The postnatal development of the canine cerebellar cortex

The cerebellum of the neonatal puppy was found to be incompletely differentiated. Its gross dimensions were less than half those of the normal adult, and although major lobules were recognizable, folial development was rudimentary. Microscopically, the cortex on the second postnatal day was distinguished by a thick subpial external granular layer of actively dividing cells. Beneath this transient layer, permanent cortical layers were largely undeveloped and myelination was not yet evident. During the first two postnatal weeks, the width of the external granular layer remained relatively constant (at slightly more than 50 μ), while other cortical layers underwent considerable growth, with histologic and cytologic differentiation. After the second postnatal week, the width of the external granular layer declined steadily, so that by the tenth week only a vestige of the layer remained. Between the second and seventy‐second postnatal days, a tenfold increase occurred in the width of the molecular layer. Major growth and differentiation also occurred in the internal granular layer, which became almost tripled in its width during the first ten postnatal weeks. Myelination proceeded in a centrifugal direction and occurred earlier in the vermis than in the hemispheres. By the end of the first month, myelination of the vermis was practically complete, and by the tenth week, the arbor vitae of the hemispheres appeared fully myelinated.