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Organized colonies of cells have evolved into multicellular organisms. In Chapter 3 we have taken a look at Dictyostelium: within an organism-like assembly, tasks were divided between different groups of cells for the purpose of motion, reproduction and dispersion. Specialisation of tissues and division of tasks among organ systems has obviously had survival value.
Multicellular organisms are, by a differentiation of tissues, equipped for three vital functions:
the inner layer (the primitive gut) takes care of nutrition (selecting and processing food)
a middle layer of cells provides a supporting framework and motility by means of contractile protoplasm
the outer layer grows cells that scan the environment for relevant stimuli; some announce nutritious objects that can be approached, other cells are sensitive to stimuli that spell danger and are be avoided.
In the vertebrate embryo we recognise the three basic functions in the germinal tissue-layers. They are called: the endoderm, mesoderm and ectoderm.
Derived from the endoderm are organs that are qualified for digesting and processing food.
In exchange for easy-to-metabolize products served up by the endodermal layer, organs derived from the mesodermal layer provide structure and motion (bones and muscles) that enable an individual to find food and keep safe
Organs derived from the ectodermal surface-layer specialise in chemo-tactile scanning of the environment and then transmitting the interpreted data to the endodermal system for appreciation, and to the mesodermal system for goal-directed locomotion.
During the phase of tissue specialisation there is a double selection of cell-types:
- by its position, that is by the environment in which a cell is (re)produced certain properties in cells are selected that are most compatible with that particular environment. Seen from afar this used to be called induction, but on close view it is evidently a process of selection
- a cell which possesses particular properties migrates to a place where these features prove to be of most advantage, and so secures itself an existence. In the course of events this leads to a close cooperation between cells of ever more diversity. A differentiation in three germ-layers takes place. The three embryonic tissue layers are the cellular ancestors of all the different organs and organ-systems in the adult individual. It is not a strict tri-partition: cells migrate during development and the various tissues are interdependent. This listing summarises what is derived from the primary germ layers:
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