The Stages of Embryonic Development in Animals

Compare the concepts of preformation and epigenesis.

Preformation – was the idea that the sperm or egg contained an embryo that was preformed and just a miniature of the adult, development was then just the enlargement of this miniature. Also the embryo contains all its descendants.

Epigenesis – proposed by Aristotle, suggested that the individual emerges slowly from a formless eggs that proceeds to develop into the individual.

Epigenesis displaced preformation.

List the two functions of fertilization.

a. to combine haploid sets of chromosomes from two individuals into a single diploid cell, the zygote.

b. Activation of the egg to trigger onset of embryonic development.

Describe the acrosomal reaction and explain how it ensures that gametes are conspecific.

Release of hydrolytic enzymes that enable the acrosomal process to penetrate the jelly coat of the egg. Allows for a specific interaction between the acrosomal process and jelly coat for recognition to ensure eggs are fertilized by sperm of same species. Leads to fusion of sperm and egg plasma membranes and membrane depolarization…fast block to polyspermy.

Describe the cortical reaction.

The fusion of sperm and egg plasma membranes results in a series of changes to the outer zone of the egg cytoplasm, which include the release of Ca++ and the subsequent release of contents of the cortical granules and the swelling of the perivitelline space and the hardening of the zona pellicuda….slow block to polyspermy.

Describe the general process of cleavage. Explain how the distribution and abundance of yolk influence this process.

Following fertilization, fusion of the sperm and egg nuclei and activation of the egg the zygote will cleave either into equal sized blastomeres or if a yolk is present equal division is not possible and the animal pole (lowest concentration of yolk) will divide the fastest.

Distinguish between meroblastic cleavage and holoblastic cleavage.

Birds – cleavage is restricted to a small disc of yolk-free cytoplasm at the animal pole of the egg cell, meroblastic cleavage.

Insects – undergo meroblastic cleavage, but the yolk-rich eggs results in the nucleus undergoing mitotic division without cytokinesis. Several hundred nuclei migrate to outer edge of egg.

Amphibians – complete division of eggs having little or moderate yolk, holoblastic cleavage

Describe the process of gastrulation and explain its importance. Explain how this process rearranges the embryo. List adult structures derived from each of the primary tissue layers.

Gastrulation results in the 3-layered embryo, ectoderm, endoderm and mesoderm. Movement of cells from the periphery to the inside of the embryo allowing the cell layers to develop into all parts of the individual.

Ectoderm – nervous system and epidermis

Endoderm – digestive tract, pancreas and liver

Mesoderm – muscle kidney, heart, and inner layer of the skin, dermis

What do the notochord, neural tube, and somites in a frog go on to form.

Notochord –from dorsal mesoderm, mesodermal cells gather to form the vertebrae and vertebral discs

Neural tube – from the ectoderm, will form the central nervous system, the brain and spinal cord

Somites – mesoderm that lies lateral to the notochord, vertebrai, muscles

Describe the significance and fate of neural crest cells.

Neural crest cells arise next to the neural tube, these cells migrate to various parts of the body to then form pigment cells of skin, bones, some muscles of the skull, the teeth, adrenal medulla and the sensory and sympathetic ganglia of the peripheral nervous system.