Development of embryo in animals
At first, the embryo is just a small mass of cells. As it develops, it has to ‘know’ where its top and bottom are, as well as its front and back, and left and right sides. Different cells of the embryo grow into their designated organs and body parts. All must grow in the right place, and to the right size. Surprisingly, many of the genes that control the early embryo development of organisms such as mice, chickens and fruit flies are very similar to Human’s.
An animal’s body results from division of cells during embryonic development.The embryonic development begins once the egg is fertilized by the sperm,it is now called as a “Zygote”.zygote then divide many times to give a bunch of cells called as “Blastomeres”.This cleavage continues till “Blastula” is formed.
Except Sponges (Porifera) in all multicellular organisms Blastula invaginates to form future gut.At this stage embryo is “Gastrula”.The invaginating layer of cells, which will give rise to the gut, form a germ layer called the endoderm. The endoderm surrounds and defines a body cavity called the gastrocoel.
The cells not involved in forming the invagination constitute another germ layer the ectoderm. The ectoderm surrounds a cavity called the blastocoel.If the invaginating gastrocoel forms a complete tube by forming a another opening then it is called “gut”.In Cnidarians no second opening is observed.all animals except sponges and cnidarians develop another germ layer apart from endoderm and ectoderm called as “Mesoderm”,making those animals Triplobalstic animals.The way in which the mesoderm forms a cavity (called a coelom) or not is important in concluding the relatedness of animal groups.
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