Human Embryology and Morphology 28

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 XXVIII. Skin and its Appendages

Stages in the Evolution of the Skin

We have already seen that the structures which are developed in the human embryo can be best explained by supposing that at one stage of evolution the ancestry of mammals lived and breathed in water. The skin of the human embryo until the end of the 2nd month of development is translucent, and has many points in common with that of the lowest gill-bearing vertebrates. It then consists of two layers — a deep or germinal, consisting of cubical epithelium and a superficial, made up of fattened cells (Fig. 481). In the 3rd month this superficial layer, known as the epitrichium or periderm, becomes horny in nature, recalhng a stage which represents the evolution into a terrestrial form of life. The appendages of the skin — its hair and glands — appear later ; they seem to be modifications of glandular and sensory structures seen in the soft skin of amphibia. The hairs are developed in groups and lines.^ Their arrangement can be best explained, according to Dr. Max Weber, by supposing that the skin of primitive mammals was covered by scales, and that the hairs sprouted out in groups at their tessellated junctions, as in certain living edentates (see Fig. 480). The human hairs are arranged in irregular series, but in most instances only the chief hair of a group is developed. In later period of foetal life, however, the chief hair has one or two subsidiary hairs planted on either side of it — making one of a group of three or five hairs.


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Fig. 480. Showing the Arrangement of Hair Groups in the Human Foetus and their Relationship to Hypothetical Dermal Scales. (Stohr.)


The skin of man, compared to other primates, is comparatively hairless. We must regard his nudity as a lately acquired character. At the 7th month of foetal life the chimpanzee and gorilla have hair only on the scalp, eyebrows and lips ; the rest of the body is nude, except for fine hairs or lanugo. This is also the condition in the human foetus at a corresponding period ; in man, although the foetal crop of lanugo is succeeded by a general outgrowth of fully developed hair, yet we may regard the human condition as representing an arrest of hair development at a stage seen in foetal apes. The human skin is also more sensitive and more richly supplied with sensory nerves than is the case in other primates. In Professor Elliot Smith's opinion the rich sensory supply to the skin must have been a factor in bringing about the large size of the human brain. In the distribution and " lie " of the hair on his body and limbs man also resembles the hairy anthropoids.


1 Stohr, VerJiand. Anat. Gesellsch. 1907, p. 153. 462


There are on record a number of cases of men and women, in whom the whole surface of the body was covered with a close covering of hair. The development of hair on the face is certainly regulated by a secretion derived from the sexual organs, for in eunuchs the beard is never developed. It is also well known that the thyroid has a direct influence on the development and growth of hair. Desquamation from the epidermis begins in the 3rd month of foetal life, and never ceases until death. In a certain disease of foetal life, named Ichthyosis, desquamation does not take place ; the unshed epidermis forms cracked cakes on the surface of the child at birth.


Development of the Skin

Considerable assistance in the understanding of the diseases to which the skin is liable and of the nature of the growths which arise from the epidermis, such as corns, bunions, and cancer, is to be obtained by studying the manner in which the skin is developed. At first the human embryo is covered by a single layer of epithelium (epiblast or ectoderm), as is the case in the adult amphioxus. By the end of the 1st month there are two layers, the lower representing the germinal or basal layer ; the upper the epitrichimn, so named because it was supposed that hairs are developed beneath it, and when they grew out in the sixth month this surface layer of flat epithelium was shed. This evanescent foetal layer is also known as the periderm.


In the 4th month we find developmental processes in full activity in the skin ; three strata are recognizable in the epidermis — all derived from the single germinal layer. These are (1) a basal layer — a single stratum of cubical or columnar cells, representing the primitive germinal epithelium (Fig. 481, B) ; (2) an intermediate or mucous stratum, several cells deep ; (3) a heaped-up superficial or corneous stratum, representing the protecting but perishing superficial covering of the skin. At the same time the opening phases in the development of hair follicles, sebaceous and sweat glands and of skin ridges and papillae are to be detected. In the 5th month the stratum lucidum becomes differentiated between the mucous and corneous strata.[1]

The epidermis rests at first on undifferentiated mesoderm, consisting of small round cells closely imbedded m a mucoid matrix. This is the normal structure of undiSerentiated mesoderm. The superficial mesodermal cells are condensed beneath the epidermis to form a corium towards the end of the 3rd month ; an areolar or subcutaneous stratum of tissue is differentiated at the same time. Connective tissue fibrils begin to develop in connection with the mesodermal cells and by the fifth month the mucoid substance has almost disappeared ; but even in adult life, when the thyroid body is diseased or removed, a mucoid substance may reappear, and a condition resembling the foetal state be thus produced. In the mucous membranes of the lips, anus and vulva, the superficial layer of epithelium does not become cornified.

