"The development of the corpus callosum occurs between the 12th and 16-20th weeks of gestation 2-3. It begins with the genu and then continues posteriorly along the body to the splenium. The rostrum is the last part to be formed. In primary agenesis parts of the corpus callosum to form before the insult will be present whereas later parts will be absent. The presence of the rostrum essentially excludes primary agenesis."