Embryology of Corpus Callosum

The corpus callosum forms between the 11th and 20th weeks. The anterior genu is the first part to form. Development then proceeds posteriorly: anterior body, followed by the posterior body and splenium. After all of this is done, development pays attention to the front again, forming the rostrum.

This is important in differentiating partial dysgenesis of the corpus callosum from a developmental injury when the splenium is not seen. Since the rostrum is the last to form, if you see a rostrum in the absence of a splenium, you know that the problem is injury to the splenium. If the rostrum is not seen, then the absent splenium is likely due to partial dysgenesis.

References

Loevner, LA. Case Review Series: Brain Imaging, 2nd edition.