is advisable to work out ectodermal, endodermal, and mesodermal derivatives separately. record should be kept as usual, and a series of small sketches

Should be made recording the iQCfttlpn Msd identification of various structures.

Ectodermal Derivatives

The brain, organa of special eenae, cranial nerves, spinal nerves, and developing sympathetic ganglia constitute the chief ectodermal derivatives which are present at this stage in addition to the epidermis. These will be considered independently. Constant reference should be made to Fig. 32.

A. EPIDERMIS. A layer of cells constituting the outer covering of the entire body.


1. VESICLES. The brain is already divided into five vesicles. These are encountered as you work down through the series in the following order:


Fig. 32. Lateral view of dissected head of a 10-mni. pig showing relation of cranial nerves to brain and organs of i-pi'i'i.'il sense.

Mesencephalon. Thick-walled. Section typically round in shape. Isthmus. Constriction between mesenc.ephalon and metencephalon.

Metencephalon. Just posterior to isthmus. Thick-walled. Widens gradually behind isthmus.

Myelencephalon. Posterior to metencephalon — tapering gradually toward spinal cord. Roof of anterior part thin-walled. Side walls thick and marked by a series of indentations, the neuromeres, which were once thought to represent primitive segmentation of the brain.

(b) SECTIONS SUCH AS THOSE AT LEVEL 4, MG. 32 Diencephalon. Associated with optic cup. Section of the brain is round at junction with mesencephalon. It gradually changes to an oval at the level of the optic cups, which are connected to the brain by the optic stalk.


Telencephalon. The telencephalon has already begun to develop the two thick-walled cerebral hemispheres which bulge laterally. Their cavity are the lateral ventricles.

2. SAGITTAL 8BCJTION. * At this Stage it is well to examine a single typical mid-sagittal section through the brain. In such a section identify:

Telencephalon. The most anterior vesicle of the brain which is marked oS from the diencephalon by two constrictions, namely the optic recess in the floor and the velum transversum in the roof.

Diencephalon. Bounded anteriorly by the optic recess and velum transversum and posteriorly by the tuberculum posterius on the floor of the brain.

Mesencephalon. This part of the brain is located in the region of the cranial flexure. It appears posterior to the diencephalon as a sudden expansion and is bounded posteriorly by the narrow isthmus.

Metencephalon. This division lies just posterior to the isthmus. It is marked by a comparatively ‘thick roof.

Myelencephalon. There is no sharp line of demarcation between this part of the brain and the metencephalon. The thin roof, however, is quite characteristic.

c. CRANIAL NERVES. Working down through the series identify the various cranial nerves, as follows:


(IV) Trochlear. Very small, inconspicuous. Leaves the brain on the upper side of the isthmus where the mesencephalon just joins the metencephalon. This nerve cannot be traced very far at this stage but it eventually innervates the superior oblique muscle of the eye.


(a) ANTORIOE TO THE OTIC VESICLE (LISTED IN ORDER ANTERIORLY) (yill) Auditory Nerve: Acoustic Ganglion. Innervates the otic vesicle.

{VII) Faded Nerve: Geniculate Ganglion. Closely associated with VIII. The nerve is directed posterolaterally and innervates the ' side of the head, its fibers passing into visceral arch II.

(V) Trigeminal: SemUunar Ganglion. Situated at the broad anterior end of the myelencephalon. The ganglion is half-moon shaped. Traced deeper into the body, three main branches of the nerve appear:

(a) Superficial Ophthalmic Ramus. Arises from anterior end of the ganglionic mass and passes toward the eye.

(b) Maxillary Ramus. Arises from mid-portion of the ganglion and passes into the maxillary process of the first visceral arch.

(c) Mandibular Ramus. Arises from posterior end of the ganglion and passes into the mandibular process of the first visceral arch.


{IX) Glossopharyngeal: Superius Ganglion; Petrosal Ganglion. Situated just posterior to the otic vesicle. Its fibers are directed deeper into the head to innervate the tongue and pharynx. The superius ganglion is found near the point of origin. The petrosal ganglion is located farther down the nerve.

{X) Vagus: Jugular Ganglion; Nodose Ganglion. Its fibers at first run parallel to those of IX but may be traced well down into the body, where they innervate the visceral organs. The jugular ganglion is locatcKl near the point of origin. The nodose is found by tracing the nerve somewhat deeper into the series.

