Human Embryology and Morphology 21

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 XXI. Circulatory System (continued)

Purkinje System of the Heart

About the middle of the nineteenth century, Purkinje, Professor of Anatomy at Breslau, discovered large peculiar muscle fibres beneath the endocardium of the heart of the sheep and of other ungulate animals. In 1906 Tawara showed that such fibres were connected with a muscular bundle, which rose in the waU of the auricle near the orifice of the coronary sinus and entered the ventricle along the upper border of the interventricular system.^ In many cases of malformed heart the primitive relations of the auriculo-ventricular (A.V.) bundle may be seen (Fig. 350). It passes along the upper border of the interventricular septum below the interventricular foramen. Its left branch descends on the septum to the musculari papillares of the left ventricle ; the right division or branch passes along the moderator band, which marks the junction of the bulbus cordis with the body of the right ventricle. When it is remembered that the ventricles arise from evaginations of the ventricular tube, it will be seen that the bundle on the upper border of the septum occupies the least disturbed part of the lumen of the primitive cardiac tube.


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Fig. 350. The Auriculo-ventricular Bundle in a Heart with open Interventricular Foramen. 1, aorta ; 2, on the site of the pars membranacea septi ; 3, left division issuing from bundle situated on the upper margin of the interventricular septum (4) ; 5, anterior wall of left ventricle ; 6, mitral valve ; 7, cut wall of left ventricle ; 8, left auricle ; 9, pulmonary artery.


The evolution of the Purkinje system may be realized from a study of Fig. 351. Gaskell found in 1883 that the auricles and ventricles were connected in j&shes, amphibians and reptiles by the musculature of the auricular canal (Fig. 351, 4, 4), and that this connection conveyed the wave of contraction from auricle to ventricle. The auriculo-ventricular muscular collar begins in a ring of peculiar muscle cells situated as shown in Fig. 351, 3, 3. In the mammalian heart the primitive muscle of the auriculoventricular canal disappears — except at the upper border of the septum, where it forms the bundle. The node in which it arises represents a remnant of the ring of peculiar muscular tissue which surrounds the auriculoventricular junction. It is true that the sinus venosus reaches the posterior endocardial cushions (Fig. 342) near the site of the node, but it is most improbable, in the light of comparative anatomy, that the node at the commencement of the bundle should represent sinus musculature.


^ For ap account of Tawara's discovery see Keith, Lancet, 1906, Aug. 11.


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Fig. 351. Section of a Generalized Type of Heart to show the Origin of the Auriculoventricular Bundle and Node, a, sinus venosus ; b, auricular canal ; c, auricle ; d, ventricle ; e, bulbus cordis ; /, aorta ; 1, 1, sino-auricular junction ; 2, 2, auricular junction with canal ; 3, auricular ring of peculiar fibres ; 4, auriculoventricular musculature ; 5, bulbo-ventricular junction.

Changes in the Circulation at Birth

(1) The outflow of the blood to the placenta by the hypogastric arteries and its return by the umbilical vein is arrested when the umbilical cord is tied.^ The umbilical vein and ductus venosus gradually become ligamentous. (2) The first breath expands not only the air spaces of the lungs, but also the pulmonary vessels, so that the pressure within them becomes less than in the aorta ; hence the blood in the pulmonary aorta passes through the lungs instead of gaining the aorta by the ductus arteriosus. A section across the ductus arteriosus and aorta (Fig. 353) shows that, before birth, the septal wall of the ductus is invaginated within the lumen of the aorta ; after birth the septal wall is bent within the lumen of the ductus, thus partly closing it.

1 For changes in vessels see A. W. Meyer, Amer. Journ. Anat. 1914, vol. 16, p. 477.


(3) The foramen ovale is closed by the pressure within the left auricle being raised by the inflow of pulmonary blood, the pressure in the left auricle then reaching a higher point than in the right auricle. The closure of the foramen is assisted by an alteration in the action of the limbic bands (Fig. 336) brought about by their indirect attachment to the diaphragm.