Formation of Dermal Papillae

Up to the end of the 3rd month the epidermis is easily detached from the corium as a flat membrane, but early in the 4:th month they become more closely united by ridges of epidermis becoming folded within corresponding furrows on the corium. About the ith month, the dermal papillae, which are grouped in lines and ridges as is well seen in the palm, are formed in the following manner : Long, linear furrows of epidermis grow down into the dermis (corium) and divide its surface into narrow ridges (Fig. 481, B). These ridges are subsequently subdivided into papillae. The down-growing nature of the ectodermal (epidermal) cells which is here exemplified, is of the greatest clinical importance. The enamel organs, we have seen, arose by a species of downgrowth of the epidermis ; so do hairs, sweat glands and sebaceous follicles. Prolonged pressure and friction welds the corneous cells into a solid plate, such as the callosities seen on the palms of manual labourers. Normal desquamation is arrested ; the cells produced in the deeper layers, unable to grow to the surface, grow inwards and produce corns. In cancer, the epithelial cells of the skin renew their youth and invade the dermis and deeper tissues.

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Fig. 481, A. Diagrammatic Section of the Skin at the commencement of the second month. B. Diagrammatic Section of the Skin at the commencement of the fifth month, a. a. a. Infoldings of the epidermis between the primary ridges.


1 For literature see E. J. Evatt, Journ. Anat. and Physiol. 1907, vol. 41, p. 66. Also paper by Walter Kidd, same volume, p. 35. M. Heidenhain, Anat. Hefte, 1906, vol. 30, p. 419. 0. Schlaginhaufen, Ergebnisse der Anat. 1905, vol. 15, p. 628. H. H. Wilder, Amer. Journ. Anat. 1901, vol. 1, p. 423. Walter Kidd, The Sense of Touch in Mammals and Birds, London, 1907 ; The Initiative in Evolution, 1920.


Sweat glands arise as buds from the ectodermal troughs (Fig. 481, B). Their ducts open on the surface of the skin in lines or rows corresponding to the primary epidermal furrows. In the 5th month the epidermis round their mouths is raised up into ridges, and it is these ridges which give rise to the papillary patterns on the balls of the fingers and elevations of the palm. It will be thus seen that the epidermal ridges correspond not to the lines of dermal ]3apillae, but to the furrows of epidermis lying between the papillae.


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Fig. 482. The more common patterns formed by the Dermal Papillae on the Tips of the Fingers. A, The Loop Pattern. B, The Triangle Pattern. C, The Whorl Pattern.


The papillary lines on the palms and fingers give security of grasp (Hepburn). They are arranged in most variable patterns, but the prevailing types in man are those arranged as loops, spirals or whorls (Fig. 482). So much does each pattern vary and so variable is the sequence of the patterns on the pulps of the digits, that no two people show exactly the same pattern in the same order counting from thumb to little finger in both hands. Hence the impress of the ten finger-tips has been successfully used in the identification of criminals — a practical discovery made and put into use by Sir Edward Henry.

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Fig. 483. The " Pad " Elevations on the Palm and Sole of a Human Foetus at the end of the 2nd month of development. (After Retzius.^)


These epidermal patterns are formed on elevations which appear on the human hand and foot at the end of the 2nd month, and which certainly correspond to the horny pads found on the feet of quadrupeds. Besides the elevations on the terminal phalanges there are five situated on the palm and sole at the base of the digits. Three others are situated on the proximal part of the palm. In the human foot the elevation corresponding to the hypothenar elevation of the palm undergoes a remarkable enlargement to cover the heel (Fig. 483).


