Human Embryology and Morphology 18

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

Chapter XVIII. Tongue, Thyroid and Structures Developed from the Walls of the Primitive Pharynx

The Tongue and its Development

Two parts are to be recognized in the tongue.[1] The buccal part (Fig. 262) is situated in front of the foramen caecum and the V-shaped groove. It is covered by papillae, concerned in mastication and liable to cancer. The second or pharyngeal part, bounding the buccal wall of the pharynx (Fig. 262), is covered by glandular and lymphoid tissue and concerned with swallowing. These two parts are not only different in function but also in origin and development.

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Fig. 262. Showing the Buccal and Pharyngeal parts of the Tongue.

The buccal part arises during the 4th week by an upgrowth — the tuberculum impar — which springs from the floor of the pharynx, in front of the united ventral ends of the 2nd and 3rd arches (see Figs. 251, 264). This outgrowth was at one time believed to give rise to the whole of the buccal part of the tongue, but researches made by Kallius and others have clearly demonstrated that in the 5th week there arise from the mandibular arch, on each side of the tuberculuni impar, riglifc and left lingual buds which, fuse with and bury the median element (Fig. 251). It is probable that this fusion has already occurred in Fig. 264, and that the tuberculuni impar already represents the buccal element. Hence the buccal part of the tongue is bilateral in origin, and as its nerve supply shows, is entirely derived from the mandibular arch. In the 7th week the tip of the tongue is bifid, because the lateral buds are imperfectly fused (Paulet). The bilateral origin of the tongue explains the occasional occurrence of a bifid tip and the formation of cysts in the median raphe. Besides the lingual nerve, the chorda-tympani — the branch of the facial nerve which enters the mandibular arch — also supplies the buccal parts with sensory fibres. Until the 7th week the buccal part of the tongue, still separated from the pharyngeal part by a depression in the floor of the pharynx, from which the thyroid bud has arisen, remains unseparated from the mandibular arch. There then occurs a down-growth of epithelium in the form of a horse-shoe plate, which separates the lingual from the mandibular tissues ; in this way the tongue becomes separated from the alveolar ridge of the mandible. In the floor space between the tongue and mandible are developed the submaxillary and sublingual glands. Not unfrequently part of this glandular field may be imperfectly separated from the tongue, and in this manner various peculiar congenital malformations of the tongue are produced (see Fig. 263).

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Fig. 263. Upper Surface of the Tongue of a Cliild in which the glandular tissue, which forms the sublingual and submaxDlary glands, has been imperfectly separated from the tongue by the down-growt,h of the mandibulo-lingual plate of epithelium. A, lower lip ; B, alveolar ridge ; C, glandular tissue (sublingual) ; D, submaxillary ; E, buccal part of tongue ; F, tonsil ; O, pharyngeal part of tongue ; E, opening of larynx.


The pharyngeal part of the tongue is derived from the fused ventral ends of the 2nd and 3rd arches, in which, as we have already seen, the body of the hyoid is developed. The glosso-pharyngeal, the nerve of the 3rd arch, or more strictly of the 2nd cleft, supplies it. The V-shaped groove (sulcus terminalis) marks the union of the buccal with the basal or pharyngeal part. The foramen caecum, at the apex of the V-shaped fissure, marks the site from which the thyroid outgrowth took place.


Musculature of the Tongue

The muscles of the tongue[2], which make up almost its entire substance, do not arise within the visceral arches, but are of extraneous origin. It has been shown that the head is probably composed of nine segments. From the muscle plates of the three posterior or occipital segments processes arise and grow downwards and forwards until they reach the mesenchymal basis of the tongue derived from the three visceral arches, carrying their nerves with them — ^the hypoglossal or 12th cranial nerve (Fig. 257). Hence, while the sensory nerves of the tongue come from the nerves of the 1st, 2nd, and 3rd visceral arches, its motor fibres are derived from the posterior cephalic segments. The primitive muscle of the tongue is the genio-hyoid ; the genio-glossus is a derivative of it, and so is the hyo-glossus. The lingual muscles are already recognizable in the 6th week, but the intrinsic muscles of the tongue, which have much to do with its fine movements, are later in point of differentiation — appearing in the fourth month. The sense of taste is present in a child born at the 8th month of development.