(4) The hypogastric arteries, beyond the origin of the vesical arteries, become reduced to cords. (5) The pressure within the aorta becomes three times that in the pulmonary arteries ; the left ventricular wall becomes three times as thick as that of the right. Before birth the ventricular pressures were equal and so were the thicknesses of the ventricular walls.


Remnants of the Foetal Circulation in the Adult

The nature of these remnants has been already described ; they need be only enumerated here. They are : (1) The Obliterated Hypogastric Arteries ; (2) The Umbilicus ; (3) The Round Ligament of the Liver ; (4) The Fibrous Remnant of the Ductus Venosus ; (5) The Eustachian Valve ; (6) The Foramen Ovale ; (7) The Fibrous Remnant of the Ductus Arteriosus.

Changes in the Position of the Heart

The alteration in the position of the heart from a subpharyngeal to a thoracic position during the 5th, 6th and 7th weeks of development is brought about by two factors. First, the heart is primarily a pump for forcing the blood through the organ of respiration ; hence in the fish it lies beneath the gills, in air-breathing vertebrates it is situated close to the roots of the lungs. Secondly, in reptiles, birds and mammals a neck is developed, the head and pharyngeal region being gradually forced forwards, while the heart and pericardium come to lie opposite the middle part of the dorsal region of the spine. The neck is difierentiated in the human foetus during the second month. All the structures in the neck become elongated — ^the oesophagus, trachea, vagus nerves, jugular veins and carotid arteries. During this change the arch of the aorta and its branches are evolved from the ventral stems of the aortae and aortic arches. In most mammals the left carotid arises in common with the aortic stem, and a reversion to this type is the commonest abnormality to which the aortic arch is liable in man (Parsons). The separation of the left carotid from the innominate m man is due to the large size of the upper aperture of his thorax. The left vertebral artery or the thyroidea ima may gain an origin from the aortic arch.



Fig. 352. Diagrammatic Section across the Head Fold of a developing Salamander to show the relationship of the Pericardial part of the Coelom to the Heart and Fore-gut. (After C. Rabl.)


Fig. 353. Section across the Junction of the Aorta and Ductus Arteriosus (viewed from behind) of a full time Foetus to show the Inflection of the Wall of the Ductus within the Lumen of the Aorta.


Final Fixation of Heart. — As may be seen from Figs. 329 and 355, the heart of the human embryo is fixed within the pericardium exactly as in a fish — being attached behind to the septum transversum by the venous mesocardium and under the pharynx, by the arterial mesocardium. By the 8th week the interventricular septum is complete and the heart has taken up its position in the thorax, being fixed within the pericardium in the same manner as in the adult (Fig. 354). The original mesocardia can still be recognized, separated by the transverse sinus of the pericardium (Fig. 354, h). The sinus is also shown in Fig. 356. The derivatives of the truncus arteriosus — -the aortic root and pulmonary arteries — ^lie within the reflections of the arterial mesocardium ; the caval and pulmonary veins reach the auricles through the reflections of the venous mesocardium. That part of the septum transversum which contained the sinus venosus and great veins has become an intrinsic part of the dorsal wall of the pericardium. The heart has so doubled on itself that the venous and arterial mesocardia are in contact, being only separated by a potential space — the transverse sinus (Fig. 356).



Fig. 354. The Heart pulled forwards to show its two Attachments by the Arterial (d, d) and Venous (e, e) Mesocardia. Fig. 355. — Diagram of the Heart of a Fish to show : (1) the Primitive Parts of the Heart ; (2) the Relationship of the Heart to the Pharynx ; (3) the Septum Transversum ; (4) the fixation of the Heart.


The venous mesocardium becomes much more extensive by the ingrowth and separation of the pulmonary veins. These grow in from the lungs, and pierce the pericardium to reach the left auricle (Fig. 356). They reach the auricle through the mesentery or venous mesocardiuni of the sinus venosus. The migration of the left pulmonary veins causes a prolongation of the venous mesocardiuni to the left side ; when the heart is removed the venous mesocardium is seen to be F-shaped in section. The oblique sinus lies in the concavity of the pulmonary venous mesocardium (Fig. 356).