Biolog. Untersuch. 1904, vol. 11, p. 33, Jena, 2g


2 See references, p. 464.

The Hairs

Hairs begin to develop in the 4th month, although in some regions, such as the eyebrows and lips, their formation begins a month earlier. Morphologically, a hair may be regarded as a dermal papilla which has sunk in the subcutaneous tissue, and become capped by a process of epidermis. Hairs appear to have been primarily touch organs, and are modifications of the touch bodies found in the skin of reptilia (Gegenbaur). These touch bodies are composed of epithelial cells, having the same shape and arrangement as those which form the taste buds in the circumvallate papillae of the human tongue. The cells which cap the hair papilla evidently represent the primary sensory cells of the touch bodies ; they are situated in line, and continuous with the basal or germinal layer of the skin. The primary function of the hairs as touch organs is seen in the vibrissae round the mouths of carnivora. Fnedenthal has found that certain of the hair-roots in the lips and eyebrows of the human foetus develop the same large sensory bulbs as are found in the roots of the vibrissae of lower mammals.

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Fig. 484. Three Stages in the development of a Hair Follicle. (After Stohr.)

A, Hair Follicle, commencing to form in a Foetus of 3 months.

B, The downgrowth of the follicle and mesodermal thickening to form papilla.

C, Invagination of Follicular Bulb by Papilla with formation of Matrix Cone.


The first stage in the development of a hair is the ingrowth of epidermis as a solid bud, which pushes in front of it the dermis to form the papilla on which the hair grows (Fig. 484). Only the two deeper of the primary layers of the epidermis are carried inwards to form the hair sheath and hair root. Three stages in the development of a hair follicle are shown in Fig. 484. As the follicle grows downwards the fundus of its shaft expands to form a bulb. Outside the follicular bulb mesodermal cells collect to become a papilla. Presently the bullj is invaginated by the papilla, which thus becomes clothed by the central cells of the basal stratum while the surrounding wall of the bulb is lined by peripheral basal cells. On the papilla and within the shaft of the follicle is produced a mass of cells— the hair cone (Fig. 484, C) — the first rudiment of a hair. The central cells on the papilla give rise to the cells which form the core or pith of the hair shaft ; from the peripheral cells arises the inner rootsheath. The outer root-sheath is formed by the lining cells of the follicular shaft. As will be seen from Fig. 484, C, a sebaceous gland is produced from the shaft of the hair follicle, while the erector muscle arises from the shaft at a deeper level.


1 See Friedenthal, Zeitschrift fur Ethnol. 1911, vol. 43, p. 974; K. Backmund, Anat. Hefte, 1904, vol. 26, p. 315 ; P. Stohr, Aiiat. Hefte, 1904, vol. 23, p. 1 ; L. Stieda, Anat. Hefte, 1910, vol. 40, p. 285.


The hairs produced at the 4th month are fine in texture (lanugo), and by the 7th month the whole body is covered by them. The hair roots of the eyebrows, eyelids, and of the lips and scalp are the first to appear. The production of hair buds goes on until birth, the later buds and hairs being thicker and stronger. After birth, new hairs are constantly reproduced within the sheaths to replace the old. Probably the manner in which new hairs are produced resembles that of teeth, viz., from processes of the original bud. Hairs appear first on the head and then on other parts of the body. The fat in the subcutaneous tissue takes the place of hair as a heat conserver. Certain sexual hairgrowths appear at puberty on the face, pubes and axilla. Morphologically, the pubic region represents the separated axillary regions, and probably th.e explanation of sexual hairs in the axilla is due to this correspondence, for there is a persistent tendency towards symmetry of development in the upper and lower extremities. The primitive mammary ridges, also sexual structures, end at the axilla and groin.

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Fig. 485. Diagrammatic Section across a Nail.

Nails

The nails are made up of three strata representing the basal layer of cylindrical cells, the stratum mucosum and the stratum lucidum of the skin, the corneous layer being lost after the 4th month of foetal life. They appear first in the 3rd month as fields of thickened epidermis on the tips of the digits (Fig. 473), but are afterwards shifted dorsally, carrying their palmar nerves with them, so that the terminal phalanx is wholly supplied from the palmar digital branches. At the end of the 3rd month the germinal layer of epithelium at the proximal margin of the nail field forms a lamina which grows into the dermis to form the root and is thus overhung by a reflection of skin — the nail fold (Fig. 486). The nail of the little toe, a digit in a retrograde phase of development, is frequently shaped like a claw, probably a reversion to a primitive form. The nail is produced on the scattered papillae (the matrix) at its root. The area of production is marked by the lunule. On the nail bed, in front of the lunule, the papillae are arranged in longitudinal rows. If the nail be pressed, as by the boot, the lateral papillae, under the nail fold (see Fig. 485) are directed downwards, and their epithelial outgrowths follow the same direction, thus causing ingrowing nail.


