Book - Comparative Embryology of the Vertebrates 4-12

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

   Comparative Vertebrate Embryology 1953: 1. The Period of Preparation | 2. The Period of Fertilization | 3. The Development of Primitive Embryonic Form | 4. Histogenesis and Morphogenesis of the Organ Systems | 5. The Care of the Developing Embryo | Figures
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This 1953 textbook by Nelsen compares development in a number of different species.

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1906 Papillary ridges | 1910 Manual of Human Embryology | 1914 Integumentary | 1923 Head Subcutaneous Plexus | 1921 Text-Book of Embryology | 1924 Developmental Anatomy | 1941 Skin Sensory | Historic Disclaimer
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Part IV - Histogenesis and Morphogenesis of the Organ Systems

Part IV - Histogenesis and Morphogenesis of the Organ Systems: 12. Structure and Development of the Integumentary System | 13. Structure and Development of the Digestive System | 14. Development of the Respiratory-buoyancy System | 15. The Skeletal System | 16. The Muscular System | 17. The Circulatory System | 18. The Excretory and Reproductive System | 19. The Nervous System | 20. The Development of Coelomic Cavities | 21. The Developing Endocrine Glands and Their Possible Relation to Definitive Body Formation and the Differentiation of Sex

The Integumentary System

A. Introduction

1. Definition and General Structure of the Vertebrate Integument or Skin

The word integument means a cover. The word applies specifically to the external layer of the body which forms a covering for the underlying structures. The integument also includes the associated structures developed therefrom, such as hair, feathers, scales, claws, hoofs, etc. The latter are important features of the body covering. The skin is continuous with the digestive and urogenital tracts by means of mucocutaneous junctions at the lips, anus, and external genitalia.

The integument is composed of two main parts, an outer epidermis and an underlying corium or dermis. Below the latter is a third layer of connective tissue which connects or binds the corium to the underlying body tissues. This third layer forms the superficial fascia (tela subcutanea or hypodermis). The superficial fascia is continuous with the deep fascia or the connective tissue which overlies muscles, bones, and tendinous structures of the body (fig. 272H).

2. General Functions of the Skin

The integument acts as a barrier between other body tissues and the external environment. Modifications of the integument serve also as an external skeleton or exoskeleton in many vertebrates. In warm-blooded forms, the skin is associated intimately with the regulation of body temperature. The hypodermal portion of the skin often serves to store reserve fatty substances. The presence of fat functions as a buffer against mechanical injury from without, as reserve food, and as an aid in temperature regulation in warm-blooded species. Still another and very important function of the skin is its intimate association with the end organs of the peripheral nervous system by means of which the animal becomes acquainted with changes in the external environment. (See Chap. 19.)

3. Basic Structure of the Vertebrate Skin in the Embryo

a. Component Parts of the Developing Integument

In all vertebrates, the integument arises from a primitive embryonic integument which at first is composed of the cells of the epidermal tube only, i.e., the primitive epidermis. Later this rudimentary condition is supplemented by a condensation of mesenchymal cells below the epidermis. Following this contribution, the primitive skin is composed of two main cellular layers:

(1) a primitive epidermal (ectodermal) layer of one or two cells in thickness and

(2) an underlying mesenchymal layer.

The former gives origin to the epidermis, while the latter is the fundament of the dermis. A little later, chromatophores or pigment cells, presumably of neural crest origin, wander into the primitive dermis and become a conspicuous feature of this layer. In the development of the vertebrate group as a whole, these two basic layers serve as the basis for the later development of the integument. As a result, these two layers undergo characteristic modifications which enable the skin to fulfill its specific role in the various vertebrate species. The marked differences in later development of these two integumentary components in different vertebrate species are associated with the needs and functions of the skin in the adult form.

b. Origin of the Component Parts of the Early Integument

1) Origin of the Epidermal Component. The epidermal component descends directly from the primitive epidermal (ectodermal) organ-forming area of the late blastula, which, as we have seen, becomes greatly extended during gastrulation and, in the post-gastrular period, is tubulated into the elongated, cylinder-like structure. The primitive epidermal tube thus forms the initial skin or outer protective investment of the developing body.

The wall of the primitive epidermal tube at first may be composed of a single layer of cells of one cell in thickness, as in the shark, chick, pig, opossum, or human (figs. 263A; 269A; 272A). However, in teleost fishes and amphibia, the primitive epidermal tube is composed of two layers of cells. For example, in the sea bass, the wall of the primitive epidermal tube is composed of two layers, the outer layer being thin and made up of much -flattened cells and the lower layer being two cells in thickness (fig. 264A, B). In the anurans and urodeles, the wall of the primitive epidermal tube is composed of two layers, each of one cell in thickness (fig. 267 A, D). The lower layer in the frog, salamander, and teleost often is referred to as the inner ectodermal or nervous layer. It is the germinative layer and thus forms the inner or lower portion of the stratum germinativum of the later epidermis (fig. 267 A, D). The outer layer is densely pigmented and forms the periderm.

In the embryo of the shark, chick, and mammal, the single-layered condition of the primitive epidermal tube soon becomes transformed into a doublelayered condition, the outer layer or periderm being composed of muchflattened cells (figs. 263B; 269B; 272B). In all vertebrates, therefore, the

Fig. 263. Developing skin of Squalus acanthias. (A) Section through differentiating somite and epidermis of 10-mm. embryo. (B) Integument of 34-mm. embryo. (C) Section of skin, showing beginning of scale formation in 60-mm. embryo. (D) Scale development in 145-mm. embryo. (E) Later stage of placoid scale, projecting through epidermal layer of skin.

Fig. 264. Diagrams pertaining to the skin of bony fishes. (A and B after H. V. Wilson: Bull. U. S. Fish Commission, Vol. 9, 1889, reprint, 1891; C after Kingsley: Comp. Anat. of Vertebrates, 1912, P. Blakiston’s Son & Co., Phila.; F from Reed; Am. Nat., 41.) (A) Section of ectoderm (primitive epidermis) of 39-hr. embryo of Serranus atrarius, the sea bass. (B) Epidermis of sea-bass embryo of 59 hrs. (C) Skin of the lungfish, Protopterus. (D) Integument of teleost fish with special reference to scales. (E) Higher power of epidermal and dermal tissue overlying scale in D. (F) Poison gland along pectoral spine of Schilheodes gyriniis.

primitive epidermal layer of the skin eventually is composed of two simple cellular layers, an outer protective periderm, and a lower, actively proliferating stratum germinativum. It is to be observed further that the periderm in the recently hatched frog embryo possesses ciliated cells (fig. 267H, I). These cilia, as in Arnphioxus (fig. 249B), are used for locomotor purposes, and also function to bathe the surface with fresh currents of water. As such, they probably play a part in external respiration.

The periderm forms a protective covering for the actively dividing and differentiating cells below. In the mammals, the periderm occasionally is called the epitrichium, as it eventually comes to rest upon the developing hair. In Arnphioxus, there is no periderm, and the epidermal tube (epidermis) remains as a single layer of one cell in thickness (fig. 250E, F).

2) Origin of the Dermal or Mesenchymal Component. In Arnphioxus, the thin lateral and ventro-lateral walls of the myotome give origin to the dermatome which comes to lie beneath the epidermal wall. From the dermatome arises the dermis or connective-tissue layer of the skin (fig. 250E, F). The origin of the embryonic dermis in the vertebrate group is more obscure than in Amphioxus, for in the vertebrates its origin varies in different regions of the developing body. Moreover, the origin of the dermal mesenchyme is not the same in all species. For example, in the head region of the frog and other amphibia, the dermal portion of the skin is derived in part from wandering mesenchyme of the head area, at least in the anterior extremity of the head and posteriorly to the otic or ear region, while immediately caudal to this area the mesenchyme of the dermis is derived from the dermatomic portion of the somite, together with mesenchymal contributions of the outer wall of the lateral plate mesoderm. In the trunk region of the body, mesenchyme from the dermatomic portion of the somite wanders off to form the embryonic connective-tissue layer of the skin in the dorso-lateral region of the embryo. In the middorsal region, sclerotomic mesenchyme appears to contribute to the dermal area. However, the dermal layer in the latero-ventral region of the body is derived from mesenchymal cells whose origin is the somatopleural layer of the hypomere (lateral plate mesoderm). The dermal layer in the tail arises from the mesenchyme within the developing end bud (tail bud).

The embryonic dermis in the head region of the chick arises from mesenchyme in the head and pharyngeal areas. In the cervico-truncal region, the dermatome of the somite contributes mesenchyme to the forming dermis on the dorso-lateral portion of the body wall (Engert, ’00; Williams, ’10; fig. 269C), whereas latero-ventrally the mesenchyme of the future dermis springs from the lateral wall of the hypomere. That portion of the developing dermis overlying the neural tube appears to receive contributions from the sclerotomic mesenchyme. The mesenchyme which forms the dermal layer of the skin in the tail descends from the mesoderm of the end bud (tail bud).

In the shark embryo, the origin of the embryonic dermis is similar to that of the amphibia. In the mammalian embryo, a small portion of the dermal tissue may arise from the dermatome; however, the greater part arises in the head and pharyngeal area from the mesenchyme within these areas, in the middorsal region of the trunk from sclerotomic mesenchyme, and in the lateroventral region of the trunk from the outer wall of the lateral plate. In the tail region, the tissue of the dermis derives from tail-bud mesoderm. Bardeen (’00) concluded that the dermatome in pig and man gives origin to muscle tissue. However, Williams (’10) doubted this conclusion. The fact remains that the exact fate of the dermatome or cutis plate of the somite in mammals, and even in the lower vertebrates, is not clear.