Primitive Relationships of the Pericardium. — Were one to restore the head and pericardium to the relative positions they occupy in the 5th week of development, then the pericardium must be lifted from the thorax and placed beneath the chin and larynx so that the septum transversum is opposite the origin of the phrenic nerve from the 4th cervical segment ; the anterior border of the umbilicus is also then opposite the origin of the phrenic nerve. In the somatopleure over the pericardium and between the mandible and umbilicus are developed the depressors of the hyoid, the sternum and sternal ribs. The pericardium is therefore the coelom of the neck ; its fibrous wall represents the deepest layer of the cervical somatopleure, corres^^onding to the fascia transversalis of the abdomen. With the elongation of the neck and separation of the pharynx and pericardium, the tissue of the branchial segments which surrounds the aortic arches is drawn out to form the carotid sheaths.


Fig. 356. View of the Interior of the Pericardium showing the Attachments of the Heart to its Dorsal Aspect by the Arterial or Venous Mesocardia.


Ectopia Cordis

Occasionally children are born with their hearts exposed on the surface of the chest. In extreme cases only the dorsal wall of the pericardium is present, and it is flush and continuous with the skin of the chest. In these cases the sternum is partially absent, or if present it is cleft, the right and left halves being widely parted. No satisfactory embryological explanation of this condition has yet been given.


Dorsal Aortae

The dorsal or descending aorta, like the heart, is bilateral in origin. At the beginning of the ith week, as somites are being demarcated in the cervical region of the embryonic plate, the right and left dorsal aortae, commencing at the upper ends of the pharyngeal arches, pass backwards side by side, supplying branches to the archenteron as they go (Fig. 25). From their terminal branches on the yolk sac commence the umbilical arteries (Fig. 25). By the end of the 4th week the dorsal aortae have fused to form one vessel from the 1st thoracic to the 1st lumbar segment. At this date the radicles of the umbilical arteries arise from the aorta opposite the 1st lumbar segment- ; by the end of the 5th week their origin has migrated backwards to the level of the last lumbar segment. Although the umbilical arteries appear to be direct continuations of the dorsal aortae in later embryonic and foetal Hfe, yet there can be no doubt that this honour falls to the middle sacral artery, for, as we have seen (p. 27), the umbilical arteries must be regarded as greatly modified vesical or allantoic branches of the aorta. The middle sacral artery is formed by the fusion of the caudal arteries — morphological continuations of the dorsal aortae. The coeliac axis,^ superior and inferior mesenteric arteries are the sole survivors of the numerous branches supplied by the paired aortae to the archenteron. At the end of the 5th week the coeliac axis arises from the aorta opposite the 7th cervical segment ; by the end of the 7th week its origin is opposite the 10th thoracic segment — its permanent position. The superior and inferior mesenteric arteries undergo a corresponding degree of migration backwards during the 6th and 7th weeks.


Formation o! Blood Vessels

The development of blood vessels and blood corpuscles can best be studied in the mesoderm which covers the yolk sac, for in the human embryo, with the exception of the chorion, the wall of the yolk sac is the site at which vessels and blood are first formed. The mesodermal cells covering the yolk sac show a differentiation into two strata — a superficial or mesothelial, representing the peritoneum and a deeper or mesenchymal, lying between the mesothelium and the entodermal lining of the sac (Fig. 357). The cells of the mesenchyme, as already mentioned (p. 40), are angioblastic or vaso-formative in nature. Originally their cell-bodies are continuous and form a syncytium, but in Fig. 357, A this continuity is disappearing and a mass of mesenchymal cells is being separated to lie within a blood space. In the wall of the space certain cells are being differentiated to form a lining membrane. The blood space, the cells within it and the enclosing endothelium constitute a blood island. In the island are to be seen certain mesenchymal cells — ^haemoblasts — which represent the parent type of all blood cells^both white and red. In the same island (Fig. 357, A) are to be seen certain haemoblasts in which haemoglobin is being formed, thus becoming erythroblasts — the parent type of red cells. They represent cells which are being set aside as oxygen carriers and are therefore to be counted units of the respiratory system. Further, in such an island (Fig. 357, A) are to be recognized large lymphocytes — or leucoblasts — the parent type of white corpuscles.

