THE ENDOCRINAL ORGANS ^^^ 



In the human embryo, the parathyroid glands are formed from the 

 epithelium of the third and fourth pharyngeal pouches. By proliferation 

 from the dorso-lateral wall of these pouches are formed masses of cells 

 which soon lose connexion with the pouches and migrate caudally. The 

 epithelial character of the cells of the parathyroid is retained during 

 the histogenesis of the gland. (Fig. 295) 



Parathyroid glands occur in all vertebrates except fishes, making their 

 first appearance in Amphibia in which the gills have begun to degenerate. 

 Comparison of lower with higher vertebrates reveals a progressive reduc- 

 tion in number but apparent increase in endocrinal importance. 



Ultimobranchial Bodies. Among the pharyngeal derivatives which 

 may have an embryonic endocrinal function are the ultimobranchial 

 bodies, called also postbranchial and suprapostcardial bodies. These are 

 paired glands which arise from or near the fifth pair of gill pouches. They 

 come to lie near the posterior border of the thyroid and attain a vesicular 

 structure. 



Ultimobranchial bodies occur in all gnathostomes except possibly 

 teleostome fishes. In Amphibia, the gland may be paired or unpaired. 

 It is not unlikely that, as its name suggests, it represents a posteriormost 

 pair of gill pouches modified as an endocrinal organ. 



Thjnnus Gland. The human thymus glands or "throat sweetbreads" 

 are paired organs located where the throat joins the chest. They are 

 pinkish in childhood, but in old age become yellow from fatty degeneration. 

 They enlarge up to puberty and thereafter gradually shrink. Contrary 

 to earlier opinion, the gland does not completely disappear in the adult, 

 but persists as a shrunken remnant within the mediastinum. The endo- 

 crine function of the thymus is doubtful, since removal of the gland is not 

 followed by disturbances of normal functions. The enlargement of the 

 gland in early life and its later atrophy suggest that its functional activity 

 may be limited to stages of growth and differentiation. 



Thymus glands first appear in a six-weeks human embryo as hollow 

 tubular outpocketings of the third pair of pharyngeal pouches, dorsal to 

 those of the parathyroid glands. 



Thymus glands are pecuhar to vertebrates but occur in all members 

 of that sub-phylum. In cyclostomes all gill pouches give rise to thymic 

 tissue and the gland persists dorsal to the gills throughout life. In fishes 

 there is a reduction in the number of thymus anlagen, since some of the 

 gill pouches make no contribution to the thymus. In amphibians the 

 thymus comes to lie at the angle of the jaw. The thymus glands of 

 reptiles are lobular organs located in the sides of the neck. In mammals 

 the gland assumes its definitive position in the mediastinum at the base 

 of the neck. The history of the thymus is thus one of reduction in the 

 number of gill pouches which contribute to the gland. 