(XI) Spinal Accessory: Froriep’s Ganglion. This nerve arises from a series of small roots, which, therefore, appear like a row of dots alongside the posterior end of the myelencephalon. The ganglion is obscure. The internal branch of the nerve runs parallel to the vagus down to the shoulder, whereas the external branch runs alongside the hind brain and parallels the spinal cord. It may thus appear as a long band of fibers adjacent to the myelencephalon, or, deeper in the series, as a minute fiber wedged in between the dorsal roots of the spinal nerves and the spinal cord.




a level such as this will show the spinal cord, base of the floor of tiie mydencephalon and the mesencephalon, or possibly the diencephalon.

• {VI) Abducent, This small nerve arises from the ventrolateral portion of the floor of the myelencephalon. Its fibers are directed forward and outward toward the eye, vdiere it will eventually innervate the external rectus muscle.

• {XII) Hypoglostal. Arises as a series of roots from the ventrolateral sides of the myelencephalon before the brain narrows to the spinal cord. Appears as a row of dots bridging the gap between the spinal cord and base of myelencephalon. This nerve innervates the tongue.


• (III) Oculomotor. The fibers of this nerve protruding posteriorly arise from the floor of the mesencephalon. It may be traced some distance toward the eye, where it will eventually innervate certain eye muscles, namely: inferior oblique, superior rectus, inferior rectus, and internal rectus.


{II) Optic. This nerve runs from the retina along the choroid fissure on the under side of the optic stalk. It can rarely be seen at this stage,


{I)Olfactory. This nerve runs from the olfactory pit to the telencephalon. It can usually (though not always) be seen. Terminal. Not present at this stage.


1. one vxsiou). Lateral to the myelencephalon. Thick>walled and elongate oval in shape. Slightly constricted into dorsal utricular portion and ventral saccular portion.

Endolymphatic Duct. Situated between the otic vesicle and the myelencephalon. Thick-walled and almost circular in outline. Joins the lower or saccular portion of the otic vesicle.

2. OFTic CUF AND LENS. Located alongside the diencephalon to which the optic cup is attached by means of the optic stalk. The lens is almost solid and located within the rim of the optic cup.

Retinal Layer oj Optic Cup. The inner layer of sensory cells of the optic cup.

Pigmented Layer oj Optic Cup. The outer layer of cells of the optic cup darkened by the deposit of pigment.

3. OLFACTOBY FIT. Located at the anterior ventrolateral surfaces of the head. Has the form of a thick-walled pit extending inward from the surface of the head.

B. SPINAL COKD. PIG. 36. At almost any level past the head, preferably one about midway through the series, typical sections of the spinal cord and associated spinal nerves can be found. Identify :

Spinal Cord. Located dorsally just beneath the epidermis. Somewhat oval in shape.

Central Canal or Neurocoel. The cavity of the spinal cord. Ependymal Layer. The inner epithelial lining of the central canal.

Note the abundant mitotic figures.

Mantle Layer. The intermediate layer composed of neuroblasts and spongioblasts — the precursors of the neurons and neuroglia cells.

Marginal Layer. The outer layer of white matter composed of the . ; . processes of neurons and neuroglia cells.

F. TTPiCAL SPINAL NERVE. Paired. Located ventrolateral to the spinal cord. Dorsal Root. The bundle of afferent nerve fibers joined to the dorsolateral wall of the spinal cord.

Dorsal Root Ganglion. A prominent mass of neurons located on the dorsal root close to the point of origin from the spinal cord and lying alongside the cord.

Ventral Root. The bundle of efferent nerve fibers joined to the ventrolateral wall of the spinal cord.

Spinal Nerve. The main trunk formed by junction of the dorsal and ventral root. It gives off three branches or rami, as follows:

Dorsal Ramus. The branch arising from the main trunk and ascending to the dorsolateral body wall to supply the skin and muscles of the trunk.

Lateral Ramus. The branch directed laterally to the ventral body wall.

Communicating Ramus. The branch directed ventrally and medially toward the ganglia of the sympathetic system.

Brachial Plexus. At the level of the anterior limb buds the lateral rami of the spinal nerves are interconnected, thereby forming the brachial plexus which innervates the limb bud.

Lumbosacral Plexus. The nerve plexus supplying the posterior limb bud.

• H. SYMPATHETIC GANGLIA. Paired gangUonic masses lying dorsolateral to the aorta. These are most readily recognized at about the level of the anterior limb buds where the dorsal aorta is still paired.


The original gut or endodermal tube has already become differentiated into the main parts of the digestive system as well as the primordia of the respiratory apparatus. Beginning at the anterior end, follow the digestive tube backward in the series, identifying the following:

Oral Aperture. The external opening of the mouth cavity whidh is bounded by the maxillary and mandibular process of the first visceral arch. The maxillary process appears as a paired lateral prominence attadied to the head bdow the eye. The mandibular process constitutes a thick- ridge just posterimr to the maxillary process.

