Part 94

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