About the end of the 7tli month the matrix of the nail root becomes differentiated, active growth sets in and the terminal margin of the nail becomes free ; it grows forwards over the corneous layer which covers the terminal row of papillae of the nail bed. The ridge of corneous epithelium under the nail-tip represents the central part of the hoof (" frog ") of ungulates (Fig. 486).

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Fig. 486. Diagrammatic Section of the Terminal Joint of tiie Digit of a Human Foetus to show the Cap of the terminal Phalanx and the Subungual Welt.


The nail is carried by the terminal phalanges. Professor Leboucq observed that the tip of the terminal phalanges of the foetus is covered by a special fibrous cap[2] (Fig. 486), which undergoes ossification directly from membrane, while the rest of the phalanx is laid down and ossified in cartilage. The terminal phalanges have thus a special element added to them for the support of the nail and for the fixation of the terminal bulb of the digits.

Sweat Glands

In the 4th month solid processes of epidermis grow into the dermis from the ectodermal troughs and also from the necks of hair follicles and produce sweat glands (Fig. 481, B). They arise at the same time and in the same manner as, and often in common with, the buds of hair roots and sebaceous glands. They are produced within the epidermal ridges, and hence the ducts of sweat glands, as may be seen on the palms and fingers, open along the summits of these. The sweat glands in the axilla arc peculiar. In section they resemble the acini of the mammary gland, also believed to be highly modified sweat glands. The axillary glands contain much epithelial debris. They appear to be sexual in nature. The wax glands of the external auditory meatus are also modified sweat glands.

Sebaceous Glands

The sebaceous glands are outgrowths from the more superficial part of hair buds (Fig. 484). Their epithelial lining is derived from the germinal layer. In hair sheaths which have become occluded after their hairs have been shed or lost, or when the mouth of a gland is blocked, the secretion is retained, and a sebaceous cyst or wen, so frequently seen in the scalp, is produced. Round the mouth, on the lips and nose, the sebaceous glands, especially in disorders of the sexual organs, are apt to retain their secretions and become inflamed, smaU pustules being thus produced. The Meibomian glands in the eyelids are modified sebaceous glands. At birth the child is covered by the vernix caseosa, which is composed of desquamated corneous epithelium and the secretion of sebaceous glands.


Mammary Glands

Evolutionary History

It is a remarkable fact that although the milk glands do not come into use until adult life and although they must be regarded as among the later evolved structures of vertebrate animals, yet they are the first of all the glands arising from the epidermis to appear during development of the embryo. In the human embryo of the 6th week or in the corresponding stage of a pig (Fig. 487), or of any other mammal, the primary mammary ridge or milk line — a mere surface thickening of the ectoderm — is seen extending along the body wall on either side from axilla to groin. Breslau[3] regards these primary ridges as representatives of the brooding organs of the ancestors of mammals, from which structures he supposes that the mammary glands were evolved. In a large number of human beings (15 %) one or more supernumerary nipples are to be found between the axilla and groin, indicating the wide distribution of ancestral glands. There is no longer any doubt that the mammary acini and ducts have been modified from sweat glands ; a mamma represents a group of sweat glands developed from a circumscribed area of skin lying under the primitive mammary ridge. Nor are there two opinions as to the stages in the evolution of the human nipple ; they are repeated in its development. In its primitive form the nipple is represented by a pocket — an invaginated area of mammary skin — on the wall of which milk ducts open. This pocket — an inverted nipple — becomes everted, chiefly by a proliferation of the tissues round the terminal parts of the duct, which raises the interior of the pocket first to the level of the surrounding skin and then above it to form a nipple — an everted mammary pocket. Further, the mammary ridge appears in both sexes alike, but this may not mean that both sexes of ancestral mammals were concerned in brooding or gave milk. The male is the father of girls as well as of boys ; it is therefore necessary to provide both father and mother with a complete sexual outfit if each sex is to provide equal shares to the making of their progeny. In females the breasts undergo a great development at puberty, while in males they retain their infantile form.