Mouth Cavity. Formedby the junction of t^stomodeum and anterior end of the fore-gut. Its posterior boundaries are indefinite.

Hypophysis. The hypophysis or pituitary body is formed from ectodermal tissues contributed by Rathke’s pocket, which is a dorsal evagination from the stemodeum, and the infundibulum.

Tongue. A flattened thick prominence on the floor of the mouth formed of tissues contributed by all three germ layers.

Pharynx. Posterior to the mouth cavity. Compressed dorsoventrally. Marked by the presence of four paired, lateral visceral pouches. These meet the corresponding visceral grooves but the closing plates do not rupture to form a row of clefts. The visceral arches are pillars of tissue between the pouches. The parts of visceral arch I form the boundaries of the mouth opening, the maxillary process being the primordium of the upper jaw and the mandibular process, the primordium of the lower jaw. Next in order posteriorly are arches II (the hyoid), III, and IV. Note the similarity to what has already been encountered in the 72-hour chick.

Auditory Ttibe. Proximal end of visceral pouch I.

Cavum Tympani. Dilated terminal portion of visceral pouch I.

Parathyroid Glands. Located at the upper (or dorsal) ends of third and fourth visceral pouches. These primordia have the appearance of irregular thickenings or buddings of the endoderm running into the adjacent mesenchyme of arches III and IV.

Tkytnus Glands. Located at the lower or ventral ends of the third and fourth visceral pouches. Their appearance is similar to that of the parathyroids.


Thyroid Gland. A ventral diverticulum arising from the floor of the pharynx at tlie level of the second visceral pouch. At this stage of development it has already sunk deeper into the mesoderm and will be much more readily identified when the aortic arches are studied.

Laryngotracheal Groove. Posterior to the fourth visceral pouch the tube narrows greatly and develops a deep longitudinal depression on its ventral surface, known as the laryngotracheal groove. The groove is prolonged posteriorly so that its distal end separates off from the main tube, thus forming the trachea and leaving the oesophagus in the position of the original tube.

Trachea. Ventral to the oesophagus. Round in cross-section.

Eparterial Bronchus. A single rounded, budlike protuberance on the right side of the trachea.

• Primary Bronchi. The rounded bifurcated ends of the trachea.

Oesophagus. Round and thick-walled in section. Forms the connection between pharynx and stomach.

Stomach. Elongate and thick-walled. Broader than oesophagus. Supported by the dorsal mesentery. Directed ventrolaterally and tows^ the ri^t. Note the omental bursa, which is a blind coelomic pocket to the ri^t of the stomach.


Duodenum. Round and thick-walled. Begins where stomach narrows posteriorly. Ends at point of origin of the bile duct and pancreas.

BUe Duct {ductus choledochus). A ventral tubular canal joining the distal end of the duodenum. Thick-walled and directed ventrally toward the gall bladder within the liver tissue,

Gall Bladder. The enlarged saccular distal end of the ductus choledochus.

Liver (Fig. 36). Both the ductus choledochus and the gall bladder are entirely surrounded by the liver tissue, which is almost spongy in appearance owing to the extensive ramifications of the blood vessels and capillaries throughout the entire structure. As a result, liver tissue is endodermal in origin as regards the secretory tissue, but also has a great deal of tissue of mesodermal origin, which forms the walls of the blood sinusoids.

Pancreas. A dorsal diverticulum from the duodenum near the level at which the bile duct enters. It forms an irregular mass lying in the dorsal mesentery distinguishable mostly on the basis of its staining reaction. Slight indication of the ventral pancreatic diverticulum is sometimes found in the form of a minute, budlike prominence just at the point of entrance of the bile duct into the duodenum. It is directed dorsally and to the right.

Intestine Proper. A thick-walled tube of small bore continuing backward from the duodenum. It is supported by the dorsal mesentery and is accompanied by the vitelline arteries and veins.

Intestinal Loop. At the level of the body stalk the intestine swings ventrally out into the body stalk and then back mto the body cavity again. The two limbs of this loop are in close proximity, and at its distal end the narrow yolk stalk, which communicates with the yolk sac, is attached.

Colon. After re-entering the body cavity the tube continues backward toward the cloaca.

Cloaca. An enlarged cavity into which the colon enters (Fig. 33).

Nora. This structure is of considerable importance and will be dealt with more fully when the nephric system is ccmi^red (page 86).

