Embryology means "study of embryos". It is the descriptive and experimental study of changes in morphogenesis (form or shape) of animals during their embryonic phase.

Each multicellulor organisms begin life as single cell, the fertilized egg, which is almost similar to all other cells but differs mainly by its potential to divide and produce all the cells that is going to make an adult organism. The cells of the embryo diverge from one another structurally and functionally during the process of development and eventually organized into an adult organism.

Aristotle originated the idea that form of an embryo gradually emerges during development. Nearly 2000 years later in the 17^th anf 18^th century many embryologists rejected this idea. They proposed that egg contains a miniature fully formed embryo. Bonnet (1745) formalized this concept in his theory of "emboitment".

Casper Wolff (1759) gave an alternative theory of development called "epigenesis", which means that adult gradually develops from a formless egg as originally proposed by Aristotle.

Nature and significance of Germ Cells remain unclear for almost a century until Schwann (1839) recognized that egg is a cell.

Schweigger-Seidel and St. George (1876) determined that sperm is a cell

Hartwig (1876) stated that fertilization results from the union of sperm and egg. He also observed that germ cell nuclei are the vehicles of inheretance.

Van Beneden (1883) gave the first detailed description of the behavior of chromosome in fertilized egg.

Wilhem Roux (1883) stated his idea that hereditary material represents different characteristics of of the organism and fertlized egg receives all of these substances which during cell division are linearly arranged on the chromosomes. These substances are then distributed unequally to daughter cells during cell division. According to his idea this "qualitative division" fixes the fate of cells and their descendents.

Roux (1888) confirmed his theory by conducting an experiment on Frog egg. He used a hot needle to destroy one of the two cell of a 2-cell frog embryo. The uninjured cell develop to form half embryo, lacking complete structures. Roux used this expt. To support his theory that the undamaged half frog embryo developed into half embryo because it lack the informations that was destroyed by killing the other half embryo.

Hans Driesch (1892) used sea urchin 2-cell stage embryo to disprove Roux's theory. He separated the 2-cell embryo by mechanical shaking. These half embryo developed normally but become dwarf. Later he separated the 2 and 4 cell embryo in calcium free sea water and found that both types of embryo develop into normal adult.

Mendel (1865) proposed that characteristics of organism are determined by factors that retain their characteristics through generations of breeding.

Thodor Boveri (1902) established precisely the role of chromosome in development.

The final proof of the "Chromosome theory of inheretance" came from the work of T. H. Mogan (1906-1940) on fruit fly .

Inheritance is the transmission of genes from parents to offspring. 2. Genes are located on chromosome 3. Each gene occupy a specific site on the chromosome 4. Genes can exists in several alternative forms 5. Allelic genes in a zygot donot blend or contaminate each other but segregate independently of each other. 6. Chromosomes and genes on the chromosome assort independently. 7. Genes are duplicated during cell division