1897 Human Embryology 25

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History
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Chapter XXV. The Epidermal System

That portion of the ectoderm which remains upon the surface of the embryo is called the epidermis; it constitutes the outer skin; for convenience the inner skin (cutis or dermis) is treated in connection with the history of the true skin in this chapter. We have also to consider the development of the following epidermal appendages; nails, hairs, and glands.

I. The Skin

Epidermis. — The ectoderm of all amniote vertebrates is at first a single layer of cells, which presents considerable variations in appearance not only in different classes, but also at different stages of the same species, and even in different parts of the same embryo. Since in all invertebrates the ectoderm consists of a single epithelial layer, we may call the first stage of the vertebrate epidermis the invertebrate stage. The appearance of the ectoderm while in this stage has been indicated by the figures and descriptions scattered through Chapters V. -XV., and until a comprehensive study of the ectoderm of amniota in the one-layered condition shall have been made, it is impossible to give a minute description of it possessing much value or any interest. The epidermis of Amphioxus and the ectoderm of the amnion never pass beyond the one-layered stage, p. 335.

In its aecond stage the epidermis becomes two-layered. The cells of the single layer become irregularly placed ; some have their nuclei nejirer the outer, others nearer the inner, surface of the ectoderm. The difference rapidly increases, and though for a time the cells stretch through the whole thickness of the layer, yet they gradually draw away, some fnim the upper, others from the lower, surface, until they have definitely arranged themselves in two distinct layers, l^ig. 30(i. This stage is established in the human embryo by the end of the first month, and i>ersists over part certainly of the embr^-o, at least until the close of the second month.

Fig "Ve —Section of ti Human Emiiiyo of Ql < nlleilloD No "" r luyw of epitleri

In stained sections the outer layer, Fig. 306, a, is composed of somewhat flattened cells, with iireja^ularly shaped, slightly granular nuclei, and are darker and thicker walls than the cells of the inner layer. These latter, Fig. 30G, 6, are larger and clearer, and have larger, more granular nuclei of round shapes. The appearance of the outer cells suggests a necrotic change, feowen's careful researches, 88. 1, render it probable that the outer layer is the epitrichium, compare below.

It is a remarkable fact that the primitive blastoderm in amphibia, teleosts, and ganoids never passes from the several-layered to the one-layered condition, but only to the two-layered condition. For description of this stage in Bombinator, see A. Goette, 75.1, and in t^le<:)st8 see M'Intosh and Prince, 90.1, 739, in Lepidosteus, Balfour and Parker, 82.1. The developnient, therefore, in this group of forms, offers a marked difference from that found in marsipobranchs and amniota, but since in Petromyzon we encounter the one-layered stage, we must cimsider the succession of stages adopted in this chapter as the primitive one, and conclude that the precocious api)earance of the two-layered stage in amphibians, etc., is a secondary modification, the cause of which is unknown. That the two layers of the epidermis are homologous throughout the vertebrate series, we have no reason to doubt (Balfour, '*Comp. Embrj^ol.,'* II., 3()U). Where the epidermis has an initial division into two layers, the inner is commonly termed the nervous layer, and it has the main share in forming all the organs derived from the epidermis ; the outer layer, according to homologies I hold to be probaole, must 1x3 identified with amniota epitrichium, although unlike the true epitrichium it disappears as a distinct layer, its cells showing themselves between those of the inner layer (Goette, 75. 1, 158).

The ectoderm of the chorion and umbilical cord never advances beyond this stage, unless we regard the formation of the chorionic cellular Liver as such an advance.

The third stage is very gradually reached by the increase in the numlx^r of layers until there are several. I consider it probable that this stage is established in two ways — one, the more primitive, involves the disiippearance of a distinct outer layer, as in amphibia; the other depends upon the preservation of the outer layer, as the epitrichium. This view can be advanced, at present, only as an hyi)othesis.

1 . The primitive method is maintained in amniota only over very limited special regions ; as such I vcmture to designate the cornea, the nasal pits, the mouth cavity and lips, and the anal ectoderm. Over these parts the distinct outer layer disappears as such, and we have developed a stratified epithelium, which never produces a true horny layer, but consists of a basal row of protoplasmatic cells, and several layers of cells above, which are clear in appearance and have thickened walls. The details of the process of differentiation have not yet l^een worked out.

'2. The secondary method of forming the several-layered epidermis is establishes! over the skin proper. It can be well seen in the human embryo of the third month. In an embryo of two and onehalf months. Fig. 307, there are four to five layers of cells. The basal layer, 6, is composed of a single row of cuboidal cells, which are rich in protoplasm, though small in size, and which have round nuclei. This basal layer persists throughout life in all amniota, and is one of the most characteristic features of the amniote epidermis.

Fig. 807.— Epidermis from the Occiput of the Human Embryo of two and one-half Months. Eptr^ Epitrichial layer; m, Malpighian layer; 6, basal layer. After Bowen.

Above the basal comes the middle layer, which varies from two to three cells in thickneas ; its cells are irregular in shape and size, and are so large that the nuclei of many of them do not appear in the section. The outermost layer, Epti\ is the epitrichiimi, and consists of a single layer of large dark cells, which from their arching up may be termed dome cells. It is probable that the epitrichium is the outer layer of the second stage preserved and modified, and that all the middle cells come from the inner layer of the previous stage, but conclusive proof of this identification is still required. The history of the epitrichium is treated in the next section.

