The authors provide an anatomic and ultrasonographic description of the fetal brain from the 16th to the 27th week of development. During the second trimester the primitive brain is smooth and homogeneous and has few sulci and relatively large lateral ventricles. The cerebral hemispheres consist largely of cells migrating from the periventricular germinal matrix to the primitive cortex. The "featureless" hemispheres yield homogeneous ultrasonographic images very different from the more complex and familiar cerebral anatomy of fetuses and premature babies in the third trimester.