ORIGIN OF THE ENDOCRINE GLANDS 881 



branchial pouches during the sixth week. These epithelial derivatives of the 

 third branchial pouch become solid masses of cells which migrate posteriad 

 into the anterior thoracic area. 



The thymus gland is found in all vertebrates, but its morphology is most 

 variable. In birds, it is situated in the neck region in the form of isolated, 

 irregular nodules. The bursa of Fabricius, previously mentioned (Chap. 13) 

 as an evagination in the cloacal-proctodaeal region of the chick, is a "thymus- 

 like organ" (Selye, '48, p. 681 ). Thymus glands in reptiles are located in the 

 neck region, and, in amphibians the two thymus glands lie near the angle of the 

 jaws. In fishes several small, thymus-gland nodules arise from the dorsal 

 portions of the gill pouches and come to lie dorsal to the gill slits in the adult. 



The function of the thymus gland is not clear. It appears to have some 

 relationship to sexual maturity. (For thorough discussion, see Selye, '48, 

 Chap. IX.) 



6. Pineal Body 



The pineal gland appears to have been first described by Galen, the Greek 

 scientist and physician (130-ca.200 A.D.), who believed it to function in 

 relation to the art of thinking. Descartes (1596-1650) considered it to be 

 the "seat of the soul." 



During development, two fingerlike outgrowths of the thin roof of the 

 diencephalon of the brain occur in many vertebrates, namely, an anterior 

 paraphysis or parietal organ, and a more posteriorly situated epiphysis. In 

 certain Cyclostomes (Petromyzon), the posterior pineal body or epiphysis 

 is associated with the formation of a dorsal or pineal eye, while the anterior 

 pineal organ or paraphysis forms a rudimentary eyelike structure. In Spheno- 

 don and in certain other lizards, the paraphysis or anterior pineal evagination 

 develops an eyelike organ. Also, in various Amphibia (frogs; Ambystoma) 

 rudimentary optic structures arise from the fused epiphyseal and paraphyseal 

 diverticula. In consequence, we may assume that a primary function in some 

 vertebrates of the dorsal, median pineal organs is to produce a dorsal, light- 

 perceiving organ. In certain extinct vertebrates, a fully developed median 

 dorsal eye appears to have been formed in this area. 



On the other hand, the epiphysis (fig. 366A) in some reptiles, in birds 

 and in mammals has been interpreted as a glandular organ. Various investi- 

 gators have suggested different metabolic functions. However, an endocrine 

 or essential secretory function remains to be demonstrated. (Consult Selye, 

 '48, p. 595.) 



Many types of cells enter into the structure of the pineal gland. Among 

 these are the chief cells, which are large and possess a clear cytoplasm. Nerve 

 cells and neuroghal elements also are present. Various other cell types pos- 

 sessing granules of various kinds in the cytoplasm are recognized. 



