Human Embryology and Morphology 20

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures

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Chapter XX. Circulatory System

Early Stages in the Evolution of the Heart

In Ammocoetes, the larval form of the lamprey, is represented the most primitive form of heart in vertebrate animals. Even in this early type the heart consists of four chambers (Fig. 311) : (1) Sinug venosus, receiving the portal blood through the liver ; (2) auricle ; (3) ventricle ; (4) bulbus cordis, from which the primitive ventral aorta passes out to distribute the blood in the branchial chamber. The primitive heart is thus a respiratory pump which forces the portal blood through a branchial system. It is clear, then, that the early evolutionary stages of the heart must be sought for amongst invertebrate forms, but these stages are as yet unknown. When the heart appears in the human embryo towards the end of the 3rd week, it is double — consisting of a right and left cardiac tube. We therefore suppose that originally there were right and left hearts, which arose as modifications of the vessels which convey the blood from the alimentary to the respiratory systems. In Fig. 312 the left side of such a primitive circulation is represented. The left heart forces the blood along a primitive dorsal aorta to the capillary system of the archenteron. An afferent (primitive portal) vessel conveys the blood back to the heart. When the head and tail folds are produced in the embryonic plate at the beginning of the 4th week (see Fig. 312), the right and left cardiac tubes are thrust under the fore-gut, where they speedily become fused into a median heart. 1 In its origin the heart is thus made up of symmetrical halves derived from the corresponding sides of the body. When formed, the heart is suspended within the anterior part of the coelomic space — which becomes the cavity of the pericardium. In Ammocoetes the pericardial and peritoneal cavities are continuous (Fig. 311).

Angioblastic Tissue

That the cardiac tube has arisen by the modification of a blood vessel is apparent by the way it commences to form in the human embryo. Late in the 3rd week certain cells become grouped under the fore-gut to form the lining membrane of the heart. At the same date similar cells in the chorionic villi, in the wall of the yolk sac and along the tracks of the future aortae, are grouping themselves in an identical manner to form the lumina of blood channels. The mesodermal cells which have this vessel-forming power pervade the whole embryonic


Fig. 312. Diagram showing the Relationship of the Heart to the Archenteron of the Developing Ovum. The outgrowth of the head fold is indicated carrying a process (fore-gut) of the Archenteron and also the Aorta and Heart. The outgrowth of the tail fold and hind-gut is also shown. (After A. Robinson.)

mass and are known as angioblastic tissue. One group of angioblasts unites with neighbouring groups, thus forming a network. Further, angioblasts not only form the lining cells and lumina of blood vessels but also produce the blood cells and plasma which fill them. A " blood island " is a group of angioblasts surrounding a brood of nucleated red corpuscles. When neighbouring islands unite the essential part of the circulatory system has come into existence. The lining of the heart arises in the same manner as a simple capillary.^

Later Stages in the Evolution of the Heart

So long as the heart is merely a pump for the gills, it retains the simple structure seen in Ammocoetes — but with the origin of a pulmonary system a series of most remarkable changes occur. The pulmonary system in the human embryo takes on its definite form during the second montli ; at the same time the heart is undergoing a series of changes, which converts it into a double pump, one for the lungs, another for the body. We know that these evolutionary changes occurred slowly, for in amphibia the heart has reached that point in evolution where a single ventricle can serve both the respiratory and systemic circulations. The evolution of a pulmonary system also led to a series of changes in the arrangements of veins. Amongst the most remarkable of these is the formation of a new passage, by which the blood of the abdomen can pass direct to the heart — the inferior vena cava. In the human embryo of the 5th week the heart and great veins are arranged as in a fish ; in the 7th week they take on the definite mammalian form.

1 Recent papers on fusion of cardiac tubes : Chung-Ching Wang, Journ. Anat. 1918, vol. 52, p. 107 ; H. W. Schulte, Amer. Journ. Anat. 1916, vol. 20, p. 45 ; Henry A. Murray, Amer. Journ. Anat. 1919-20, vol. 26, p. 29.

^ For recent papers on angioblasts : J. L. Bremer, Amer. Journ. Anat. 1914, vol. 16, p. 447 ; Florence R. Sabin, Contrib. to Embryology, 1920, vol. 9, p. 213.

Fixation of the Heart

At the beginning of the 4th week (Fig. 279) the heart lies free within the pericardium, with its two extremities fixed to the wall of that cavity (Figs. 330, 333). Its anterior or arterial extremity perforates the dorsal wall of the pericardium to give of! the aortic arches in the floor of the pharynx (Fig. 248). The venous or posterior end is fixed to the septum transversum, the embryonic partition, which is formed between the pericardial and peritoneal cavities (Fig. 279). The fate of the aortic arches, which convey the blood from the ventral to the dorsal aorta, has been already traced (p. 251). We now propose, before surveying the complicated changes which ensue in the heart itself, to trace the evolution of those great venous channels which convey the blood to the heart — the venae cavae.


Fig. 313. The Superior Vena Cava of the Adult.


Fig. 314. The Embryonic Venous Trunks out of which the Superior Vena Cava is formed. The arrow is in the communication between the pericardial and pleuro-peritoneal cavities. (See Fig. 315.)

1. The superior vena cava arises from the following foetal vessels (Figs. 313, 314) :

(a) The part above the entrance of the vena azygos is the terminal part of the right anterior cardinal (primitive jugular) vein ;

(6) The part below the entrance of the vena azygos major represents the right duct of Cuvier. The condition of these venous trunks, the anterior and posterior cardinal veins and ducts of Cuvier, in a human embryo of the 4th week, is shown in Figs. 314, 315. The condition shown in these figures is retained permanently in fishes.

The anterior cardinal, which drains the anterior half of the body on each side, with the posterior cardinal vein, which drains the posterior half of the body, receive a tributary (segmental vein) from each body segment. In Fig. 315 the anterior and posterior cardinal veins on each side are shown uniting to form the duct of Cuvier, which conveys the blood to the sinus venosus — a contractile chamber opening into the primitive auricle. The sinus venosus remains as a separate chamber of the heart in lower vertebrates, but in the course of mammalian development it becomes partly merged in the right auricle of the heart.

