Walls of the myelencephalon have swellings called neuromeres. The walls and floor of the myelencephalon will become the medulla. The auditory vesicle lies beside neuromere IV. The small hole first seen is the endolymphatic duct. It opens into a larger chamber which will subdivide as the vestibule, cochlea, and semicircular canals. Only the semicircular canals can be pinpointed.

Ganglia of nerves V, VII, and VIII (which are fused for part of their length) appear next to the myelencephalon somewhat farther back in the series.

Return to the metencephalon. Its walls will become the cerebellum, and its floor will become the pons. The cavities of metencephalon and myelencephalon will become the fourth brain ventricle and will fill with cerebro-spinal fluid. This fluid is secreted by the pia mater which becomes closely associated with the ectoderm of the roof of the myelencephalon. This vascular membrane is called the posterior choroid plexus.

The mesencephalon joins the metencephalon away from you. At about the point of juncture nerve III, the motor oculomotor nerve to some of the eye muscles can be seen reaching away from you. These nerves emerge from ventral swellings (floor of mesencephalon) which are called motor nuclei. The mesocoel becomes a relay between ventricles III and IV and is the cerebral aqueduct (of Sylvius).

Continue moving backward in the slide until the mesencephalon disappears and the diencephalon becomes apparent. The floor of this brain region becomes the infundibulum. The mouth, with Rathke's pocket extending toward the infundibulum, is a triangular cavity which soon enlarges.

The pineal body is seen as a small round structure between the diencephalon and the telencephalon.

The sensory and pigmented layers of the retina are obvious as is the lens vesicle. Covering the lens are ecto-mesoderm which will become the cornea. Find where the optic stalks are given off from the diencephalon.

The last brain region, the telencephalon, will appear toward you as two vesicles. These will be the cerebral hemispheres and the coels will become brain ventricles I and II. Ventricle III is made from median telo and diocoels. Olfactory pits occur in the same sections but their nerves have not developed yet.

A study of the ganglia should be carefully made. The semilunar ganglion of V, the trigeminal, is huge and three branches can be followed from it. It branches to the maxilla, the mandible, and has a sensory branch to the eye muscles. The trigeminal is a mixed nerve.

Nerves VII and VIII have an acousticofacialis ganglion. The acoustic, which is sensory, can easily be followed to the otic vesicle. The facial nerve can be seen for some time in the second visceral arch as a small, round, pink bundle of fibers.

Above (away from you) nerves IX, the glossopharyngeal, and X, the vagus, appear as a streak.  X is above IX and has a dorsal part, the jugular ganglion, a short nerve, and a ventral larger ganglion, the nodose.

Nerve IX also has a dorsal superior and a ventral petrosal ganglion. The nerves and ganglia must be followed by going backwards through the series. Both nerves are mixed. IX can be followed in the third visceral arch and X persists for a long time in the developing fourth visceral arch. IX is a mixed nerve to the tongue and X is a mixed nerve to the fourth arch and the viscera. It has fibers which enter the autonomic system.
 

Gut and Arches- Visceral arch I can be located by finding the mouth. Above the mouth is pouch I. Below the mouth is visceral arch I. The lateral part of pouch I will become the middle ear cavity.  Mesoderm on either side of the pouch will be ear ossicles. The openings of pouch I into the pharynx will become the eustachian tube.

The stomodeal plate has disappeared so the mouth opens to the outside. Sessel's pocket appears toward you and Rathke's pocket appears in the next section or two. Maxilla and mandible are found as mesodermal masses on either side of the mouth.

Pouches II and III are the same as in the 48 hour chick. The thyroid gland comes off ventrally from pouch II.

The laryngotracheal groove appears when the pouches are passed. Soon the dorsal part becomes the esophagus and the ventral part the trachea. Lung buds are endodermally lined and mesodermally covered. The epiploic folds around the lung buds have a coelomic slit, the pulmoenteric recess.

The gut swings to the left as the stomach, which in the region of the liver, becomes small intestine. OM veins collect blood from the liver sinusoids, enlarge and fuse. The space between sinusoids is occupied with endodermal hepatic cords. In this region, the mesoderm around the liver touches the body wall as septum transversum. Veins in the body wall here are allantoic or umbilical veins.

The bile duct enters the duodenum. A dorsal pancreas can be seen here in some specimens.

Soon the anterior intestinal portal opens. This area is the mid gut which is open. Far back is a posterior intestinal portal where the gut gives off an allantoic vesicle. The allantoic vesicle expands into the coelom and fuses with serosa to become a vascular exchange membrane called the chorioallantois. Gaseous exchange will occur through this membrane rather than through the yolk sac. The chorioallantois has an endodermal lining and mesodermal covering.

The allantoic stalks connect to the large intestine. The gut at about this point is the cloaca and where the cloaca touches the body wall is the cloacal membrane which will rupture to produce the anus.
 

Heart and Blood Vessels- The heart has four chambers. The right atrium is best developed and receives the sinus venosus. The right common cardinal makes up the bulk of the sinus venosus. The OM veins from the liver also contribute to the sinus. A partial septum separates right and left atria. The left common cardinal (coronary sinus) lies in the body wall anteriorly, then crosses the septum transversum and enters the sinus venosus farther forward than the right common cardinal. The sinus enters the right atrium through the sinoatrial canal. The atrioventricular canal can be distinguished by the presence of the endocardial cushion.

The ventricle can easily be distinguished because of the developing musculature, the trabeculae carneae which has come from the myocardium. The truncus arteriosus can be seen to leave the ventricle and lie beneath the digestive tract. Farther forward the truncus loses much of its musculature and becomes a blood vessel, the ventral aorta.

Moving forward stepwise, aortic arches III, IV, and VI can be found in corresponding visceral arches. The aortic arches can be seen to enter the dorsal aorta. Aortic arches I and II are largely gone except for small parts that have been incorporated into the internal carotid system. Visceral arch III will be the proximal component of the internal carotid. The dorsal aorta anterior to it becomes the middle component and the embryonic distal component completes the carotid. The distal component can be seen in many places around the anterior parts of the brain. Around the mesencephalon the distal component becomes part of the Circle of Willis. External carotids leave the ventral aorta and extend into the mandibular process.

Very far back in the embryo, the OM arteries are given off from the dorsal aorta. They enter the yolk sac where they function in nutrition. Allantoic arteries are difficult to distinguish except where they lie in the body wall close to the hind limbs.

Anterior cardinals can be followed for many sections anteriorly until they enter into the common cardinals. Post cardinals are seen in the same position as the anterior cardinals past the point where the common cardinals become obvious. They are associated posteriorly with the mesonephros. The subcardinal veins lie ventral to the mesonephros.
 

Coelom- The ventricle lies in the pericardial coelom. The lungs lie in the pleural coelom. The septum transversum can be seen to cross the body in the region of the sinus venosus.

Somites can best be seen in the most posterior sections. The dermatome is part of the skin and is indistinguishable. The sclerotome is forming the vertebrae, ribs, and sternum. The myotomes appear as light pink bodies in the sides of the body throughout the embryo.
 

Kidneys- The new kidney is the mesonephros. The pronephric duct remains and is now called the mesonephric duct or Wolffian duct. It becomes the vas deferens in the male. Glomeruli are medial to the duct and are drained by nephrons, with Bowman's capsule being the most proximal part. The nephrons become the epididymis in the male. The duct enters the cloaca.