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Fig. 264. Showing the Origin of the Tongue in the Floor of the Primitive Pharynx. The condition represented is from an embryo in tiie 6tli week. (After His.)

Lingual Papillae

The filiform papillae are the first to appear, then the fungiform, a few of which, along the posterior border of the buccal part, become enlarged and sink to form circumvallate papillae, round the bases of which taste buds are developed. The papillae are confined to the buccal or masticatory part of the tongue. It will be observed that the taste papillae are situated at the brink of the pharynx (Fig. 262), at which the food is seized and carried away by the involuntary muscles. At the lateral margins of the buccal part of the tongue, just in front of the anterior pillars of the fauces, the fungiform papillae are arranged in a series of laminae, recalling and corresponding to the papillae Soliatae of low primates and of rodents. Between the papillae foliatae occur taste buds. On the under surface of the tongue at birth, on each side of the sublingual papillae and over the position of the ranine artery, are two fimbriated folds of mucous membrane, the plicae flmbriatae, structures which are well developed in lemurs, serving as tooth-combs[3] (Wood Jones). A remnant of the plicae fimbriatae can commonly be seen on the under surface of the human tongue.

The Epiglottis

The origin of the larynx, trachea, bronchi and lungs as a depression and bud from the floor of the pharynx will be dealt with later (p. 270) ; but the origin at the 4th week of the furcula (Fig. 251), a process from which the epiglottis is derived, may be noted here. Both epiglottis and thyroid cartilage arise from the 4th visceral arch. The superior laryngeal is the nerve of the 4th arch, hence it supplies the epiglottis and upper part of the larynx.


The epiglottis and palate are peculiar to mammals.[4] They separate the respiratory passage from the mouth. In all mammals the epiglottis lies within the naso-pharynx in contact with the soft palate, but with the acquisition of speech in man this relationship is lost.

Origin of the Salivary Glands

[5] In the depression between the tongue and the mandible, formed by the opening out of the linguo-mandibular plate of epithelium, there appear two linear furrows (Fig. 265). From the inner or mesial of these two furrows arises the submaxillary gland ; from the outer or lateral, at a rather later date (7th week), grows the sublingual. While the latter arises by a series of buds from the entodermal lining of the groove, the former — the submaxillary — is developed by the depth of the entodermal furrow being enclosed in the mesoderm in the shape of a cord, which later becomes canaHculized and opens as a duct at the sublingual papilla, while the gland itself arises by a process of budding from the distal end of the enclosed entodermal cord. The submaxillary ganglion is made up of nerve cells carried out from the geniculate ganglion during the outgrowth of the chorda tympani. The parotid gland, which is the first of the salivary glands to be developed (6th week), springs as a bud of entoderm from the lateral or bucco-alveolar recess of the primitive mouth (Fig. 265). Its duct is formed first as a groove, which later becomes enclosed to form a canal. It grows backwards in the connective tissue over the masseter, and at birth is comparatively superficial in position, but as the mandible and external auditory process grow, it sinks inwards to surround the styloid process, pushing the deep cervical fascia beneath it. In this way the stylo-mandibular ligament is formed from the fascia pushed in front of it. Its nerves are derived from the 3rd division of the fifth (auriculo-temporal). Salivary glands are accessory to the function of mastication, and hence are developed only in mammals.


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Fig. 265. Showing the Origin of the Submaxillarv and Sublingual Glands from furrows between the gum and tongue during the 7th week. The tongue projects between the maxillary folds into the nasal cavity. After Wilhelm His (1831-1904)

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Fig. 266. Showing the position of the Visceral Clefts in the Adult. The lines only indicate the approximate positions of the clefts. For instance, the soft palate is made up largely from the 3rd arch. See also Fig. 272.