It is important to notice how the ducts of Cuvier reach the sinus venosus (see Figs. 315, 330, 280). They pass from the dorsal to the ventral surface of the body in the somatopleure or body wall, and enter the septum transversum to reach the sinus venosus, thus encircling the coelomic passages passing from the pericardial to the pleuro-periteal cavities. Thus the exit from the pericardial cavity to the pleural passage is surrounded by the great venous channels — the ducts of Cuvier ; hence the exit is sometimes named the iter venosum or pericardio-pleural passage. Ultimately, by the end of the 6th week, the part of the coelom lying in front of the ducts of Cuvier and septum transversum is cut off from the rest ; the part so cut ofi forms the pericardium. In the 4th week the dorsal end of the septum transversum is situated opposite to the 2nd cervical segment ; by the end of the 6th week, the embryo being about 10 mm. long, it has shifted backwards so as to lie on a level with the 3rd thoracic segment, in this way bringing the duct of Cuvier into an oblique position (Fig. 330). Thus the ducts of Cuvier are instrumental in separating the pericardial from the pleural cavity. If the primitive pleuro-pericardial communication (iter venosum of Lockwood) persists between them, it occurs as a foramen in the pericardium behind the part of the superior vena cava derived from the duct of Cuvier^ On the left side the duct of Cuvier atrophies, and the iter venosum^ if it persists, is then represented by an aperture in the pericardium in front of the root of the left lung (Fig; 316). The ducts of Cuvier, and the folds of the somatopleure in which they liCj are separated from the body wall and buried deep in the thorax by the development of the lungs and pleurae.


Fig. 315. Diagram to show the manner in which the Ducts of Cuvier encircle the Coelom at the junction of the Pericardial and Peritoneal Passages at the 4th week. (After His.)

2. The Vestigial Fold and Oblique Vein of Marshall.— In the human embryo, during the 4th week, and for two weeks afterwards, there is a right and left duct of Cuvier and corresponding cardinal veins (Fig. 319). A left superior vena cava is present and may persist (Fig. 317). The vestigial fold and oblique vein of Marshall (Fig. 318) are all that usually remain of the left superior vena cava. The right superior vena cava.


Fig. 316. Heart of a Child, showing an Abnormal Aperture in the Pericardium in front of the root of the Left Lung, representing a patent Iter Venosum or Pericardio-pleural passage of the Embryo. The left auricle is seen within the aperture.


Fig. 317. Abnormal Heart of a Child seen from behind, showing Persistence of the Left Duct of Cuvier, absence of the Inferior Vena Cava, and with the Pulmonary Veins terminating in the Sinus Venosus. A similar condition is seen in certain fishes.

within the pericardium, passes in front of the right pulmonary vessels, and is bound to them by a mesentery or fold of serous pericardium ; the left has a similar relationship (Fig. 318) ; when it disappears the pericardial reflection remains in front of the left pulmonary vessels as the vestigial fold. The intra-pericardial part of the left vena cava or duct of Cuvier becomes the oblique vein : it turns round the left auricle to terminate in the left horn of the sinus venosus (coronary sinus). The extrapericardial part of the left duct of Cuvier joins the superior intercostal vein (Fig. 318). Both right and left superior venae cavae persist in some lower mammals, and occasionally this is also the case in man. The left superior vena cava begins to atrophy when the common auricular chamber is divided into a right and left compartment in the 6th and 7th weeks.

The left superior intercostal vein represents the following embryonic vessels (see Fig. 318) : (a) Anterior part of the left posterior cardinal vein ; (6) The extra-pericardial part of the left duct of Cuvier ; (c) The terminal part of the left primitive jugular vein.

3. Left Innominate Vein opens up as a channel of communication between the two primitive jugular veins, the left superior vena cava undergoing a simultaneous process of atrophy (Fig. 318).

4. Subclavian Veins are developed in the 5th week with the outgrowth of the fore-limb buds ; they are developed from the vein of the 7th cervical segment and at first end in the posterior cardinal vein. As the neck and thorax become demarcated in the 2nd month, the termination of the subclavian veins is shifted until it ends in the anterior cardinal (primitive jugular) vein.

5. Jugular and Cerebral Veins. — In the 6th week each anterior cardinal vein commences in a corresponding primitive head vein (see page 133). Each primitive head vein passes along the base of the skull, receiving the veins from the fore-, mid- and hind-brains, and makes its exit by the jugular foramen, where it becomes the jugular or anterior cardinal vein.


Fig. 318. The Remnants of the Left Superior Vena Cava, derived from the Structures shown in Fig. 69.


Fig. 319. Diagram of the Sinus Venosus and Ducts of Cuvier of the Human Embryo about the 4th weeks.

Posterior Cardinal Veins and their Derivatives

In Figs. 320, 321 a schematic representation of the origin of the inferior vena cava and azygos veins is given. In the 5th week the posterior cardinals commence by the union of the veins from the limb buds and sacrum and passing forwards, dorsal to the developing Wolffian ridge, receive as they go a tributary from each somite — tributaries which will become the lumbar, intercostal and lower cervical veins, and end in the veins of Cuvier. With the development of the nephric or Wolffian system, a large tributary — the subcardinal vein — appears on the mesial side of the system or body, collecting blood from and pouring it into the posterior cardinal vein at the cephalic end. ol the nephric body. Ultimately the subcardinal veins extend their origin in a caudal direction and effect a connnunication with the hinder part of the posterior cardinal veins (Fig. 320). There is thus established a reno-portal system comparable to that seen in amphibia (Fig. 322). A wide cross channel (intern ephric) opens between the subcardinals.

^ Florence Sabin, Gontrib. Embryology, 1915, vol. 3, p. 5 ; Huntington and M'Clure, Anat. Record. 1908, vol. 1, p. 36; ibid. 1920, vol. 20, p. 1.

From the right posterior cardinal vein are formed (1) the vena azygos major ; (2) the ascending lumbar vein.


Fig. 320. Scheme of the arrangement of body veins about the end of the 6th week of development. The sites of new channels are stippled.


Fig. 321. Scheme showing the derivation of the body veins of the adult.

From the left cardinal arise (Figs. 320, 321) — (1) Part of the left superior intercostal vein ; (2) left superior azygos vein ; (3) left inferior azygos ; (4) the left ascending lumbar vein.