The fonrtn stage is characterized by the presence of a homy layer {stratum cornetun). The stratum comeum presents marked variations in structure, and it is probable that, as explained in the following paragraph, at least two morphologically distinct layers have been confused under a comnion name. Unfortunately almost nothing is known conc*eming the genesis of the horny layer, Bowen 's observations, 89.1, render it probable that it arises from the epitrichium, but if tliis view he aiiopted we encounter certain difficulties which our present knowledge cannot remove. If Bowen 's hypothesis is correct, we must define the fourth stage as characterizeil by the cornification of the thickened epitrichium. Concerning the process of cornification we possess some information, which is referred to more fully under the head of nails, ]). 555. When the homy layer is produced the skin is considerably thickened and the number of layers of cells which it comprises is much increased. The line of division between the homy layer and the underlying mucous or Malpighian layer becomes quite sharp. It must be assumed that cells of the deep layer are added to the homy layer.

They?/f/i .s/af/(^ is estiiblished by the development of the stratum lucidum, Bowen has made the important discovery that the stratum lucidum of the human embryo lies immediately underneath the epitrichium, and is directly continuous with the nail, and the epitrichium is continuous with the homy layer outside the stratum lucidum. Bowen suggests, 89. 1, 440, that, whore there is no epitrichial layer nor characteristic stratum lucidum (Zander's Typus B, 88.1), the stratum really extends over the Malpighian layer, l)eing modified and constituting the horny layer of those parts. The essential characteristic of the stratum lucidum is that its cells are solidlv comified, their nuclei being obliterated. When the epitrichial cells comify they acquire thickened walls, but remain hollow (Zander's Typus A, 86.1, 88.1). The histogenesis of the stratum lucidum is described in the section on the nails, p. 555, the process having scarcely been studied except in connection with the investigation of the nailu.

The ridges {retes d'Henle) on the under or dermal side of the epidermis b^in to appear on the hairless parts, according to Blaschko, 87.1, about the fourth month, but on the haiir parts, where they are always rudimentary, they do not appear until toward the end of fcetal life. There are primary and secondary ridges. The foi-mer are the first developed, and from them the solid outgro^vths to form the sweat glands originate. Fig. 308 represents a section across the primary ridges: the epidermis is some seven or eight cells thick, its outer surface irregular, but not thrown into folds or ridges ; the structure of the superficial layer is indistinct but the cpitrichium seems to have disappeared ; the dermal surface is thrown up into regular rounded equidistant ridges, Ri, from which grow out here and there the solid anlages of sweat-glands, S. These ridges do not arise all at the same time, biit their formation si)reads from sundrj- centres, nor do the ridges run in straight lines altogether, but on the contrarj' in parallel curves. The ridges under the nails appear first (three and one-half months) under their distal and lateral bonlers, later under their central and proximal portions; additional ridges appear between those first formed (F. Curtis, 89.2, 1*9). In the next stagje, which is assumed by the epidermis only upon the palms and soles, the outer surface forms a low i-idge over each of the inner ridges. The external ridges with the openings of the sweat glands upon them are easily seen upon the adult hand. When the external ridges are developed there appear also secondari' riilges on the dermal side, bet^^■een the primarj- ridg;es. The secondan,- are much smaller than the primary ridges and underlie the grooves separating the external ridges.

The origin of epidermal pigment has been already discussed, p. 419.

Epitrichlum. — The external layer of the skin is known to be stratified in all amniota, but the homologies of the strata have never been satisfactorily determined. That the mucous or Malpighian layer is the same in all classes is evident, but that the homy layer comprises two distinct strata is, I thjnk, extremely probable, as stated above. One stratum may be homologized with the stratum lucidum, the other with the epitrichium. Where the epitrichium is lost (nails and hairy skin) the stratum lucidmn forms the superficial layer of the epidermis, but when the epitrichium is preserved, it forms the outer layer and the stratum lucidum underlies it. the history of the epitrichium is the key to the morphology of the amniote epidermis.

The epitrichium was discovered by "Welcker, 84, 1, in the embryos of a sloth (Bradypus), where it forms a continuou.'j membrane overlying the hairs. Welcker found the layer in several mammals, including man, and demonstrated that it belongs to the epidermis, becoming separated from the rest of the outer skin, when the hairs grow forth. In the sloth it forms, so to speak, an extra fcetal envelope, which we find mentioned by Eschricht and Ebsen (Miiller's Arch., 1837, 41) and and by Simon (Miiller's Arch., 1841. 370-372), but these authors did not ascertain its origin. Kerbert, 77. 1, demonstrated the epitrichium in reptiles; Jeffries, 83.1, and Gardiner, 84.1, in birds — the latter author adding also considerably to our knowledge of the layer in mammals. KoUiker failed to recognize the layer in man(see his "Entwickelungsges.," 1871', and" Gewebelehre," Gte Anfl., 204). Minot, 86, showed that the layer is present in the human embryo at certain stages and is absolutely distinct from t^e underlying homy layer. The bistorj- of the human epitrichium has been quite fully worked out by J. T. Bowen, 89.1.

