Embryonic Development: Protostomes, Deuterostomes, and Metamorphosis

Basic Development Models

Protostomes

Protostomes: In protostomes, which translates to "mouth first", the mouth of the new individual develops separately from the blastopore, the first opening of the embryo.

Deuterostomes

Deuterostomes: In deuterostomes, which translates to "mouth second", the mouth opens later in a different place than the blastopore.

Extraembryonic Membranes

Extraembryonic membranes originate from the embryo but are not part of it. They form with input from all embryonic layers and play important roles in development:

• Yolk Sac: The first embryonic membrane that forms. It surrounds the mass of yolk, where one exists.
• Amnion: A membrane surrounding the embryo that secretes amniotic fluid, providing an aqueous environment.
• Chorion: Surrounds the amnion and the yolk sac. It limits water loss from the embryo and controls gas exchange between the embryo and the external environment.
• Allantois: Forms a bag in which to store the metabolic wastes generated by the embryo.

Organogenesis

Organogenesis is the process where tissues are organized to build organs. The fate of embryonic layers is as follows:

• From the ectoderm forms the epidermis of the skin and its appendages, neural tissue, and receptor cells of sensory organs.
• From the endoderm originates the epithelial lining of the digestive and respiratory systems, digestive glands, and urinary bladder.
• From the mesoderm forms the dermal layer of the skin, the lining of internal cavities, the heart and other organs of the circulatory system, kidneys, gonads, and the skeletal and muscular systems.

Metamorphosis

Simple or Incomplete Metamorphosis

Simple or incomplete metamorphosis takes place in amphibians, echinoderms, annelids, mollusks, and many insects. Larvae gradually change shape to become adults without going through a stage of inactivity. In insects, changes coincide with the detachment (molting) of the outer shell.

Complicated Metamorphosis

Complicated metamorphosis is typical of many insects. After reaching its maximum size, the larva stops its activity and becomes a nymph or pupa. The nymph stops eating and, in most cases, immobilizes. In this phase, various internal processes take place that transform it into an adult.

Post-Embryonic Development

Post-embryonic development starts with birth in viviparous animals and egg hatching in oviparous animals. The new individual begins a period of growth that continues until it reaches the adult stage and reproductive maturity.

Direct Development

Direct development is typical in oviparous animals with abundant yolk. In this case, post-embryonic development is a process whereby growth reaches adult size and sexual maturity.

Indirect Development

Indirect development occurs when egg yolk reserves are low. The embryo cannot complete its development and gives rise to a free-living larva. The transformation of this larva into an adult is done through a series of profound changes that constitute metamorphosis.