Inferior Vena Cava

The transformation of the cardinal system and the development of a new caval channel to convey the blood in the systemic veins of the abdomen direct to the heart, take place during the 2nd month as the pulmonary system begins to expand. With the evolution of lungs, respiratory movements of the body wall were introduced — a new force which was utilized to assist the return of the venous blood to the heart. The development of pleural cavities made the old or cardinal route circuitous and difficult and hence a new or direct passage became necessary — the inferior vena cava. It became fashioned thus ; a retrohepatic anastomosis between the right subcardinal and terminal part of the right vitelline vein (Fig. 320) opened up and thus the blood of the subcardinal system could pass straight to the heart. The pre-renal or retrohepatic part of the inferior vena cava is formed out of this new channel. The post-renal part is formed from the right subcardinal vein. A cross channel (presacral) opens up between the right and left cardinal systems — forming the greater part of the left common iliac vein — and so all the blood from the pelvis and pelvic limbs passes to the right subcardinal as it becomes converted into a permanent channel. The left subcardinal ^ vein persists not unfrequently — giving rise to a double or divided inferior vena cava. The internephric channel becomes the left renal vein, the terminal parts of both subcardinal veins persist as communications between the renal and azygos veins (Fig. 321). From Figs. 320, 321, it will be seen that part of the common iliac veins are derived from the hinder part of the cardinal system of veins.


Fig. 322. The Arrangement of the Cardinal, Umbilical and Inferior Caval Veins in Lower Vertebrates. The venous blood from the posterior part of the body passes through either the renal or hepatic circulations before reaching the heart. (After Hochstetter.)

Portal Vein

The Portal Vein is formed out of the terminal parts of the two vitelline veins. They end in the posterior chamber of the tubular heart of the embryo — the sinus venosus. The vitelline veins, right and left, arise from ramifications on the yolk sac and pass in the ventral mesentery of the fore-gut to the sinus venosus (Fig. 323). The nutriment within the yolk sac is thus carried to the heart and distributed by the heart to the tissues of the embryo and yolk sac. With the differentiation of the gut from the yolk sac, the parts of the vitelline veins, at first situated on the yolk sac, fuse together in the dorsal mesentery. Thus while the terminal parts of the vitelline veins lie in the ventral mesentery of the fore-gut, the three tributaries of the portal vein — the splenic vein from the fore-gut, the inferior mesenteric from the hind-gut, and the superior mesenteric from the mid-gut (Fig. 323) — lie in the dorsal mesentery. They are developed as tributaries of the vitelline veins, for we have already seen that the veins of the yolk sac may persist as a cord which joins the superior mesenteric vein below the pancreas (see Fig. 301). The duodenum forms a loo]:) between the vitelline veins (Fig. 324), and hence on either side of the 1st and 3rd stages of the duodenum the vitelline veins remain separate, while in front, between and behind these stages, they are united by anterior, middle and posterior junctions (see Fig. 321:).

1 For literature and description of cases of abnormal development of the posterior cardinal veins, see Dr. Gladstone's article in Journ. Anat. and Physiol. 1912, vol. 46, p. 220 ; J. Cameron, ibid. 1911, vol. 45, p. 416 ; T. B. Johnston, ibid. 1913, vol. 47, p. 235 ; H. Rischbieth, ibid. 1914, vol. 48, p. 290 ; W. E. Collinge, ibid. 1916, vol. 50, p. 235.


Fig. 323. The Left Vitelline Vein of an Embryo of the 5th week.

The portal sinus in the transverse fissure of the liver is formed out of the anterior junction of the right and left vitelline veins in the ventral mesentery (Figs. 324, 282) ; the part of the portal vein in the gastrohepatic omentum (ventral mesentery), and behind the 1st stage of the duodenum, is formed from the right vitelline vein ; the corresponding part of the left vein disappears ; the commencement of the portal vein — in the neck of the pancreas — represents the middle junction of the two vitelline veins (Fig. 324) ; the terminal part of the superior mesenteric vein, which in the adult lies in front of the 3rd stage of the duodenum, represents a part of the left vitelline vein — ^the corresponding part of the right disappears (Fig. 283). To understand the transmutation which leads to the formation of the portal vein, it must be remembered (1) that the duodenum forms at first a free loop, the right surface of which afterwards becomes applied to the posterior wall of the abdomen ; (2) the pancreas is developed in its dorsal mesentery ; (3) the ventral mesentery, in which the liver is developed, is attached to the anterior part of the loop (Fig. 323).

Hepatic Veins are formed out of the terminal parts of the vitelline veins. These veins end at first in the sinus venosus (Figs. 282, 283, 324). The liver is developed between and around their terminal parts (see p. 273). Thus it comes about that the vitelline veins are transformed into the veins of the portal and hepatic circulation. All the foetal and umbilical blood is at first poured through the liver.


Fig. 324. Diagram showing the Formation of the Ductus Venosus, and the fate of the Umbilical and VitelKne Veins. The arrows show the parts of the vitelline veins which become the portal vein.

Ductus Venosus is a new channel formed in the 5th week between the portal sinus and the terminal part of the right vitelline vein, whereby the greater part of the umbilical blood is short-circuited to the sinus venosus without passing through the liver. After birth, when a short circuit is no longer required between the placental circulation and heart, it becomes reduced to a fibrous cord.^ It occupies the posterior part of the longitudinal fissure of the liver and lies within the hepatic attachment of the gastro-hepatic omentum (Fig. 325).

Umbilical Veins

The umbilical vein at birth consists of two parts : (1) A part within the umbilical cord ; (2) another within the body, enclosed in the falciform ligament and anterior half of the longitudinal fissure of the liver. It joins there the ductus venosus and portal sinus (Fig. 32.5). The arrangement of the umbilical veins in a human embryo of the 3rd week is shown in Fig. 25, and of the 5th week in Fig. 326. The vessel from which the umbilical veins have been evolved — the lateral vein of lower vertebrates — is illustrated in Figs. 27 and 322. In the body stalk the umbilical veins have already fused (Fig. 326), but in the body wall and ventral mesentery, in which they pass to reach the sinus venosus, they remain separate. With the differentiation and closure of the umbilicus, the parts of the body wall in which the umbilical veins are situated are drawn out to form the umbilical cord. The intra-embryonic parts then lie within the ventral mesentery of the fore-gut, lateral and ventral, to the vitelline veins. By the umbilical veins the blood is returned from the placenta to the heart. In nearly all vertebrate embryos the vitelline veins are the first of all the vessels of the body to be developed, but in the Higher Primates, including man, this appears not to be the case. Professor Eternod found that in a human embryo, of about 21 days, the umbilical veins and the venous sinuses of the chorion were already in process of formation, while the vitelline veins had not yet appeared (Fig. 25). We have already seen (Chap. II.) that the Higher Primates are remarkable for the precocious development of the chorion ; this early differentiation of the chorion is attended by an equally early formation of the umbilical vessels, which return the blood from the chorion to the heart.