Allantoic stalk. A partially collapsed duct opening into the ventral side of the cloaca. Runs through the body stalk to communicate with the bladderlike allantois, which is usually removed before sectioning.

Mesodermal Derivatives

MESBNCHTME. The loose reticular tissue which fills spaces between developing organs throughout the body of the embryo. This material is of great importance in that it constitutes the forerunner of the various types of connective tissue. It is composed of cell bodies each containing a nucleus, and protoplasmic processes which extend from cell to cell forming a delicate reticulum, in the interstices of which a more or less fluid matrix is developed.

NOTOCHORD. A cylindrical rodlike structure forming the skeletal axis of the embryo. Circular in crosssection. Located ventral to the spinal cord. Extends from region of mesencephalon to tail.

SOMITES. In the 10-mm. pig embryo there are approximately forty-four somites. These have already undergone the early differentiation corresponding to that studied in the chick, and consequently in a typical somite, occurring about midway through the body, we may recognize:

Dermatome. A dense strip of cells situated lateral to the spinal cord just beneath the ectodermal epidermis.

Myotome. The proximal hooklike end of the strip of cells constituting the dermatome but bent under or ventral to it and hence located closer to the spinal cord.

Sclerotome. The main bulk of the somite consuting of more loosely arranged cells.

Myocoel. The cavity of the somite. Located beneath or ventral to the dermatome and usually obliterated.

Nephrotome. The intermediate mesoderm is already differentiated into the complicated mesonephros which will be considered under the excretory system.

Lateral Mesoderm. The lateral sheets of mesoderm are already differentiated into a somatic and splanchnic layer. Identify:

Somatic Layer. In close association with the ectoderm of the outer body wall.

SpUmchnic Layer. Covering the mesonephroi and also forming the mesentery supporting the gut and associated structures.

COELOM. In the 10-mm. pig the pleural cavities and pleural grooves are already present at the level of the lung buds. In the region of the cloaca the backward extensions of the coelom form a saddleshaped pelvic recess beneath the urogenital sinus of the cloaca.

Within the trunk a blind coelomic pocket has been formed in the mesentery owing to the shifting of the position oHhe stomach. As a result the dorsal mesentery is wrapped around the surface of the stomach, as it were, and so forms a blind pouch, the omental bursa. This communicates with the coelom by means of the epiploic foramen (foramen of Winslow).

PARTITIONING OF COELOM. The transverse partitioning of the coelom into thoracic and abdominal regions has begun in the 10-mm. pig with the development of the septum transversum, which forms a sort of semicircular shelf attached to the ventral body wall situated between the heart and liver.

MESENTERIES. During early embryonic development the splanchnic mesoderm envelops the gut. As a result the digestive tube is attached to the dorsal coelomic wall by the dorsal mesentery and to the ventral wall by the ventral mesentery. Only the dorsal mesentery persists, however, and Hie usual regions may be recognized:

Mesogaster — supporting the stomadi.

Mesodttodemem — supporting the duodenum.

Mesentery ptoper — supporting the main body of the gut.

Mesocolon — supporting the colon.

In addition the gastrohepatic omentum is a persisting porticm of the ventral mesentery joining the gut to the livier.



PBONEPH^. In the pig a transitory pronephros is found in the most anterior nephrotomes. It has already degenerated and been supers^ed 1^ the mesonephros at the 10-mm. stage.

MBSONEPHSoe. The mesonephroi which constitute the functional kidney of the 10-mm. pig are very conspicuous and almost disproportionately large in comparison with the rest of the body. They are located posterior to the liver and develop in behind the splanchnic mesoderm; hence they are said to be retroperitoneal in position. They are so Im'ge, however, that they protrude down into the coelom. Well-marked differentiar tion of parts is apparent at this stage, but it should be remembered that individual mesonephric tubules make up the entire structure. Select a mid-body section (Fig. 36) and identify:

Bowman's Capsule. The enlarged, double-wdled chamber situ ated at the terminal end of the tubule and containing a glomerulus. Usually located near the mid-line. These structures are usually quite irregular in shape.

Glomerulus. The knot of capillaries contained within the inner cup of the Bowman’s capsule. Irregular in shape, compact in appearance. Located near the mid-line.

Mesonephric Tubule. The tubules lead from Bowman’s capsule to the mesonephric duct. The tubules are hollow, curving tubes. Usually situated laterally.

Mesonephric Dtict. The common collecting duct leading back ward to the cloaca. Located on the ventrolateral border of the mesonephros. Thick-walled and prominent. Usually flattened oval in cross-section.

