formation of papillæ, which is constituted by the upward growth of the
dermis, or the sensitive and vascular parts of the skin. Then the skin
immediately around the papilla sinks downward, so that later the papilla
is inclosed in a follicle of the skin. The epidermis over the papilla is
the same as over the rest of the surface. The horny outer layer of the
epidermis forms for the growing feather a protective sheath which is
cast off as the feather is formed. The feather proper develops from the
underlying germinative layer, which as the feather develops, forms a
cylinder of cells. The lower part of the cylinder is in touch with the
papilla; this later becomes the quill (Fig. 80, No. 2). The upper part
of the cylinder develops the web portion of the feather. As soon as the
feather is fully developed the papilla, which has projected into the
quill and nourished it, is withdrawn, and the quill becomes filled with
a pithlike material forming septa, which extend in different directions.
Once a year, usually in the late summer or in the fall, the entire
feather coat is changed. This process is called _molting_. During this
time the bird appears in a somewhat depressed condition, the hen almost
always ceases laying. Birds also molt in the spring, to a limited
extent. The male at this time takes on the so-called breeding plumage,
which is the most beautiful of the year. It has recently been
established by Rice, that the young fowl, in reaching a stage of egg
production, molts five times before the laying period begins.
The =structure of the skin= of fowls is similar to that of the skin of
mammals. The skin consists of two layers the outer portion, or
_epidermis_, and the inner true skin, the _cutis_, _corium_, or
_dermis_. If we study a section of skin from the shank region of a fowl
we find the outer portion is differentiated into two distinct regions,
the rete Malpighii and the stratum corneum. The _stratum corneum_ is the
outer horny layer. The cells making up this portion are fusiform,
flattened, and in regular rows. The nuclei in these layers of cells are
not pronouncedly visible, and the outline of the cells not clear in a
section such as used in our ordinary methods of study. The corneum is a
compact mass of remnant cells which have lost the appearance and their
texture of living cells. It thus becomes modified into scales upon the
skin surface.
Between the stratum corneum and the stratum Malpighii there is another
zone which consists of the cells undergoing a transitional stage from a
cubical shape to a more flattened appearance, and gradually becoming
more granular and hyaline.
The various parts of the epidermis are in close genetic relationship to
one another. The upper layer of epithelium is constantly being
desquamated. This casting off is compensated for by a continuous upward
pushing of its lower elements. Cell proliferation occurs in the basal
cells and in adjacent cellular strata of the stratum germinativum, or
stratum Malpighii, where the elements are often seen in process of
mitosis, or cell division. The young cells are gradually pushed upward.
During their course they assume the general characteristics of the
elements composing the layers through which they pass. This process is
as follows: each cell changes first into a cell of the stratum
Malpighii; then, when it commences the formation of keratohyalin, it
changes into a cell of the stratum granulosum; later still, into a cell
of the stratum lucidum; and finally into an element of the stratum
corneum, where it gradually loses its nucleus, cornifies, and at last
drops off.
The mesodermic portion of the skin consists of loose, subcutaneous
connective tissue containing some fat. The amount of adipose tissue in
the subcutaneous layer is subject to great variation; there are a few
places in which there is little or no fat. The upper portion of this
layer contains a few elastic fibers, which interlace and run in all
directions. Numerous round or oval cells are found in the upper region.
The lower and middle portions of the corium are richly supplied with
blood-vessels terminating into capillaries, which penetrate the portions
bordering the epidermis. Nerves giving off terminal branches also occur.
The various _colors_ of the skins of fowls are due to the distribution
of various quantities of two colors, orange-yellow and brownish-black.