1 See Scammon and Norris, Anat. Bee. 1918, vol. 15, p. 165,


Fig. 325. Diagram of the Remnants of the Umbilical Vein in the Adult — as seen on the dorso-ventral sm'face of the liver.

The outgrowth of the liver-bud within the ventral mesentery breaks up not only the vitelline veins, but also the umbilical at their junction with the sinus venosus (Figs. 282, 283). The intra-embryonic part of the right umbilical vein atrophies, while the left enlarges. With the terminal parts of the vitelline veins the opposite is the case. Thus the umbilical blood as well as the vitelline comes to be poured into the liver. The termination of the left umbilical vein is gradually transferred during the 6th and 7th weeks from the sinus venosus to the portal sinus (p. 273). The left umbilical vein thus comes into communication with the ductus venosus (see Figs. 324, 325).

The Heart as a Placental Pump

Having thus traced the origin of the great veins which conduct the blood to the heart we now turn to the development of this organ. In the 4th and 5th weeks the umbilical veins are fully established (Fig. 326) and the heart is receiving the major part of its blood from the chorion, and its chief task is to serve as the pump of that organ. Hence the large size of the heart and pericardium when compared with the actual dimensions of the embryo itself (Fig. 326) — or the individual organs such as the stomach. Angioblastic cells are being transformed into vascular structures at the end of the 3rd and beginning of the 4th weeks, and although vascularization proceeds at an extremely rapid pace, it is late in the 4th week before an effective circulation has been established.


Fig. 326. Diagram of the Right Umbilical Vein of a 5th week: Embryo before the Outgrowth of the Liver Trabeculae. (Modified from His.)

Cardiac Tubes and Pericardium

In Fig. 327 is shown a coronal section of the forward projection of the head region of a human embryo in which the neural canal is still open and in which only five body segments are demarcated — about the beginning of the 4th week. The cardiac tubes are seen in process of fusion. Under the fore-gut is seen the angioblastic cells — representing the endothelial lining of the heart ; the walls of the tubes clearly represent foldings of the visceral layer of the mesoderm — for they are seen to be still continuous with the mesodermal covering of the fore-gut. The pericardial part of the coelomic space is already formed. It came into existence during the latter part of the 3rd week — by a process of cleavage which separated the mesoderm lying under the fore-gut into visceral and somatic layers. While the heart tubes are separated from the somatic or parietal layer of mesoderm, they remain attached to the floor of the fore-gut by the dorsal mesocardium. No ventral mesocardium is formed. Sections showing the evolutionary origin of the pericardium and of the mesodermal wall of the heart are shown in Figs. 149, 327 and 352.

In Fig. 328 a corresponding section of an embryo a few days older is represented. The process of fusion is complete and already the cardiac tube has become elongated and bent so that it is laid open in the section at two places — near where it enters the floor of the fore-gut, to which it is bound by the dorsal mesocardium — and across the segment which will become the ventricles. The angioblastic mesenchyme now forms the endothelial lining of a narrow cardiac lumen ; the outer wall — derived from the visceral mesoderm — represents the muscular and epicardial strata, but as yet, although its cells are contractile, they are still in a premuscle state. Between endothelial and mesodermal strata is interposed a thick subendothelial reticulum. Into this subendothelial tissue the myogenic cells wfll proliferate and establish a myocardial sponge-work. The spaces of the reticulum are laden with fluid ; there is, then, at this time, under the myocardial wall, a fluid subendothelial cushion.


Fig. 327. Coronal section of Pericardial Region of a Human Embryo with 6 somites — beginning of 4th week. (After Tandler.)


Fig. 328. Coronal section of Pericardial Region of a Human Embryo with 15 somites — end of 4th weelk. (After Tandler.)

Arterial and Venous Mesocardia

The manner in which the tubular cardiac pump is fixed to the wall of the pericardium in a human embryo in the 4th week of development is shown in Fig. 329. The myocardial wall has been stripped off, showing the endothelial lining of the tube. The heart is fixed at two points only — behind at the place where its first chamber, the sinus venosus, is embedded in the septum transversum — and in front, where its terminal segment, the truncus arteriosus, perforates the roof of the pericardium to enter the wall of the pharynx. At these two points of attachment the epicardial covering of the cardiac tube becomes continuous with the lining membrane of the pericardial cavity ; the posterior reflection, on the sinus venosus, is the venous mesocardium, the anterior, enclosing the truncus, is the arterial mesocardium. The rest of the heart is free within its bursa — the pericardial cavity. For a brief interval there is a dorsal mesocardium, but by the middle of the 4th week only a trace remains on the dorsal wall of the pericardium between the two points of attachment (Fig. 329). At no time is there a ventral mesocardium. The iter venosum leading from the pericardial to the pleuro-peritoneal cavity is still open ; the cardiac tube has grown in length and assumed certain definite bends and twists.


Fig. 329. The Attachments of the Cardiac Tube — merely its lining membrane is depicted — in a Human Embyro 2"1 mm. long and in the 4th week of development. After Wilhelm His (1831-1904)


Fig. 330. The Attachments of the Heart in a Human Embryo 4.2 mm long and in the 5th week of development. As in the preceding figure, only the endothelial lining is represented. After Wilhelm His (1831-1904)

A week later, as shown in Fig. 330, the arterial mesocardium has shifted backwards along the roof of the pericardium and become approximated to the venous mesocardium. There have also been changes in the hinder attachment, for the septum transversum, which is also migrating backwards, has taken up a more oblique position, being now partly on the dorsal wall. The iter venosum, which is reduced in size, is now crossed by the vein or duct of Cuvier, in a slanting direction. By the 3rd month the mesocardia have approximated and the heart has become fixed in its final position (Fig. 354).