The epitrichium becomes well marked during the third month, as a single layer of cells of large size, and each arching up from the surface, Fig. 307, Eptr. Over the hairy paris of the skin the development does not seem to progress beyond this stjige. The cells of the epitrichium enlarge and gradually flatten down, but before they are completely t Humao EmbiTo of tfas flattened there intervenes a le'SS^'***'™*^'^'* condition in which the expanded cells are flattened except in their central part, which forms a dome-like projection on each oeB; into this dome the nucleus and protoplasm of the cell are found withdrawn and degenerating. Later the cells are very large. Fig. ;j(Jll,

Fig. sni.— Epitrichium of ft Pirth Month. a.b. Cells of tr

three to six times the diameter of the underlying epidermal cells; there are no transitional forms, as KoUiker has erroneously maintained, between the epitrichial and the underlying cells. The outlines of the cells are polygonal and very distinct ; in the middle of each cell is an irregular lump of degenerated protoplasm, in which the nucleus can sometimes be distinguished. The epitrichium overlies the hairs; those hairs which project from their follicles lie between the epitrichium and the rest of the epidermis.

Over the hairless parts of the skin the epitrichium probably persists and becomes several-layered, except that it disappears in great part over the nails, see p. 555. Thus, in an embr^^o of three months, there appear on the palms several layers of cells, all of which have the vesicular character and dark look of the cells of the single-layered stage. It is unknown how this growth of the epitrichium is effected ; the primitive epitrichial cells have so much the appearance of degenerating tissue, that it is improbable that they proliferate, hence we must assume that the growth is effected by the addition of cells from the deeper layers. It was indicated alx)ve that in other parts the many-layered epitrichium probably undergoes comification according to Zander's Typus A, and forms the stratum corneum of authors, which is found overlying the stratum lucidum. This probability rests chiefly upon Bowen's observation that the epitrichium over the developing nail is continuous with the homy layer. If we accept this interpretation, we must say that the epitrichial cell cornifies so as to form a thick-walled vesicle, while the underlying cells coroify so as to form solid scales (Zander's Typus B, 88.1). That the epitrichium in birds and mammals may become homy was demonstrated by Gardiner's careful researches, 84.1.

Dermis. — Although the dermis or cutis is of exclusively mesenchymal origin, it is convenient to consider its development in connection with that of the external skin. In very early stages the mesenchyma extends to the ectoderm, but shows no trace of a special layer under the epidermis. This layer is, however, well marked in embrs'os of two months bv the condensation of the dermal mesenchyma, the cells becoming flattened in a plane parallel with the surface, and hence they appear somewhat elongated in vertical sections of the skin. Fig. 3^)0, c; the nuclei are granular, the protoplasm forms a rich network of great delicacy. Later the protoplasm is, I find, more condensed around the nuclei, and the cells have more individuality; at the same time the protoplasmatic network becomes coarser and simpler in character. During the third month (KoUiker, "Entwickelungsges.," 1870, p. 774) the primitive dermis becomes differentiated into two layers, the true dermal (corinm, Lederhant) and the subdermal (UnterhantzeUgewehe), the tissue being more condensed in the former and more fibrillar in the latter. During the latter half of the fourth month fat cells arise in the subdermal layer and steadily increase thereafter in both number and size, and by the end of the fifth month the whitish fat islands are conspicuous to the naked eye. The skin now comprises. Fig. 310, the epidermis, Ep, the dermis or cutis, Cu, and the fat-layer F; below is loose connective tissue, c. The hairs grow to the bottom of the fatty laj'er. The origin of the columnae adiposaj (J. C. Warren, 77.1), calls for invest iKation. The papilke of the dermis can be first seen during theBixthmoDth (Kolliker, /.c.) on the band and feet, foiming a doable

row between everj' pair of primary ridges. Fig. 30i*, Iti. Tbe elastic fibres apijear during the seventh month (KoUiker," Entwickelungsges.," ate Aufl., 770).

11. Nails aad Hairs.

Nails, — A nail U a modifie*! area of the stratum lucidum, situated upon the upper Bido of the terminal joint of a digit and laid bare by the loss of the overlying epitrichium. This definition is essentially different from that hitherto current, and is Iwised on Bowen's discoveries, 89.1.

The first indication of the nails may lie seen in the human embryo at the l^eginning of the thii-d month an a thickening of the epitrichium over the end of tbe digit. In most mammals this [Htsition is permaneni. and there is develope«l a terminal claw, but in man, as discovered by Zander, 84.1, the tenninnl position is transitorj-, and the ungual arta migrates on to the dorsal side of the digit. The change of {xisilion is attributed by KoUiker, 88.2, "^5, to the growth and exjiausion of the palmar side of the finger-tip. A set-oudarjresult of the miration of the nail is the transfer nf the terminal branches of the two digital ner^-es of the mlmar 8urfa<-e to the back of the finger (t<te) tips. Zander, 84. 1. The nail area is marked out <|uite dehnitely by a limiting groove or dejiression which [lersists more or }i:>m di»tinctly throughout life.

As so'>n as the nail area has reaclie<I its dorsal permanent jKtsition, there appeiirs at its jiroximal edge an obli<{ue ingrowth of the Malpigbiaii layer of the ei)idermi«. bi form the so-called nxit of the nail. The epitrichimn over the nail is much thickened — see Bowen, I.e., Fig. :i— but is thickf^t near and beyond the distal edge of the nail. The primar>- ridges of the Malpighian layer now apjwar. but only over the (lidmar surface of tlio finger or toe tip. and as they do not appear until much later under the nail, they establish a marked difference between the epidermi.s suiTounding and that covering the nail area. The epitrichial layer over the area has received the special name of eponychium f rom Unna, 76.1. Until the fourth month there is little change except that the anlage of the root of the nail grows considerably, and at the same time becomes more and more inclined toward a horizontal position, a change which progresses until by the eighth month the nail-root is horizontal, i.e,^ in the same plane with the nail-bed proper — compare Fig. 311.

