allantoic stalks - 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.

allantoic veins - Veins in the body wall in the region of the liver and septum transversum are allantoic, or umbilical, veins.
 

allantois - The allantois is a membranous sac, which becomes highly vascular, extends outside the embryo, and becomes involved in nutrient and oxygen absorption.  It fuses with the chorion to form part of the placenta, the chorioallantois.  It is an outgrowth of the hind gut composed of endoderm overlain by mesoderm.

amnion - The amnion is the inner of the two fetal membranes enclosing the developing embryo (the outer being the chorion or serosa).  The amniotic cavity, which lies between the embryo and the amnion, is filled with amniotic fluid.  This prevents adhesions between the embryo and parts of the egg.

anterior cardinals - 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, but occur past the point where the common cardinals become obvious.  They are associated posteriorly with the mesonephros. The subcardinal veins lie ventral to the mesonephros.

anterior intestinal portal - The anterior intestinal portal opens near the duodenum.  This area is the mid gut which is open.  Far back is a posterior intestinal portal where the gut gives off an allantoic vesicle.

aortic arches - 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 portions 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.

atria - The left and right atria are two of the four chambers of the heart.

auditory vesicle - The auditory vesicle lies beside neuromere IV, against the myelencephalon.

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

brain ventricle III - Ventricle III is made from median telo and diocoels.

carotid artery - The carotid artery is the main supplier of blood to the head.  It branches to form the internal and external carotid arteries.

chorioallantois - 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.

choroid plexus - The choroid plexus, a highly vascular membrane and part of the roof of the brain, consists of both anterior and posterior portions and functions as secretor of cerebrospinal fluid.

cloaca - The posterior cavity of the digestive tract into which intestinal and urogenital ducts open.

cornea - 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.

coronary sinus - 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.

cranial nerve V - 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.  Nerve V, the Trigeminal, is mixed and subdivides into maxillary, mandibular, and ophthalmic parts.

diencephalon - Continue moving backward in the slide until the mesencephalon disappears and the diencephalon becomes apparent.  Between the diencephalon and the telencephalon is the pineal body, a small round structure.  The roof of the diencephalon becomes the cerebellum and the floor becomes the pons.

dorsal aorta - 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.

dorsal ganglion - 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.

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

ectoderm - Ectoderm is the outermost of the three primary layers of cells of the embryo, and gives rise to epidermis and nervous tissue.

endocardial cushion - The atrioventricular canal can be distinguished by the presence of the endocardial cushion.  The same is true for the ventricle.

endolymphatic duct - 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.

epiphysis - The epiphysis of the brain (also known as the pineal body) is seen as a small round structure between the diencephalon and the telencephalon.

epiploic folds - The epiploic folds are made of mesoderm.  The endodermal lung buds lie inside the folds, which have a coelomic slit, the pulmoenteric recess. 

esophagus - The dorsal part of the laryngotracheal groove becomes the esophagus and the ventral part the trachea.

floor of metencephalon - the walls of the metencephalon will become the cerebellum, and its floor will become the pons.

fourth brain ventricle - The cavities of metencephalon and myelencephalon will become the fourth brain ventricle and will fill with cerebro-spinal fluid.

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

infundibulum - The floor of the diencephalon becomes the infundibulum.  The infundibulum becomes the neurohypophysis or the posterior pituitary.  The mouth, with Rathke's pocket extending toward the infundibulum, is a triangular cavity which soon enlarges.

interatrial septum - A partial septum which separates the right and left atria.

intersegmental arteries - Intersegmental arteries arise between the somites as small paired branches of the dorsal aorta.

intestine - The gut swings to the left as the stomach, which in the region of the liver, becomes small intestine.  The intestine has been reached when bile and pancreatic ducts become obvious.

jugular ganglion - Nerve X is above nerve IX and has a dorsal part, the jugular ganglion, a short nerve, and a ventral larger ganglion, the nodose.

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

larynx-trachea - The laryngotracheal groove appears when the pouches are passed. Soon the dorsal part becomes the esophagus and the ventral part the trachea.

left atrium - The heart has four chambers. The right atrium is best developed and receives the sinus venosus.  A partial septum separates right and left atria.

left common cardinal - 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.

lens - Covering the lens are ecto-mesoderm which will become the cornea.  The lens began as an ectodermal placode.

limb buds - Limb buds are simply the beginnings of limbs.

liver - 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.

lung bud - Lung buds are endodermally lined and mesodermally covered. The epiploic folds around the lung buds have a coelomic slit, the pulmoenteric recess.

mandible - Maxilla and mandible are found as mesodermal masses on either side of the mouth.  Nerve V, the trigeminal, branches to the maxilla, the mandible, and has a sensory branch to the eye muscles.  Extending into the mandibular process as they leave the ventral aorta are the external carotids.  The mandibular and maxillary processes (with the mouth between) are the first visceral arch.

maxilla - Maxilla and mandible are found as mesodermal masses on either side of the mouth.  The semilunar ganglion of nerve V, the trigeminal, branches to the maxilla, the mandible, and has a sensory branch to the eye muscles.

mesencephalon - 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.

mesenchyme - Mesenchyme is embryonic mesoderm that gives rise to blood, skeletal tissues, and connective tissues.

mesocoel - The mesocoel becomes a relay between ventricles III and IV and will become the cerebral aqueduct (of Sylvius).

mesonephric duct - Duct of the mesonephros, or middle kidney.  Formerly the pronephric duct, and then referred to as 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.

mesonephric tubules - Tubules of the intermediate form of the excretory organ, known as the mesonephros, or mid-kidney, which possess glomeruli.

mesonephros - The new kidney is the mesonephros.  Post cardinals are associated posteriorly with the mesonephros. The subcardinal veins lie ventral to the mesonephros.