The Female Breast is composed of two embryological elements :

(a) Glandular tissue derived from the ectoderm by a process of inbudding
(b) An intricate arrangement of connective tissue derived from the mesodermal subcutaneous tissue over the pectoralis major.


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Fig. 487. Embryo of a Pig, showing the Mammary Ridge extending from Axilla to Groin. (After Schultze.)

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Fig. 488. Diagram to show the Position in which Supernumerary Nipples are usually found. (After Merkel.)


Seven stages may be recognized in the developmental history of the glandular mammary tissue. Four of these take place before birth :

  1. The stage represented by the ectodermal ridge passing from axilla to groin — formed during the 6th week (Fig. 489, A).
  2. The production of a bulb-like downgrowth of ectoderm from the pectoral part of the mammary ridge. This downgrowth represents the pocket form of nipple (Fig. 489, B).
  3. From the deepest stratum of the ectodermal bulb arises a number of solid buds, exactly similar to those of sweat glands (5th month). The stalks of these buds form the epithelial lining of the lactiferous ducts (Fig. 489, C).
  4. The lobular buds, for each bud develops into a lobe, subdivide at their growing extremities. At first solid, they begin to canaliculize (7th to 9th months). At or about birth the pit or depression, from which the lobular buds originated, is raised, evaginated, and forms the surface of the nipple (Fig. 489, D). Thus the ducts come to open on the apex of the nipple. An ampulla is developed in each duct within the base of the nipple. It is normal for the glandular tissue of the newly born child to secrete milk during the two weeks following birth (Roger Williams).


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Fig. 489. Showing the various Stages in the Development of the Mamma. A, during the 2nd month ; B, at the commencement of the 3rd month : C, at the 5th month ; D, at birth. A = Ectoderm; B = Subcutaneous tissue (mesoderm) ; c = Pectoralis major.

Stages after Birth

Stage 5 occurs at puberty ; the latent infantile lobular buds again undergo a rapid growth, and give rise to the minor lobules and acini.

Stage 6 occurs towards the end of pregnancy, and consists of a renewed production of glandular tissue.

Stage 7 sets in with the menopause, and is characterized by an atrophy of the glandular tissue formed in the later stages of development.


In the process of subdivision, minor buds of adjacent lobes frequently unite together. Hence it is found difficult, during dissection, to separate the gland into its primary lobes. In any of the three later stages a localized and invading hypertrophy of the cells of the glandular tissue may take place. In this manner cancer is produced. The part played by the lymphatics, which are situated in the mesodermal tissue of the gland, in the spread of this disease, makes their study important.

Origin of the Capsular or Mesodermal Part of the Gland

As the glandular buds grow into the subcutaneous mesodermal tissue, which reacts and hypertrophies around the invading processes, they divide it (see Fig. 490), into {a) superficial, and (6) deep layers, these being joined together by (c) interstitial septa. The superficial and deep layers are fused in {d) the circum-mammary tissue in which the final glandular buds terminate. The processes as they grow outwards also take on (e) perilobular and periductal sheaths. The deep and superficial layers are also connected with the anterior sheath of the pectoral muscles and the skin — for they are all parts of the same subdermal layer.


Lymphatics

We have already seen (p. 337) that during the 3rd month the skin and subcutaneous tissues become invaded by the developing system of lymph vessels, the pectoral system lying chiefly in the zone arising in connection with the jugular lymph sac. As each part of the capsule carries with it lymph vessels of the pectoral subdermal area it will be seen that the arrangement of the parts of the capsule is an important matter in both the physiology and surgery of the gland. The periductal and perilobular lymphatics communicate through the septal or interstitial vessels with the superficial mammary and deep (retromammary) lymphatics (Fig. 490). The superficial communicate with the subcutaneous ; the deep with those in the pectoral siieath, and thus it will be seen that mammary cancer may spread to the skin or pectoralis major. The deep and superficial join in the circum-mammary lymphatics, and from these pass efferent vessels to the pectoral and central glands of the axilla. The lymph passes from these to the deep axillary and inferior deep cervical glands — all of which are involved in late stages of cancer of the breast. Other efferent vessels pass from the circum-mammary to the anterior intercostal glands of the upper four spaces ; one or two vessels may go to the cephalic gland. During the mammary hypertrophy, which takes place at the end of pregnancy, there is a further formation of lymphatic glands in the axilla (Stiles).