About the beginning of the fourth month tnere appear, KoUiker, 88.2, 4, granules in the uppermost cells of the Malpighian layer. The granules are rounded in form, variable in size and have a decided affinity for coloring matters, especially for acid fuchsin, Zander, 86.1, 285. Very soon the cells form a stratum lucidum, which appears first in the distal part of the ungual area and is very thin, thence spreads proximalward, and, last of all, appears in the nailroot, being there also preceded by the granular cells. . By the middle of the fourth month the stratum lucidum is present over the whole nail and also extends on to the palmar surface. Fig. 311, s.L The granules have been supposed to be identical with eleidin, but on this }>oint there has been some discussion, which is summarized by Kolliker ("Gewebelehre,"' 6te Aufl., 21G) ; Ranvier C'Traite technique d'Histologie," 886) was the first to observe that the granules differ somewhat from true eleidin. There can be little question, if any, that the granules are directly connected with the cornification of the cells to form the nail proper. The granules were described by Brook in *1883, in a paper (Schenk's " Mitth.," II., 159), which I have not seen, and their relation to keratosis was more fully studied by Zander,

86.1, whose results have been in the main confirmed by Kolliker,

88.2, and F. Curtis, 89.2. The walls of the granular cells gradually l)ecome thickened (marginal keratinization of Curtis), the cell becomes flattened, its nucleus disappears, the walls unite, and there is thus produce<l a horny scale in the place of the cell. By the transformation of additional cells, the homy stratum lucidum is constantly thickened on its underside — compare Fig. 163, in Kolliker 's '* Gewel)elehre," 6te Aufl. During the fifth month the development of the stratum gradually extends beyond the nail area over the rest of the finger-tips, and more slowly into the nail-root.

The epitrichium disappears over the nail at about five months, first in the centre, then toward the base, sides, and distal end, but a small band persists as the pei'ionix across the root of the nail. Fig. 311, Ep\ and a large mass, Ep'\ forms a conspicuous ridge after the fifth month, across the distal end of the nail, and is continued over the palmar surface of the digit, as a considerable homy layer covering the stratum lucidum, sJ. The nail, iV, although the direct continuation of the stratum lucidum, has, of course, its surface exposed. The epitrichium varies greatly in appearance, for it may either preserve more or less the vesicular form of its cells, or its cells ma}" l>e more or less comified and flattened. It is probably owing to the frequency of the latter modification that the nature of the layer has been overlooked. The cornification of the epitrichium is precoded bv the appearance of eleidin granules in its cells, Curtis, 89.2, 17.

The final step in the development of the nail is the change by which its diBtal edge hecomee free, according to KoUiker, 88.2, 7, by desquamation of the stratum lucidum at the point where the nail passes distally into the stratum of the pahnar surface.

Fig. 311. -Lonaltudinal Sect loo of the Nail ot the Gnat Toe of a Hum«E Embtyo oTfli Months, Mlnot LWl, No, B5. iji. RBUinanl of epllrlehiuui; K(j', distal riifire "f u|jUrichiiini: .^ nail; i.l. Hiratuiu luL'lilum: b. bone. Fruiu aBncClnn bf l>r, Bowen. atainal with aclil ruchsin.

Moypholofiif. — The diacoverj' that the nails are modified portions of the stratum lucidum (jives the question of their evohitJon an entirely new asi«xrt. It renders it probable that the claws and hoofs are also derived from the stratum lucidum, and that the development and changes of tliis layer of the epidermis will have to be carefully investigatetl in the lowest amniota before we can hope to understand the origin of claws.

It may 1« safely assumefl that the nail is a modified claw. Zander, 84.1, having obserred the primitive terminal position of the nail area {Sagelfeld) in the human embrj-o, and its subsequent migration to the dorsal side of the digit, concluded that the human nail re])TORonted a terminal claw flattened oiit, and that the centre of the nail must correspond to the point of the claw. Boas, 84.1, from comjKtrative anatomicjil studies on claws, hoofs, and nails, estahlisheil a distinction )>etween the ^Tilar side aii<l the palmar side of claws anil hoofs, and lioinologize<I the nail with the volar side of a claw, which may therefore be termed the iiaij-plate {Sogelplotfe) \ Boas further inHintaincd that the palmar side (sole-plate, Soblenlioni) of the claw lieiximes nidimentarj'in man, and l)elieved that its rejiresentativp is the small area of epidermis under the edge of the nail in the a<lult; this area probably corresponds to that which in the embryo is covered by the epitrichial ridge. Fig. 311, Ep", at the distal edge of the nail. This interpretjition has l)ecn adopted by (Jegeiibaur, 86.1, in whose lahora ton,' Boas' researches were carried out. In view of our present knowledge it Beeins to mo that Bitas' conci>I>tion must be acceptetl, with the modifiontion, however, that the stratum lucidum covere<l by epitrichinm over the end of the digit innst Ih' considered the homoli^ie of the solc-plato (■Sd/ilcnhoni), and that not merely the epitrichial ridge at the edge represents the sole-plate. To decide the question, we must acquire exact knowledge of the relation of the sole-plate to the stratum lucidum in clawed and hoofed mammals.

Hairs. — A hair is a long downgrowth of the mucous layer of the epidermis into the cutis, Fig. 312, A; into the enlarged end of the downgrowth extends a papilla, p, of mesenchymal tissue ; the downgrowth separates into two parts, the axial or hair proper, Hy which grows upward and projects above the surface, and a peripheral part or follicle, /. At the base of the hair, the hair itself and the follicle unite.