Bends, Twists and Primary Chambers

In the previous paragraph we have seen how the arterial and venous niesocardia become approximated, thus bringing together the ends of the original simple cardiac tube. We are now to see that a similar process takes place in the cardiac tube itself, whereby its auricular (atrial) segment is brought in contact with its terminal or aortic segment. The bends, twists and evaginations of the cardiac tube are easily understood if the reader keeps in mind the manner in which the curvatures of the stomach are produced — namely, by unequal growth. The greater curvature of that organ is due not only to its growth being more rapid than that of the lesser curvature but also to the localized expansion or evagination of the fundus. In some animals there is an actual reduction — an absorption — of the lesser curvature which brings the pylorus in contact with the oesophagus. The bends, twists and evaginations of the cardiac tube are produced in a similar manner ; they are expressions of asymmetrical growth leading up to the stage reached in the fully developed heart.


Fig. 331. The Primitive Divisions of the Embryonic Heart.

In Fig. 331 the embryonic heart, early in the 4th week of development, is seen on its ventral aspect and already the primitive ventricular segment of the tube shows a greater curvature towards the right and a sharply bent lesser curvature towards the left. These curvatures are better shown in Figs. 329 and 330 ; the ends of the primitive ventricular segment are being approximated. The limb of the ventricular loop nearest the beginning of the heart — the proximal limb — will give rise to the 3rd or ventricular chamber of the heart ; the distal limb will produce the 4th chamber of the heart — the bulbus cordis. Besides the ventricular, there is another important curvature at the junction of the auricular with the ventricular segment. The lesser curvature — the sharp angle — of this auriculo-ventricular bend is on the right and ventral aspect of the tube (Fig, 331). The 2nd chamber of the heart — the auricular or atrial — is scarcely marked in the early part of the 4th week (Figs. 331, 329), but by the 5th week evaginations are produced on its dorsal side — at the side opposite to the auriculoventricular bend (Figs. 330, 332). The sinus venosus or 1st chamber of the heart is partly embedded in the septum transversum in the 4th week, while the truncus arteriosus or 5th segment of the cardiac tube, which is elongated in the 4th week, is greatly shortened by the 5th (Fig. 329). Further, it will be observed that as early as the 4th week (Fig. 329) there are two constricted segments in the endothelial lining of the cardiac tube — one between the auricular and ventricular segments — the auricular canal, and one between the bulbus and truncus — the bulbar canal. All of these five parts of the cardiac tube are to be seen in the heart of a fish (Fig. 332) such as the shark. The sinus venosus serves as a blood reservoir ; the auricle acts as a pump to feed the ventricles, the ventricle is the pump of the gills and body ; the bulbus, which becomes incorporated in the right ventricle of the mammalian heart, feeds the gills in diastole, the truncus serves purely as a canal.


Fig. 332. Diagram of the five Segments of the Primitive Cardiac Tube. I. The venous segment. II. The auricular segment ; on its dorsal aspect the auricle proper is developed ; the venous valves are shown between the venous and auricular segments. III. The ventricular segment, the ventricle proper being developed from its ventral aspect. IV. The bulbus segment. V. the truncus, conus or aortic segment. It is separated from the last by the aortic and pulmonary valves.

The Sinus Venosus

The sinus venosus, the first chamber of the foetal heart, is formed by the union of the vitelline veins ; the umbilical veins and ducts of Cuvier come subsequently to open in it (Fig. 333). The ducts of Cuvier reach it from the somatopleure by passing round the coelomic passages (Figs. 329, 330) and entering the septum transversum. In fish and in the human embryo the sinus serves as a reservoir during systole of the auricle ; the systolic wave always commences in the sinus venosus. The right and left venous valves (Fig. 335) at the juncture of the sinus and auricle prevent the regurgitation of blood during systole of the auricle. These valves become more or less atrophied when the right and left sides of the heart are completely separated by the formation of septa.

Fate of the Sinus Venosus

(Fig. 334). — Since the sinus venosus plays such a dominant part in the physiology of the heart of lower vertebrates, it is extremely important that we should follow its fate in the human heart. It becomes submerged chiefly in the right auricle, the sulcus terminalis (see Fig. 337), marking the line at which it became included by the upgrowth of auricular tissue. Already, at the end of the 5th week, its orifice has come to occupy a position in the posterior or dorsal wall of the right part of the common auricle (Fig. 335). The part which it forms of the right auricle is indicated by the entrance of the following vessels which primarily terminate in the sinus : (1) The superior vena cava (the right duct of Cuvier) ; (2) The inferior vena cava, which also opens into the sinus ; (3) The oblique vein of Marshall (left duct of Cuvier), which opens into the left horn of the sinus venosus. The left horn of the sinus becomes the coronary sinus. The sulcus terminalis is marked on the interior of the right auricle by a strong muscular band (taenia terminalis), which runs down on the anterior wall of the right auricle from the superior to the inferior vena cava, and indicates the junction of the prinutive auricle with the sinus venosus (Fig. 336). The musculature which surrounds the terminal part of the superior vena cava, and that contained in the wall of the coronary sinus, represents the musculature of the sinus. Elsewhere the nmscle of the sinus appears to be replaced by that of the auricle.


Fig. 333. Showing the two chief Bends which occur in the Heart during the 4th week.

The Valves of the Sinus Venosus

Right and left lateral valves (venous valves) guard the entrance of the sinus to the primitive auricle and prevent the regurgitation of blood when the auricle contracts (Fig. 335). The valves meet above and form a superior fornix in front of the superior caval opening ; they meet below in an inferior fornix, which, owing to the great shortening of the ventral part of the auricular segment, reaches the base of the ventricle, and actually fuses with the posterior endocardial cushion (Fig. 342). This has an important bearing on the origin of the auriculoventricular (A.V.) bundle within the auricular canal. Along the base of each valve is arranged a band or taenia of the auricular musculature. Thus each valve consists of a membranous marginal part and a muscular basal part. The right valve in the adult heart becomes (Fig. 336) (1) the Thebesian and (2) Eustachian valves ; (3) the musculature at its base forms the taenia terminalis. The left valve becomes (1) a fretted membrane on the septal margin of the inferior caval orifice, (2) a band of musculature accompanying this remnant (Fig. 336).