3) Origin of Chromatophores. Chromatophores or pigment-bearing cells occur in relation to the epidermis and the dermis. Dermal chromatophores are numerous in vertebrates from man down to the fishes. Pigment also appears in the epidermal cells, hair, feathers, and certain epidermal scales. This pigment is derived from melanoblasts or chromatophores which lie in the basal area of the epidermis or in the zone between the epidermis and the dermis

(Dushane, '44). Experimental embryology strongly suggests that these chromatophores are derived from the neural crest cells which in turn take oilgin from the primitive ectoderm in association with the neural tube at the time of neural tube closure. From the neural crests, the mesenchymal cells, which later give origin to chromatophores, migrate extensively throughout the body and to the skin areas (Dushane, ’43, ’44; Eastlich and Wortham, ’46).

B. Development of the Skin in Various Vertebrates

1. Fishes

a. Anatomical Characteristics of the Integument of Fishes

The epidermal layer of the skin of fishes is soft, relatively thin, and composed of stratified squamous epithelium (figs. 263E; 264E; 265). Cornification of the upper layers is absent in most instances. However, in those fishes which come out of the water and spend considerable time exposed to the air, cornification of the surface cells occurs (Harms, ’29). Unicellular mucous glands are abundant, and multicellular glands also arc present (fig. 264C). A slimy mucous covering overlies the external surface of the epidermis. Poison glands may occur in proximity to protective spines or other areas (fig. 264F).

Fig. 265. Development of phosphorescent organ in Porichthys notatus, (From Greene: J. Morphol., 15.) (A) Rudiment, separating from epidermis. (B) Section of ventral organ of free-swimming larva. (C) Section of fully developed ventral organ.

The dermal layer of fishes is a fibrous structure of considerable thickness. The layer of dermal tissue, immediately below the epidermis, is composed of loosely woven, connective-tissue fibers, copiously supplied with blood vessels, mesenchymal cells, and chromatophores. Below this rather narrow region is a thick layer, containing bundles of fibrous connective tissue. Between the latter and the muscle tissue is a thin, less fibrous, subcutaneous layer (fig. 263E).

Scales are present generally throughout the group and are of dermal origin in most species. However, both layers of the skin contribute to scale formation in the shark and ganoid groups of fishes. Scales are absent in some fishes as, for example, in cyclostomes and certain elasmobranchs, such as Torpedo. In certain teleosts, the scales are minute and are embedded in the skin. This condition is found in the family Anguillidae (eels).

Highly specialized, phosphorescent organs are developed in deep-sea fishes as ingrowths of masses of cells from the epidermis. (Consult Green, 1899.) These epidermal ingrowths (fig. 265 A) separate from the epidermal layer and become embedded within the dermis (fig. 265B, C).

b. Development of the Skin in the Embryo of the Shark, Squalus acanthias

1) Epidermis. In shark embryos up to about the 15-mm. stage, the integument consists of an epidermis composed of one layer of cells, one cell in thickness (fig. 263A). The shapes of these cells may vary, depending upon the area of the body. In some areas, especially the dorso-lateral region of the trunk, they are flattened, while along the middorsum of the embryo they are cuboidal. In the pharyngeal area they arc highly columnar.

By the time the embryo reaches 25 to 35 mm. in length, two layers of cells are indicated in the epidermis, an outer periderm of much-flattened cells and a lower, basal, germinative layer, the stratum germinativum (fig. 263B). The stratum germinativum retains its reproductive capacity throughout life, giving origin to the cells which come to lie external to it. Eventually the epidermis is composed of a layer of cells, several cells in thickness. The outer cells may form a thin squamous layer, covering the external surface (fig. 263D).

2) Dermis. The dermis gradually condenses from loose mesenchymal cells which lie below the stratum germinativum of the epidermis (fig. 263B, C). The dermis gradually increases in thickness and becomes composed of scattered cells, intermingled with connective-tissue fibers. Deeply pigmented chromatophores become a prominent feature of the dermal layer, where they lie immediately below the germinative stratum (fig. 263D, E).

3) Development of Scales and Glands. In the formation of the placoid scale of the shark, masses of mesenchymal cells become aggregated at intervals below the stratum germinativum to form scale papillae (fig. 263C). Each papilla gradually pushes the epidermis outward, especially the basal layer (fig. 263D). The cells of the outer margin of the papilla give origin to odontoblasts or cells which secrete a hard, bone-like substance, resembling the dentine of the teeth of higher vertebrates (fig. 263D). This substance is closely related to bone. The cells of the basal epidermal layer, overlying the dentine-like substance, then form an enamel organ, composed of columnar ameloblasts which produce a hard, enamel-like coating over the outer portion of the conical mass of dentine (fig. 263D). As this scale or “tooth-like” structure increases in size, it gradually pushes the epidermis aside and projects above the surface as a placoid scale (fig. 263E). Some are small, while others are large and spine-like. Many different shapes and sizes of scales are formed in different areas of the body (Saylcs and Hershkowitz, '37).

As the epidermis increases in thickness, unicellular glands appear within the epidermal layer (fig. 263D). These glands discharge their secretion of mucoid material externally, producing a slimy coating over the surface of the skin. Multicellular glands appear at the bases of the spines which develop at the anterior margins of the dorsal fins and in the epidermis overlying the claspers of the pelvic fins of the male.

c. Development of the Skin in the Bony Ganoid Fish, Lepisosteus (Lepidostcus) osseus

The development of the epidermis and dermis in Lepisosteus is similar to that of the shark embryo. Consideration, therefore, is confined to the development of the characteristic ganoid scale.

In the formation of the ganoid scale of Lepisosteus, a different mechanism is involved than in that of the placoid scale of the shark embryo. Most of the scale is of dermal origin; the epidermal contribution of enamel substance is small and restricted to the outer surface of the spines of the scale (fig. 266D-F).

The scale first appears as a thin calcareous sheet, secreted by the dermal cells in the outer portion of the dermis (fig. 266A). Unlike the formation of dentine in the shark skin, the calcareous material comes to enclose some of the scleroblasts (osteoblasts) or bone-forming cells (fig. 266B). This process continues as the scale increases in mass, and the scleroblasts become distributed as bone cells within the hard, bony substance of the scale. These cells occupy small spaces or lacunae within the bone-like substanee, and small eanals (canaliculi) traverse the hard substance of the scale to unite with similar canals from neighboring, bone-cell cavities (Nickerson, 1893, p. 123).

Spine-like projections (fig. 266F) appear on the surface of the bony scales. These spines are secondarily developed and form in a manner similar to the placoid scale of the elasmobranch fish. That is, a dermal papilla is formed externally to the already-formed dermal scale. This papilla pushes outward into the epidermal layer, and a dentine-like substance appears on its outer surface (fig. 266D). As development of the spine proceeds, this cap of dentine gradually creeps basalward and unites secondarily with the dentine of the scale (fig. 266F). The papillary cells thus become entirely enclosed within the spines of dentine, with the exception of a small canal, leading to the exterior, at the base of the spine (fig. 266F). As the dentine-like spine develops, an enamel-like substance is deposited upon its outer surface by the epidermal cells.

Fig. 266. Formation of the scale in Lepisosteus (Lepidosteus) osseus. (After Nickerson; Bull. Mus. Comp. Zool. at Harvard College, 24.) (A) Section through posterior end of scale of fish, 150 mm. long. (B) Section through po.sterior end of decalcified scale of fish, 300 mm. long. (C) Section through scale of fish, 300 mm. long. (D) Section showing developing spine. (E) Outlines of .scales viewed from surface. (F) Section of scale spine attached to scale.

Another characteristic of scale formation in Lepisosteus is the deposition of ganoin upon the outer surface of the scale (fig. 266B, C). This ganoin appears to have many of the characteristics of the enamel. It previously was considered to have been formed by the lower layer of epidermal cells, but Nickerson (1893) concluded that it is of dermal origin. The outer, ganoincovered surface of the scale eventually lies exposed to the exterior in the adult condition and, therefore, is not covered by epidermal tissue.

Much of the external surface of the body of the bony ganoid fish, LepL sosteus osseus (common garpike), is covered with these plate-like scales, and, consequently, the epidermal layer of the skin tends to be pushed aside by this form of scaly armor. In Amia calva the epithelial (epidermal) covering is retained, and cycloid scales, similar to those of teleosts, are developed. The “ganoid” scales of Amia lack ganoin. They protect the head (fig. 316D).

d. Development of the Skin in the Teleost Fish

The early development of the epidermis and dermis in the teleost emdryo resembles that of the shark embryo, and a soft glandular epidermis eventually is formed which overlies a thick, connective-tissue-layered dermis, containing numerous scale pockets, each containing a scale (fig. 264D, E). Consideration is given next to the development of the teleostean scale.

The development of the scale in teleost fishes is a complicated affair (Neave, ’36, ’40). It arises in the superficial area of the dermis in relation to an aggregation of cells. This aggregation of cells forms a dermal pocket or cavity. The latter contains a fluid or gelatinous substance. The scale forms within this cavity. A homogeneous scale rudiment of compact, connective-tissue fibers, the fibrillary plate, is established within the gelatinous substance of the scale pocket. A little later, calcareous or bony platelets are deposited upon this fibrous scale plate. The scale continues to grow at its periphery and, thus, stretches the dermal cavity. At the posterior margins of the scale, the dermal cavity becomes extremely thin. Further growth of the scale posteriorly pushes .the epidermis outward, but the epidermis and the thin dermal cavity wall normally retain their integrity (fig. 264D).