The yellow pigment is probably carotin and xanthophyll, two pigments
contained in association with the chlorophyll of plants, which the bird
obtains in its feed. These coloring matters were formerly called
lipochrome; but as lipochrome may be any coloring matter of fat, it is
not sufficiently definite. This yellow pigment, when present, is
diffused through all parts of the cell. When dilute, it gives a yellow
hue; when concentrated, orange. It is found in the epidermis and in the
fatty masses of and beneath the corium, and is probably identical with
the yellow color of fat in other portions of the body and in the yolk of
the egg. A yellow shank, in a heavily laying hen, soon loses a part of
its pigment; this is also noted of the coloring matter in other parts of
the body. This fact indicates that the coloring matter of the fatty part
of the egg yolk is from the same source as that of the fat. The draught
of this substance is more than normally and the reserve is being drawn
upon. Feed rich in zanthophyll and carotin cause intense yellow colored
yolks, and feeds poor in these substances cause the yolks to be a pale
yellow. Cotton seed meal contains two pigments, one a yellow crystalline
substance and the other a brownish resinous substance. Both of these
pigments are probably deposited in the egg yolk. The eggs of some hens
contain a large quantity giving a light brownish-yellow color to the
yolk.
The brown or black-brown pigment is carried in microscopic pigment
granules, which may be scattered through the ordinary cells or may be
confined to special pigment cells. The former are confined to the
epidermis; the latter may be found in both layers, but infrequently in
the epidermis. When granules are present in the flattened cells of the
corium, they occupy the nuclear region. They lie in short thin lines
while those of the under portion of the Malpighian layer occur in oval
groups. Where these granules occur in the rete layer, they cluster
around the nuclei. In the colored skin there are dark pigment granules
found in the corium and to a less extent in the rete layer. Hanau has
described a definite cellular body which he found densely packed among
the black-brown granules. There is a central body which sends out
branches in all directions. In very dark skinned shanks these ramifying
strands interlace, and form a compact network, which in many cases is so
thick as to give the impression of a homogeneous mass. Here and there
occur round or oval pigmented bodies, which Hanau concluded were the
star-shaped cells with their pseudopod-like appendages contracted.
Pigment cells commonly lie around blood-vessels, clearly indicating
their course. They frequently form a compact tube, but more often are
limited to fragments which only partly enclose the vessels. Pigment
cells occur in several well-defined localities: in the upper portion of
the cutis among the closely interwoven strands of connective tissue; in
the region bordering the blood-vessels; in proximity to nerves, in nerve
endings and in surrounding fat masses. Isolated granules are frequently
scattered throughout the lower section of the corium. Barrows concludes
that the lower bodies of pigment play little or no part in the color of
the external shank, as they lie far below the opaque connective tissue.
Melanin pigment granules are always contained in the pigment cells. When
found in the Malpighian layer, the pigment cells are of an oval form.
Immediately below the epidermis in the shank skin, extends a space less
in width than the row of columnar cells, which is devoid of pigment. The
brown pigment is melanin, which in large quantities takes on a black
hue.
White skin does not contain superficial pigment. Melanin has been
observed in the study of white shank skin, but it lay at considerable
depth or in quantities insufficient to be noticeable. In some breeds of
fowls, as the Mottled Houdan, there are areas in which much melanin is
irregularly deposited, which circumstance gives the leg its mottled
appearance.
The yellow color is the result of a deposit of the yellow pigment in the
fat of the shank. This may be deposited in both layers of the skin, or
in the corium alone. When present it is diffused throughout the entire
cell as well as throughout the intercellular substance. In young birds
the Malpighian layer contains much yellow pigment. Old hens have only
small quantities in the corneum. In breeds with normal yellow shanks old
hens that have never laid eggs show a deep orange color in both the
dermis and epidermis.
In blue shanked birds melanin is present only in the corium. The black
pigment is seen through the semi-transparent Malpighian stratum, making
it appear bluish instead of brown or black.
The black shank color results when melanin appears in the epidermis. Two
forms of black pigment occur in the epidermis: granules in both layers
and pigment cells in the rete Malpighii.
The green shank is produced where there is pigment in the epidermis and
numerous melanin pigment cells in the upper corium. It is an optical
effect due to melanin lying under the semi-transparent yellow epidermis.
There is no melanin in the epidermis.
In the beak the corium of the skin is represented by a thin layer
located between the =periosteum= and the stratum Malpighii. Numerous
blood-vessels pass into it, and in the soft horn-like skin, occur
sensory nerve fibers.
