At the end of gastrulation, embryo develops into three germinal layers. Formation of different into three germinal layers. Formation of different tissues and organs from germinal layers is called histognensis.
Fate Of Ectoderm :
Ectoderm gives rise to epidermis of skin and epidermal derivatives like hair, nails, sweat glands, conjunctiva, cornea, lens, retina, internal and external ear, enamel of teeth, nasal cavity, adrenal medulla, stomodaeum proctodaeum, posterior pituitary and entire nervous system.
Fate Of Mesoderm :
Mesoderm forms all types of muscles, connective tissues, dermis of skin, adrenal cortex, circulatory system including heart, blood vessels and blood, lymphatic vessels, middle ear, dentine or teeth.
Fate Of Endoderm :
Endoderm develops into epithelium of mid-gut (pharynx to colon), glands of stomach and intestine, tongue, tonsils, lungs, trachea, bronchi, larynx, urinary bladder, vagina, vestibule, liver and pancreas, anterior lobe of pituitary, thyroid gland, parathyroid, thymus, eustachiam, tube, epithelium of urethra including associated glands.
Monozygotic/ identical twins- They are derived from a single zygote that splits at an early stage during development. They have same genetic makeup and are of same sex.
Dizygotic (Fraternal) twins- In rare cases two ova are released simultaneously. They are fertilized by two different sperms. Each fertilized ovum develops into embryo. It could be of the same sex or of the different sex and they are genetically dissimilar.