Apical ectodermal ridge (AER) is a structure that forms from the ectodermal cells at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb. After the limb bud induces AER formation, the AER and limb mesenchyme—including the zone of polarizing activity (ZPA)—continue to communicate with each other to direct further limb development.
An acid is a substance that donates hydrogen ions. ... Now there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is acidic
Early protists were first eukaryotic organisms and had several membrane bound organelles. The anaerobic protists evolved by acquiring purple bacteria. The purple bacteria have the ability to carry out aerobic cellular respiration. The endosymbiont transformed into mitochondria in due course of evolution. Similarly, the photosynthesizing protists were evolved by endosymbiosis of cyanobacteria which in turn was transformed into chloroplast during further evolution.
