Principles of BiologyDevelopmentStages of DevelopmentCleavageBlastulaSlide 6GastrulaSlide 8DifferentiationEmbryologic Origin of Body StructuresSlide 11Slide 12The EndPrinciples of BiologyByFrank H. Osborne, Ph. D.DevelopmentDevelopmentFertilization•Fertilization is the beginning of life. The sperm and egg unite to form a fertilized egg which is known as a zygote.•The zygote begins to develop by undergoing a rapid series of cell divisions known as cleavage divisions.Stages of DevelopmentCleavage•After fertilization, the egg becomes a zygote.•The zygote begins a series of rapid cleavage divisions. During the cleavage divisions, the embryo remains about the same size as the original egg. The result is called a morula, which is a solid ball of cells.•As the cell divisions continue, the cell sizes continually become smaller in these stages.Blastula•Cell division continues in the embryo. The blastula is a hollow ball of cells that results from these divisions.•Early differentiation is seen because there are larger cell at one end of the embryo.Blastula•The end with the larger cells is called the vegetal pole. The opposite end with the smaller cells is called the animal pole.•The hollow interior part of the blastula is called the blastocoel.Gastrula•Gastrulation is the infolding of the embryo at the vegetal pole. The result is the gastrula.•The outside layer of the gastrula is called the ectoderm. The inside layer is called the endoderm.Gastrula•As cell division and differentiation continue, the interior space fills up with cells of a third type called mesoderm. The mesoderm forms between the other two cell layers.Differentiation•Differentiation is the process of a cell or cells becoming different from the other cells in the same embryo. The cells become different as the embryo develops. The primary germ layers; ectoderm, endoderm, and mesoderm; give rise to the structures of the body.Embryologic Origin of Body StructuresEctoderm Skin Epidermis, skin glands, hair, nails, and corresponding parts of other animals (fur, hooves, claws, scales, etc.) Lining of the Mouth Enamel of teeth, glands, covering of tongue and lips, anterior pituitary gland Nervous system Brain, spinal cord, cranial and spinal nerves, pineal gland, sensory parts of the sense organs Others Lens of eye, eye muscles, pineal gland, anal canal liningEmbryologic Origin of Body StructuresMesoderm Muscles All types except eye muscles Skeleton Bone, cartilage, connective tissue Excretory organs Kidneys and their ducts Reproductive organs Gonads and related structures Circulatory system Heart, blood vessels, blood, lymphatic system, blood-forming tissues Others Dentine of teeth, dermis of skin, adrenal cortex, lining of body cavities, other parts of the eyeEmbryologic Origin of Body StructuresEndoderm Endoderm forms linings of other organs. Each organ below is composed of tissues derived from mesoderm except for the lining which is derived from endoderm. Alimentary canal Pharynx, esophagus, stomach, intestine, liver, pancreas, most of the colon Pharyngeal derivatives Larynx, trachea, lungs, middle ear, thyroid, parathyroids, thymus Others Bladder, utethraThe EndPrinciples of
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