Fig. 334. Showing the part of the Right Auricle formed from the Sinus Venosus.


Fig. 335. Section of the Heart of a 6th weelc Human Embryo showing the Right and Left Venous Valves which guard the Entrance of the Sinus Venosus into the Primitive Auricle. After Wilhelm His (1831-1904)

The Limbic Bands

Two inflections of the wall of the sinus venosus are formed (a) between the superior and inferior caval orifices, (6) between the inferior caval orifice and that of the coronary sinus. In these inflections bands of auricular musculature cross, forming the upper and lower limbic bands (Fig. 336). Thus the mechanical valves which prevent regurgitation in auricular systole are replaced by a muscular mechanism which serves the same purpose. In amphibians and reptiles, where the division of the heart is incomplete, over-pressure in the right side is relieved by the escape of blood to the left side of the heart ; but in birds and mammals such an adjustment is impossible, hence the mechanical venous valves are replaced by a " safety mechanism," which will allow regurgitation from the auricles to the veins if the right side becomes over-distended.


Fig. 336. Diagram of the Right Auricle thrown open to show the position and relations of the Uight and Left Venous Valves and the manner in which they are broken up by the Superior and Inferior Limbic Bands.

Sino-auricular Node

The musculature of the sinus venosus of fishes is made up of small peculiar fibres rich in nuclei and in nerve supply. It has, more than all the musculature of the heart, the power of automatic rhythmical contraction. In human and mammalian hearts the sinus musculature is replaced by fibres similar to those of the auricle — all but at the sulcus terminalis, which marks the junction of the sinus and auricle. In the sulcus, just in front of the termination of the superior vena cava (Fig. 337), an area of primitive fibres persists — the sino-auricular node. In lower mammals like the mole, the sino-auricular tissue is more extensive ; it extends along the greater part of the sulcus terminalis, and passes towards the pulmonary veins. In amphibia and reptiles it extends to the part of the left auricle (vestibule), in which the pulmonary veins terminate. In the lowest mammals — monotremes — the muscular tissue of the node assumes a peculiar form.^ Thus the higher in the animal scale one ascends, the greater is the reduction of the sino-auricular nodal tissue. It is in reality a neuro-muscular tissue, and is well defined by the 5th month of development. Dr. T. Lewis found that the contraction of the heart spread from the sino-auricular node, and gave it the name of the " pace-maker " of the heart.

1 Keith, Proc. Anat. Soc. Nov. 1902 ; Lancet, Feb. 27th, March 5th and 12th, 1904 ; Journ. Anat. and Physiol. 1905, vol. 42, p. 1.

2 Keith and Flack, Journ. Anat. and Physiol. 1907, vol. 41, p. 172; W. Koch, Verhand. Deutsch. Path. Gesellsch. 1909, vol. 13, p. 85.


Fig. 337. Human Heart at the beginning of the 3rd month of development to show the position of the Sino-auricular Node. The unsubmerged strip of sinus venosus is seen between the superior and inferior venae cavae.


Fig. 338. The Posterior Wall of the Common Auricle of an Embryo of the 5th week, showing the Left Extension of the Sinus Venosus. (His.)

Formation o£ the Right Auricle

The right auricle or atrium is formed by the combination of three parts : (1) the right primitive auricle which appears as a diverticulum from the right dorso-lateral aspect of the auricular segment of the cardiac tube (Fig. 332) ; it forms the appendix and all that part of the right auricle which is furnished with musculi pectinati. (2) The auricular canal (Fig. 332) which forms the inner layer of the right auriculo-ventricular valve, and the smooth part of the auricle above the base of that valve. The morphological and physiological junction between the auricle and ventricle is at the lower or free margin of the auriculo-ventricular cusps. (3) The sinus venosus which forms the part of the right auricle between the remnants of the right and left venous valves (Fig. 336).

1 Dr. Ivy Mackenzie, Verhand. Deutsch. Path. Gesellsch. 1910, vol. 14, p. 90.

Formation of the Left Auricle

The left auricle is also formed by the combination of three parts : (1) the vestibule which arises as an extension round the terminal parts of the pulmonary veins (Figs. 339, 340), (2) the left primitive auricle, and (3) the auricular canal, all of which arise in a manner similar to the corresponding part on the right side. In the human heart the vestibule forms a large part of the left auricle, the primitive auricle being reduced to form merely the appendix (Fig. 340). The vestibule is marked off from the rest of the auricle by a prominent muscular fasciculus — the taenia terminalis sinistra.

Origin of the Vestibule of the Left Auricle

The representative of the pulmonary veins in fishes — ^viz. the vein of the swim bladder — ends directly or indirectly in the sinus venosus, a condition which may reappear as an abnormality in the human subject. In the Dipnoi, in which the swim bladder serves as a real lung, the pulmonary vein passes along the left wall of the sinus venosus to open in the left auricle near the base of the left venous valve in a manner almost identical to that shown in some abnormal human hearts (see Fig. 371). In the human embryo the pulmonary veins meet in the venous mesocardium, and open by a single orifice as in the Dipnoi. As the lungs develop they grow round and overlap the heart ; the right and left pulmonary veins separate ; their orifices move apart ; later the right and left veins subdivide. With these changes the venous mesocardium is widened, and the part of the auricle in which the veins end is greatly extended to form the vestibule (compare Figs. 339, 340). It is highly probable that the vestibule of the left auricle also represents an extension of the sinus venosus. The late Professor His, who laid our knowledge of the development of the human embryo on a sure foundation of fact— he died in 1904— believed this to be the case. It is certainly so in the heart of amphibians. In Fig. 338 the sinus area will be seen to extend into the posterior wall of the left auricle. It is on this left extension that the venous channel from the lung buds opens.


Fig. 339. Reptilian Heart, viewed on its Dorsal Aspect, to show (1) the manner in which the Auricles arise from the Cardiac Tube, (2) the Auricular Canal, (3) the Sinus Venosus and Great Veins, (4) the Common Pulmonary Vem, which, at its termination, is embraced by the sinus venosus.