The hairs arise in man as solid processes of the epidermis, the ends of which very soon expand. Fig. 313, 5, 6, and acquire the dermal papilla, 7. In other cases, as has been observed by Alexander Goette 68.1, and also, it is said, by Keissner and Feiertag, the papilla is formed first, as a slight projection of the dermis into the Malpighian layer of the ectoderm; the overlying epidermis then forms a downgrowth, which carries the papilla with it; in other respects the hair develops as in man. O. Hertwig (**Entwickelungsgeschichte," 3te Aufl.,43G) regards the type of development in which the papilla appears first, as the more primitive ; this view is plausible, and enables us to assume that the hairs were evolved by modifications of the epidermis, overlying special dermal papillae. Hertwig fortifies his hypothesis by comparison with the teeth, which in the lower vertebrates are developed from dermal papillae, while in the higher forms there is a deep ingrowth of the epidermis before the mesenchymal papilla of the dental germ appears.

The hair anlages appear in the human embryo at about three months, and can be first seen over the forehead and eyebrows, but very soon (sixteenth to seventeenth week) are developed over the entire head, and a little later the rest of the body, so far as it is ever hairy — on the limbs the hairs appear about the twentieth week. By the end of the fifth month, all the hairy areas are marked out. From the third to the seventh month at least — my observations do not go further — new hair anlages continue to arise, so that one finds various stages at once. It is thus possible to study in one preparation the gradual differentiation of the hair. In embryos of five to seven months, which have died and been retained in utero, the epidermis is usually loosened and may be isolated.* Such a piece of epidennis stained with alum haematoxylin and viewed from the under side is represented in Fig. 313. I distinguish two kinds of nuclei, those which are more darkly stained and those which are lighter. Some of the light nuclei appear dark because of the epitrichial cells underlying them. The darkly stained nuclei all belong to cells which participate in the formation of hairs. At first the dark nuclei make a little cluster, as at 1 and 3 ; the clusters grow in size — one a little larger is seen just to the left of that numbered 2, one a good deal larger is shown at 3. Sections show that such clusters are on the under-side of the epidermis and form slight protuberances or rudimentiiry papillae; the papilhe lengthen out and acquire rounded ends, 4 ; they grow rapidly down into the cutis, and by the contrac

  • Tlip prrmosa may be imitattHl by soaking the skin of a f«i»tii» for Heveral days in a 0.75 per cent salt solution to which a little ttiyuiol has been added to render it aseptic.

tion of their upper part become club-shaped, 5 and 6. The next step is the formation of the dermal paplllee of the hair, 7 ; a little notch arises at the thick end of the epidermal ingrowth, and the tissue filling this notch is the so-called dermal papilla. The figure presents also a well-developed hair; here the axial portion of the papilla has

formed the 1 a r / h le tl e cort cal port n has formed tJ e follicle, /; the end f 1 e I a r tl ckei ed / as tl e so called 1 r bulb; the soliaeeous gland, Gl, haa begun to grow out from the follicular walls. In the upper ])art of the follicle the hair lies quite free, hence in several places where the hairs have Iteen forcibly torn oJf the upper part of the follicle, jP, still remains, while the lower part attached to the hair is gone.

The differentiation of the hair in the axis of the downg^rowth begins about three to five weeks after the anlage appears, when the anLages are from 0.25--0.40 mm. long, and before the dermal papilla is recognizable. Two changes mark the commencing differentiation of the hair and the follicle : 1, the axial cells elongate in the dii-ection of the future hair : 2, the outermost layer of cells assumes the character of a cuboidal epithelium. The next step is the formation of the papilla, Fig. 313, 7, which is followed by the separation, in anlages of O.G-0.7 mm., of the axial mass of elongated cells into a smaller darker central portion, the hair proper. Fig. 312, H, and a lighter portion, which constitutes the inner follicular sheath, s. It is at this stage that the sebaceous glands. Fig. 313, Gly and Fig. 312, A, flf/, buds from the follicular tissue. At the enlarged base of the hair the laj^ers all merge into one another. The hair proper grows in length very much, in diameter very little, and by its elongation penetrates the epidermal layers, being accompanied by the inner follicular sheath. As all the hair anlages descend obliquely, the hair penetrates the epidermis obliquely and within the epidermis is bent down. By its continued elongation it finally reaches the surface of the skin, and its tip remains covere<l only by the epitrichium (Minot, 83), and when that disappears the hair is free. The detaile<l history of the hair follicles calls for much further study. I have observed the following details : In a longitudinal section of a fully developed hair. Fig. 312, A, the upper part of the follicle, F^ is seen to have a central cavity, which is partly filled by the fragments of the broken-down inner follicular sheath; on the lower side of the hair, and at the end of the hollow division of the follicle, is the anlage of the sebaceous gland, gl; from this ix)int down there is no space between the wall of the follicle and the hair ; immediately below the gland is an eminence, lu.i^ which is forme<l by a thickening of the follicle, and serves for the insertion of the slender muscle, miisCy the erector pili. How this muscle arises is unknown. The thickening of the follicle where the muscle is attached is not mentioned in the text-books I have consulted. From repeated observations I conclude that it is a typicid feature of the human hair. It has been described and figured by Unna, 76.1. Below the muscular insertion the follicle is differentiated into three layers, which are bettor sliown under a higher power. Fig. 312, D; there is an innermost sharply limited honij' layer, S, with no trace of cellular structure, a middle layer of granular cells, c, and an outermost layer of clear epithelioid cells, J?p, having their nuclei in their bases toward the hair, h. The follicle is incased in a fibrous mesenchymal tunica propria, in. Returning to Fig. 312, A, the two outer layers of the follicle are seen to merge into one another toward the base of the hair, and to thin out and disappear ; the inner sheatli, .«?, on the contrary, thickens, becomes more and more distinctly cellular, and finally expands as the hair bulb around the papilla. The liair proper, //, is of nearly uniform diameter until it reaches the bulb, where it expands to embrace the papilla, pa, and fuses with the inner follicular sheath. A network of blood-vessels, t\ in the tunica propria is spun around the bulb, but vessels have not, in the stage figured, penetrated the papilla itself, pa.