The mature scale consists of a hard fibrous substrate, upon the upper posterior margins of which are embedded calcified plates. These calcified plates fuse together basally as development proceeds. Most of the scale is embedded deeply in the tissue of the dermal or scale pocket. At the anterior, deeply embedded end of the scale, small, hook-like, retaining barbs or teeth develop along the inner margins of the scale which serve to fasten the scale within the pocket (fig. 264D).

2. Amphibia

a. Characteristics of the Amphibian Skin

The amphibian skin is soft, moist, and slimy. It is devoid of scales, with the exception of the Gyrnnophiona which possess patches of small scales embedded within pouches in the dermal layer of the skin (fig. 267J). However, some of the Gyrnnophiona lack scales entirely. Unicellular and multicellular glands of epidermal origin are a prominent feature of the amphibian skin (fig. 267F, G). Specialized poison glands also are present (Noble, ’31, p. 133). Glands are developed in some species which attract the members of the opposite sex during the breeding season. In Cryptobranchus, the epidermal layer may be invaded by capillaries which penetrate almost to the surface of the skin in the region of the respiratory folds, located along the lateral sides of the body (Chap. 14). Cornification of the outer epidermal cells is the rule during later stages of development, in some species more than in others. For example, the development of a cornified layer is characteristic of the skin of toads, whose wart-like structures on the dorsal surface of the body represent areas of considerable cornification. Horny outgrowths of the epidermis are common in certain species.

The dermal layer in general is delicate and characterized by the presence of many pigment cells (chromatophores) of various kinds. The scales within the skin of the Gymnophiona are of dermal origin. In frogs, the dermis is separated from the deeper areas of the body along the dorso-lateral region of the trunk by the presence of large lymph spaces.

Fig. 267. Developing integument of amphibia. (A after Field: Bull. Mus. Comp. Zool. at Harvard College, 21; F after Dawson: J. Morphol., 34; H and 1 after Assheton: Quart. J. Micr. Sc., 38; J from Kingsley, 1925: The Vertebrate Skeleton, Blakiston, Philadelphia, after Sarasins.) (A) Section of skin of frog embryo in neural plate stage. (B) Section of skin of 10-mm. frog embryo. (C) Skin of 34-mm. frog embryo. (D) Skin of Necturus embryo, 6 mm. long. (E) Skin of Necturus embryo, 20 mm. long. (F) Structure of mature skin of Necturus. (G) Structure of skin of Rima pipietis of section through head shortly after metamorpho.sis. (H) Frog embryo, 3 mm. long, showing water streams produced by cilia. (I) Semidiagrammatic figure through suckers of frog embryo, 6 to 7 mm. long. (J) Section of skin of the Gymnophionan, Epicrium.

b. Development of the Skin in Necturus maculosus

The newly formed, epidermal tube of a 6-mm. embryo of Necturus consists of two layers of epidermal cells, an outer periderm and an inner stratum germinativum (fig. 267D). In the ventro-lateral region of the trunk, however, these two layers are flattened greatly and may become so attenuated that only one layer of flattened cells is present. Unicellular glands appear in the head region and represent modifications of cells of the outer ectodermal (peridermal) layer.

In larvae of 18 to 20 mm. in length, the epidermis is 3 to 4 cells in thickness, with the outer layer considerably flattened (fig. 267E). The dermis consists of a mass of mesenchymal cells, with large numbers of chromatophores lying near the epidermis. Chromatophores also lie extensively within the epidermal layer; some even approach the outer periphery. According to Eycleshymer (’06), some of the pigment cells of the epidermis represent modified epithelial cells, while others appear to invade the epidermis from the dermis. Dawson (’20) believed these epidermal pigment cells to be entirely of an epidermal origin in Necturus. Dushane (’43, p. 124) considered the origin of epidermal pigment cells in Amphibia in general to be uncertain but suggested “that these cells also come from the neural crest’’ via the dermal mesenchyme.

Later changes in the developing skin consist in an increase in the number of epithelial cells and in a great increase in the thickness of the dermis, with the formation of bundles of connective-tissue fibers. Associated with these changes, two types of multicellular alveolar glands arise as invaginations into the dermis from the stratum germinativum. One type of gland is the granular or poison gland, and the other is the mucous gland. The latter type is more numerous (fig. 267F). Mixed glands, partly mucous and partly granular, also may appear (Dawson, ’20). Large club-shaped cells or unicellular glands may be observed in the lower epidermal areas, while flattened cornified elements lie upon the outer surface of the epidermis.

The dermis is arranged in three layers as follows:

(a) a thin, outer, compact layer between the lower epidermal cells and the dermal chromatophores,

(b) below this outer compact layer, the intermediate spongy layer, containing some elastic, connective-tissue fibers as well as white fibers, and

(c) below the spongy layer, the inner compact layer.

The chromatophores located in the outer part of the dermal layer are of diflferent kinds (see p. 591).

c. Development of the Skin in the Frog, Rana pipiens

The development of the skin of the common frog resembles closely that of Necturus. The primitive epidermal tube consists of two layers of ectodermal cells, an outer periderm and a lower nervous layer or stratum germinativum (fig. 267A). The cells of the periderm contain pigment granules, and unicellular glands also are present, particularly in the head region. At the 10-mm. stage, the outer, pigmented, peridermal layer begins to flatten, while the stratum germinativum assumes the normal characteristics of the reproductive stratum of the epidermis (fig. 267B). The cells are cuboidal and closely arranged. A condensation of mesenchyme, immediately below the thin epidermal layer, represents the rudiment of the future dermis. Chromatophores are prevalent in the dermal area. In figure 267C are shown the characteristics of the skin of the head area of the 34-mm. tadpole, while figure 267G represents the skin of the head region of the newly metamorphosed frog. In this area of the body, the dermis is compact and dense, but in the dorso-lateral area of the trunk, large lymph spaces are present in the dermis.

3. Reptiles

a. Characteristics of the Reptilian Skin

Most reptiles are land-frequenting animals. The land type of habitat dictates the development of a mechanism which keeps the lower layers of the epidermis soft and moist. The problem of epidermal drying is not encountered to any great extent in the fishes and most amphibia because of the moist conditions under which they live. To circumvent the drying effects imposed upon land-living animals, the outer layers of the skin become cornified. A superficial or outer stratum corneum, therefore, becomes a prominent feature of the epidermis of reptiles, birds, and mammals.

Aside from its role of protecting the lower epidermal layers of cells against loss of moisture, the cornified layer also functions as a protective mechanism against mechanical injury. Foot pads, friction ridges, and all calloused structures are evidence of this function. The cornified stratum represents flattened, dead, epithelial cells, infiltrated with a protein substance, keratin, present abundantly in all horny structures, such as claws, scales, etc.

Both epidermal and dermal layers are thickened considerably in reptiles, while epidermal glands, so prominent in fishes and amphibia, are absent, with the exception of certain specialized regions in the oral and anal areas, between the carapace and plastron of some turtles, and between the scales in certain areas of the skin of crocodiles and alligators.

b. Development of the Turtle Skin

The turtle is an example of an armored animal, possessing a “shell” consisting of a dermal skeleton, the carapace, and the plastron, composed of a series of interlocking bony paltes, associated with an outer cover, the epidermal skeleton, composed of horny scutes. The latter comprises the so-called tortoise shell of commerce. The dorsal carapace and ventral plastron are united along their lateral edges by a bony ridge, and the carapace is firmly fused with the vertebrae and ribs of the endoskeleton. The skin of the head, neck, tail, and legs is fortified with thick horny plates placed at intervals (fig. 268E). Between these horny plates, the stratum corneum is highly developed (fig. 268F).

Fig. 268. Development of turtle skin. (A) Section through turtle embryo, showing early division of epidermis into periderm and germinative stratum. (B) Section showing two-layered condition of epidermis in slightly older embryo. (C) Section through dorsal area of embryo, 1 1 mm. long. (D) Higher power drawing of epidermis of 11-mm. embryo. (E) Section of skin of turtle, after hatching, to show horny plates. (F) Higher power sketch of skin shown in square in (E). (G) Section of skin of turtle just before hatching, showing epidermal scales of carapace, dermal mesenchyme, and vertebrae.

At the 11- to 15-mm. stage, the condensation of dermal mesenchyme already is thickened greatly in the dorsal region of the embryo in the future carapace area. This thickened condition and the intimate association of the mesenchyme with the trunk vertebrae and ribs are shown in figure 268C. The rudiment of the plastron begins to appear in the ventral region at this time.

After the young hatch from the egg, ossifications occur within the dermal mesenchyme of the carapace and plastron. The bony ossifications of the carapace gradually fuse with the flattened trunk vertebrae and the flattened ribs. In figure 268G is shown a longitudinal section through a part of the middorsal area of a turtle just before hatching. It is to be observed that the epidermal horny scales or scutes are well formed, while the dermal mesenchyme of the carapace is wrapped intimately around the flattened, dorsal, spinous processes of the vertebrae.

Epidermal scales and thickened horny skin pads, together with an armor of bone, in turtles, demonstrate the types of dermal and epidermal differentiations which form a protective coat in the reptilian group. The “shed skin” of the snake represents a sheet of horny epidermal scales which is periodically cast off. New scales are reformed repeatedly throughout the life of snakes. The rattles on the terminal end of the tail in the rattlesnake represent horny rings, developed proximal to the horny spine, prevalent as the end piece of the tail of many serpents. Lizards are well protected with thick epidermal scales, and in some species these scales are reinforced with dermal bony plates. The crocodiles are tough-skinned animals, possessing thick epidermal scales; the dorsal scales are supported underneath by corresponding dermal bony plates. Horny claws develop upon the digits of the appendages ^n turtles, crocodiles, and lizards.

