For the same reason that an egg does not resemble an adult rooster, or a seed a tree, or a frog egg the adult frog, or a human sperm cell an adult human. All of these are in different stages of their life cycle. The fertilized egg is the first stage in the development of all animal species. It is not visible through the naked eye in tigers and humans. The egg is fertilized when a male sperm unites with the female egg. The process of life begins. Through the microscope the egg looks like a tiny circular ring with a visible border. When it is fertilized cell division within it begins. All the information for development and growth are contained within the fertilized cell. The cells keep on dividing for about seven to ten days and then attach to the uterus to become an embryo. Attempts are now being made to clone and bring back the Tasmanian tiger that was hunted to extinction during the first half of the twentieth century.