metencephalon - The metencephalon joins the myelencephalon away from you. It is much thicker than the myelencephalon, which can be distinguished by its thin roof.  Walls of the myelencephalon have swellings called neuromeres. The walls and floor of the myelencephalon will become the medulla.  The cavities of metencephalon and myelencephalon will become the fourth brain ventricle and will fill with cerebrospinal fluid.

mouth - The mouth, with Rathke's pocket extending toward the infundibulum, is a triangular cavity which soon enlarges.  Rathke's pocket becomes the anterior pituitary or the adenohypophysis.  

myelencephalon - The chick's first section shows the myelencephalon which can be distinguished by its thin roof.  Walls of the myelencephalon have swellings called neuromeres. The walls and floor of the myelencephalon will become the medulla.  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.  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.

nerve III - At about the point of juncture of mesencephalon and metencephalon, nerve III, the oculomotor nerve, which innervates 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.  Nerve III is a motor nerve.

nerve IV - Cranial nerve IV, the Trochlear nerve, leaves the brain and innervates the superior oblique muscle of the eye.  It is a motor nerve.

nerves IX & X - 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.

neuromeres - Walls of the myelencephalon have swellings called neuromeres.  The auditory vesicle lies beside neuromere IV.

notochord - The notochord is an elastic skeletal rod that runs along the back beneath the nerve cord.  It induces the nervous system.

olfactory pits - Olfactory pits occur in the same sections (as the last brain region, the telencephalon, which appears as two vesicles that will be the cerebral hemispheres and the coels that become brain ventricles I and II), but their nerves have not developed yet.

OM veins - OM veins come into the embryo from the yolk sac.  They collect blood from the liver sinusoids, then enlarge and fuse.

OM arteries - OM arteries branch from the dorsal aorta to supply arterial blood to the yolk sac.

optic stalks - Optic stalks are given off from the diencephalon and run from the sensory layer of the retina inward..

otic vesicle - Nerves VII and VIII are an acousticofacialis ganglion. The acoustic, which is sensory, can easily be followed to the otic vesicle.

pericardial coelom - The pericardial coelom is the body cavity which contains the ventricle (of the developing heart).

peritoneal coelom - The main part of the body cavity surrounding the viscera of the abdomen.

pigmented retina - The sensory and pigmented layers of the retina are obvious as is the lens vesicle.  The sensory part is thick while the pigmented layer is thin.

pleural coelom - The body cavities which surround the lungs.  These are also known as the pulmoenteric recess.

posterior cardinal veins - Posterior 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.

pouch I - 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. 

pouches II and III - Pouches II and III are the same as in the 33 hour chick. The thyroid gland comes off ventrally from pouch II.

Rathke's pocket - The mouth, with Rathke's pocket extending toward the infundibulum, is a triangular cavity which soon enlarges.  Rathke's pocket becomes the anterior pituitary.

right atrium - The heart has four chambers.  A partial septum separates right and left atria.  The right atrium is best developed and receives the sinus venosus.  The sinus enters the right atrium through the sinoatrial canal.

right common cardinal - The right common cardinal makes up the bulk of the sinus venosus.  The allantoic veins from the body wall are seen to join it.

roof of the myelencephalon - The chick's first section shows the myelencephalon which can be distinguished by its thin roof. The metencephalon joins the myelencephalon away from you. It is much thicker than the myelencephalon.  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.
 

sensory retina - The sensory and pigmented layers of the retina are obvious as is the lens vesicle.  The sensory layer is thick and the pigmented part is thin.

sinus venosus - 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.  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 septum transversum can be seen to cross the body in the region of the sinus venosus.

somites - 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.

spinal cord - The spinal cord is the main nerve trunk and runs the length of the back.  It eventually becomes enclosed by the vertebrae.

spinal ganglia - The spinal ganglia are the portions of the nervous system which contain the mass of nerve cells that constitute spinal nerves.  Ganglia are aggregations of nerve cell bodies outside the central nervous system.  Neuron cell bodies inside the CNS are called nuclei.

stomach - The gut swings to the left as the stomach, which in the region of the liver, becomes small intestine.

superior ganglion - 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.  Nerve 10, the vagus, serves the fourth visceral arch and the viscera as part of the autonomic nervous system.

tail bud - The tail bud is the beginning of the outgrowth of a tail.

telencephalon - 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.

trabeculae carneae - The ventricle can easily be distinguished because of the developing musculature, the trabeculae carneae which has come from the myocardium.

truncus arteriosus - 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.

ventral aorta - The truncus arteriosus loses much of its musculature and becomes a blood vessel, the ventral aorta.  External carotids leave the ventral aorta and extend into the mandibular process.  The aortic arches III, IV, and VI come off the ventral aorta like the fingers of a hand.

ventricle - The heart has four chambers.  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.  The ventricle lies in the pericardial coelom.

visceral arch I - Visceral arch I can be located by finding the mouth. Above the mouth is pouch I. Below the mouth is visceral arch I.  It is composed of maxillary and mandibular processes.

visceral arch II - The facial nerve (cranial nerve VII) can be seen for some time in the second visceral arch as a small, round, pink bundle of fibers.  The facial nerve is a sensory nerve which serves visceral arch II.

visceral arches III & IV - In the nervous system, nerve IX can be followed in the third visceral arch and nerve 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.  In the developing vascular system, aortic arches III, IV, and VI can be found in corresponding visceral arches.  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.

walls of myelencephalon - The walls of the myelencephalon have swellings called neuromeres. The walls and floor of the myelencephalon will become the medulla.  The thin roof joins with the pia mater to form the vascular choroid plexus.