Fig. 269. Development of skin in the chick. (C after Williams: Am. I. Anat., 11.) (A) Epidermis of 48-hr. chick. (B) Epidermis of 72-hr. chick. (C) Dermal mesenchyme, arising from dermatome of embryo of 40 somites. (D) Skin of chick embryo, incubation six days. (E) Skin of eight-day embryo, showing beginning of feather rudiment. (F) Eleven-day embryo, feather rudiment. (G) Section of mature skin between feather outgrowths. Observe that the epidermis is thin, and that the dermis is composed of two compact layers separated by a vascular layer.

4. Birds

a. Characteristics of the Avian Skin

The skin of the bird is more delicate than that of the reptile. The epidermal layer is thin with a highly cornified external surface. The dermis is composed of an outer compact layer below the epidermis, and beneath the latter is a vascular layer. Below the vascular layer is another compact layer of connective-tissue fibers, and between this layer and the deep fascia is the characteristic adipose (fatty) layer (fig. 269G). Extensive cutaneous glands are not developed. However, the two uropygial or preening glands at the base of the tail are common to most birds, although they are not present in the ostriches. In certain gallinaceous birds, such as the common fowl, modified sebaceous glands are present around the ear. Scales, resembling the reptilian type, are developed on the distal parts of the legs, while feathers present a feature characteristic of the avian skin.

1) Kinds of Feathers. Feathers arc of many kinds, but they may be grouped under three major categories:

(1) plumae (plumous or pennaceous feathers), the most perfectly constructed type of feather, filling the role of contour feathers,

(2) plumules (plumulae or plumulaceous feathers), making up the under feather coat or down, and

(3) hloplumes or hair feathers.

Of all the epidermal structures developed in the vertebrate group, feathers appear to be the most ingeniously constructed. They possess to a high degree liic qualities of lightness, strength, and toughness which serve to protect a delicately constructed skin from cold, moisture, and abrasion.

2) General Structure of Feathers: a) Pluma or Contour Feather. The plumous feather consists of a rachis (shaft or scape) and a vane. The proximal portion of the rachis or shaft is the quill or calamus. The latter is hollow but may contain a small amount of loose pith. It has an opening, the inferior umbilicus, at its base. The quill resides in a feather follicle, a deep pit surrounded by epidermal tissue projecting downward into the dermal part of the skin (fig. 270D, E). Above the quill is the expanded “feathery” portion of the feather, called the vane. At the junction of the quill and the vane is a small opening, the superior umbilicus, to which is attached, in some contour feathers, a secondary, smaller shaft, the aftershaft or hyporachis, together with a group of irregularly placed barbs.

The shaft of the vane of the feather is semisolid, with its interior filled with a mass of horny, air-filled cavities. Extending outward from the shaft in this area are lateral branches or barbs (fig. 270E). The barbs form two rows, one on either side of the shaft. From the barbs, smaller branches extend outward; the latter are the barbules (fig. 270E). An interlocking system of hooks, the barbicels, enables the barbule of one barb to connect with a barbule of the next barb. If these interlocking hooks are disrupted mechanically, the bird restores them while preening its feathers.

Fig. 270. Diagrams of developing feathers in chick. (A) Nestling, down-feather rudiment of chick of about 12 days of incubation. (B) Feather rudiment, 12 to 14 days of incubation, showing beginning of definitive feather rudiment. (C) Nestling down rudiment and definitive feather rudiment of chick shortly before hatching. (D) Relation of nestling down feather to definitive feather shortly after hatching. (E) Later stage in definitive feather development; nestling down feather is attached to distal end of first definitive feather. (F-H) Cross sections of nestling down rudiment diagrammatically shown in (B). (I) Cross section of definitive feather rudiment shown in

(D). (J) Cross section of definitive rudiment shown in (E). It is to be noted that the sheath around the developing feather extends for a considerable distance beyond the surface of the skin during development. This area is shortened considerably in E for diagrammatic purposes. F-l based on data from Jones (’07).

b) Plumae OR Down Feather. The plumules or down feathers form an inner feathery coat which lies below the contour feathers in the adult bird. They constitute the main insulating portion of the feather coat. In the down feathers of the adult, the barbs arise in bouquet fashion at the distal end of the quill. On the other hand, the nestling or first down feathers of the chick or newly hatched birds of other species do not possess a quill, for the barbs are attached to the distal ends of the apical barbs of the definitive feather (fig. 270E). Therefore, two types of down feathers are found:

( 1 ) the nestling down feather without a quill and

(2) the later down feather which possesses a quill.

The barbules in down feathers do not interlock, and a vane is not formed (fig. 270D, E).

c) Filoplume or Hair Feather. The filoplume or hair feather possesses a long slender shaft which generally is deprived of barbs, although a tuft of barbs may be present at the distal end.

d) Distribution of Feathers on the Body. Feathers are not evenly distributed over the surface of the body but arise in certain definite areas or feather tracts, the pterylae. Between the pterylae are the apteria or areas where the number of feathers are reduced or absent altogether. When feathers are present in an apterium, they consist mainly of a scanty distribution of downy and filoplumous feathers.

b. Development of the Avian Skin

1) Development of the Epidermis, Dermis, and Nestling Down Feather.

When the epidermal tube in the chick embryo begins to form, it consists of a single layer of cells of one cell in thickness. As development proceeds, this single-layered condition becomes transformed into a double layer, so that at 48 to 72 hours of incubation a two-layered epidermis is realized. This condition consists of an outer layer or periderm, considerably flattened, and an inner layer or stratum germinativum (fig. 269 A, B). At 96 hours of incubation in most parts of the developing integument, a primitive dermis is present as a loose aggregate of mesenchyme below the two-layered epidermis. The origin of a part of this mesenchyme from the dermatome is shown in figure 269C. At six days of incubation, mesenchyme is present as a definite dermal condensation (fig. 269D).

Between the sixth and eighth days of incubation, the epidermis and dermis increase in thickness, and small, mound-like protuberances begin to appear in certain areas (fig. 269E). Each elevation is produced by a mass of cells, known as the dermal papilla, which pushes the epidermal layer outward (fig. 269E). The initial dermal papillae represent the beginnings of the feather rudiments. At eleven days of incubation, many feather rudiments have made their appearance. Each rudiment consists of a central, mesenchymal (dermal) core or pulp, surrounded externally by epidermal cells. The dermal pulp is supplied copiously with small blood vessels (fig. 269F). The epidermal cells at this time are beginning to be arranged into longitudinal columns of cells. These longitudinal cellular columns represent the initial stages of barbrudiment development (fig. 270A). This condition of the developing feather marks the beginning of the first or the “nestling down” feathers.

At 12 to 14 days of incubation, the feather rudiment increases considerably in length and begins to invaginate into the dermal layer at its base (fig. 270B). This invagination of the base of the feather rudiment marks the beginning of definitive feather formation (Jones, ’07). In the developing feather from 14 to 17 days of incubation, two general regions are indicated. These regions of the developing feather are:

(a) a region from the surface of the skin to the distal end of the feather germ where the barbs and barbules of the nestling down are being formed (fig. 270B) and

(b) a proximal region below the surface of the skin where the barbs and barbules of the definitive feather begin to differentiate (fig. 27()B).

After the seventeenth day, the differentiation of the definitive feather proceeds rapidly (fig. 270C, D).

From the fourteenth to the seventeenth days, the barbs of the nestling down feathers elongate slightly by adding new ridge material at the basal end of each ridge (fig. 270B, C). The length of the barb rudiments of the down feather thus increases as the feather rudiment grows outward from the surface of the skin. As the barb rudiments elongate, they differentiate into the barbs and barbules (fig. 27 IB, C). (See Davies, 1889; Strong, ’02.) At about eighteen days of incubation, such a feather may be removed, and the distal portion of the horny sheath may be ruptured with a needle. Following the rupture of the horny sheath, the enclosed barbs will spread out as shown in the distal part of the developing feather in figure 270D.

At eighteen to twenty days of incubation, feather development in the chick may be represented as shown in figure 270C and D. A distal or nestlingdown-feather region and a proximal definitive-feather area are present. Barbs and barbules of the definitive feather differentiate in fhe-proximal area. A real quill is not established at the base of the nestling down feather, although a horny cylinder may intervene between the base of the down feather and the barbs of the definitive feather (fig. 270D). (See Jones, ’07.) Thus, in the chick and most birds, the first or nestling down feather and the succeeding definitive feather are developed as one continuous process, and cannot be regarded as two separate feather growths (Jones, ’07, p. 17). When me chick hatches, the outer horny sheath around the differentiated down feather dries and cracks open, and the barbs and barbules of the down feather spread out into fuzzy tufted structures (fig. 270D). Later, as the definitive feather emerges from the surface of the skin, the down-feather barbs appear as delicate tufts, attached to the distal ends of the barbs of the definitive feather (fig. 27 OE).

