Part 93

Note the auditory vesicles and whether or not they are closed sacs.

Note that the heart has grown in length and has become doubled on itself. The two ends are fixed. Note the relation of the heart to the afferent and efferent blood-vessels. Note that two, and possibly three, visceral pounches are visible. Note whether or not they are ventral to the midbrain. The first, the hyomandibular pouch, is bounded in front by the first visceral, or mandibular, and behind by the second visceral, arch. The second pouch is bound in front by the hyoid, and behind by the third visceral arch. The third pouch is bounded in front by the third visceral arch, and behind by the fourth. Note the number of mesoblastic somites and the condition of the mesoblastic segmental plates.

Note how far back the foregut is closed. Locate the head fold of the amnion, and note how far back it is closed. Note changes that have taken place in the vascular area. In studying the sections it will be found that a section cut transversely to the trunk will pass horizontally through the forebrain and through the midbrain.

Study a section through the trunk a short distance behind the heart. Observe the elevation of the axis of the body. Note the way in which the lateral folds, or the lateral limiting sulci, in the somatopleure, delimit the embryonic from the extra-embryonic area.

Note the appearance of the mesenchyme, the approximation of the two dorsal aortæ, the appearance of the amniotic folds. Observe in the mesoderm the posterior cardiac veins, and the myotomes, or muscle plates. The sclerotome is made up of the mass of mesenchyme between the myotome, on the one hand, and the neural tube and the notochord, on the other. Note the folding of the splanchno-pleure, and note if there is present the Wolffian duct, or the nephrotome.

Study sections through the optic vesicles. Note if there is the beginning of the lens. Do you note the diverticula of the pharynx? Can you identify the closed amnion and the chorion?

Study sections through the auditory pit. Note fusion of the gill pouches with the ectoderm. Note the blood-vessels. Study sections through the region of the heart, through the roots of the vitelline veins, and through the primitive streak, if it is still present.

For this study it will appear that the anterior end has developed in advance of the posterior end. The tail fold has probably just begun.

Write a description of the pharynx, and of the circulation at this stage.

4. THE EMBRYO SIXTY-EIGHT TO SEVENTY-TWO HOURS OLD WITH CERVICAL 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.