Lanugo is the term applied to the first coat of hairs in the embryo. This coat is a conspicuous feature at seven months. It is to be regarded as the embryonic reproduction in man of an ancestral simian characteristic (Darwin, "Descent of Man," Chap. I.). The hairs are fine, compared with those of the adult, and are therefore usually described as woolly hairs ; they are lost from most parts of the body, and replaced by larger and coarser hairs. Over the face the lanugo persists throughout life, but owing to its fineness and loss of color is not usually noticed.

Loss and Renewal of Hairs, — The length of life of a single hair is not long, for, as is well known, the hairs are continually shed. In many mammals the shedding is an annual process, but in man it takes place constantly. As the number of hairs, except in cases of baldness, does not diminish sensibly, it follows that new hairs must be continually formed.

The loss of hairs begins during foetal life. The hairs shed by the foetus fall into the amniotic fluid and are sometimes swallowed by the embryo and found in the meconium, see Chap. XXIX. Immediately after birth the shedding of the lanugo occurs, its place being taken in certain parts by coarser hairs. The shedding of the hair'is initiated by changes in the hair bulb, or expanded end of the hair fitt,ed over the papilla; the multiplication of cells in the bulb, by means of which the growth of the hair is maintained, ceases, and the bulb atrophies, separates from the papilla, and breaks up into a bundle of fibres ; the hairs in which the bulbs have become fibrous are the Kolbenhaare of J. Henle, the Beethaare of P. Unna, 76.1. That these hairs which have no papillae cannot grow has been demonstrated experimentally by L. Ranvier. For a time the hair is still retained in place by the sheath of the follicle pressing against it. It is finally either pulled out by some outside force, or pressed out by the secondary hair (Ersatzhaare) ; there is also an actual shortening of the follicle of the atrophying hair, a fact observed by von Ebner, 76.1, and confirmed by Kolliker (" Gewebelehre/' 6te Aufl., 241.

As to the development of the secondary or replacement hairs (Ersatzhaare) ^ authors are not agreed. That there is a long continued production of new hair-germs during foetal life is well known, and that the process is continued after birth has been maintained by several writers, but such hairs cannot be regarded as secondary but only as primary hairs. The true secondary hairs are those which ari^ from the follicles of previous hairs. According to some authors, the old papilla is preserved and the new hair is formed over it, but this opinion does not appeiir to me to rest upon satisfactory obser\^ation8. Far better founded is the view of Kolliker (" Gewebelehre," 5te Aufl., 1807, p. 137), that the new hairs are developed from buds, which spring from the base of the old follicles soon after the old hair bulb has atrophied ; the buds are small in diameter, and lengthen out the old follicle ; the cells show, at first, no differentiation, the bud resembling closely a yoimg hair germ; in it a new hair is developed in the same way as in the primary hair germs. The figure showing the development of the secondary liairs given by Kolliker in his **Gewebelehre," 5te Aufl., have been reproduced by him in the sixth edition, Figs. 18G and 187, also in his '* Entwickelungsgeschichte/' 1879, Figs. 476, 477.

Sebaceous Olands. — As the sebaceous glands are outgrowths of the hair follicles, they are appropriately treated here. They appear as thickenings of the follicles of the hair germs, about the time the hair proper reaches the level of the epidermis. The thickenings are solid, and as they enlarge become somewhat lobulated, Fig. 312, A, gl; they usually are situated on the under side of the hair. Fig. 312, A, C, but sometimes spring laterally. Even before the lobulation begins, the anlage is seen to be differentiated into an outer layer. Fig. 312, C, coVy in which the cells retain their original character, and are small and granular : and a central mass of larger modified cells, Sb. The latter increase in number until they find an exit into the ca,vity of the follicle, Fig. 312, A, gl. According to Kolliker C'Entwickelungsgesch.," 1879, p. 797) the central cells contain fat globules and are discharged into the follicle, thereby becoming the secretion of the gland ; the cortical layer persists as the germinating bed of new fatty central cells. In specimens hardened in alcohol, stained in alum cochineal, and cut in paraffine, the central cells of the sebaceous glands of the foetus present a highly characteristic appearance; they are rounded or oval, and much larger than the cortical cells, Fig. 312, C. Under a high power, Fig. 312, B, each cell is seen to be separated from its fellows, to have a distinct outline, a coarse intracellular network and a finely granular rounded nucleus, lying in a perinuclear space, which is darker than the rest of the cells. The further development of the gland consists principally in the addition of lobules, which arise as buds of the cortical layer, the fatty central cells developing later in each bud (alveolus) ; the neck connecting the lobules with the hair follicle becomes the duct of the gland. The growing gland spreads around its hair follicle, but the position of its duct permanently indicates its origin from the under side of the hair.

