CHAPTER 21 



THE PINEAL SYSTEM OF BIRDS 



In birds there is no parietal eye and no parietal foramen. Also there is 

 usually no indication of a parietal spot. The pineal organ, moreover, 

 is only represented by the proximal part, or epiphysis. The epiphysis, 

 however, in many forms undergoes an important and characteristic 

 transformation, which although foreshadowed in lower classes of verte- 

 brates is not definitely evolved. This change consists in the outgrowth 

 from the primary embryonic diverticulum of hollow epithelial buds the 

 cavities of which are at first continuous with the lumen of the primary 

 diverticulum (Fig. 196). At a later stage of development these often 

 become cut off from the main stem and form independent vesicles lined 

 by columnar epithelium. The vesicles or follicles are at first separated 

 by a considerable amount of interfollicular connective tissue containing 

 blood-vessels, and on section the organ has a glandular appearance 

 resembling in some respects the thyroid gland. The epithelium lining 

 the follicles in the pineal organ of birds is, however, ependymal in type 

 and composed of tall, columnar cells as contrasted with the low, cubical 

 cells of the thyroid gland ; and the content of the follicles of the pineal 

 organ of adult animals seems to correspond to that of the ventricles of the 

 brain, with a small amount of cell detritus, there being no colloid material. 

 The whole organ is at first pear-shaped and the hollow epithelial buds 

 at first communicate with the central lumen or one of its branches. Later, 

 however, when the follicles are separated, the stalk becomes elongated 

 and eventually its lumen disappears. The stalk may persist as a fibro- 

 vascular cord connecting the main organ with the roof of the brain or 

 it may become ruptured and the organ freed from its primary connection 

 with the diencephalon. Thus all escape of secretion into the cavity of 

 the third ventricle is prevented. Up to this period the epithelium lining 

 the follicles is in a sound, healthy condition, the cell outlines being well 

 defined and the alignment of the cells being quite even (Fig. 197). Later, 

 however, degenerative changes take place in the cytoplasm and nuclei, 

 and cell detritus accumulates in the cavities of the follicles (Fig. 198). 

 The follicles also diminish in size and come nearer together, the whole 

 organ forming a dense mass of closely packed small vesicles in many of 

 which the lumen has become greatly reduced in size, or has completely 



disappeared. 



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