METANBPHROS. The metanephros has already begun to develop at this stage. It arises from two sources, as follows: The secretory portions of the tubules take their origin in the non-segmented nephrogenous tissue posterior to the mesonephros and the collecting portions and duct develop as evaginations from tfie mesonephric ducts. They may be recognized in the following manner after tracing the mesonephric ducts backward to the cloaca.

Ureter or Metanephric Duct. A small evagination from the meso nephric duct at its point of entrance into the cloaca. Heavywidled. Directed dorsally at first, then anteriorly. Often bifurcated several times to give the primordia of the collecting tubules of the first and second order. Its cavity represents the primary pelvis of the future kidney.

Primordvum or the Secretory Tubules. Heavy condensations of nephrogenous mesoderm forming a cap over the end of the ureter or the collecting tubes originating from it.


As a key poifit for this study, select a section well back toward the posterior end of the body^ which goes through the genital prominence and tail as shown in Fig. 33a,c. Beginning at the surface of the

Fw. 33. a. Lateral view of lO-mm. pig embryo showing location of the genital prominence, O.P. b, Reconstruction of the cloaca and related structures: Mes. duct, mesonephric duct; Met. duct, mctanepbric duct; Nepb. tissue, nephrogenous tissue; P.C.G., poet cloacal gut; Rect., rectum; U.G.S., urogenital sinus, c, Cross-aection at level x-v, Fig- 33a, showing closing plate of cloaca, C.P.

genital prominence and tracing deeper into the series, the following structures may be recognized:

Cloacal Plate. The closing plate of the cloaca formed by apposition of its lateral walls. In section it has the appearance of a heavy, median, streaklike thickening (Fig. 33C).

Rectum, When traced deeper the lateral walls of the cloaca gradually separate, forming a naiyow, elongate chamber. The dorsal portion of this constitutes the primordium of the rectum and it receives the colon. The terminal portion of the gut continues beyond the rectum as the post cloacal gut.

Urogenital Sinus. The ventral portion of the cloacal diamber becomes the urogenital sinus. Traced deeper it widens laterally, forming a broad, saddle-shaped cavity. It receives the allantoic stalk in the mid-ventral line, and, at its lateral an^es, the mesonephriaand metanephric ducts.

Note. The pelvic recess of the coelom extea^^beneath fhe urogenital emus and may be confused with it because in cross-se^n both are curved, arcMdi^ped cavities. However, the cavity of the coelom lies udfAtn the ate of the urogenital sinus and between it and the gut. ^


The gonads of the embryo are represented in the 10-mm. pig only by the merest rudiments in the form of thickened genital ridges on the ventromesial side of the mesonephros. These structures are located about midway back along the mesonephroi. Identify:

Germinal Epithelium. The irregular, thickened splanchnic meso derm forming the surface layer.

Primordial Germ Cells. Prominent cells with large vesicular nuclei situated in the tissue underlying the germinal epithelium. Gonoducts. Since the sex of the individual pig is quite uncertain at this stage, the gonoducts have not yet been differentiated. However, if the animal is to be a male, the mesonephric duct will eventually function as a ductus deferens.

External Genitalia. No differentiation of the external genitalia can be recognized, but the genital tubercle is the primordium of these structures.

Fts. 34. PyiT tf on tbrou^ the head of a 10-mm. pig at the level of origin of the internal carotid arteries.


At tihis stAge of development the circulatory system has become very complex, for the early stages of. deviriicvxnent corresponding to those of the 33- to 72-hour chick have already been completed. As ft ooxh^uence. it wyi be found h^ly desirable to work out the circulatory system in separate units:



In the pig, just as in other vertebrates, the aortic arches develop as a series of six pairs of blood vessels associated with the visceral arches. They also do not all develop at once and the ones formed first soon begin to disappear. As a result, at the 10-mm. stage, aortic arches III, IV, and VI alone persist. These constitute the basis on which the adult plan of the circulation is established. Consequently they should be carefully worked out in detail.

As a key point for tracing these blood vessels, select a section approximating level 3, Fig. 32, as illustrated in Fig. 34. Such a section is significant because it shows the junction of the radix of the aorta, the third aortic arch, and the internal carotid artery. The characteristic relation of these vessels is clearly shown in the figure (subject to slight modifications, of course) as well as the typical association with the nodose ganglion of the vagus nerve X. Trace out the courses of these vessels, as follows:

(a) Internal Carotid Artery. This vessel can be traced forward in the head to the side of the diencephalon. In this region it becomes broken up into a multiplicity of capillaries running out over the brain. The main channel, however, can be picked up again beneath the floor of the mesencephalon. Here the two vessels unite to form the basilar artery which continues backward beneath the floor of the brain as a somewhat uncertain channel to the beginning of the spinal cord. At this level the basilar flows into the paired vertebrals which continue backward, paired, to the level of the limb buds. Here they anastomose with the subclavian arteries but also continue backward and eventually fuse to form the spinal artery which flows beneath the spinal cord.