2) Development of the Contour Feather. The development of the contour feather is more complicated than that of the nestling down feather described above. Its development may be divided into early or primary and later or secondary phases (Lillie and Juhn, ’32). The formation of barbs during the early phase consists in the elaboration of barb and barbule rudiments without a shaft rudiment. This type of development resembles somewhat that of the down feather. The secondary phase of contour-feather development is concerned with the formation of a shaft, as well as the barb and barbule rudiments.

a) Formation of Barbs During the Primary or Early Phase of Contour-feather Formation. During the first phase of contour-feather formation, the barbs are formed in two different orders. The first order of barb rudiments arises more or less simultaneously (Lillie and Juhn, ’32); they are practically of the same size, about equal in number on either side, and dorsally placed. After this first set of barb rudiments is formed, a second order of barb rudiments arises in seriatim with the youngest barb rudiments, located more ventrally. (See first and second sets of barb rudiments in fig. 270D.) Both of these sets of barb rudiments eventually give origin to the barbs at the apical or distal end of the feather. As a shaft is not formed during the period when these two sets of barb rudiments are developing, i.e., during the first phase of definitive, contour-feather formation, these barbs later become associated with the forming shaft as the latter develops during the next or second phase of feather formation.

b) Secondary Phase of Contour-feather Formation. Following the formation of the barb rudiments mentioned above, the second phase of feather formation is initiated. It consists in the formation of the shaft and the further development of barb ridges and barbules. The development of the shaft is effected by the migration dorsalward of the collar cells (fig. 270E), which produces a continuous concrescence and fusion in the middorsal line of the two dorsal ends of the barb-bearing collar. This fusion of the collar cells forms the rudiment of the shaft as indicated in figure 270D. This concrescence of cells, however, establishes only the rudiment of the shaft, for it is apparent that the development of the shaft results from two sets of processes:

(1) the concrescence of a segment of the shaft rudiment at a particular point in the middorsal line of the feather rudiment and

(2) the elongation and growth of the rudiment material thus established.

As the shaft is laid down progressively from apex to base, the continuous concrescence of the collar cells and gradual formation of the shaft rudiment along the middorsal plane of the feather germ bring about the formation of the shaft (Lillie, ’40; Lillie and Juhn, ’32, ’38), beginning at its apex and progressing baseward.

As the collar material is fed into the developing shaft rudiment dorsally, the bases of the barbs, which are located in the collar or germinative ring, are carried continuously dorsalward and eventually become located along the sides of the shaft (fig. 270E). Also, the first set of barbs, which was formed in the first phase of contour-feather formation, becomes attached along either side of the developing shaft in the same way that the later barbs become attached.

In the formation of the barb, the apical or distal end of the barb is laid down by cellular contributions from the collar. Following this, more basal or proximal portions of the barb are elaborated by cellular deposition from the collar cells. The base of the barb thus remains attached to the collar as the barb rudiment elongates, while the apex maintains its position in the midventral line. As the base of the barb and the collar material to which it is attached move dorsalward toward the forming shaft, as observed in the previous paragraph, the base of the barb comes in contact with and fuses with the rachis or shaft, whereas the ventral extremity, i.e., the distal end of the barb, remains associated with the mesodermal pulp along the ventral aspect of the developing feather (fig. 271 A). The barb thus comes to form a half spiral around the developing feather within the external horny sheath (fig. 270E).|As successive barb rudiments are laid down, the previously formed barbs are moved progressively distad along with the mesodermal core.

c) Formation of the Barbules and the Feather Vane. During the period when the barbs are being formed, the side branches of the barbs or barbules are developed by the formation of groups of cells along either side of the barb (fig. 27 IB, C). Each of these groups of barbule cells differentiates into a barbule. A barbule thus represents a group of cells, specialized to form an elongated structure as shown in figure 27 ID. After the distal end of the feather extends markedly beyond the surface of the skin, the horny sheath breaks, and the barbs and barbules expand to form the vane of the feather. In doing so, the barbules interlock by means of barbicels which develop on the barbules, located on the side of the barbs facing toward the apex of the feather (fig. 271D).

d) Later Development of the Feather Shaft. During its development, the shaft gradually enlarges in the direction of the base of the feather. When the feather approaches its mature length, the shaft has enlarged to the extent that it comes to occupy the entire basal portion of the feather rudiment. As the last condition develops, barb formation becomes less exact until finally it is suppressed altogether. When this stage is reached, the contained dermal pulp within the base of the shaft begins to atrophy, starting at the end nearest the proximally placed barbs. As a result, a series of horny, hollow cells are formed within the base of the developing feather shaft. This hollow, basal end of the feather shaft forms the quill or calamus. The quill has a proximal umbilicus or opening through which the dermal pulp extends into the interior of the quill in the intact feather (fig. 27 lA). A distal umbilicus, from which the after feather emerges, may also be present in some feathers at the point where the ventral groove of the shaft meets the upper end of the quill.

Fig. 271. Diagrams of feather development. (A from F. R. Lillie: Physiol. Zool., 13; C and D redrawn from Strong: Bull. Mus. Comp. Zool. at Harvard, ’40.) (A)

Semidiagrammatic drawing of the pulp (papilla) of a regenerating feather. The axial artery of the feather is shown traversing the pulp to the distal end. The veins of the pulp (not shown) consist of a series of central and peripheral veins which connect with venous sinuses at the base of the pulp and, from thence, communicate with the cutaneous veins. (B) Part of transverse section of a feather follicle, showing the developing barbs and barbules. (C) Transverse section of a feather rudiment of the tern. Sterna hirundo. Pigment cells, within the barb substance, send out processes which distribute melanin to the cells of the developing barbule. (D) Middle portion of wing-feather barbule, showing pigment within individual barbule cells together with the distal barbicels with their booklets; comification is not complete.

3) Formation of the After Feather. The after feather emerges from the upper end of the quill of the contour feather. It is well developed in the unspecialized, contour feather but may be absent or represented merely by a few barbs in flight and tail feathers of the fowl (Lillie and Juhn, ’38). For a description of the after feather and its distribution in birds, reference may be made to Chandler (’16).

As observed above, when the rachis or shaft reaches a certain size, the development of barbs tends to be suppressed. A stage is reached ultimately when the barbs are irregular and not well formed. Consequently, the barbs near the quill lose all tendency to form a vane and are placed in an irregular fashion along the shaft. As this distortion of barb development occurs dorsally, some of the developing barbs on the ventral side of the enlarged shaft become physiologically and morphologically isolated from those which are moving dorsad in the normal fashion along the collar. As a result, they remain on the ventral surface and, in this position, they endeavor to form a twin feather. In doing so, they become attached in their isolated position to the ventral aspect of the forming quill. The superior umbilicus marks this point of attachment.

The degree of development of the after feather varies from the presence of a few barbs to a condition where a well-formed, miniature, secondary feather is developed. The secondary or after feather in this condition possesses a secondary rachis or aftershaft, known as the hyporachis, and is attached to the main rachis at the superior umbilicus.

4) Development of the Later Down and Filoplumous Feathers. The development of the later down or undercoat feather is similar to that of the nestling down feather, with the exception that a basal shaft or quill is formed to which the barbs become attached at the distal end of the quill. In the formation of the hair feather or filoplume, an elongated shaft of small diameter is formed to which a few small barbs may be attached at the distal end.

5. Mammals

a. Characteristics of the Mammalian Skin

The adult skin of manimals is characterized by a highly cornified, outer layer of the epidermis, together with the presence of numerous glands and hair. Hair, a distinguishing feature of the mammalian skin, is present in all species, with the exception of the Cetacea (whales) and the Sirenia (sea cows). Various types of horny structures are associated with the epidermis, while the dermis may develop plates of bone in certain instances. Both epidermis and dermis are of considerable thickness.

b. Development of the Skin

1) Development of the Skin in General.

As in other vertebrates, the primitive mammalian integument is formed by the epidermal tube which, when first developed, consists of a single layer, one cell in thickness (fig. 272A). Later it becomes double layered, having an external flattened periderm and an inner stratum germinativum. As in other vertebrates, the germinative stratum is the reproductive layer. Mesenchyme condenses below the germinative stratum, and the rudiment of the future dermis is formed (fig. 272B).

In the further development of the epidermal layer, a third layer of cells, the stratum intermedium, appears between the periderm and the stratum germinativum (fig. 272C). The stratum germinativum or deep layer of Malpighi may appear to be several cells in thickness as development proceeds. The cells of the germinative stratum, in contact with the dermal surface, are cuboidal or cylindrical (fig. 272C, D). During later developrpent, the epidermis becomes highly stratified, and the outer or external layer is converted into a cornified layer, the stratum corneum (fig. 272D). Cornification occurs first on the future contact surfaces of the appendages, such as the volar surface of the hand, plantar surface of the foot, and foot pads of the cat, dog, etc. Pigment granules (melanin) appear in the deepest layers of the epidermis in the region of the basal, cylindrical cells of the stratum germinativum during later fetal development and after parturition (birth).

In the meantime, the dermal mesenchyme increases in thickness, and various types of connective-tissue fibers, white and elastic (see Chap. 15), appear in the intercellular substance between the mesenchymal cells. Pigment cells make their appearance in the dermis during later fetal development. These cells descend, probably, from cells of neural crest origin, although other mesenchymal cells possibly may contribute to the store of pigment-forming cells. Fat cells occur in the deeper layers of the dermis.

2) Development of Accessory Structures Associated with the Skin:

a) Development of the Hair.

The first indication of hair development is the formation of a localized thickening and invagination of the epidermal layer, particularly the germinative stratum (fig. 272E). This thickened mass of epidermal cells pushes inward, accompanied by an increase in the number of epidermal cells in the area of invagination (fig. 212V). Adjacent mesenchymal cells of the dermis respond to this epidermal activity by aggregating about the invaginating mass (fig. 272E, F). As the germinative stratum with its central core of cells continues to push downward in tangential fashion into the dermis, the surrounding mesenchyme forms a delicate, enveloping, connective-tissue sheath around the epidermal downgrowth (fig. 272G).

