Principles of Biology By Frank H. Osborne, Ph. D. Development

Fertilization 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 Development

Cleavage 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 Structures Ectoderm SkinEpidermis, skin glands, hair, nails, and corresponding parts of other animals (fur, hooves, claws, scales, etc.) Lining of the MouthEnamel of teeth, glands, covering of tongue and lips, anterior pituitary gland Nervous systemBrain, spinal cord, cranial and spinal nerves, pineal gland, sensory parts of the sense organs OthersLens of eye, eye muscles, pineal gland, anal canal lining

Embryologic Origin of Body Structures Mesoderm MusclesAll types except eye muscles SkeletonBone, cartilage, connective tissue Excretory organsKidneys and their ducts Reproductive organsGonads and related structures Circulatory systemHeart, blood vessels, blood, lymphatic system, blood-forming tissues OthersDentine of teeth, dermis of skin, adrenal cortex, lining of body cavities, other parts of the eye

Embryologic Origin of Body Structures EndodermEndoderm 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 canalPharynx, esophagus, stomach, intestine, liver, pancreas, most of the colon Pharyngeal derivativesLarynx, trachea, lungs, middle ear, thyroid, parathyroids, thymus OthersBladder, utethra

The End Principles of Biology Development