Median Pharyngeal Recess

In the middle line of the roof of the pharynx, just under the basi-occipital, there is a depression or recess of mucous membrane which receives this name. In Fig. 266 it is erroneously named SeesseVs pocket which, as has been mentioned on page 107, disappears in the human embryo during the development of the pituitary gland. Its embryological significance is doubtful, but the site of its appearance corresponds to the point at which the notochord remained unseparated from the dorsal wall of the embryonic pharynx (Fig. 131). Lymphoid tissue is developed in its walls immediately after birth, and in the mucous membrane round it. It is developed behind the oral plate. The adenoid tissue of the naso-pharynx continues to increase in amount until the age of puberty, when it begins to undergo a slow process of atrophy (Symington).


The Tonsil

The tonsil[6] arises early in the 3rd month of foetal life from that part of the second cleft recess which is left between the soft palate and the tongue (Fig. 270, B). In the 4th month eight or ten isolated buds of entoderm push out from an elevation or tubercle situated in this recess or pocket, and grow into the mesodermal tissue in the wall of the pharynx (Fig. 268). The buds afterwards canaliculize and form the crypts and glandular tissue of the tonsil. Follicles of lymphoid tissue — for the tonsil must be regarded as a lymphoid structure — begin to collect round these glandular buds in the 5th month of foetal life.


Concerning the origin of the lymphoid cells, both of the tonsil and the thymus, there are two quite distinct theories. The more recent (Gulland's) is that the epithelial entodermal cells, which form the glandular buds of the tonsil, give rise to broods of lymphoid cells ; the older and the better founded, that these lymphoid cells arise from the blood or surrounding connective tissue, creep in and form follicles round the glandular entodermal buds.


A fold of mucous membrane, the plica triangularis (Fig. 267), passes from the lower part of the tonsil to the anterior pillar of the fauces. It represents the anterior part of the elevation or tubercle in which the glandular buds develop. Although present in the foetus, it commonly disappears in the adult. Its attachment to the tonsil marks a line of separation between an anterior and posterior group of tonsillar outgrowths (Fig. 268). The recess above the tonsil, sometimes crossed by a fold — the plica semilunaris — is a remnant of the recess of the second cleft in which the tonsil is developed (Fig. 267). In many mammals the tonsillar recess assumes the form of a funnel-like process resembling the finger of a glove, the blind end reaching almost to the angle of the jaw.

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Fig. 267. The Tonsil in a Human Foetus of 8 months. (Hett and Butterfleld.)

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Fig. 268. Section across the 2nd cleft recess showing the Outgrowth of the Tonsillar Buds. The elevation between the anterior and posterior groups forms the lowei part of the plica triangularis. (After Hammar.)


The tonsil is part of a great lymphoid system stationed along the alimentary canal. It reaches its fullest growth in youth, as is the case with the lymphoid system generally ; when active growth of the system is over, and especially in the years of decay, it becomes markedly reduced in size. The upper part of the 2nd cleft recess is included with the 1st in the Eustachian tube (Frazer). The lower part of the 2nd recess, containing the tonsil, is separated from the Eustachian part by the growth forwards of tissue of the 3rd arch to help in the formation of the palatal folds in the latter part of the second month. Occasionally the tonsillar recess projects outwards, and comes in contact with a tubular fistula representing the cervical sinus (see Fig. 253).

The Pharyngeal Recess and Pharyngeal Tonsil

At each side, the roof of the pharynx is produced outwards, behind the Eustachian tube and levator muscles of the palate, to form the lateral recesses of the pharynx (Fig. 266). In the recess, and especially on the posterior wall of the pharynx between the recesses and also in and round the median pocket, there is developed a submucous carpet of lymphoid tissue, the pharyngeal tonsil, which often becomes hypertrophied to form adenoids in youth.


The Lingual Tonsil

That part of the tongue (pharyngeal) produced between the 2nd and 3rd arches is studded with mucous glands which are surrounded by nodules of lymphoid tissue — the collective glandular mass receiving the name of lingual tonsil. It will thus be seen that from the 2nd cleft and its neighbourhood is produced a circum-pharyngeal ring of lymphoid tissue of great physiological and pathological importance.