(b) Radix of Doreal Aorta and Third Aortic Arch. These vessels should be traced deeper into the animal. It will be found that' the paired radices maintain their original position in the section relative to the spinal cord, etc., and that the paired third aortic arch moves steadily away from the radix. The two third arches eventually meet in the mid-line at a point which represents their origin from the ventral aortal Near this level sharp watch should be kept on the radix aortae for the junction point of the fourth aortic arch.

t Note. This is the point at which the developing •thyroid gland is readily identified as a small caplike condeDBaliDn of endodennal tissue above the end of the ventral aorta.

(c) External Carotid Artery. At the level where the two third arches arise from the ventral aorta, the small external carotids are found coursing obliquely forward and outward into the mandibular arch.


(d) Fourth Aortic Arch. This vessel usually persists through only a very few sections, and often its origin from the ventral aorta and junction with the dorsal aorta can be seen in the same section. This gives it a characteristic U shape.

• (e) Sixth Aortic Arch. A few sections posterior to the level at

which the fourth arch can be identified the junction points of the sixth and dorsal aortae are encountered. When traced deeper these vessels are seen to arise not directly from the ventral aorta but rather from a separate trunk — ^the pulmonary trunk — which has been separated off from the ventral aorta. At this level it is usual to speak of the ventral aorta as the systemic trunk. Note the characteristic Y shape of the sixth arches at their point of separation from the pulmonary trunk.

(f) Pulmonary Arteries. These arteries which are quite small arise from a point about midway along the sixth aortic arch and may be traced deeper in the body toward the lung buds.


Bulbus Arteriosus. Follow the systemic and pulmonary trunks deeper to their point of origin from the bulbus arteriosus. This channel is very thick-walled and will be found to show a tendency toward division into two channels near the ventricular region of the heart. This is indicated by the presence of a ridge-like thickening on both its dorsal and ventral wall. The ridge is the beginning of the septum aorticopulmonale. The potential pulmonary trunk will be found to communicate with the right ventricle and the systemic trunk with the left ventricle.

The heart is already divided into atrial and ventricular regions and these in turn are incompletely partitioned into right and left halves, thus forming the beginnings of the adult four-chambered heart. As the bulbus arteriosus is traced backward in the series, the parts of the heart become clearly evident in association with it, as follows:

Atria. Thin-walled and characterized by a number of saccular projections on their upper lateral surfaces.

Sinus Venosus. Associa^ with the dorsal surface of the ri^t atrium and communicating with the heart by an opening guarded by a pair of thin valves which project into the atrium.

Interatrial Septum. The thm partition between the stria.

Foramen Ovale. The opening in the interatrial septum which allows communication between right and left atria.

AtrioventriculaT Canah. The passagewa 3 rB forming a communication between atrium and ventricle on each side. The tissues of the rim of the right canal wiU form the tricuspid valve, those of the left, the bicuspid valve.

Ventriclee. Thick-walled and characterized by the presence of a reticular syncytium of myoblasts in their walls.

Interventricular Septum. The thick partition between the ventricles.

Interventricular Foramen., The canal allowing communication between ri^t and left ventricles.

Endocardial Cushion. Thick masses of mesodermal tissue in the dorsal and ventral wall of the ventricle of the heart at the level of the atrioventricular canal.

3. ARTBRiEB OF THE BOOT. As the dorsal aorta courses backward in the body, a series of branches are given off, as follows:

Jntersegmental Arteries. Small paired vessels. Arise from the domolateral angles of the aorta. Run between the somites and to the neural tube.

Subclavian Arteries. Run out laterally into the anterior limb buds where they spread out as a capillary network.

Renal Arteries. Numerous small vessels arising laterally from the aorta. Supply the ^omeruli of the mesonephroi.

Coeliac Artery. Arises from ventral side of aorta at about the level of the stomach.

Superior Mesenteric Artery. Arises from ventral side of aorta dightly posterior to the coeliac artery. Runs out along the yolk stalk.

Allantoic or Umbilical Arteries. Lateral vessels arising from the aorta at the level of the posterior limb bud. Run out along the allantoic stalk into the umbilical cord.

Iliac Arteries. Branch off the allantoic arteries. Run out laterally into the posterior limb bud.

