get well established during the processes of gastrulation and neurulation. In the next stage the primary organ rudiments subdivide into secondary organ rudiments. These rudiments get differentiated into various organs and organ systems.
The development of ectodermal organs The neurula of frog has three kinds of ectodermal tissues namely, epidermal ectoderm, neural ectoderm and neural crest cells.
Epidermal ectoderm The epidermal derivatives are the skin, olfactory sense organs, ear,lateral line sense organs, median fins, external gills and lining of mouth and anus
Neural ectoderm This layer of cells form the central nervous system and peripheral nervous systems.
Development of eye The eye is a photoreceptor. It is an ectodermal derivative. Its development begins even at the gastrulation stage. However the first sign of eye formation appears with the Development of two optic vesicles from the lateral walls of the embryonic diencephalon.
Formation of optic cup The eyes develop as two lateral outgrowths of the prosencephalon called optic vesicles. The cavity of the optic vesicle is called optocoel. The connection of the optic vesicle with the brain becomes a narrow stalk like structure called optic stalk. The optic stalk becomes connected with the ventral side of the optic vesicle rather than at its centre. The optic vesicles extend outward and reach the ectoderm. The wall of the optic vesicle next to the ectoderm is gradually flattened and later invaginates to form a double walled cup called optic cup The optic cup consists of two layers. The inner layer (derived by invagination) gives rise to the nervous region, the retina. The outer layer will be a thin, black pigmented layer (tapetum – nigrum) for the absorption of light. Initially the opening of cup is very large. Soon its rim bend inward and converges, so that the opening is reduced. This opening is called the pupil. The rim of the optic cup surrounding the pupil becomes the iris. Later on, large amount of pigment is deposited in the outer epithelial layer of iris. A groove extends along the ventral side of the optic cup. It is called the choroid fissure. It extends to the middle of optic stalk. It serves for the entry of blood vessels and mesenchyme cells into the posterior chamber of eye. The retina develops a membrane on its inner most surface called the internal limiting membrane. The outermost cells of the neurosensory retina differentiate into rods and cones. The inner cells of the retina differentiate into neuroblasts or nerve cells.
Development of lens : When the lateral surface of the growing optic vesicle comes in contact with the ectoderm it gives off stimulus of some kind, which causes the ectodermal cells to elongate, forming a disc shaped thickening. This is called the lens placode or lens rudiment. It curves into a cup and finally separates from the ectoderm. The free edges of the cup fuse to form a globular hollow lens vesicle. The lens vesicle comes to lie in the cavity of the optic cup. The cells of the inner side of the lens vesicle elongate, become columnar and are finally transformed into long fibres. Their nuclei degenerate and cytoplasm becomes hard and transparent making it refractile. These cells are called lens fibres. The outer layer of the lens remains unchanged and becomes the lens epithelium. The junction between the lens fibres and the lens epithelium represents the growing point of lens. Here the epithelial cells are continuously transformed into lens fibres. When the lens is formed, the free margin of optic cup touches the edges of the lens and grows in front forming iris. Thus lens hangs in the opening of optic cup Soon after the development of lens the overlying ectoderm closes over and differentiates to become the cornea. It is continuous with the skin. The transformation of the skin into cornea is caused by an induction arising from the optic cup and lens. The ectodermal cells covering cornea form an extremely thin, transparent membrane. This is known as conjunctiva of the eye ball. In adult this becomes continuous with the inner lining of upper and lower eyelids. The space between lens vesicle and the overlying presumptive anterior epithelium of cornea represents the anterior chamber. It contains cellular material called anterior vitreous body. The choroid and sclerotic coat of eye develop from the mesenchyme cells accumulating around the eye ball. The interior layer of mesenschyme cells give rise to a net work of blood vessels surrounding the pigmented retinal layer and is called choroid coat. The outer layer of mesenchyme form fibrous capsule, the sclerotic coat or sclera around the eye. The sclera provides protection to eye and the eye muscles
The ectoderm from above and below the original lens placode region grows out as two folds. These folds grow over conjunctiva and come to touch each other forming a complete layer of ectoderm. At a later stage these folds separate along the line of fusion to form the regular upper and lower eye lids.
The development of mesodermal organs The mesodermal derivatives are the limbs, endoskeleton, heart, blood vessels, kidney, coelom and reproductive organs.
Development of heart in Frog The heart is the first organ to start functioning because it has to
circulate blood which brings nourishment from the endodermal cells to the other parts of the embryo.
The initial development of heart is similar in all vertebrates. The first rudiment of heart develops after neurulation in the region beneath the pharynx, from the mesenchyme and mesoderm. The heart of frog develops at the ventral side of pharynx. It is formed by mesenchyme cells. The mesenchyme cells accumulate at the mid-ventral line of pharynx and form a longitudinal strand of tissue called the rudiment of endocardium. Soon the endocardial cells get arranged in the form of a thin walled tube, the lumen of which later develops into the cavity of the heart. While the endocardial tube is being formed, the mesoderm of both sides unite above and under the endocardial tube and a cavity called pericardial coelom is formed around the latter. The visceral layer of mesoderm envelops the endocardial tube and gives rise to its muscular walls or myocardium and also its close fitting covering called the visceral pericardium.
Differentiation of Heart The heart thus formed is a straight tube, situated longitudinally in the middle line below the pharynx and surrounded by the pericardium on all the sides. Its wall is formed of three layers, outer epicardium, middle myocardium and inner endocardium. The myocardium grows in thickness. The tubular heart elongates rapidly in antero-posterior direction. The small size of pericardial cavity and the anterior and posterior attachments do not permit its increase in antero-posterior direction. The increase in size causes the originally straight tube to curve and fold forming
S- shaped coiled tube.The anterior part is folded ventrally and posteriorly and the posterior part becomes folded dorsally and anteriorly. At the same time different regions of the tube grow differently in thickness and constrictions appear demarcating different presumptive chambers of the adult heart. The posterior most limit of the heart, where the vitelline veins converge, is the sinus venosus. This is followed by a thin walled atrium which opens infront into a thick walled ventricle. Initially the atrium is single chambered. Later on, a partition grows downward from the dorsal wall of the atrium dividing it into larger, right and smaller left atrial chambers. The partition forms inter-atrial septum. As the septum is developed on the leftside of the opening of the sinus venosus, the sinus venosus communicates only with the right auricle. The pulmonary veins which are also formed at this stage of development now pour the blood into the left auricle. The valves of the heart arise as folds of endocardium.
The development of endodermal organs The predominant endodermal organs are the organs of the alimentary canal, lungs, pancreas and urinary bladder.
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