In the early developing embryo during the gastrulation phase, a trilaminar embryo (trilaminar germ disk) is being formed. The three germ layers include the ectoderm, mesoderm, and endoderm, which give rise to all the tissues and organs.
The ectoderm is the outermost of the three germ layers. The derivatives of the ectoderm may be divided into two major classes: surface ectoderm and neuroectoderm.
Surface ectoderm gives rise to:  epidermis and its derivatives (hair, nails, sweat glands, sebaceous glands, and the parenchyma and ducts of the mammary glands),  cornea and lens epithelia of the eye,  enamel organ and enamel of the teeth,  components of the internal ear,  adenohypophysis (anterior lobe of pituitary gland), and  mucosa of the oral cavity and lower part of the anal canal.
Neuroectoderm gives rise to:  the neural tube and its derivatives, including components of the central nervous system, ependyma (epithelium lining the cavities of the brain and spinal cord), pineal body, posterior lobe of pituitary gland (neurohypophysis), and the sensory epithelium of the eye, ear, and nose;  the neural crest and its derivatives, including components of the peripheral nervous system (cranial, spinal, and autonomic ganglia, peripheral nerves, and Schwann cells); glial cells (oligodendrocytes and astrocytes); chromaffin (medullary) cells of the adrenal gland; enteroendocrine (APUD) cells of the diffuse neuroendocrine system; melanoblasts, the precursors of melanocytes; the mesenchyme of the head and its derivatives (such as pharyngeal arches that contain muscles, connective tissue, nerves, and vessels); odontoblasts; and corneal and vascular endothelium.
Mesoderm is the middle of the three primary germ layers of an embryo. It gives rise to:  connective tissue, including embryonic connective tissue (mesenchyme), connective tissue proper (loose and dense connective tissue), and specialized connective tissues (cartilage, bone, adipose tissue, blood and hemopoietic tissue), and lymphatic tissue;  heart, blood vessels, and lymphatic vessels, including their endothelial lining;  kidneys and the gonads (ovaries and testes) with genital ducts and their derivatives (ureters, uterine tubes, uterus, ductus deferens);  mesothelium, the epithelium lining the pericardial, pleural, and peritoneal cavities; and the adrenal cortex.
Endoderm is the innermost layer of the three germ layers. In the early embryo it forms the wall of the primitive gut and gives rise to epithelial portions or linings of the organs arising from the primitive gut tube. Derivatives of the endoderm include:  alimentary canal epithelium (excluding the epithelium of the oral cavity and lower part of the anal canal, which are of ectodermal origin);  extramural digestive gland epithelium (e.g., the liver, pancreas, and gallbladder);  lining epithelium of the urinary bladder and most of the urethra; respiratory system epithelium; thyroid, parathyroid, and thymus gland epithelial components; parenchyma of the tonsils; lining epithelium of the tympanic cavity and auditory (Eustachian) tubes.