As the development of the sebaceous glands begins at a definite stage of the hairs, and as the hair germs continue arising throughout fcetal life, so we encounter, at any time after the fifth month, glands in various stages. The first glands, according to Kolliker, appear on the head at about four and one-half months, on the body at about five months.

Vernix Caseosa. — As we have learned from their development the sebaceous glands l)egin their secretory activity by the end of the fifth montli. Their fatty secretion is discharged on the surface, and, together with the shed portions of the epidermis, usually forms a more or less extensive coating of the embryo. Minot, 83, has suggested that the persistence of the epitrichium may l>e a factor in the formation of the coating, whicli is known as the vernix caseosa (smegma embryonum, Kiisefim iss, Fruchtschmiere) . Simon (** Mod. Chemie, " II. , 48G) is said to have been the first to show that the vernix consists entirely of sebaceous cells, fat globules and epidermal cells, and therefore could not be a pro<luct, as some of th(» older writers imagined, of the amniotic Huid. Quantitatively the epidermal cells are the chief components. The vernix becoines coDSpicuoua during the sixth month and increases until birth. It is extremely variable in amount. Kulliker states that Buck ("De vemice caseosa," Halis, 1844) found it mi^ht increase to 3.5 drachms in weight. In other cases it is almost entirely absent. Elsasser (Schmidt's Jahrbiicher, Bd. VII., 1S33) found that about half the children of both sexes are bom without vernix caseosa, the other half with a varying amount. On the chemical composition of the vernix see Davy (London Med. Gazette, 1844) and Buck ("De vemice caseosa," Halis, 1844). The vemix contains nine to ten per cent fats and seventy-eight to eighty-four per cent water.

III. Glands op the Skin.

The development of the sebaceous glands of the hair has been described, p. 562 ; concerning the development of the other sebaceous glands, such as those of the external ear and of the prepuce, little is known ; the glands of the eyeballs and eyelids are treated in Chapter XXVIII. ; there remain to be considered here the sweat glands and the mammary glands.

Sweat Q-lands. — They arise as solid ingrowths of the Miilpighian layer of the epidermis, somewhat similar at first to young hairgerms. They may be distinguished from hair-germs by their descending perpendicularly instead of obliquely, and by appearing in the fresh state — not whitish, like hair-germs, but yeUowish. They appear on the hairless parts (soles and palms) early during the fifth month, but not until much later on the hairy parts. KSlliker, 88.2, 15, has observed that the sweat glands are developed earlier on the under than on the upper side of the digits, and earlier on the third digrit than on the others. The ingrovrais arise on the soles and pilms from the primary ridges. Fig. 308, S. The lower end is somewhat thicker than the upper part of the ingrowth, which rapidly elongates, passes through (^ the dermis proper, y>_\ and when it reaches x^T the fatty layer or sub-dermal tissue, the anlage of the gland begins to assume a contorteil course, the end of the gland rolling over toward the Fio, SH,-S«tlun of the sole of Ihe Foot of . F-EtuB of the pnidnrmiR Viv 314 "'•*! MoDlh. to hIiow ihe Swest Glands, which arise from the inepKlermiS. Ulg. Jl*. oer<„juiplKhiaL layer or the cpidem.la.

Ihe lumen of the gland can l>e readily distinguished at this time, but does not extend through the epidermis until later — on the foot, not until the seventh month (Kolliker). This fact is important, because it sets aside the notion, formerly advanced, that the sweat glands produce the liquor amnii. At the time of birth, the glands are longer, more coiled, and their ducts take a spiral course, hut the spirm turns are by no means so close together or so numerous as in the adult.

Mammary Glands

The milk glands vary id position. It is probable that there were typically two rows of glands, and that different portions of these rows are preserved in different mammalB, e. g., the headward portions in primates, the tailward portions in ruminants.

According to 0. Schultze, 82.1, the first trace of the mammfie may be observed, in pig embryos of 15 mm. and rabbit embryos of thirteen to fourteen days as a continuous ridge-like thickening {MilchliHie) running from the fore limb to the inguinal fold. lu the next stage ( ^0 mm } the ridge is speLially tbickened^in the pig at o-" )x>ints at each of w hich a mamma is developed; each local thickening becomes separate and assumes a rounded form. The local thickening of the epidermis is the anlage of a milk gland, and this anlage has been long known and mnrks the site of the future nipple. In man the thickening may be ob'»er\ed toward the end of the second month It is at first very slight, though it causes a discernible external protuberance. Later it projects from the epidermia into the dermis. The thickening commences when the epidermis is two-layered and solely at the expense of the inner layer, the outer layer persisting for a time as a distinct epitrichium, Fig. a 15, A, Eptr. The epithelial ingrowth, Fig. ;J15, B, Ep.in, enlarges, and the cells in its central portion gradually comify and fall out, so that the ingrowtti becomes hollow; but the excavation progresses very slowly and sometimes is not completed until after birth. Soon after the hollowing has begun the ingrowth sends out buds, which resemble, in their appearance and early development, true sweat glands. The oi^an may be said to be now in the monotreme stage. ^ C. Gegenbaur showed in 188fi that

in Echidna the mamma is a deF o s i De opmrD h* M mmftr; pressed area of the skin, from which B'"Sb? "^ "w*" * Embryo o nun spring a number of lacteal glands mm itepriitmFpvrm resembling the sweat glands in ?'k(t«nd"'ISjw"'^«- BR.^ " K. appearance. The depressed area ron s and A and B f Jegenbaur tenus the Driisenfeld

(gland area). It seems to me be. on I pos I le quest on tl at the thickening of the outer skin to form the depressed area by a subsetiuent loss of cells in no wise militates against the liomology here maintiiined, and which was first advanced by (iegeubaur.

