Anton van Leeuwenhoek made another very important advance. He discovered the single celled organisms we know as protazoa. He was also the first to see, understand and explain red blood cells and sperm cells in both humans and animals. In another subject Leeuwenhoek continued his studie on the lifecycles on insects and the structure of plants. To conclude, Leeuwenhoek made many important discoveries during his lifetime about cells and many other things.