1 Broman, Anat. Hefte, 1908, vol. 36, p. 405.

^ For recent literature on origin of blood and vessels : see H. E. Jordan, Amer. Journ. Anat. 1916, vol. 19, p. 227 ; C. R. Stockard, ibid. 1915, vol. 18, pp. 227, 525 ; R. D. Lillie, ibid. 1919, vol. 26, p. 209 ; Vera Danchakoff, ibid. 1918, vol. 24, p. 1, Anat. Bee. 1916, vol. 10, p. 415 ; Florence Sabin, Contributions to Embryology, 1917, vol. 6, p. 61 ; 1920, vol. 9, p. 213.


The blood islands scattered over the yolk sac become confluent by the union and canaliculization of intervening endothelial cells. In this manner a vascular network is produced on the yolk sac ; the manner in which the blood islands are united is typical of the manner in which new blood channels are formed. Within the body of the embryo mesenchymal cells assemble in vasoformative groups, become canaliculized and unite with neighbouring groups to form both arteries and veins. The endothelial cells of capillaries retain throughout life the vasoformative power which characterizes them during the period of development and growth. The cellular processes at the growing point of a capillary are permeable at first to the plasma only, subsequently the lumen becomes large enough to allow the blood cells to pass.


Fig. 357. A, Section of the Wall of the Yolk Sac to show the constitution of a Blood Island. (H. E. Jordan.) a, Haemoblast, dividing ; b, Erythroblast, dividing ; c, Blood-space ; d, Haemoblast ; e, Endothelial cell ; /, Leucoblast. B, Wall of a Blood-space, showing Blood Cells arising from its Endothelium. (H. E. Jordan.) a, Endothelial Cell ; 6, Haemoblast being produced from Endothelial Cell ; c, Haemoblast arising outside Blood-space from Endothelium.


Formation of Blood

In the development of each system of the human body the various parts appear in the same order as they are seen to occur in ascending the scale of the animal kingdom. In many invertebrates the blood is formed by only a fluid living intercellular substance — the plasma ; when the human heart beats first, its lumen contains no blood cells, only plasma. In amphioxus nucleated uncoloured corpuscles appear in the plasma ; the cells which appear first (during the 4th week) in the circulation of the human embryo are the red nucleated corpuscles (erythroblasts) formed in blood islands. In all vertebrates, with the exception of amphioxus, nucleated white as well as nucleated red cells appear in the blood ; in the human embryo the white cells (leucocytes) appear somewhat later than the red. In mammals only do the nuclei disappear or become extruded from the er}i;hroblasts, red blood corpuscles (erythroplastids) being thus formed. The erythroplastids begin to appear in the blood of the human embryo before the end of the 2nd month, and gradually replace the erythroblasts, which cease to appear in the circulating blood some days after birth (Ham). At every period erythroblasts are formed as derivatives of the endothelium of vascular walls. The mode in which blood cells arise from the endothelial lining of blood spaces is illustrated in Fig. 357, B.


The Germinal Centres for Red Blood Corpuscles

At every period of life the red blood corpuscles (erythroplastids) arise from erjrfchroblasts. These are formed first in the blood islands of the chorion, of the yolk sac and within vascular extensions of the vasoformative cells throughout the body. The formation of blood corpuscles in the liver commences at the beginning of the second month of development, and ceases in the later months of foetal life.^ The parent erythroblasts lie side by side with the liver cells. The splenic blood spaces are also sites of blood formation in the latter half of foetal life. About the middle of foetal life the capillaries of bone marrow begin to be invaded by angioblastic outgrowths, and from birth onwards the capillaries of the red bone marrow become the breeding ground of erythroblasts, from which the red corpuscles arise by disappearance of their nuclei.


