History of the field Coming to the realization that there is history embedded in the development of an embryo was a profound achievement in biology. This revelation is on par with that experienced with the advent of modern molecular biology. There is history embedded in DNA and protein structure. A brief, and truncated historical account concerning the 1800's debate over the priniciples governing the evolution of morphology. Graded series of types arises by terminal addition of structures in ontogeny, and Vertebrate embryongenesis is the classic example.
Development proceeds from the general features, to the specific.
The general features of a large group of animals appear earlier in the embryo than the special features. Each embryo of a given species, instead of passing through the stages of other animals, departs more and more from them. The development of Arthropods is governed by several large complexes of genes that are linearly arranged in the genome. E.g., reverting to the millepede like ancestral condition. Thus many genes have been added much in the way layers of an onion cover the embryonic onion plant.