Fig. 272. Diagrams of developing hair. (A from Johnson: Carnegie Inst., Washington, Publ. No. 226, Contrib. to Embryol., 6; C and D from Pinkus, Chap. 10, The development of the integument, Keibel and Mall, 1910, Vol. 1, Lippincott, Phila.) (A) Section through epidermis of 24-somite human embryo. (B) Section through developing skin of 15-mm. cat embryo. (C) Section through 85-mm. human embryo, shov^ing threelayered epidermis. (D) Human skin, eight months, showing well-developed stratum corneum. (E) Early hair germ in human skin. (F) Later hair germ in human skin. (G) Still later hair germ, showing hair cone, sebaceous-gland rudiment, and epithelial bed. Observe that the hair cone arises as a result of the proliferative activity of the cells of the epithelial or hair matrix which overlies the mesenchymal papilla. Compare with fig. 273A.

As development continues, the distal portion of the germinative stratum forms a bulbous enlargement, the hair bulb. The mesenchymal rudiment of the papilla pushes into this bulb at its distal end to form the beginnings of the knob-like, definitive papilla of the future hair (fig. 272G). The hair rudiment then is formed by the proliferation of the epidermal cells, immediately overlying the knob-like papilla. The epithelial cells, overlying the papilla, form the epithelial matrix of the bulb (fig. 272G). The cells of the matrix soon produce a central core within the hair follicle, known as the hair cone (fig. 272G). The latter is a conical mass of cells which extends upward from the bulb into the center of the cellular material of the epidermal downgrowth. The hair cone thus gives origin to the beginnings of the hair shaft and the inner hair (epithelial) sheath (fig. 272G). The peripheral cells of the original epithelial downgrowth, which now surround the hair shaft and inner hair sheath, form the outer sheath (fig. 272G).

When the growing shaft of the hair reaches the level of the epidermal layer of the skin, it follows along a hair canal or opening in the epidermal layer and .finally erupts at the surface of the skin.

As the foregoing changes are effected, two epithelial growths appear along the lower surface of the obliquely placed, hair follicle (fig. 27 2G). The upper growth is the rudiment of the sebaceous gland which with certain exceptions generally is associated with hair development. The lower epithelial outgrowth forms the epithelial bed. This bed represents reserve epithelial material for future hair generations. The arrector pili muscle arises from adjacent mesenchymal cells and becomes attached to the side of the follicle (figs. 272G; 273). This muscle functions to make the hair “stand on end,” so noticeable in the neck-shoulder area of an angered dog.

The first hair to be developed is known as the down hair, fine hair or lanugo. In the human, the body is generally covered with lanugo by the seventh to eighth fetal month. It tends to be cast off immediately before birth or shortly thereafter. The lanugo corresponds somewhat to the nestling down of the chick, for the replacing hairs develop from the same follicles as the down hairs after the follicles have been reorganized from cells derived from the epithelial bed. However, some replacing hairs appear to arise from new hair follicles.

The hair on the face of the human female, exclusive of the eyebrows, nostrils, and eyelids, and also on the neck and trunk is of the fine-haired variety and resembles the lanugo of the fetus, whereas hair on the face of the human male is of the fine-haired type, exclusive of the eyebrows, eyelids, nostrils, and beard. Hair on various other regions of the male body may be of the fine-haired or lanugo variety.

b) Structure of the Mature Hair and the Hair Follicle.

The general structure of the mature hair and its follicle is as follows: The hair itself consists of a shaft and a root (fig. 273A). The hair shaft is composed, when viewed in transverse section, of three regions of modified cells or products (fig. 273B). The innermost, central (axial) portion of the shaft is the medulla. It is composed of shrunken, cornified cells separated by air spaces. Surrounding the medulla, is the cortex, constructed of a dense horny substance interspersed with air vacuoles. External to the latter is the cuticle, made up of thin, cornified, epithelial cells with irregular outlines. The cuticle is transparent and glassy in texture. The pigment or coloring substance is contained within the cortical and medullary portions of the hair. Hair color is dependent upon two main factors:

  1. the nature and quantity of pigment present and
  2. the amount of air within the cortex and medulla.

In some hairs, a distinct medullary portion may be absent.

Fig. 273. Diagrams of hair and follicle. (B redrawn from Maximow and Bloom, 1942, A Textbook of Histology. Saunders, Phila., slightly modified.) (A) Diagrammatic representation of the hair shaft and follicle in relation to skin. (B) Transverse section of hair shaft and follicle in skin of a pig embryo.

Fig. 274. Diagrams of nails, claws and hoofs. (A redrawn and modified from Pinkus, Chap. 10. The Development of the Integument, from Keibei and Mall, 1910, Vol. I, Lippincott, Phila.) (A) Longitudinal section of index finger of human fetus of 8.5 cm. (B) Longitudinal section of human finger, showing relationships of fully developed nail plate. (C) Claw of the cat, (D) Cloven hoof of the pig. (E) Developing hoof of pig. (F) Uncleft hoof of horse, lateral view. (G) Uncleft hoof of horse, ventral view.

While the shaft of the hair represents a cornified modification of epidermal cells, the root contains the cells in a viable condition before transformation into the cornified state. The root of the hair consists of the hair papilla, composed of dermal mesenchymal cells, blood vessels, nerve fibers, and a cup-shaped epithelial matrix which overlies the papilla (fig. 273A). The hair shaft and the internal root sheath are derived from the modification of the cells of the hair matrix. The internal root sheath is composed of the inner sheath cuticle, together with Huxley's and Henle's layers (fig. 273B). The internal sheath disappears in the upper regions of the follicle near the entrance of the sebaceous gland. External to the internal root sheath is the external root sheath. The latter represents the wall of the epithelial follicle and is the downward continuation of the epidermal layer of the skin around the root of the hair. The external root sheath thus forms a pocket-like structure, extending from the distal margin of the hair matrix to the epidermis of the surface skin. A sheath of dermal cells and fibers lies around the external root sheath and acts as the skeletal support of the hair.

During development, hair first appears in the region of the eyebrows and around the mouth. Later it develops over the surface of the body in a regular pattern. This pattern tends to have a definite relationship to scales when present.

3) Development of Nails, Claws, and Hoofs.

Resembling and closely linked to epidermal scales are the nails, claws, and hoofs of mammals. The claws of reptiles and birds belong to the same category of terminal protective devices for the digits. Nails are flattened discs of horny material, placed on the dorsal surfaces of the terminal phalanges (fig. 274A, B). Claws are similar and represent thickened, laterally compressed, and pointed nails (fig. 274C). Hoofs are composite structures on the terminal phalanges of the digits, but, unlike nails and claws, they are composed of two much-thickened nails, one dorsal and one ventral.

The distal protective device of the human digit is composed of a dorsal structure, the nail plate or unguis. A formidable, horny subunguis or ventral nail plate is absent, although a subungual region, consisting of an area of extreme cornification of the stratum corneum of the skin, is present (fig. 274B), The claw of the cat or dog is similar, with the nail plate compressed laterally, and the subungual cornification is greater. On the other hand, hoofs possess a dorsal nail plate (unguis) and a well-developed ventral nail plate (subunguis). Hoofs may be further divided into two general groups. In one group are the hoofs of cows, sheep, deer, etc., which form two, nail-forming mechanisms at the terminus of the digit, one dorsal and one ventral, from which the dorsal and ventral nail plates arise. In the other group are the hoofs of horses, donkeys, zebras, etc., which develop a dorsal, nail-developing mechanism, forming the dorsal nail plate, and two ventral, nail-producing structures. One of the latter generative devices gives origin to the frog and the other to the ventral nail plate. Thus, embryologically, nails and claws belong to one group, whereas hoofs form another.

A better appreciation of the above-mentioned facts relative to claws, nails, and hoofs can be gained by considering the development of a relatively simple, terminal structure of the digit, the human finger nail.

The nails on the terminal digits of the developing human finger begin to form when the embryo (fetus) is about three months old. In doing so, a thickened epidermal area arises on the dorsal aspect of the terminal end of the digit. This general, thickened, epidermal area constitutes the nail field. The proximal portion of the nail field then invaginates in a horizontal direction, passing inward into the underlying mesenchyme toward the base of the distal phalanx. This invaginated epidermal material forms the nail fold or groove, and it lies within the mesenchyme, paralleling the overlying epidermis (fig. 274A). The nail fold, when viewed from above, is a crescent-shaped affair with the outer aspect of the crescent facing distally; it may be divided into a deeper layer, the nail matrix, and a more superficial layer. The nail matrix is confined almost entirely within the nail fold or groove. The distal edge of the lunula marks its greatest extension distally along the nail field.

At about the fifth month, the upper cells of the nail matrix begin to keratinize, and the keratinized cells gradually fuse into the compact nail plate, ^as new material is added to the nail plate from the cells of the matrix, the distal portion of the plate is pushed progressively toward the end of the digit (fig. 274A). Although that portion of the nail field between the terminal end of the digit and the lunula takes no part in the formation of the cornified material of the nail plate, the underlying dermis below the nail field does form elongated ridges which push upward into the epidermis of the nail field. These ridges secondarily modify the already-formed nail plate by producing fine, longitudinal lines or ridges.

The claw or nail plate of the cat is compressed laterally to form a narrow, sickle-shaped structure. Three main factors are responsible for this peculiar form of the nail plate in the cat. One factor is the laterally compressed form of the distal phalanx. This condition results in a nail-fold invagination which is laterally compressed. The nail matrix thus is elliptical in shape, dorsoventrally, instead of flattened as in the human finger. A second factor responsible for the extreme, claw-shaped form of the nail plate in the cat is the more rapid growth in the middorsal portion than in the lateral areas of the nail plate. This discrepancy in growth results in the highly pointed midregion at the distal end of the nail plate. Ventrally, the two lateral sides of the nail plate tend to approach each other. The area between these two sides is filled with a cornified mass of subungual material. A final factor governing the extreme pointedness of the cat’s claw is the fact that the claw-distalphalanx arrangement, relative to the middle phalanx and tendons, makes the claw retractile when not in use, thus preserving its pointed distal end (fig. 274C).