Fig 490 Diagrammatic Section of the Breast to show the Arrangement of its Capsule and Lymphatics

Fig. 490. Diagrammatic Section of the Breast to show the Arrangement of its Capsule and Lymphatics. The lymphatic vessels are represented by thin wavy lines.

Peripheral Remnants

Besides accessory nipple ingrowths, which are to be found in most foetuses of the 3rd month, isolated or semi-isolated small masses of glandular substance may be found situated in the circummammary tissue, beyond the body of the gland. Some may pierce the sheath of the pectoralis major, and become a source of recurrent cancer.


The presence of glandular remnants is explained by the fact that, when the primary budding takes place, the subdermal tissue is shallow and of small extent ; in the subsequent growth of the thorax, the tissue in which the mamma is developed is widely spread out.


Fat begins to be deposited in the subcutaneous tissue during the 5th month of foetal life. It forms a large element of the mammary gland after puberty. The subcutaneous tissue, out of which the capsule of the gland is formed, normally contains much fat. After lactation, when the glandular tissue atrophies to a considerable extent, a growth of fat replaces it. If no fat is deposited, or if it be absorbed, then the breast loses its plump form and hangs on the chest.


The mammary nerves (secretory) come from the 3rd, 4th and 5th intercostals ; the nipple is supplied from the same nerves. The nipple contains non-striated muscle, and is covered with touch papillae, and surrounded by modified sweat and sebaceous glands.


Dermis and Subcutaneous Tissue

The subectodermal tissues, out of which the dermis and subcutaneous stratum are differentiated, is at first composed of cells of rounded outline embedded in a homogeneous jelly-like matrix — a syncytium. Mall regarded the matrix as a living substance in which, quite independently of the cells, connective tissue fibres are differentiated, both white and yellow. Processes are certainly developed from the cells, but it is doubtful if these ever become detached and form independent fibres.


Fat Cells

Certain granular cells of the connective tissue, especially of the subcutaneous layers, have the property of secreting fat, which appears first as diffuse droplets. These ultimately run together and produce the characteristic outline of adipose cells. Fat cells appear first in the subcutaneous tissue during the 5th month of foetal life ; later it appears in the subserous tissue of the body wall. It reaches its greatest normal development just before and after birth. Two theories are held regarding the origin of fat cells : (1) that they are cells of the connective tissue differentiated and set aside permanently to form and store fat ; (2) they are ordinary connective tissue cells temporarily laden with fat.[4] There is present at birth a sharply differentiated mass of fat and lymphoid tissue in each posterior triangle of the neck and extending on each side beneath the trapezius muscle. Hatai regards this mass as the representative of the interscapular gland of hibernating mammals (see p. 339).


Touch Bodies and Sense Organs

The cells of the ectoderm in the simpler forms of invertebrate animals not only protect the body but many of them become specially sensitive or nervous in nature, developing processes which link them with neighbouring or even distant cells and thus are able to afford the animal knowledge of its surroundings. In the development of the olfactory mucous membrane, of the auditory cells and of the taste buds of the human embryo, this specialization of areas of the ectoderm is seen. The retina, the brain, spinal cord and nerves are also areas of the ectoderm which have been highly specialized and set aside for the purpose of correlating the organism with its surroundings. Such cells may migrate and become grouped in central masses of the nervous system. Dart and Shellshear have shown that the dermal origin of neuroblasts is a factor of importance in the origin of the nerve system. Although the various forms of touch bodies, such as the Pacinian corpuscles and those of Krause and Meissner, have not been traced developmentally, there can be little doubt that they arise directly from the epidermis beneath which they are situated.



  1. Comparative anatomy of epidermis: see F. K. Studnicka, Anat. Hefte, 1909, vol. 39, p. 1.
  2. F. A. Dikey, Journ. Anat. and Physiol. 1906, vol. 40, January. 2 P. Diem, Anat. Hefte, 1907, vol. 34, p. 187 ; C. Schoeppler, Anat. Hefle, 1907, vol. 34, p. 429.
  3. The Mammary Apparatus of the Mammalia, with Introduction by Prof. J. P. Hill, London, 1920,
  4. J See article by Batty Shaw, Journ. Anat. and Physiol. 1902, vol. 36, p. 1.


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