48 SOURCES OF KINETIC AND METABOLIC HORMONES 



Pin 



Fig. 2-13. Diagrammatic sagittal half of the head and pharynx of 

 a tadpole, Rana. An early stage in development of the brain and 

 stomodaeum (St) shows the adenohypophysis (Hyp) growing up 

 to meet the infundibulum (Inf), or neurohypophysis, from the 

 floor of the fore brain (F) ; together they form the pituitary body. 

 Optic chiasma (OC), pineal organ (Pin), spinal cord (CNS) and 

 notochord (Nch). The pharynx, leading to oesophagus (Oes), is 

 shown at a later stage \ I to VI, visceral arches; III to VI with 

 branchial arteries running behind gill slits to dorsal aorta (DA) ; 

 THYROID (Th) is mid-ventral; a series of ventrolateral epithelial 

 thickenings form the carotid gland (C) on III, the parathyroids 

 (P) on IV and V, and the ultimobranchial body (U) on VI. 

 Dorsolateral thickenings on II and III (and also on IV and V in 

 other animals) form the thymus gland (Thym). 



compact and encapsuled. It receives a rich blood supply from the 

 carotids, and its nerve supply is mainly vasomotor. 



In most vertebrates the histological character of the gland is 

 fairly constant. Its cells form a cubical epithelium surrounding 

 spaces, which become filled with a colloidal material secreted 