The Thymus

The thymus[7] arises in the same manner as the tonsil, only from the 3rd instead of the 2nd cleft (Fig. 270). The 3rd cleft is represented in the adult by the space in front, and on each side, of the epiglottis. It is crossed by the posterior pillars of the fauces, which represent a continuation of the palatal processes (Fig. 266). In the 6th week the 3rd pharyngeal pocket has assumed the form shown diagrammatically in Fig. 270, B, where its lower and hinder wall is represented as extended in the form of a flask-like process, lined by thickened entoderm, the embryological basis of the thymus. On the dorsal part of the same pocket there is another thickening representing the lower parathyroid' or epithelial body, while the original mouth of the pocket has been drawn out to form a tubular process or duct. In Fig. 270, A, is represented another view of the 3rd pocket, during the 5th week of development. By the 7th week the ectodermal covering, shown in Fig. 270, A, has been invaginated to form the cervical sinus and vesicle, the latter being continuous with the thymic outgrowth. The neck of the glandular thymic pocket becomes separated from the pharynx in the 7th week and usually disappears, but a strand of tissue frequently persists and represents the stalk of the outgrowth (Fig. 271). By a species of secondary budding the thymic entodermal outgrowth becomes broken up into islands or separated acini. The epithelial acini proliferate and give rise to a meshwork of united cells (syncytium), in which broods of lymphoid cells appear during the 3rd month. The lymphoid cells — lymphocytes — become aggregated into follicles, where the production of lymphocytes is continued. All trace of the original epithelial cells disappears. The concentric bodies, known as the corpuscles of Hassall,[8] were at one time supposed to represent remnants of the epithelium, but they are now known to be produced from single cells, which divide without a separation of the daughter cells thus formed. Hassall's corpuscles also arise from capillaries, some of which, after invading the thymus, become broken up into segments. The endothelial cells lining those segments may proliferate, occlude the lumen, and thus give rise to a Hassall's corpuscle (Nussbaum) (see also p. 335). The surrounding mesoderm supplies the connective tissue stroma and capsule of the thymus. The lateral lobes come together under the ventral aortae and pericardium during the 7th week, and ultimately assume a thoracic position along with these structures. The pointed upper extremity of each lateral lobe can be traced upwards in the fully developed foetus, under the lateral lobes of the thyroid towards the thyro-hyoid membrane (Figs. 171, 253). These apical strands represent the stalk of the thymic buds. Thymus buds also arise from the 4th pouch (Fig. 270), and from the cervical sinus, but these never proceed beyond a rudimentary stage in the human embryo.

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Fig. 269. Showing the origin of the Tonsil, Thymus and Thyroid from the Internal Cleft Recesses during the 5th week. After Wilhelm His (1831-1904).


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Fig. 270, A. The Lining Membrane of the Pharynx of a Human Embryo, 5 mm. long (5 weeks old), seen on its ventral aspect and showing the external configuration and relationships of the pharyngeal pockets. (After Grosser.)


Fig. 270, B. A schematic representation of the pliaryngeal pockets and the glandular structures rising from them in the 6th week of development. (After Grosser.)


'^ Dr. E. T. Bell defends the theory of their Epithelial origin, Amer. Journ. of Anat. 1906, vol. 5, p. 30.

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Fig. 271. Diagram of the Thyroid and Thymus. The position of the parathyroids on the posterior aspect of the lateral lobes of the thyroid is indicated.


While Beard regards the thymus as the parent source of all the white blood corpuscles of the body, many interpret the appearances in quite an opposite manner and are of opinion that the leucocytes are brought within the epithelial element of the thymus along with the mesodermal invasion. Professor Bryce has demonstrated that, white blood corpuscles appear in vertebrate embryos before any are seen in the thymus.


The thymus reaches its fullest growth in early childhood (3rd or 4th year), and continues large as long as the body is in a state of active growth. It begins to shrivel up when maturity is reached, and only a remnant is left as a rule, less remaining in men than in women. It receives its blood supply from the 4th aortic arches through the internal mammary. In manner of origin it resembles the tonsil ; indeed it may be regarded as a buried tonsil. There is a profuse production of lymphoid cells in the gill clefts of fishes, many of which wander out, and by their phagocytic properties help to keep the gill surfaces clean. This fact throws some light on the origin of so mucli lymphoid tissue from the second and third cleft recesses in higher animals.

