During development, how do embryos distinguish their posterior (tail) versus anterior (head)? Dr. Geraldine Seydoux’s lab uses the small worm C.elegans as a simple model to study this question. In her first video, she introduces how the sperm divides the egg into distinct anterior and posterior domains shortly after fertilization to create the body axis. Her lab discovered that the sperm introduces microtubules that reorganizes the distribution of a network of polarity regulators, called PAR proteins. The PAR proteins segregate into two non-overlapping domains that define the anterior and posterior axis of the worm.