For mitosis experiments, onion root tips are used. The root tip cells are actively dividing meristematic cells and mitosis can be observed in them very easily.
Cells in the testes are germ cells. And meiosis can be demonstrated only in germ cells; especially the stages of prophase I like crossing over and chiasmata can be easily seen in actively dividing cells of testes.
Whitefish blastula cells are used for studying mitosis. These cells are constantly dividing cells just like meristematic cells of plants. So, for studying mitosis in animal cells, generally whitefish blastulas are used. Please note that if you will use human blastula instead of whitefish blastula; the human chromosomes will not be clearly visible even at high power magnification. So, it’s better to use whitefish blastula.
