Embryology

Overview
Embryology is the study of the development of an embryo. An embryo is defined as any vertebrate in a stage before birth or hatching. Embryology is more specific as it only encompasses the modern definition: an animal that is undergoing early development including the formation of primitive organ systems, the creation of fundamental tissues, and cleavage; especially involving the development of human individuals from the moment the blastocyst is implanted until the end of the eight week after conception. Past the eight week, the developing animal is called a fetus.

History
After the 1950s, with the DNA helical structure being discovered by James D. Watson and Francis Crick, (in collaboration with Rosalind Franklin and Maurice Wilkins) and the increasing knowledge in the field of molecular biology, developmental biology emerged as the field of study that correlates the genes and such morphological changes; in other words, which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.

Vertebrate and invertebrate embryology
Many principles of embryology apply to both invertebrate animals as well as to vertebrates. Therefore, study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time looks like an offspring of its parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, some insects proceed directly from egg to adult form whereas others develop through an elaborate sequence of changes.

Neuroembryology
Neuroembryology refers to the development of an embryos nervous system. At the middle of the third week the neural plate appears which originates from the ectoderm.