Caudal Arteries. Backward extension of the dorsal aorta into the tail.

Not?. It is desirable at this point to consider briefly the significance of the status of the aortic arches in the 10-mm. pig. The third aortic arches represent parts of the future internal carotid arteries. The dorsal aortae between third and fourth arches are obviously dwindling in size and importance and will eventually be eliminated. Of the paired fourth arches, the left side alone will persist while the right, along with the radix of the aorta or. that side, will disappear. The sixth arch on the right side will persist to the point of origin of the pulmonary artery, while the left wdll persist in its entirety until the time of birth, as the ductus arteriosus, thus maintaining a sort of “by-pass” to the dorsal aorta so that Uttle blood actually goes to the hmgs.


The venous system of the 10-mm. pig, like the arterial system, is already quite complicated, and it is desirable, therefore, to work it out in a similar manner by studying individual units. In the pig, just as in tile chick, the cardinal system constitutes the basic series of veins draining the body proper in the early embryo. Moreover, the vitelline veins bring back blood from the yolk sac, which, in the pig, is large and of conad^able importance as a nutritional organ. To these must be added the allantoic veins, which are very large and prominent, since they constitute the drainage from the placenta. Finally a new veasd, the poet caval vein, is in process of being established as one of the main drainage channek.


’ , afater^. cardinal veins are broad sinudike vessels which course backward along the sidra of ialelel to the otw vedde. At this level they are easily identified (Fig. 34). When traced forward toward the forebrain they soon break up into a capillary network anaatomosmg with branches of the internal ciurotid artery which cover the surface of the brain. These ramifications are difiScult to follow and give little information of importance. Traced posteriorly, however, they soon take up a position dorsal and lateral to the paired dorsal aortae which tiiey accompany posteriorly.

Note. (1 ) The typical close association of these vessels with the ganglia and fibers of the vagus nerve X.

(2) Intersegmental veins, draining independent areas of the side of the headj brain, and spinal cord, can be seen at regular intervals emptying into the cardinals.

(3) At the level of the visceral arches fairly large but uncertain vessels join the cardinals laterally and ventrally. These represent beginnings of the external jugulars.

(4) At the level of the limb buds a number of vessels draining the appendages join the anterior cardinals. From the most prominent of these the subclavian vein will develop.


The anterior cardinals ultimately trace backward to join the common cardinals'which fiow into the heart.

The right common cardinal will be seen to communicate directly with the right atrium of the heart by way of the sinus venosus. Note the prominent valves of the sinus venosus at the point of entry of the vein.

The left common cardinal behaves in a different manner. Instead of flowing directly into the heart, it will be found to narrow down greatly, assume a glose contact to the back wall of the left atrium, and tJien loop across to the right side (still closely applied to the heart), where it communicates with the postcaval.

Relation of Anterior Cardinals and Common Cardinals to Precaval, Postcaval, and Coronary Veins. The conditions encountered in the 10-mm. pig are highly significant, since they foreshadow the development of the precaval and coronary veins. The precaval vein represents a shift in drainage of the venous blood into the ri^t side of the. heart. Its main trunk, is formed from the right common cardinal vein plus a small portion of the proximal part of the ri^t anterior cardinal. The tributary duumeb on botb sides are the subclavian veins and the internal and external jugulars. Blood from the left inde the head ultimately will flow across to the ri^t by the develi^nMnt of a new ohaond, the ledft innondiate vein, which will form an anastomosis from the left to the ri^t (Fig. 35). As yet these develcpineiitte, are not definitely under way. However, the striking manner in which the left common cardinal is associated with the heart at this stage indicates that it is being taken over to drain the wall of the heart, for, along with small portions of the proximal ends of the left anterior and posterior cardinals, it thus becomes the coronary vein.

Fio. 35. Changes occurring in the cardinal veins of the pig during development. Left, primitive condition; Right, adult condition. ' Vessels added , vessels dropped


At this stage the pulmonary veins are too uncertain and too poorly developed to trace successfully.


There are four main venous channels to consider in working out the drainage of the posterior part of the body.

1. Posterior Cardinals. These are vessels of dwindling importance. They originally drain the dorsolateral portion of the body wall and adjoining organs and hence are located dorsal to the meso<

Fra. 8S. Section through midrbodjr of the lO-mm. pig.

ni^roa. Beginning at the point of junctiim with the common cardinal, they may be traced backward • ifei^eriiito liie series and will be found to maintain their original dorsolateral position with reference 19 thi9 floral aorta. * As they are followed backward, a level is finally reached where the anterior tip the mesonephros suddenly appears lying amMKi the vessel. The cardinal is thus divided into a dorsal posterior cardinal and a ventral subcardinal, each of which receives munerous tributaries from the mesonephros. The posterior cardinals from this point back gradually dwindle Mid may even be obliterated for a time. Farther back in the body they may be picked up once more, again in a position dorsal to the mesonephros, and from this point they may be traced back into the tail.