The Thyroid

The site at which the thyroid gland[9] arises is shown in Fig. 269 — on the floor of the pharynx behind the mandibular arch and ' exactly in the middle line. The entoderm of the retro-mandibular furrows . gives rise to a saccular diverticulum almost as soon as the fore-gut becomes differentiated — early in the 4th week of development. Immediately in front of the thyroid evagination arises the buccal part of the tongue ; behind it the pharyngeal part, the foramen caecum in the sulcus terminalis remaining to mark the site of origin. The entodermal vesicle thus formed grows downwards and backwards through the tissue in which the body of the hyoid will be formed and as it extends, bifurcates. The stalk of the . evagination, at first hollow and representing a duct, quickly becomes solid, breaks up, and by the 6th week has disappeared. The epithelium of the evagination proliferates, and in the 7th week forms a transverse plate ventral to the larynx (Fig. 252). The plate is invaded and broken up into reticulating columns by the surrounding mesoderm. In the 3rd month the epithelial cells become arranged as follicles ; these at a later date are converted into vesicles. The original plate assumes a bent or horse-shoe form, the middle part forming the isthmus, the side parts the lateral lobes (Fig. 271).


The thyroid is present in all vertebrates and, although it arises in a manner which suggests that at one time it was a gland of the mouth, yet in no animal does the duct persist. Its early origin in the embryo and its universal distribution in vertebrates point to the antiquity and importance of its function. We now know that duct glands have a double function, producing both external and internal secretions. It is reasonable to suppose that, as regards the thyroid, its excretory function has been lost. Thyro-glossal Duct. — In the great majority of subjects the thyroglossal duct or stalk completely disappears at the commencement of the 2nd month of development ; the foramen caecum marks one extremity, while a ligament or a pyramid of thyroid tissue prolonging the isthmus towards the hyoid bone often marks the other extremity (Fig. 271). The pyramid of the isthmus may carry on it a detached part of the thyrohyoid muscle — the levator glandulae thyroideae. The body of the hyoid bone is developed in the tract of the thyro-glossal duct (Figs. 271, 272) and splits it up. Remnants of the duct or of secondary detached acini of the thyroid may persist and form cysts or thyroid tumours in the base of the tongue above the hyoid, and commonly between the genio-glossus muscles. They may also occur between the hyoid and thyro-hyoid membrane. The supra-hyoid or infra-hyoid bursae may also become cystic, and may be mistaken for thyro-glossal cysts (see Fig. 272).


In lower vertebrates the lateral lobes of the thyroid are stationed under the mandible. It is not uncommon to find in the right submaxillary region of man a thyroid tumour or cyst, evidently arising from an arrest in the descent of a part or of the whole of a lateral lobe. Aberrant masses of thyroid are often met with in the neck, and frequently become the site of cystic tumours. Occasionally the lumen may persist in the median thyroid and open as a fistula in front of the larynx (Fig. 272).

Ultimate Branchial Bodies

In Fig. 270, B, is represented the entodermal outgrowth from the 5th or ultimate pharyngeal pouch. At one time it was supposed that the entodermal outgrowth — the ultimate branchial body — gave rise to the greater part of the lateral lobes of the thyroid. They do give rise to tissue which is thyroidal in structure, often containing tube-shaped vesicles. The tissue so produced is applied to the dorsal aspect of the lateral lobes of the thyroid, but forms a very small part of their glandular mass. Like the thymic buds they lose their connection with the embryonic pharynx by the 7th week. The pyriform fossa, within the ala of the thyroid cartilage marks their point of origin (Fig. 272). The blood supply suggests a double origin for the thyroid gland, for while the superior arteries supply the area formerly assigned to the median outgrowth, the dorsal parts of the lateral lobes are nourished by the inferior thyroid branches of the 4th aortic arch.

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Fig. 272. Section of the Pharynx to show the Track of the Median Thyroid Outgrowth. In rare cases there is a flstvila connected with the thyroid, which opens in front of the larynx. The point of origin of the thymus outgrowth from the 3rd cleft may be marked by a recess containing lymphoid tissue as is represented in the figure. The pyriform fossa occurs at the site of the 4th and 5th clefts. The group of mucous glands in front of the epiglottis may give rise to cystic tumours.