The dog's claw or nail on the ordinary digits is compressed laterally less than that of the cat, with the result that the subungual cornification is broader and more pronounced and the distal end of the claw not as pointed. However, the claws upon the vestigial first digit, the so-called dewclaws, are pointed and cat-like. The fact that the claw of the dog is non-retractile is a factor in reducing its pointedness, for it, unlike the cat's retractile claw, is worn down continually.

The cloven hoof of the pig or cow is produced by the formation of two nail plates, one dorsal and one ventral, around each of the distal phalanges of the third and fourth digits (fig. 27 4E). The dorsal nail plate is rounded from side to side and meets the lower nail plate ventrally, with which it fuses along the lateral and distal portions of the lower plate. The unsplit hoof of the horse is produced by a somewhat similar arrangement of dorsal and ventral nail plates around the hoof-shaped phalanx of the third digit (fig. 274F, G). A third nail plate or growth center produces the frog or cuneus.

4) Development of Horns.

The horns of cattle arise as two bony outgrowths, one on either side of the head, from the area of the parietofrontal bones of the skull. In most instances the frontal bone alone is involved. Each bony outgrowth pushes the epidermis before it. The epidermis then responds by producing a highly keratinized, horny substance around the outgrowing bone. The result is the formation around the bony outgrowth of an unbranched cone (or horn) of cornified epidermal material (fig. 275 A). This type of horn grows continuously until the mature size is reached. If removed, this type of horn will not regenerate. Horns of this structure are found in sheep, goats, cattle, and antelopes.

The horns of the pronghorn, Antilocapra americana, are somewhat similar to those of cattle, with the exception that the external, keratinized, slightly branched, horny covering, overlying the bony core, is shed yearly, to be replaced by a new horny covering (fig. 275B).

On the other hand, the antlers of the deer offer a different developmental procedure. A new bony core is formed each spring which grows and forms the mature antler. As this hard, bony antler matures during late summer and early autumn, the outside covering of epidermis (i.e., the velvet) eventually atrophies and drops off, leaving the very hard, branched, bony core or antler as a formidable fighting weapon for use during the breeding season (fig. 275C). When the latter period is past, the level of the male sex hormone falls in the blood stream, which brings about a deterioration of the bony tissue of the antler near the skull. This area of deterioration continues until the connection to the frontal bone becomes most tenuous, and the antlers fall off, i.e., are shed. (See Chap. 1, p. 27.)

The horns of the giraffe are simple, unbranched affairs which retain the velvet or epidermal covering around a bony core. The horns of the rhinoceros are formidable, cone-shaped, median structures (one or two), composed of a keratinized, hair-like substance. These horns are located on the nasal and frontal bones. (For a discussion of horns in the Mammalia, see Anthony, ’28, ’29.)

Fig. 275. Horns of mammals. (A) Cow. (B) Prong-horn antelope. (C) White-tailed deer.

5) Development of the Skin Glands.

Three types of glands develop in relation to the skin in mammals:

  1. sebaceous or oil glands,
  2. sudoriferous or sweat glands, and
  3. mammary or milk glands.

a) Sebaceous Glands.

Sebaceous glands generally are associated with the hair follicles (figs. 272G; 273 A), but in some areas of the body this association may not occur. For example, in the human, sebaceous glands arise independently as invaginations of the epidermis in the region of the upper eyelids, around the nostrils, on the external genitals, and around the anus. When the sebaceous gland arises with the hair follicle, it generally takes its origin from the lower side of the invaginated hair follicle, although this condition may vary (fig. 272G). The sebaceous-gland rudiment originates as an outpushing of the germinative stratum and differentiates into a simple or compound alveolar type of gland. The secretion originates as fatty material within the more centrally located cells of the gland, with subsequent degeneration of these cells and release of the oily substance. Since the secretion forms as a result of alteration of the gland cells themselves, this type of gland is classified as an holocrine gland. New cells are formed continuously from that portion of the gland connected with the germinative stratum. The oil produced is discharged to the surface of the skin through the opening of the hair follicle when a relationship with the hair is present. If not connected with a hair follicle, the gland has a separate opening through the epidermal layer.

b) Sudoriferous Glands.

Sweat or sudoriferous glands most often develop independently of hair follicles, but in certain areas they form on the sides of these follicles. Whenever formed, they represent solid, elongated ingrowths of the epidermis into the dermis. Later these cellular cords coil at their distal ends to form simple, coiled, tubular glands (fig. 276).

The outer wall of the forming sweat gland develops so-called myoepithelial cells; the latter presumably have the ability to contract. The cells lining the lumen of the gland secrete (excrete) the sweat, the distal ends of the cells being discharged with the exudate. Hence, this type of gland is called an apocrine gland. The secretion is watery and contains salts, wastes, including urea, and occasionally some pigment granules and fat droplets. In the cat, dog, and other carnivores, sweat glands are reduced in number.

c) Mammary Glands.

Mammary glands are characteristic of the mammals. The first indication of mammary-gland development is the formation of the milk or mammary ridges (fig. 24 ID, E). These ridges represent elevations of the epidermis, extending along the ventro-lateral aspect of the embryo from the pectoral area posteriad into the inguinal region. The ridges are developed in both sexes and represent a generalized condition of development. In the human embryo, the mammary ridge is well developed only in the pectoral region, but it is extensive in the pig, dog, and cat. In the cow, horse, deer, etc., its greatest development is in the inguinal area.

Fig. 276. Diagram of sudoriferous (sweat) gland.

Only very restricted areas of each mammary ridge on either side are utilized in mammary-gland development. In the pig or dog embryo, a series of localized thickenings begin to appear along the ridge. In the sheep, cow, and horse, these thickenings are confined to the inguinal region, whereas in the primates and the elephant, they are found in the pectoral area. In the human, one thickening in each ridge generally appears, although occasionally several may arise. These thickenings represent the beginnings of the nipples and result from increased proliferations of cells (fig. 277B). Eventually, each thickened portion of the ridge becomes bulbous and sinks inward into the dermis (fig. 277C). Gradually, solid cords of cells push out from the lower rim of the solid epidermal mass into the surrounding dermal tissue (fig. 277D). These cords of cells represent the rudiments of the mammary-gland ducts. Secondary outpushings appear at the distal ends of the primary ducts. Later, lumina appear in the primary ducts. Further development of these ducts, with the formation of the terminal rudimentary acini, occurs during late fetal stages, resulting in the formation of an infantile state. This condition is found at birth in the human, dog, cat, etc. Under the influence of hormones present in the blood stream of the mother (see Chap. 2, p. 103), these acini may secrete the so-called “witch’s” milk in the newborn human male and female. While the occurrence of this type of milk secretion is not uncommon, the gland as a whole is in a rudimentary, undeveloped state. It remains in this infantile condition until the period of sexual development when, in the female, the mammary-gland ducts and attendant structures begin to grow and develop under the influence of estrogen, the female sex hormone. (See Chap. 2.) It should be observed that the rounded condition of the developing breast in the human female at the time of puberty (fig. 277F) is due largely to the accumulation of fat and connective tissue and not to a great extension of the duct system of the glands, although some duct extension does occur at this time.

As the original epithelial thickening of the nipple rudiment sinks inward, the center of the thickened area moves downward to a greater extent than the margins. Some disintegration of the central cells also occurs. As a result, a slight cavity or crater-like depression is formed in the middle of the epithelial mass of the rudiment (fig. 277C). In the cow and rat, this depressed area continues in this state, while the edges of the cavity and adjacent integument grow outward to form the nipple (fig. 277E). This type of nipple is called an inversion nipple. The ducts of the gland thus open into the bottom of the nipple (teat or mammilla). In the human, the original depression and the openings of the primary ducts of the gland gradually are elevated outward to form the type of nipple or mammilla indicated in figure 277 A. This type of nipple is called an eversion nipple.

Fig. 277. Diagrams showing mammary-gland development. (A) Human nipple showing mammary duct openings. (Modified from Maximow and Bloom, A Textbook of Histology, after Schaffer, 1942, Saunders, Phila.) (B) Transverse section of early nipple rudiment of 20-mm. pig embryo. (C) Transverse section through developing nipple of pig embryo of 70 mm., showing epidermal invagination into the dermal area of the skin. (D) Section through nipple of mammary gland of human male fetus, eight months old. (After Pinkus, Keibel and Mall: Manual of Human Embryology, Vol. I, 1910, Lippincott, Phila.) (E) Section through developing nipple of newborn rat. (Redrawn and modified from Myers, ’16, Am. J. Anat., 19.) (F) Development of human mammary gland from birth to maturity.

As indicated above, the distribution of nipples and mammary glands along the ventral abdominal wall varies greatly in different mammalian species. In lemurs and fruit bats, the mammary glands are developed in the axillary region; in the human and in primates, they are pectoral; in the cat, they are best developed in the pecto-abdominal area; in the dog and pig, they are mainly well developed in the abdominal and inguinal areas; in the cow and horse, inguinal nipples only appear; and in whales, the mammary glands are located near the external genitals.