In the stage of Fig. 315, C, all the parts of the adult gland may be recognized. The tissue around the epitheUal ingrowth, Ep. in, is destined to form the protuberant nipple, of which the dermal tissue is clearly differentiated during foetal life, although the nipple does not usuaUy become protuberant, according to Rein, 82. 1, until after birth. The boundary of the dermal tissue of the nipple is marked by a distinct layer of smooth muscle fibres, msc. (Jutside or below the muscular layer is the fibrillar connective-tissue stroma, str, into which the glands grow, and within which they are differentiated.

The next stage of development is reached by a series of changes, of which the most important are: 1, the obliteration of the depression, which arose by the hollowing out of the epithelial ingrowth; 2, the development of branches from, and cavities in, the milk glands proper; 3, the development of the fat layer under the gland; and, 4, the growth of the nipple. The branching of the glands begins with the seventh month, and even at the time of birth is very slightly advanced. The lumen of the glands appears first in their enlarged lower ends, not long before birth, and then extends toward the mouth of the glands. In each gland we can distinguish : 1, the terminal branched glandular portion, and, 2, the duct. The duct consists of a wide part, sinus lacteus of authors, next the secretory portion, and a narrow part, which extends into the nipple and opens there on the apex ; the orifice of the duct is funnel-shaped, and hence is termed thenars infundibularis. The fat layer is a continuation of that of the skin, locally thickened ; about five or eight years after birth fat develops also in the stroma of the mamma between the gland tubules.

The course of development has been shown by Rein to be essentially the same in several classes of mammals as in man. There are, however, noteworthy secondary differences, particularly in ruminants ; in them the nipple is precociously developed and the epithelial ingrowth carried up on to its apex before the gland buds appear; the central cells of the ingrowth disappear as in man, but the depression left by their loss is not obliterated, but is permanent. Moreover, there is only a single gland bud developed, which grows out to a considerable length to attain the base of the long nipple or teat, where it branches. Consequently in ruminants there is but a single duct through the nipple, instead of several as in man and most mammals. In the horse (Rein, 82.2, 685) the epithelial ingrowth forms two buds, hence there are two ducts in the adult.

Milk at Birth. — Although the mammary gland is immaturely developed at birth, yet, as is well known, there is frequently a secretion (Ijschar^eil from it for a few days after birth. Scanzoni, de Sinoty, and Rein, 82.1, 4G4, have shown that this secretion is true milk. It is known in German as Hexenmilch,

Montqomenfs glands have been shown by Rein, 82.1, 470, to be accessory rudimentaiy milk glands.

Evolution of the Mammary Gland. — That the mammary gland arose through specialization of a group of epidermal glands, is a necessary deduction from the facts of comparative anatomy and embryology'. Several authors have thought that the milk gland was evolved from the sebaceous glands, others from the sweat glands.

The latter opinion rests upon strong evidence, the former principally upon the analogy of there being considerable fat in both the sebaceous and lacteal secretions. Haideuhain (Hermann's '* Physiologie, " Bd. v., 380) has shown that in the milk ghmds there is no fatty metamorphosis of the central cells, as in sebaceous glands, but a secretion from the gland walls, as in the sweat glands, so that there is nothing in the structure or function of the adult ghmd to justify a comparison with the sebaceous type. As regards the embryological development, the primary epithelial ingrowth, Fig. 315, A, Ep.iuy is, I think, to be regarded merely as the result of a modified method of developing the depressed glandular area (Dn'iftenfeld) ; the glands, sensn stn'cfu, arise as solid, long, slender ingrowths of the Malpighian layer, and resemble closely the true sweat-gland anlages and not the sebaceous glands. Another point of importance is the resemblance, which has been obsei'ved by Gegenbaur, 86.1, between the milk glands of the lowest mammalia and the sweat glands. The derivation of the milk glands from the sweat glands is indicated bv the structure and mode of secretion of the adult mamma, by the development of the gland, and by the structure of the gland in the Echidna.

Gtegenbaur, 75.1, 86.1, has maintained that there are two types of milk glands, one type modified sweat glands, the other type modified sebaceous glands ; he has maintained, also, that there are two types of nipple. The embryology of the organ shows that both the nipple and the gland are of one type, certainly in most, probably in all, mammalia. Gegenbaur's conception that there are two morphologically distinct forms of nipple w^as based upon Huss' observations, w^hich are inaccurate in several important respects.


Our knowledge of the development of the mammae was derived chiefly from the observations of Langer, 52.1, and of Kolliker(*'Gewebelehre,"1867), until Huss in 1873, 73.1, greatly widened our acquaintance with the early stages in man and ruminants. Huss' memoir contained important errors, especially as to the origin of the ruminant teat, and these errors led Gegenbaur, 73.1, 75.1, to his notion of tw^o types of teats — a notion which has passed into the text-books, although shown by Rein to Ik? untenable. H. Klaatsch, 84.1, argues against Rein in favor of Gegenbaur, but does not, it seems to me, invalidate either Rein's observations or conclusions. Rein's investigations, 82.1, 82.2, easily take the first place. Creighton's jwiper, 77.1, added but little, how little may l^e judged from his conclusion that the glands are developed from the mesoderm.

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