2. Hepatic-Portal System. The hepatic-portal system is derived from the paired vitelline veins of the younger embryo. These vessels originally course inward along the lateral walls of the gut from the yolk sac to the heart.

With the increase in size of the developing liver primordium, which arises originally as a ventral diverticulum from the gut, the closely associated vitelline vessels become broken up into a series of capillary channels running through the liver. By the time the 10-mm. stage is reached the liver has become a large and prominent organ; modification of the vitelline vessels is under way; and the beginnings of the hepatic-portal system are apparent.

Posterior to the liver a series of transverse anastomoses, together with the occlusion of intervening portions of the original channels, transforms the vitelline veins into a vessel tending to spiral around the intestine. It will be found that the right vitelline forms the upper or anterior end of this spiral and that at this point the vessel, now the portal, suddenly moves across from the gut into the liver (Fig. 37).

1^0. 37. Changes occurring in the vitelline and umbilical veins of the pig during development. Left, primitive condition shovmg relation of vessels to liver; Right, condition at birUt: vitelline veins give rise to the hepatic portal system, umbilioab,

to the ductus venoeus which is occluded after birth. Vessels added — ,

vessels dropped

In Studying out this system it is best to begin with a section taken throu^ the levdi at whkh this occurs. The condition is shown with almost diagrammatic exactness in such a section i» F^. 36. From this point Ihe vessels should be traced forward and then backward in the series. The blood: Iron: the portal is distributed by capillary sinusoids throujdiout the liver and is picked up vergence of another group of sinusoids in amain vessel represMiting the upper end

This is the hepatic vein, and it usually joins the develqjm^ postesvsil vein hhlbiKC

heart. The proximal portion of the left vitelline vein contributes little save a few capillary sinusoids in the liver.

3. UmbUicdl Veins (transitory). The blood distributed by the umbilical arteries to the allantois is returned to the heart by the umbilical veins. These vessels course inward along the body stalk as a pair of broad venous channels. At the level of which the umbilical cord joins the ventral surface of the body, they move into the adjacent liver and become invdved with it as a series of capillary sinusoids in a manner recalling the condition affecting the vitelline veins (Fig. 37).

It is evident in the 10-mm. stage that the right umbilical vein is losing its importance and dwindling in size. The left, on the other hand, becomes large and prominent and takes over a series of capillary sinusoids in the liver to make a direct path across the liver from left to right toward the heart. This shortcut through the liver is the * ductus venosus. It usually joins the postcaval vein before entering the heart.

In working out these vessels a section at a level approximating Fig. 36 should be chosen as a starting point and the umbilical veins traced forward to the heart and back into the umbilical cord where they are fused into a single vessel.

• 4. Postcaval Vein. In the adult animal most of the blood is returned to the heart by way of the postcavel vein. This vessel has already begun to establirfi itself in the lO-mm. pig. The greatest impetus to its development seems to be the fact that blood from the mesonephros requires a more direct passage to the heart than the circuit through the original posterior cardinals. As a result a pair of somewhat uncer

Fio. 88. Development of the postcaval vein in the pig. Left, subcardinnls added to drainage of mesonephroi in 10-mm. pig, in addition to original postcardinais; Ru/ht, fusion of eubcardinals to form postcaval. Vessels added- vessels dropped

tain channels, ventral and medial to the mesonephros, first appear (Fig. 38). These are the subcardinals. They receive tributaries from the mesonephros but empty into the upper ends of the posterior cardinals. At the 10-mm. stage these upper vessels connecting with the posterior cardinals are still both present but dwindling in importance, and the two subcardinals have begun to join across the mid-line by a series of anastomoses. This establishes whit amounts to a singde vessel which can be traced forward into the heart by way of a new addition to its anterior end. The whole channel is the developing postcaval vein. It should be noted Ibat other vessels also contribute to the development of the postcaval but at this stage they cannot be clearly demonstrated.

In order to work out the postcaval vem and its relationships, it is convenient to start with a section about midway back through the body, such as Fig. 36, and work forward and back in the series.

5. Mesonephric Veins. Small vessels arising from the mesonephros which communicate with the postcaval vein.

6. Intersegmentdl Veins. Small vessels draining the dorsolateral body wall and flowing into the posterior cardinals.