Fig. 340. Heart of Adult viewed from behind to show the Vestibule and the other parts of the Left Auricle. The auricle was in a systolic condition. The remains of the left superior vena cava (vein of Marshall) and the attachment of the pericardium are also indicated.

Auricular Septa

During the 6th week the auricular part of the heart becomes separated into right and left chambers by the formation and union of the three following elements : (1) the endocardial cushions, (2) the septum primum, (3) septum secundum. Two endocardial cushions arise as thickenings of the endocardium, one on the dorsal or posterior wall, the other on the ventral or anterior wall ; they meet and fuse, and thus divide the common auricular canal into the right and left auriculo-ventricular orifices (Fig. 342). In amphibians the endocardial cushions form the dorsal and ventral cusps of the common auriculo-ventricular valve ; in reptiles these two cusps become united, and thus divide the common auriculo-ventricular orifice into right and left channels ; in birds and mammals their fusion is complete. The lower fornix of the venous valves (Figs. 335, 342) becomes implanted on the posterior cushion ; thus the sinus comes almost to reach the ventricular chamber. The septum primum (Fig. 341) appears at the beginning of the 6th week as a crescentic fold on the roof of the primitive auricle, and while it may actually grow downwards yet appears to be produced mainly by the expansion of the two auricular chambers (Fig. 332). Its lower margin, which is covered by a thickening of endocardial tissue, is attached to both endocardial cushions ; the adjacent margins of the septum and endocardial cushions fuse, but occasionally the fusion is incomplete, an inter-auricular foramen (foramen primum) being left between the bases of the auriculo-ventricular valves below and septum ovale above (Figs. 341, 348). In mammals and birds the upper part of the septum primum breaks down, the foramen ovale being thus formed. The part which remains forms the septum ovale. The septum secundum (Fig. 341) is formed by an inflection of musculature from the roof of the auricle to the right of the septum primum. It forms the annulus ovalis (limbic bands) (Fig. 336) and the musculature of the septum above the foramen ovale (Fig. 336). The foramen ovale thus becomes bounded above by the septum secundum, below by the septum primum. In 25 per cent, of people, according to Fawcett's statistics, the foramen ovale fails to close within the first year after birth, but even when an opening remains blood could pass from the right to the left auricle only when the pressure was greater in the right than in the left. The foramen ovale is an adaptation to the foetal type of respiration ; by it the purer blood returning from the placenta can pass from the right to the left side of the heart without passing through the lungs, which are then only partially pervious.


Fig. 341. Diagram of the opened Right Auricle and Ventricle to show the parts which enter into the Formation of the Septum.


Fig. 342. Coronal Section of the Heart of a Rabbit, illustrating the condition of parts in the 6th week of Human Development. (Born.) The cushions labelled " aortic " should be marked " bulbar."

Division of the Truncus Arteriosus

While the auricular segment of the cardiac tube is undergoing division during the 6th. week a similar process is taking place in its terminal segment — ^the truncus or conus arteriosus, leading to the separation of the pulmonary from the systemic aorta. We have seen that the truncus becomes shortened during the 5th week (Figs. 329, 330) and at the same time the ventral aorta (Fig. 331) is being cleft into right and left vessels. In the 6th week the process of cleavage has reached the origin of the 6th pair of aortic arches from which the pulmonary arteries arise (Fig. 360) so that there now remains but a short segment of the common aortic stem to undergo division and give rise to the intrapericardial part of the aorta and common pulmonary artery. The first step in the division is the appearance of four endocardial cushions at the commencement of the common aortic trunk (Fig. 343, A) the two larger cushions being placed right and left. As is shown in Fig. 343, these cushions become, for the chief part, converted into the aortic and pulmonary valves — but two of them, the right and left, become fused and assist in forming the spiral septum wticli separates the aortic from the pulmonary passage. By the end of the 6th week a process of cleavage has reached the lateral cushions and henceforth the pulmonary artery and aorta form distinct channels.


Fig. 343. The Origin of the Semilunar Valves. A. The four Endocardial Cushions of the Truncus Arteriosus. B. The division of the Lateral Cushions to form two Aortic and two Pulmonary Semilunar Valves.

Bulbus Cordis

We have seen how the first chamber of the heart — the sinus venosus — becomes included in the auricles. In a somewhat similar manner the fourth chamber of the heart — the bulbus cordis — becomes submerged in the ventricles — principally in the right ventricle. In Figs. 34:4: and 345 the heart of a human embryo and that of a shark are placed side by side. In both the truncus arteriosus (ventral aorta) are present (1) ; the bulbus cordis (2) ; it is lined with valves in the shark and surrounded by cardiac musculature ; the bulbus is distinctly marked oS from the ventricle at 4, and from the truncus at 3. The ventricle (5) in the shark has the shape of a stomach ; in the embryonic human heart a diverticulum or evagination indicating the left ventricle has already appeared (4th week) ; the auricular canal (6), the left and right auricles (7) (8) are also present. Thus in the human embryo all the parts of the primitive vert-ebrate heart are represented.

1 See Greil, Morph. Jahrb. 1903, vol. 31, p. 123 ; Keith, Lancet, 1909, Aug. 7, 14, 21 ; Thompson, Journ. Anat. and Physiol. 1907, vol. 42, p. 159 ; Prof, D. Waterston, Tra-ns. Roy. Soc. Edin. 1918, vol. 52, p. 257.


Fig. 344. Heart of an Embryo of 4 weeks seen from the front. Explanation in text.


Fig. 345. Heart of a Shark viewed from the front. After Wilhelm His (1831-1904)

Fate of the Bulbus Cordis

The fate of the bulbus cordis is most easily understood by a reference to such a diagram as is represented in Fig. 346, A, B. The bulbo-ventricular part of the heart in the human embryo resembles the stomach ; there is a greater and a lesser curvature. In the second month the lesser curvature, represented in the diagram by a heavy black line, undergoes a process of atrophy. The result is (Fig. 346, B) that the cavity of the bulbus becomes thrown into that of the ventricle and the auriculo-ventricular and aortic orifices are brought side by side. At this time, when the lesser curvature is disappearing, the cavities of the ventricles are appearing by an evagination or enlargement of the ventricular wall, leaving the interventricular septum between the evaginations (Figs. 288, 298). The conus or truncus arteriosus is dividing then into systemic and pulmonary aortae. Thus it comes about that the cavity of the bulbus cordis is converted into the infimdibulum of the right ventricle, merely a trace extending across to the left ventricle above the interventricular septum. The importance of recognizing the bulbus cordis as a separate constituent of the heart will be realized when it is remembered that 95 per cent, of the cases of congenital malformation are the result of its imperfect transformation to form the infundibulum of the right ventricle of the heart. In nearly every case of what is described as congenital stenosis of the pulmonary orifice, a cavity of variable size will be found under the malformed valves representing the bulbus cordis. In fishes the bulbus is connected witli the blood supply to the gills ; its derivative, the infundibulum of the right ventricle, has to do with the regulation of the blood supply to the lungs, but in neither case do we know the exact function of this part of the heart.