The nails originate from the epidermis, of which they are a
modification. The nails of the toes are bent downward. They present a
convex dorsal surface and are concave ventrally. The dorsal surface
consists of a horny plate, which is set in a nail matrix. The ventral
portion merges with the sides of the upper half, and, as the lower
portion is the softer and wears faster, the nail has a sharp point and
edge. The matrix is formed by a growth of the Malpighian layer of the
cutis. A fold of skin lies over its posterior part. The epidermic cells
of the dorsal, or nail, part, and the base of the ventral part grow
fast. The outer cell layer gradually becomes horn-like.
The _spurs_ are conical with a flat base. The basal part rests upon an
enlargement of the shank bone. Soft structure is found between the horny
spur and the bone. The upper cells, like those of the nails and of the
skin, are constantly being worn or cast off, and new cells push up from
the lower layers of cells. These newly formed flattened cells soon
become cornified. The oldest formation is found at the tip and the
youngest at the base.
EMBRYOLOGY OF THE CHICK
That a new individual may be brought into existence, there must be
accomplished the union of the male element, or _spermatozoon_, with the
female element, the _ovum_. This union is called _fertilization_. In the
fowl this fertilization is accomplished at the anterior portion of the
oviduct, after the calyx has ruptured and discharged its yolk, and
before the albumen has been formed around it. The blastoderm is found on
the surface of the yolk. One spermatozoon is all that is required; in
fact, only one can be used in this union.
=Spermatogenesis.=—The spermatozoa are formed by the seminiferous
tubules of the testis. From these cells, called the _spermatogonia_, are
formed other cells called _spermatocytes_, which in turn form the
_spermatids_, the immediate forerunners of the spermatozoa. During the
period of multiplication the spermatogonia divide repeatedly by mitosis.
Numbers of small cells are thus produced, each containing in its nucleus
the number of chromosomes typical of the somatic cell of that fowl. In
the second period the cells become larger and spermatocytes of the first
order are formed. Then comes the period of maturation, during which two
succeeding divisions rapidly occur. The first division results in the
formation of two cells exactly alike. These are spermatocytes of the
second order. They differ from the somatic cells in that they contain
only one-half the typical number of chromosomes. The second division
produces two similar spermatids from one spermatocyte of the second
order. Therefore four spermatids exactly alike may be formed from one
spermatocyte of the first order. From these spermatids the spermatozoa
are formed (Fig. 55). The heads of the spermatozoa contain the nuclei
derived from the spermatids; the necks contain the centrosomes; and the
tail, consisting of three parts is probably formed from the protoplasm.
Three parts of the tail are as follows: first, the pars conjunctionis,
which unites the tail to the neck; second, the pars principalis, which
constitutes the main length of the tail; and, third, the pars
terminalis, which consists of an axial filament which transverses the
entire tail and is surrounded by a protoplasmic sheath.
=Oögenesis.=—The ovum during its formation passes through three stages.
[Illustration:
FIG. 81.—Section of ovum in a hen. 1. Nucleolus. 2. Nucleus. 3. Liquor
folliculi. 4. Stratum granulosum. 5. Follicular cells. 6. Theca
folliculi. 7. Peripheral stroma.
]
The first stage, that of _division_, takes place before the chick is
hatched, and, according to Bradley, comes to an end about the time of
hatching. This stage consists of the rapid formation of ova in the
female chick. In the second stage, which begins about the time of
hatching, there is an increase in the size of the units of the ovary,
accompanied by _yolk formation_. At this time each ovum is in its own
follicle (Fig. 81), and is surrounded by a layer of cuboidal cells and a
_theca_. The theca is formed from the adjacent fibrous stroma. The third
stage, that of _maturation_, commences during the development of the
yolk and is complete after it has escaped to the oviduct. Maturation
consists of each cell’s dividing into two unequal parts. In each
division the cell is split into a small cell known as the _polar body_,
which is cast off and disappears, and a larger cell, which is the ovum
proper. In this process half of the original chromosomes are cast off.
=Fertilization.=—The sperm travels rapidly; experiments have shown eggs
to be fertile laid twenty-four hours after service by a male. When the
ovum is discharged into the ovarian pocket it is surrounded by
spermatozoa.