The development of supernumerary mammary glands, i.e., hypermastia, is rare, but the formation of extra nipples, i.e., hyperthelia, is common in both male and female. In female mammals, such as the bitch, it is not uncommon for the breasts to remain in an undeveloped condition in the pectoral area, whereas those in the inguinal and abdominal areas are normal. When the mammary glands continue in an undeveloped or regressed state as, for example, in the anterior pectoral region of the bitch, the condition is known as micromastia. On the other hand, the abnormal development of the mammary glands to an abnormal size is known as macromastia. The latter condition often is found in cattle and occasionally in the bitch and human.

C. Coloration and Pigmentation of the Vertebrate Skin and Accessory Structures

1. Factors Concerned with Skin Color

The color of the skin and its accessory structures is dependent upon five main factors:

  1. the color of the skin itself,
  2. its opacity or translucency,
  3. the presence of pigment granules and special, pigment-bearing cells,
  4. the capillary bed of blood vessels which lies within the dermal portion of the skin, and
  5. the color of the accessory structures.

The color of the skin itself varies considerably in different species, but it tends to be slightly yellow, resulting from the presence of fatty tissue, fat droplets, and constitutent, connective-tissue fibers in the dermis. The property of opacity or translucency is an important factor for upon it depends transmission of light waves through the skin from deeper lying structures, such as blood vessels, pigment droplets, pigment-bearing cells, etc. The presence of definite types of pigment granules within or between the cells of the epidermis and dermis determines the course and kind of light waves which are reflected. The richness or paucity of blood vessels, ramifying through the dermal area, also affects the skin’s color in many instances.

The color of the accessory structures, particularly the structures derived from the epidermis, greatly conditions the color pattern of the species. The color of these accessory structures is dependent upon three main factors:

  1. presence or absence of pigment,
  2. presence of air, and
  3. iridescence.

Pigment and air are dominant factors, for example, in the color exhibited by hair and feathers. The presence of air diminishes and distorts the effects of the pigment which may be present. The property of iridescence is to be distinguished from the color effects due to the presence of certain pigments; the latter absorb light rays and reflect them, whereas iridescence is dependent upon the diffraction of light waves from irregular surfaces. Iridescence is important in the color effects produced by the plumage of a bird or the skin surface of many fish, reptiles, and amphibia.

2. Color Patterns

In the vertebrates whose manner of life dictates a close association of the body with the environmental substrate, the underparts have less color than the parts exposed to the light rays coming from above. Also, within the general, colored areas, there are certain spots, lines, bars, and dark and light regions which follow a definite pattern more or less peculiar to the variety, subspecies, or species. These color patterns tend to be fixed and are determined by the heredity of the animal. Consequently, they are related to the genic complex in some way. However, in many species the tone of the color patterns may be changed from time to time by changing environmental conditions as mentioned on page 594.

3. Manner of Color-pattern Production

a. Role of Chromatophores in Producing Skin-color Effects

Work in experimental embryology has demonstrated fairly conclusively that the pigments necessary for color formation are elaborated principally by certain cells known as chromatophores. Chromatophores are pigment-bearing and pigment-elaborating cells. Various cells may produce pigment, but chromatophores are cells specialized in the function of pigment elaboration.

The distribution and activities of chromatophores vary in the different vertebrate groups. For example, in fishes, amphibia, and many reptiles, three or probably four kinds of chromatophores are present in the dermis, namely, melanophores, lipophores, guanophores, and (possibly) allophores (Nobel, ’31, p. 141). By their presence and arrangement, the chromatophores produce specific color patterns. Moreover, the expansion and contraction of the pigmented cytoplasm of some or all the chromatophores effects changes in color, for the contracted or expanded state determines the types of light rays which will be absorbed or reflected. The rapid color changes in certain tree frogs and lizards are due to this type of chromatophoric behavior. The slower changes of color in other amphibia and fishes also are due to this type of chromatophoric activity. It thus appears that dermal chromatophores are responsible largely for the color effects found in the lower vertebrates. On the other hand, in the bird group and in mammals, the chromatophores present are mainly of one type, known as a melanophore. Melanophores produce pigments, known as melanins (Dushane, ’44, p. 102). The melanin granules, elaborated by the bird melanophore, have a wide range of color from yellow through orange to reddish-brown to dark brown. The melanophores in the bird deposit the melanin-pigment granules within the feather as it develops (fig. 271C) . Melanophores also deposit melanins in the bill of the male sparrow at breeding time under the influence of the male sex hormone (Witschi and Woods, ’36). Hair color in mammals is due, mainly, to pigmented granules deposited in the hair by melanophores. The skin color of various races of the human species is determined largely by the amount of melanin deposited within the lower epidermal layers by melanophores resident in the upper dermal area. In other words, the color of the skin and its appendages in the higher vertebrate groups is due, to a considerable extent, to diffuse granules deposited in the epidermis and epidermal structures by melanophores, whereas, in lower vertebrates, dermal chromatophores are responsible for color pattern and color change.

b. Activities of Other Substances and Structures in Producing Color Effects of the Skin

In the common fowl, the presence of carotenoids (lipochromes) in the Malpighian layer (stratum germinativum) mainly is responsible for the color of the face, legs, and feet. Orange-red, lipochromic droplets have been found in the germinative stratum of the head of the pheasant, and these droplets plus the capillaries in the dermis produce a brilliant red coloration (Dushane, ’44, p. 102). The color of the combs and wattles of the common fowl is conceded generally to be due to the presence of a rich capillary plexus in the dermis alone. In the ear regions of the fowl, the blood capillaries are reduced in the dermis, and the presence of certain crystals of unknown chemical composition produces a double refraction of the light waves. Hence, the ear region appears white in reflected light.

c. Genic Control of Chromatophoric Activity

The transplantation of small pieces of epidermis and its adhering mesoderm from one early chick embryo to another is possible. Under these conditions, the donor tissue with its donor melanophores governs the color pattern of the feathers developed in the are^ of the transplant (Willier and Rawles, ’40). That is, melanophores from a Black Minorca embryo, transplanted to a White Leghorn embryo, will produce a Black Minorca color pattern, in the White Leghorn in the area of transplant, at least during the development of nestling down and juvenile feathers. Barred Rock melanophores produce barreo feather patterns in White Leghorn, New Hampshire Red, Black Minorca, etc. These results demonstrate that the introduced melanophore produces the color pattern in the feather in the immediate area of the implant.

Various genetic studies (see Dushane, ’44, for references) have demonstrated that the Barred Rock factor is dominant, and that it is sex-linked. For example, if a Barred Rock hen is crossed with a Rhode Island Red cock, the Fi male will contain two sex chromosomes, one from each parent. That chromosome from the female parent will have a Barred Rock factor, whereas that from the male parent will not. The Fj cock, therefore, is heterozygous for barring, and, as the barring factor is dominant, the Fi cock will show barred feathers. The Fi female, however, derives its single sex chromosome from the male parent; as this chromosome does not contain the barring factor, the Fi female is black.

Willier ('41) presents evidence concerning the transplantation of melanophores from Fi heterozygous males and Fi heterozygous females of this Barred Rock cross. Transplanted melanophores from an Fi male into White Leghorn hosts always produce barred contour feathers in either sex, whereas Fi female melanophores transplanted to White Leghorn hosts always produce non-barred or black regions. Danforth (’29) demonstrated that the barring factor in the skin of the male donor at hatching, when transplanted to a female host at hatching which lacked the barring factor, produces barred feathers in the female host in the area of the transplant. The results obtained by Danforth suggest that the barring gene acts independently of the sex hormone, although the feather type present in the graft assumes the female characters of the host and, hence, is affected by the female sex hormone. The results of these experiments by Willier and Danforth suggest that the barring gene in poultry acts directly upon the melanophore and not upon the environment in which the melanophore functions. (For extensive description, references, and discussion of these phenomena, consult Danforth, ’29; Willier, ’41; and Dushane, ’44.)

d. Examples of Hormonal Control of Chromatophoric Activity

In the indigo bunting, the male resembles the female during the non-breeding season. During the breeding season, however, the male develops a brilliant, purple-colored, highly iridescent plumage. Castration experiments and gonadotrophic hormone administration suggest that this nuptial plumage is dependent, not upon the male sex hormone, but upon gonadotrophic hormones elaborated by the pituitary gland in the male. In the female, however, the presence of the female sex hormone inhibits the effects of the pituitary gonadotrophins; hence, she retains the sexually quiescent type of plumage (Domm, ’39, p. 285). Also, in certain cases where the color of the bird’s bill is a sex-dimorphic character appearing during the breeding season only, it has been shown that the pigmentation of the bill is dependent upon the presence of the male sex hormone (Domm, ’39).

e. Environmental Control of Chromatophoric Activity

The above-mentioned instances of color-pattern development are concerned with the elaboration and deposition of pigment within the epidermis and epidermal structures. On the other hand, other observations demonstrate that the contraction and expansion of chromatophores and, hence, the production of different tones of color patterns, may be effected by a variety of environmental stimuli in lower vertebrates. In some cases this may be due to direct stimulation of the chromatophores by light or darkness or by changes in temperature; in other instances the causative factor is a secretion from certain glands, such as the pituitary or adrenal glands. The latter secretions in some forms appear to be aroused by light waves to the eye, from whence the stimulation is relayed through the nervous system to the respective gland or glands. In still other instances the light waves to the eye may cause a direct stimulation of the chromatophores by means of nerve fibers which reach the chromatophores. Other examples suggest that certain neurohumoral substances, elaborated by the terminal fibers of the nerves some distance away from the chromatophore, slowly diffuse to the chromatophore, causing its expansion or contraction (Noble, '32, pp. 141-147; Parker, '40).


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