Fig. 358. Section of a tubular part of the Thymus of a Frog, showing (1) the Production of Lymphocytes from the Thymic Epithelium ; (2) the Production of Hassall's Corpuscles. In J. a leucocyte within the wall of a capillary has become enlarged and shows concentric striae ; in B the nucleus of the leucocytes has undergone division ; it completely fills the lumen of the capillary, the nuclei of which are seen in the periphery of the body. (After Nusbrum and Machowski.)


Origin of White Blood Corpuscles

The reticular tissue which underlies the epithelial lining of the alimentary tract corresponds to the mesenchymal angioblastic stratum of the yolk sac and, from the 5th month of foetal life onwards, is the seat of a prolific production of lymphocytes. The apparent production of lymphocytes direct from entodermal cells (Fig. 358), such as are represented in the tonsillar and thymic outgrowths, is probably due to the fact that such outgrowths always are in the closest apposition to this lymphocyte-producing mesenchymal stratum.

^ See MoUier, ArcJiiv. fiXr Mikroscopic Anat. 1909, Bd. 74, p. 474.

^ See Retterer et Lelievre, Journ. d'Anat. et Physiol. 1912, vol. 48, pp. 14, 194 ; r. Weidenreich, Ergebnisse der Anat. 1909, vol. 19, p. 527. See also references on p. 261.


Leucocytes are also profusely produced from (1) the endothelium of serous cavities — such as the peritoneum and pleura ; (2) from the endothelium of lymphatic vessels ; (3) from leucoblasts of bone marrow ; (4) from the endothelium of capillaries, and possibly (5) from connective tissue cells. As yet, however, these statements must be accepted with reserve, for there is still a degree of uncertainty regarding the genetic relationship of one form of leucocyte to other forms. Mollier, who has recently studied the development of the blood corpuscles, describes the liver as the chief source of white blood corpuscles during foetal life ; later the site of their formation is shifted to the blood spaces in marrow. He regards both basophile and eosinophile leucocytes as arising in the liver from the same parent cells (haematoblasts) as give origin to the red nucleated corpuscles.


Lymphatic System

In all vertebrate animals the plasma or lymph from the tissues of the body is drained into the veins by a special system — the lymphatic vessels. In amphibia the lymph collects in large spaces lined by endothelium, from which it is forced into the venous system by two pairs of lymph hearts — one pair situated in the angle between the jugular and subclavian veins, the other pair between the internal and external iliac veins. In mammals the lymph hearts disappear ; they are no longer required, for the negative pressure in the veins of the thorax, set up by the evolution of a separate respiratory cavity, is suflS.cient to draw the lymph into the venous system. It is remarkable, however, that Miss Sabin who, by a paper ^ published in 1902, inaugurated our knowledge of the development of the mammalian lymphatic system, found that the lymph vessels appear first at those four points where the amphibian lymph hearts are situated.


Recent enquiries by American embryologists have thrown quite a new light on the origin of the lymphatic system. They have established that the formation of lymph vessels begins at definite centres and from such a centre vessels spread outwards, vascularize and drain a definite area. If the starting centre is excised, then there is no outgrowth and vessels from neighbouring areas invade and drain the one thus deprived. While the angioblasts of the blood system are everywhere and have established a complete vascularization of the embryonic tissues before the end of the 4th week, the angioblasts of the lymphatic system do not become manifest until the end of the 6th week, when they form a capillary network in the centres of initiation. The greatest and earliest centre is situated in the angle between the jugular and subclavian veins, where the termination of the thoracic duct is afterwards formed. By the end of the 8th week the capillary network of lymph vessels have fused and formed the extensive lymph sac shown in Fig. 359. In the 3rd month outgrowths from the jugular sac on each side of the neck spread and invade the tissues of the neck, head and arm — all save the central nervous system and voluntary muscles. These are not drained by the lymphatic system. The great lymph sacs are merely temporary structures ; their cavities are filled by reticular lymphoid tissue produced by the lymphatic endothelium which lines the sacs. As soon as formed, the jugular lymph sac effects a union with the jugular vein, the orifice being guarded by valvular folds.