After the male pronucleus has united with the female pronucleus in the
single-celled ovum, there is a cleavage of the cell in the long axis of
the egg, making two cells; and then a cleavage at right angles, which
progressively continues, makes the mulberry-like mass. The remaining
content of the egg consists of food for the development of the embryo.
From this mass of cells before the egg is laid the _blastoderm_ is
formed. The cells of the blastoderm are differentiated into two layers.
The superficial layer is the _ectoderm_ and the lower layer is the
_entoderm_. In the newly laid egg the blastoderm may be observed. It is
about 4 millimeters in diameter. It has a transparent central area, the
zona pellucida, which is located over the subgerminal region. There is a
peripheral, less transparent area called the zona opaca.
In the fertile egg, as soon as it is subjected to the proper
temperature, cell multiplication in the blastoderm begins. The first
signs of such change are noted in the pellucid area of the blastoderm
where embryonal traces appear in the form of the parallel lines called
the _plicæ primitivæ_, which diverge to form the cephalic dilatation. At
about this time takes place the formation of the _myelencephalous
columns_, in which the blood lakes expand in the surrounding halones and
in the tracts along which pass colorless blood particles. These tracts
extend from below the cephalic expansion to the peripheral sinuses, as
the _proto-vertebræ_, which begin to appear at the sides of the myelon.
The red color is acquired by the blood, and the heart by its movements,
is made more manifest as the _punctum saliens_. A distinct membrane, the
serous layer, is formed upon the germ and the blastoderm. The cephalic
end of the embryo rises from the surface of the blastoderm, and then,
curving down, sinks into it, forming for itself a kind of hood of the
serous layer. This hood gradually extends from the margin of the fossa
over the body, and, meeting a similar fold formed by the projecting and
incurved tail, closes over the germ on the upper side, making a
circumscribed cavity, which is the _amnion_. The progress of
differentiation of layers of the blastoderm has, meantime, gone on
beneath. The serous layer is in part reflected from the vascular and
from the mucous layer. The mucous layer is concerned in the formation of
the intestinal canal; and beyond this part, which is at first an open
groove, the mucous layer expands over the yolk, which it ultimately
incloses, the margins of the _vitellicle_ so formed contracting and
uniting at the side opposite the embryo at a sort of cicatrix, to which
the last part of the abdominal yolk adheres. The vitellicle is richly
vascular, and the surface next to the yolk is augmented by rugæ.
The fowl’s egg, at about the fortieth hour, shows the buds from which
the limbs are developed. A vesicle is seen to protrude near the anal end
of the intestine, which, rapidly expanding, spreads over the embryo,
acquiring a close adhesion to the amnion, but remaining distinct from
the vitellicle, over which it spreads. It finally encloses the albumen
and interposes itself between the latter and the lining membrane of the
shell. Umbilical vessels are associated with this membrane. Hunter
called this membrane the _allantois_ from its containing urine, and Owen
states that the sac which surrounds the albumen acts as the chorion or
_placenta_; for it is most probable that from this surface the albumen
is absorbed and the chick supported on its developmental food. The
external part of the sac apparently acts as lungs as it comes into
contact with the shell of the egg through pores of which there is an
exchange of air. Oxygen is consumed and carbon dioxide is given off. The
blood in the vessels of this membrane is in color more like arterial
blood and that in the interior more like venous blood.
The embryonic mass of the incubating egg always floats to the top side.
As the embryo grows it turns upon its left side, exhibiting a profile
view; it then indents the yolk, and finally almost divides it into two
parts.
The peripheral layers of cells rise from the margin of the germ mass,
and extend and contract toward the opposite pole. This tract of germ
substance is the _primitive streak_. Along the median line it next forms
a furrow, which stops short of the ends of the streak. This streak
terminates opposite the point from which the germ begins, and swells
into the head. The median furrow expands upon it. The cephalic borders
are next united by a thin layer of epithelial cells above the furrow,
converting it into a cavity, or ventricle. The myelonal furrow is
similarly covered by a layer, uniting the lateral columns. The embryonal
trace becomes longer, narrower, and bends round the vitellus. A layer of
epithelial cells forms a network over the whole dorsal surface of the
embryo. Oblique striæ appear in the broadening germ mass radiating from
the primitive streak. These indicate divisional segments. These
beginnings of aponeurotic septa probably accompany and support nervous
productions from the myelon columns.
