There is a marked evolution in the ultrastructure of the chicken pineal gland after hatching. The typical large follicular cavities tha tare visible just after hatching disappear with age, giving the pineal a large number of small-caliber cavities which are practically filled by the prolongations of the pinealocytes. This gives the organ an apparently solid aspect. There are still two large cell types in the follicular wall: the A and B pinealocytes, with the B type being considerably more abundant.