text: Good question.  Just FYI, embryo is used to cultivate virusses because the environment can be easily controlled, unlike that of an animal.\n\nEmbryo really don't have any disadvantages after you've discovered the method.\n\nAnimals have several however.  He're a brief listing:\n\n- immune system response time (from infection to plasma b cell action)\n- immune system strength (number of plasma b cells)\n- immune system aggression (number of antigens created by plasma b cells)\n- inconsistant host cells\n\nControl is the key.  Embryo cultivation allows for this.  That's why it's used.