Fig. 346, A. Diagrammatic Section of the Embryonic Heart in the 3rd week. B. Diagrammatic Section of the Foetal Heart at the 3rd month. 1, sinus venosus ; 2, auricle ; 3', 3", left and right ventricles ; 4, bulbus cordis ; 5, common aorta ; bulbo-ventricular junction ; 7, bulbo-aortic junction ; 8, auriculo-ventricular junction.

Bulbar Cushions

During the transformation of the bulbus in the 6th week, there appear within it two endocardial cushions — evolved from the series of valves which line the bulbus of the primitive heart (Fig. 345). The part taken by them in building up the interventricular septum can best be realized when the infundibular part of the right ventricle is exposed as in Fig. 341. The line of fusion between the posterior and anterior bulbar cushions is seen to descend in the septal wall of the infundibulum from the pulmonary valves to the site of the interventricular foramen. When the bulbar cushions fuse at the end of the 6th week the small subaortic part of the bulbus becomes separated from the main part included in the infundibulum of the right ventricle (Figs. 346, A, B). The bulbar cushions at an early stage of development are shown in Fig. 342, where they are wrongly labelled aortic cushions.


Fig. 347. Section of the Ventricles of the Foetal Heart, showing the Muscular Sponge-work within their Cavities. (After His.)

Formation o£ the Ventricles

Along the lateral and convex aspects of the ventricular tube the musculature grows rapidly, forming a dense superficial layer and a deep sponge-work system of trabeculae, which almost fill the ventricular chamber. In the hearts of fishes and amphibians the sponge-work persists, but in birds and mammals the ventricular chambers are formed as diverticula by the absorption of the sponge-work.

Between the right and left excavations, however, part of the sponge-work is left to form the interventricular septum (Fig. 347). In front the musculature of the septum is attached to the anterior ciishion of the bulbus arteriosus (Figs. 341, 342) ; behind, it is attached to the posterior of the two endocardial cushions in the auricular canal. On its upper free crescentic margin is a thickening of endocardial tissue. The closure of the interventricular foramen completes the separation of the left from the right ventricle of the heart. It is bounded below by the margin of the interventricular septum ; above, by the bulbar cushions and behind by the auricular endocardial cushions (Figs. 341, 350). The pars membranacea septi, which is found beneath the base of the septal cusp of the tricuspid, and below the septal cusps of the aortic valve, is formed towards the end of the 7th week, by the fusion of the endocardial margins of the interventricular foramen.^ The foramen is thus closed by that process to which the name of zygosis has been given (p. 287). Only in mammals and birds is the interventricular foramen closed, the foramen ovale opened and the venous valves replaced by a muscular mechanism.

^ For full details regarding the formation of the interventricular septa, see Prof. Frazer's research, Journ. Anat. 1917, vol. 51, p. 19.

Abnormalities of the Heart

Thus five elements enter into the formation of the septum of the heart, the two interauricular septa, the two endocardial cushions of the auricular canal, the interventricular septum, the endocardial cushions of the bulbus and the cushions of the truncus arteriosus (Fig. 34:1). Abnormalities may result from their non-union, but by far the commonest defect found is a patency of the interventricular foramen (Fig. 350). This is accompanied in nearly every case by an arrest in the expansion of the bulbus cordis and a stenosis or narrowing at the orifice of the pulmonary artery (congenital pulmonary stenosis). The blood of the right ventricle, in such cases, is pumped into the aorta, through the interventricular foramen ; blood is supplied to the lungs through the ductus arteriosus or by the bronchial arteries from the aorta.


Fig. 348. Abnormal Heart of a Child with the Left Auricle and Ventricle laid open, a, left ; b, right pulmonary veins ; c, septum primum ; d, d', posterior and anterior endocardial cushions ; e, interventricular septum ; /, left ventricle ; g, left auricular appendix ; h, aorta ; i, sup. vena cava.


Fig. 349. Same Heart from above, a, the orifice of pulmonary artery with fusion of septal cusps ; 6, valves of aorta, with the coronary arteries risLug above septal cusps ; c, d, e, f, continuity of the tricuspid and mitral valves across the upper border of septum.

Aurieulo-Ventricular Valves

At first the auricular canal is exposed on the surface of the heart (Fig. 294), but it soon becomes enveloped by the upgrowth and excavation of the bases of the ventricles (Fig. 338). The auricular canal, with an attenuated envelopment derived from the ventricle, thus comes to hang within the ventricular chambers and forms the lateral cusps of the tricuspid and mitral valve (Fig. 347). The septal cusps are formed from processes of the endocardial cushions (Fig. 349). The chordae tendineae, musculari papillares, columnae carneae, trabeculae and moderator band are derived from the muscular sponge-work of the ventricles.

1 For a fuller account of development of ventricles see F. P. Mall, Amer. Journ. Anat. 1912, vol. 13, p. 249 ; Frazer, Journ. Anat. 1917, vol. 51, p. 19.

2 A. Keith, Journ. Anat. and Physiol. 1912, vol. 46, p. 211 ; F. T. Le-svis and Maude Abbott, Bulletin Med. Museums, 1916, vol. 6, p. 1.

Various maldevelopments of the heart throw light on the nature of the auriculo-ventricular valves. In Fig. 348 an abnormality of this kind is represented. The anterior and posterior endocardial cushions have not united, hence the tricuspid and mitral valves are continuous across the upper border of the septum (Fig. 349). The aperture seen above the interventricular septum is the foramen 'primum— not the interventricular foramen.

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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures