FLEXURES FORMED.
In a study of the living embryo note the changes visible to the naked
eye since forty-eight hours old. Note the difference in the
blood-vessels of the vascular area. Name the arteries and veins. Note
the beating of the heart.
In a study of the entire mount note that a second, the cervical flexure,
has appeared in the head. Note that the tail fold is well formed. Note
the position of the embryo on the blastoderm. Determine if the amnion is
completely closed. Note the olfactory pits on the ventral surface of the
head, a short distance in front of the optic stalks. Note the
telencephalon, a rudiment of the cerebral hemispheres and an extension
of the primary forebrain. It is bilobed anteriorly. The optic stalks are
attached to the floor near the anterior end of the thalamencephalon.
Note the infundibular region, which is the depressed region behind the
optic stalks. In the roof of the thalamencephalon there is a short
diverticulum, the epiphysis. The mesencephalon, or midbrain, forms the
apex of the cranial flexure, and is united to the hindbrain by a narrow
isthmus. The metencephalon, or rudimentary cerebellum, appears as a
thick portion on the most anterior division of the hindbrain. The rest
of the hindbrain is provided with a transparent roof and constitutes the
myelencephalon, or the rudimentary medulla oblongata. Observe the inner
and the outer wall of the optic cup, the lens, the choroid fissure, and
the posterior, or vitreous, chamber.
Note the form of the otocyst, or auditory sac. Note above which visceral
arch it lies. Note the number of visceral clefts. The visceral arches
are formed by the thickening of the walls of the bounding clefts. The
visceral arches are as follows: the first is the mandibular, or
hyomandibular, arch, which is in front of the first cleft. From this
there is developed the lower jaw. Note if there is a maxillary process
arising from the dorsal angle of the arch. The second arch is the hyoid
arch, which is located behind the first cleft. Then follow in order the
third, the fourth, and the fifth visceral arches. Note above the
mandibular arch the rudimentary trigeminal ganglion, and above the hyoid
arch the rudimentary acoustico-facialis. The latter is in contact with
the anterior walls of the auditory sac. The rudimentary
glosso-pharyngeal ganglion is noted above the third visceral arch. The
vagus, or pneumogastric, ganglion is located above the fourth and the
fifth. Note the form and the position of the heart. Note the anterior
and the posterior limb rudiment in the trunk.
In a study of the sections it is found that cuts transverse to the trunk
pass about horizontally through the forebrain. At this age the following
sections of the embryo should be studied:
First, through the hindbrain, at which level will be noted the auditory
sacs, the neuromeres, the trigeminal, the acoustico-facialis, and the
glosso-pharyngeal and the vagus ganglion.
Second, through the upper part of the pharynx, at which level will be
observed the midbrain, the hindbrain, the visceral pouches, the nerves,
and the blood-vessels.
Third, through the choroid fissure of the optic cups. Note the parts of
the eye, and, on the other side of the section, the heart.
Fourth, a study of a section through the olfactory pits.
Fifth, a study of a section through the pancreatic and the hepatic
diverticula.
Sixth, a study at the beginning of the allantois through the hind-gut.
At this age it is of interest to study the systems of organs. Observe
the manner in which the splanchno-pleure folds to form the walls of the
intestine. Note the commencement of the mesentery. Note that the
foremost part of the alimentary tract is formed from the stomodeal
invagination of the ectoderm. The hypophysis is formed from a dorsal
outgrowth of this. Note its relation to the brain.
The following structures are formed from outgrowths of the ectoderm at
this stage:
First, the visceral pouches.
Second, the median rudiment of the thyroid. This is an outgrowth from
the pharynx between the two hyoid arches.
Third, the rudimentary lungs, which develop in a pair from a median
ventral diverticulum of the alimentary tract, just behind the last
visceral pouch. The esophagus is just posterior to this. The esophagus,
very short at this stage, is continuous with a slightly wider part that
develops into the stomach.
Fourth, the first liver diverticulum, and, at a short distance posterior
to this, the second liver diverticulum.
Fifth, the pancreas is at a point where the intestine opens ventrally.
It first appears as a slight thickening of the dorsal angle of the
intestine.
Sixth, the ventral wall of the hind-gut forms a wide evagination.
Seventh, the beginning of the allantois.
After the whole series of transverse sections have been studied and
drawn, construct a longitudinal section of the fetus, including a
reconstruction of the alimentary tract.
At this stage the =heart= is a simple tube. The following divisions are
distinctly visible: the auricular portion, the ventricular portion, the
sinus venosus, and the bulbus arteriosus. The union of the two ductus
Cuvieri and the ductus venosus form the sinus venosus. The two ductus
Cuvieri are formed by the union of the anterior and the posterior
cardinal veins. The ductus venosus is formed by the union of the small
right and the large left vitelline vein. These latter veins return the
blood from the yolk sac. The sinus venosus empties into the single
auricle above which it is located. The single auricle is later divided
into two chambers, the right and the left auricle. At this stage it is
widest in the lateral direction. The auricle empties directly into the
ventricle. The ventricle lies ventrally and behind the auricle. This
location is due to the bending of the heart at this stage of
development. Its hindmost portion forms the future apex of the heart. If
the series of sections be studied from the posterior forward, the
ventricle will be first to appear in the sections. Just beneath the
auricular portion of the heart there is the bulbus arteriosus. The
bulbus arteriosus soon divides into a number of aortic arches. There is
an ascending pair in each of the visceral arches. The dorsal aorta is
formed by the union of the aortic arches above the visceral arches. The
aortic arches are first continued a short distance forward as the
carotid arteries. The dorsal, or posterior, aorta passes backward under
the notochord. The dorsal aorta divides into two parallel aortæ which
give off on each side the vitelline arteries. Note other branches of
this aorta.
The veins at this stage consist of the anterior and posterior cardinal,
the ductus venosus, the ductus Cuvieri, and the vitelline veins.
Make drawing of the circulatory system after a completion of the study
of the series of sections.
Make a study of the =nervous system= according to hints already given.
The dorsal and the ventral roots of the spinal nerves are given off
separately, and secondarily unite. From the neuroblasts of the cord
there are at regular intervals outgrowths representing the ventral
roots. From the neural crest there develop segmental collections of
neuroblasts which form the spinal ganglia, from which the dorsal spinal
nerve roots develop. In fact this developmental stage can be observed in
embryos only forty-eight hours old, first appearing as a line of cells
springing on each side from the angle between the neural canal tube and
the external epiblast. In the section from the embryo seventy-two hours
old there are observed the rudiments of the development of these spinal
nerves. Four primary ganglia develop in the neural crest of the head.
These ganglia are as follows: the acoustico-facialis ganglia, which is
located over the hyoid arch; the ganglia of the trigeminus, which is
located over the mandibular arch; the ganglia of the glosso-pharyngeus,
which is located over the third visceral arch; and the ganglia of the
vagus, which is located over the third and the fourth visceral cleft.
In this stage of development the trigeminal and the acoustico-facialis
are clearly visible.
On each side and dorsal to the aorta is noted the Wolffian body, or
mesonephros. The Wolffian body (Fig. 82, _E_) consists of a series of
tubules imbedded in the mesenchyme. The openings into the Wolffian duct
lie just beneath the cardinal vein.
In a study of the =Wolffian duct= determine just how far anteriorly and
how far posteriorly it extends. Note whether it empties into the cloaca.
Each tube beginning in a blind extremity is later dilated. It has a thin
wall, and is situated near the median portion of the Wolffian body. The
tubule proper passing transversely, opens into the duct. The upper wall
of the thin walled part is invaginated by a mass of mesenchyme that
receives a small vessel from the dorsal aorta. The Malpighian corpuscle,
consisting of a glomerule and Bowman’s capsule, is thus established. In
the study of the four days old chick note the further development of
these parts.
In a study of a chick four days, or ninety-six hours old, note to what
extent the yolk is covered by the blastoderm. It will be noted that the
embryo lies in the extra-embryonic cavity. This cavity is bounded above
by the chorion and below by the splanchno-pleure. In removing the amnion
from the embryo, note the relation to the somatic umbilicus. Note the
relation of the splanchnic umbilicus to the splanchno-pleure. What
relation has the allantois to the above?
In examining the head, locate the cerebral hemispheres, and note their
development. Locate the pineal gland. Note changes in the olfactory pit
and the eye. Locate the lens and the choroid fissure. Note that the
maxillary process of the mandibular arch lies beneath the eye and behind
the olfactory pits. Note the otocyst and the relations of the other
arches to the above structures.
In a study of the =trunk=, note the tail, the allantois, Wolffian
ridges, the heart, and the condition and the position of the rudimentary
limbs.
Make drawing of embryo from the side view. Carefully cut off the head
immediately behind the last visceral arch, and study and draw the
structures observed on the ventral side. Note the maxillary processes,
the mandibular and the hyoid arch, the nasal pits, and the fronto-nasal
process, which is just beginning its development.
In a study of transverse sections observe from your drawing at what
level the section is made. Study and draw a section made through the
region of the anterior limbs. Note the spinal ganglion, the muscle
plate, or myotome, the condensation of mesenchyme around the notochord,
the pancreas, the liver, and the intestine; and note the distribution of
the mesenchyme. Note the ventral roots of the spinal nerves, and the
neuroblasts in the spinal cord.
In sectioning the embryo from before backward, the first sections will
pass horizontally through the hindbrain and the midbrain region. Note
the parts, including the auditory vesicle. In the first series of
sections also locate the ganglia of the pneumogastric, or vagus, the
acoustico-facialis, the trigeminus, and the glosso-pharyngeal nerves.
Note the notochord and the cardinal veins. After the disappearance of
the ear, we observe the midbrain, which, is located at one end of the
section, and the cord at the other end. The region between lies just
above the pharynx. In the following series study the visceral arches.
Note the third pair of cranial, or motor ocular, nerves. This latter
nerve springs from the floor of the midbrain. From the ventral
prolongation of the floor of the thalamencephalon there arises the
infundibulum. The hypophysis is located just beneath the infundibulum.
At this stage the hypophysis appears as a tube to empty into the mouth.
This is an ingrowth of the oral epithelium.
In a study of a section through the center of the eye we should observe
the lens and optic stalk. This is in the region of the optic chiasm.
Note the choroid fissure and the pineal gland, the latter appearing just
beyond the eyes. Just forward lies the telencephalon, or rudiments of
the cerebral hemispheres.
In a study of the =alimentary tract= we note that the mouth is bounded
by the mandibular arches. Note the maxillary processes, and the ventral
surface of the head. A finger-like diverticulum, extending from the roof
of the ruptured double membrane, formerly separating the pharynx from
the arches, forms the hypophysis. The great development of the visceral
pouches makes the pharynx rather complex. In studying sections
horizontally through the pharyngeal region note the various visceral
arches and pouches. Note the arteries of the thyroid diverticulum.
In the series note the changed development of the lung rudiments, the
glottis, the esophagus, the trachea, and the bronchi. The bronchi appear
in pairs. Note that the liver has assumed proportions, and that it
surrounds the common trunk of the vitelline veins, which it divides into
two parts. The sinus venosus lies close to the heart. The ductus venosus
is also surrounded by the liver. Above the tip of the ventricle we note
a dilatation which represents the stomach. The hepatic, or bile duct is
located immediately behind the stomach. This duct is formed by the
fusion of the right and the left duct. Locate, draw, and describe the
pancreas; trace the intestine; locate the splanchnic umbilicus, or yolk
stalk; locate the allantois stalk, and trace its connection with the
hind-gut.
In the series, locate and study the Wolffian ducts, the beginning of the
Müllerian duct, the embryonic kidney, or mesonephros, the permanent
kidney, or metanephros. The urino-genital ridge is made up of all the
above except the last named. The urino-genital ridge forms a rounded
projection on each side of the mesentery into the dorsal angles of the
body cavity.
The Wolffian ducts empty into the cloaca. There are two ducts which may
be traced far forward, and which are found to extend backward along the
lateral margin of the ridge to the cloaca. Along the greater part of
their length we note tubules emptying into them.
Beginning near the anterior end of the urino-genital ridge, we note that
the Müllerian ducts arise from a thickened line of epithelium. The
greater part of the ridge is formed by the mesonephros. This is made up
of a series of tubules in each of which we may distinguish two parts as
follows: a tuft of capillaries from the aorta forming the glomerules,
surrounded by a thin walled invaginated capsule, making up the
Malpighian corpuscle; and the tubules proper. The tubules lead from the
corpuscles, or glomerules, to the Wolffian duct.
The germinal epithelium constitutes the essential portions of the gonad,
or ovary, or testis. The germinal epithelium arises from a thickening of
the peritoneum of the median wall of the ridge. The gonad is found near
the anterior end of the ridge. At about this age of the embryo there
should appear the primitive ovary or testis. Near the posterior
termination of the Wolffian duct and from the dorsal diverticulum there
arises the ureter, or metanephros duct.
In these series there should be studied the heart and circulation, which
will be found similar to the sections from the embryo seventy-two hours
old.
Work out a summary of the relations of the allantois and the yolk stalk
to the intestines; the relations of the Wolffian ducts to the
intestines; the origin of the ureters from the Wolffian ducts; the
relations of the lungs, the liver, and the thyroid gland; the relations
of the blood-vessels; a study of the muscle plates; the relations of
epiphysis, hypophysis, infundibulum, mouth, and pharynx; the relations
of the vagus, or pneumogastric, trifacial, acoustico-facialis, and
glosso-pharyngeal nerves to the visceral arches.
In addition to the study of embryos at the end of each of the first four
days of incubation, a study should be made of preserved museum specimens
each of which represents the development of a day up to and including
the twenty-first day.
THE DERIVATIVES OF THE GERM-LAYERS
From the three primary germ-layers are developed the various tissues and
organs of the body by metamorphoses which may be referred to the two
fundamental processes of specialization, or the adaptation of structure
to function, and of unequal growth, which latter results in the
formation of folds, ridges, and constrictions.
From the ectoderm are produced:
The epidermis and its appendages, including the nails, the epithelium in
connection to the feathers and the feathers.
The infoldings of the epidermis, including the epithelium of the mouth,
epithelium of the salivary glands and the anterior lobe of the pituitary
body, or hypophysis.
The epithelium of the nasal tract with its glands and communicating
cavities.
The epithelium lining the external auditory canal, including the outer
stratum of the membrana tympani.
The lining of the anus.
The epithelium of the conjunctiva and of the anterior part of the
cornea, the crystalline lens.
The spinal cord, the brain with its outgrowths, including the optic
nerve, the retina, and the posterior lobe of the pituitary body.
The epithelium of the inner ear.
From the entoderm are produced:
The epithelium of the respiratory tract.
The epithelium of the digestive tract, from the back part of the pharynx
to the anus, including its associated glands, the liver, and the
pancreas.
The epithelial parts of the middle ear and of the eustachian tube.
The epithelium of the thymus and the thyroid bodies.
From the mesoderm are developed:
Connective tissue in all its modified forms, such as bone, cartilage,
lymph, blood, fibrous and areolar tissue.
Muscle tissue.
All endothelial cells, as of joint-cavities, bursal sacs, lymph sacs,
blood-vessels, pericardium, and endocardium, pleura, and peritoneum.
The spleen.
The kidneys and ureters.
The testicles and the system of excretory ducts.
The ovary and oviduct.
PREPARATION OF STRUCTURES FOR STUDY
It is hoped that the following suggestions will be helpful in the
laboratory work.
=Directions for Dissecting Muscles.=—The muscles that are brought into
great play in movements of the bird’s limbs are dark carmine in color,
while those which are not brought greatly into use are pale or white in
color.
The tendons are made up of white fibrous connective tissue, are very
dense, and pearly white in color.
In securing a bird for dissection of muscles it is best to select one in
rather poor flesh, as the fat is annoying. The bird may be chloroformed
or killed in a bell jar by aid of illuminating gas. After the bird is
dead pluck all the feathers and immerse it in a 10 per cent. solution of
formaldehyde or of 80 per cent. alcohol. It is best to puncture the
abdominal wall so that the fluid may at once fill the abdominal cavity
and more readily gain access to the chest cavity; and to puncture the
skin at various points so that the liquid may more quickly become
disseminated among the muscular structures. Post-mortem changes quickly
take place if these precautions are not taken. The liquid surrounding
the carcass should be at least twice the quantity of the bulk of the
carcass.
The first dissection should be to lay bare the dermal muscles. The
dermal muscles are of two kinds, true dermal and dermo-osseous. The
dermal muscles have their origin and insertion in the skin, and control
the movements of the different groups of feathers. The dermo-osseous
have their origin on some part of the skeleton, and insert to the
integuments.
The dermal muscles vary with the characteristics of the bird, we do not
find all the known dermal muscles in any one specimen. A cock of the
Cornish breed will show these muscles best developed. Birds possess an
enormous system of minute muscles divided up into an infinite number of
fasciculi, to act harmoniously upon the feather quills and to agitate
collectively the plumage. By the aid of a low-power lens the action of
the feather muscles in the large quill butts of the wing or the tail may
be studied.
The muscles may be studied in groups as outlined in the text. Make an
incision through the skin down to the bone on the superior part of the
head, parallel and close to the base of the upper mandible, and
extending completely across. From the outer end of this make an incision
backward and down to the skull and posteriorly. The muscles of the upper
part of the neck will then be exposed. The straight incision should
extend to about a half inch on the inside of the upper eyelid of the
same side. Reflect the flap of skin from the top of the skull, and
carefully examine the under side of it in the median line, where it
overlies the frontal region. The dermo-frontalis will be observed if it
be present. In many birds, especially in females, it may not be
discernible, and may be considered absent. To expose the circumconcha
make an incision completely around the ear; then carefully dissect to
the ear base. A dermal circular muscle should be observed. To expose the
dermo-temporalis extend the longitudinal incision down the back of the
neck to a point between the clavicular heads, carrying it just through
the skin and about one-fourth of an inch to the side of the median line.
Remove the skin from the throat and the anterior portion of the chest.
Lay open the alar and parapatagial duplicatures of the skin. This
exposes a number of dermal muscles. The dermo-temporalis is now observed
to extend from a small depression just above and anterior to the
temporal fossa. It makes slight attachments to the temporal muscle,
which it covers, and extends backward as a thin ribbon-shaped muscle,
the fibers blending with those of the cleido-trachealis, and becomes
lost upon the skin in front and opposite the shoulder-joint. At times
its fibers blend with those of the dermo-tensor patagii.
As the musculature of the fowl is loosely arranged, the rest of the
dissection is easily done if care be exercised.
The ligaments may be dissected after the completion of the study of
muscles, using the same subject or, a two pound broiler be prepared by
killing in the gas chamber, plucking the feathers and parboiling just
till the flesh becomes tender and is easily removed, it will be observed
that all structures can be removed from the points exposing the
ligaments distinctly. The ligaments appear swollen and more easily
observed for study.
=Directions for the Study of the Viscera.=—Carefully remove the right
and the left abdominal and thoracic walls, allowing a strip of tissue to
remain in the median line to hold the organs in their normal position.
To open these cavities it is necessary to use the bone saw and the
scalpel. The organs may now be studied from each side. To make a
longitudinal section through the median line, select a small bird, one
weighing not more than 2 pounds, kill, and preserve in a 10 per cent.
solution of formaldehyde for three days. Then with a sharp, thin,
long-bladed knife make an incision at one sweep through the median line
of the body down to the back bone, and with the bone saw section through
the vertebræ. If it is difficult to cut through the breast-bone, saw
through before making the incision. In small birds the entire cut may be
made without the aid of the saw.
=Directions for the Study of Arteries.=—Arteries should be injected.
Veins are usually more or less filled with blood so that the tracing of
these is not so difficult as the tracing of uninjected arteries. Nerves
are white and no difficulty is usually encountered in tracing them.
The courses of arteries, veins, and nerves are side by side, and many of
them, as in mammals, are arranged in the order of veins, arteries,
nerves, the veins being in front.