Blastula is a stage of embryonic development. It occurs after the formation of the morula, and is followed by the gastrula. It consists of a ball of cells that is hollow in the middle. It non-mammals, the blastula undergoes gastrulation, to turn into the gastrula. The gastrula eventually differentiates and turns into various more complex forms to turn into the complex organism. Various signaling mechanisms help this process to occur, and biologists have spent some time trying to figure out the signaling mechanisms in the process.
The process of gastrulation leads to the formation of three different germ layers, which give rise to different parts of complex organisms. The three germ layers are known as the ectoderm, mesoderm, and endoderm. More complex animals, including humans, have form three germ layers, however, other more simple animals have two germ layers, and others (like sponges) only have one germ layer.
The ectoderm, as its name implies, gives rise to part on the outside of the organism. These include things like hair, skin and nails. However, it also forms parts of the central nervous system including the brain and nerves. It also forms things like tooth enamel and other tissues that are on the surface of the body and the lining of various parts of the body.
The mesoderm forms tissues that are essentially in the middle of the body. These include things like the skeletal system, muscles parts of the vascular system such as blood vessels and heart, and some organs in the genito-urinary system.
The endoderm is the innermost germ layer and forms the inner most organs. These organs include most of the inner organs such as the pancreas, stomach, bladder, lungs and others. Typically the organs formed by the endoderm are found deep inside the body cavity and are also among some of the last to form.