Para-thyroids

There are usually two para-thyroid[10] or epithelial bodies on each side, an upper and a lower (Fig. 271). Both are usually applied to the deep or posterior aspect of the thyroid body, the upper being situated amongst the terminal branches of the superior thyroid artery, the lower amongst the branches of the inferior. They are flattened bodies, about 6 to 8 mm. in diameter, yellowish in colour when contrasted with the substance of the thyroid, but they cannot be recognized with certainty except by their microscopical structure. Their origin is shown in Fig. 270 ; the lower bodies arise from the dorsal recess of the 3rd pair of pouches ; they are drawn into a low position by their attachment to the stalk of the thymus (see Fig. 270). The upper para-thjo-oids arise from the 4th pair of pouches (Fig. 270), and become more or less united to the ultimate branchial bodies, and with these are applied to the dorsal aspect of the lateral masses of the thyroid. In structure they are made up of reticulating columns of cells, with vessels arranged between the columns, thus resembling in structure the carotid body, and probably also in nature and origin the medullary part of the supra-renal. Their presence is essential to the function of the thyroid body.

Carotid Bodies

The carotid body lies at the inner side of the fork between the internal and external carotid arteries. The commencement of the internal carotid represents the artery of the 3rd arch ; that of the external carotid, the ventral aortic trunk. The body is developed near the 3rd pharyngeal pouch with the thymus (Fig. 253). In the carotid fork nerve cells assemble which are derived from the superior cervical ganglion ; the body is linked to the superior cervical ganglion by numerous nerve fibrils. It is essentially parasympathetic in nature, being made up of chromaffin cells, similar to those of the medulla of the adrenal bodies.




  1. E. Kallius, Anat. Hefte, 1910, vol. 41, p. 177, etc. ; J. L. Paulet, Archiv. f. mikro. Anat. u. Entioick. 1911, vol. 76, p. 658 ; also reference on p. 226, under Frazer.
  2. See Warren H. Lewis, Keibel and Mall's Manual of Human Embryology, 1910, vol. 1, p. 518.
  3. Prof. Wood Jones, Journ. Anat. 1918, vol. 52, p. 345.
  4. J. Schaffer, Anat. Hefle, 1907, vol. 33, p. 455 (Evolution of Epiglottis).
  5. See reference under Paulet, p. 235 ; also W. Rubashkin, Anat. Hefte, 1912, vol. 46, p. 343.
  6. For an account of the comparative anatomy and development of the tonsil see paper by Seccombe Hett and Butterfleld, Journ. Anat. and Physiol. 1910, vol. 44, p. 35.
  7. T. H. Bryce, Journ. Anat. and Phjsiol. 1906, vol. 40, p. 91 ; P. Stoehr, Anat. Hefte, 190o, vol. 31, p. 409 ; J. A. Hammar, Ergebnisse der Anat. 1909, vol. 19, p. 1 : Anat. Hffte, 1911, vol. 43, p. 201 (Thymus) ; Fraser and Hill, Phil. Trans. 1916, vol. 207 (B) p. 1 ; B. F. Kingsbury, Amer. Journ. Anat. 1915, vol. IS, p. 329.
  8. r. E. T. Bell defends the theory of their Epithelial origin, Amer. Journ. of Anat. 1906, vol. 5, p. 30.
  9. Edgar H. Norris, Amer. Journ. Anat. 1916, vol. 20, p. 411; 1918, vol. 24, p. 443.
  10. For a full account of the comparative anatomy of the para-thyroids see Dr. Forsyth's Memoir in Journ. Anat. and Physiol. 1908, vol. 42, pp. 141, 302. He found that the para-thyroids are irregular in number and often aberrant in position, and that it is very difficult to distinguish microscopically between embryonic thyroid tissue and adult para-thyroid tissue. The para-thyroids were discovered by Sandstrom in 1880. See also F. D. Thompson, Phil. Trans. 1911, vol. 201, Ser. B, p. 91.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
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