Two transverse constrictions begin to divide the cephalic enlargements
into three lobes, the second and the third of which expand into
vesicles. An accumulation of cells at the side of the middle expansion
appears to add greatly to its breadth. This forms the basis of the eyes.
The differentiation and the confluence of the cell constituents of the
primitive streak have led to the formation of a pair of albuminous cords
along the sides of the median furrow, forming the _myelon_ proper. The
cells exterior to and above them are converted into muscle and fibrous
septa; and beneath the column is a jelly-filled cylinder, with a
transversely striated sheath, pointed at both ends, forming the
_notocord_. Its anterior point passes a little in advance of the
acoustic vesicle. Beneath the notocord and surrounding the blastema is
stretched the vegetative, or mucous, layer of cells, in contact with the
yolk. Both the head and the tail of the now cylindrical embryo are
liberated from the surface of the yolk. A fold of the blastema,
reflected from the under part of the head, sinks like a pouch into the
yolk, and soon includes the rudiment of the heart, like a bent cord,
which begins to oscillate about the seventh day. From the midline of the
inferior surface of the embryo, or its mucous layer, two longitudinal
plates descend, diverging into the yolk-substance, and form the
_primitive intestinal_ groove.
The _ophthalmic vesicle_ elongates and curves outward until the two ends
almost come into contact. Between these two ends and beneath the
delicate tegumentary layer connecting them the _crystalline lens_ is
formed. About the same time, the _otoliths_ appear in the acoustic
vesicles, which have now acquired a cartilaginous case. The _cerebral
lobes_ begin to be formed by a small fold, rising laterally and
overlapping the forepart of the second enlargement, which has expanded
to greater breadth. The _olfactory cavities_ appear as small cutaneous
follicles.
The two myelonal columns, expanding between the ear sacs and receding so
as to show the notocord beneath, bend upward and inward, and unite, to
be continued into the posterior of the _optic lobes_, thus commencing
the cerebellar bridge across the epencephalic ventricle. The encephalic
vacuities have begun to be filled by the granular basis of the
cerebellar substance.
The _intestinal groove_ begins to be converted into a canal at its two
ends. Beneath the anterior end, and behind the heart, there gradually
accumulates the cellular basis of the liver.
The commencement of the development of the _organ of hearing_ is by a
superficial depression of the cephalic blastema to meet the process from
the encephalon, which forms the acoustic nerve. The lining of the
depression becomes, on closure of the slit, the proper tunic of the
labyrinth.
The vesicle of the labyrinth swells into four dilatations, of which
three are ampullar and the fourth cochlear. The _ampullar dilatations_
extend into very slender canals, at first almost in the same plane, by
which they are brought into mutual communication. As the canals expand
and elongate, they assume their characteristic relative positions as
external, superior, and posterior, the posterior end of the external
canal being extended beneath the posterior canal. The cochlear
dilatation curves as it elongates. An inner layer becomes distinct from
the common membrane and forms the _acoustic lamina_.
As in the development of the ear, so in the development of the eye, the
production of the nerve process from the cerebral center is the first
step; the infolding of the superficial blastema to meet the nerve is the
next. The so-called _cutaneous follicle_ becomes a circumscribed sac or
vesicle, in which the changes and the development next proceed,
converting the vesicle into an acoustic labyrinth or into an eyeball. In
each case neural elements of two vertebræ become modified to lodge and
to protect the sense organs, forming respectively the recess called
_otocrane_ and that called the _orbit_. The one is located between the
occipital and the parietal vertebræ, and the other between the frontal
and the nasal vertebræ. The part of the outer blastemal layer of the
head which sinks to meet the process from the mesencephalic dilatation,
rapidly changes its follicular into a vesicular state. The vesicle thus
formed elongates, bending around the cell mass in which the _crystalline
lens_ is formed, and the meeting of the two ends results in forming the
choroid fissure at the lower part of the eyeball.