The ontogeny of the cerebellar fissures in the chick embryo.

We have studied the chronology of appearance and the cortical changes which precede the fissures appearing between stages 34 to 40. In this paper we demonstrate the structural modifications of the cerebellar cortex defined as the anlagen of the fissures. The anlage of a fissure begins in a well-determined place of the cerebellar cortex. It begins with a thickening of the internal cortical cell layer, which finally folds. These modifications precede other similar ones which occur in the overlying external granular layer. On the other hand, these cortical structural modifications precede the transversal projection of the fissures. Chronological order of appearance of the anlagen of the fissures, related to the appearance of fissures on the surface of the cerebellum is also given.