1 Florence R.. Sabin, American Journ. of Anat. vol. 1, 1902, p. 367. In nearly every subsequent volume will be found some of the important contributions made to our knowledge of the development of lymphatics by modern American embryologists. F. T. Lewis, Amer. Journ. Anat. 1909, vol. 9, p. 33 ; Florence R. Sabin, Amer. Journ. Anat. 1909, vol. 9, p. 43 ; Geo. S. Huntington and C. F. W. McClure, Amer. Journ. Anat. 1910, vol. 10, p. 177 ; F. T. Lewis, Amer. Journ. Anat. 1905, vol. 5, p. 95 ; G. S. Huntington, Anat. Anz. 1911, vol. 39, p. 385; E. R. Clark, Amer. Journ. Anat. 1912, vol. 13, p. 347; G. S. Huntington, Amer. Journ. Anat. 1914, vol. 16, p. 259 ; Ch. F. W. McClure, ibid. 1915, No. 4 ; E. and E. Clark, Contrib. to Embryology, 1920, vol. 9, p. 447.


Fig. 359. The Main Lymphatic Vessels and Sinuses of the Human Foetus at the beginning of the 3rd. month. (After Prof. Florence Sabin.)


Another pair of lymph sacs appear in the pelvis — related to the corresponding iliac veins, into which they at first open (Fig. 359). From the pelvic or iliac sacs outgrowths invade the hind limbs and tissues of the pelvis and buttocks. In the mesenchymal tissue in which the dorsal aorta is embedded there appear a series of endothelial-lined lymphatic spaces, which become united and place the posterior or iliac sacs in communication with the jugular sacs. In this way two thoracic ducts are formed at the end of the second month. Two other retroperitoneal centres appear, one at the root of the superior mesenteric artery, from which arises the system of vessels which drains the alimentary tract ; the receptaculum chyli is also formed from a special centre. The lymphatic system is just as mucli a " closed " system as is the haemal system ; everywhere its walls are lined with endothelium. Nowhere does it open on " tissue spaces."

Lymphatic Glands make their first appearance during the fourth month at the site of the lymph sacs and along the leashes of vessels leading to these sacs. They appear first as follicles which are developed within the lumina of the vessels so that the lymph passing along is exposed to the lymphocytes developed in the reticular tissue of the node. Lymphocytes arise by proliferation of the cells lining lymphatic spaces and vessels. The lymphatic glands and nodes grow in size and number during each month of foetal life. They serve as germinal centres for the production of lymphocytes.


Inter scapular Gland

Under this name has been included the mass of peculiar tissue which occupies the posterior triangle of the neck, and extends under the trapezius towards the posterior border of the scapula. It represents the hibernating gland of insectivora and bats. It begins to form in the 2nd month of foetal life at the site of the jugular lymph sac. It is composed of a stratum of three tissues — lymphoid, haemolymph (blood-forming) and fat.


Haemolymph Glands

In the subperitoneal fat of many mammals numerous red bodies may be seen which difier from lymphatic glands in the following points : (1) the sinuses contain red blood corpuscles ; (2) instead of aiierent and efferent lymphatic vessels, arteries and veins open into the sinuses. They occur in the human foetus, and apparently serve the same function as the spleen (W. B. Drummond).


Bone Marrow

Until the 5th month of foetal life the marrow is composed of branched cells embedded in a jelly-like matrix (primary marrow) ; it then assumes the appearance of lymphoid tissue, and contains leucoblasts ; in the 6th month erythroblasts and erythrocytes appear in the dilated capillaries forming red marrow in the centres of ossification (Hammar). At birth the marrow of all the osseous tissue is red ; during the years of active growth the marrow of the shafts of bones is gradually replaced by fat cells, yellow marrow being thus formed (Hutchison). From birth onwards the red marrow forms the only tissue in which red blood corpuscles are produced.

^ For an account of this structure see Bonnot, Journ. Anat. and Physiol. 1909, vol. 43, p. 43.






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

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