206 



BONY FISHES 



the first time to breed are called maidens. If they have spent only one 

 and a half years in the sea they are called grilse and may then return 

 to the sea as kelts. Others ascend for the first time after three years 

 or more at sea. 



It is well established that salmon nearly always return to breed in 

 the river in which they were born, and it is certain that they may 

 journey for considerable distances in the sea. The mechanisms by 

 which these migrations are initiated and guided are only partly known. 



Fig. 125. Pituitary gland of the primitive teleost Elops, showing the persistent 

 Rathke's pouch in the form of a hollow bucco-hypophysial canal, piercing the 



parasphenoid bone. 



ah. adenohypophysis ; b.h.c. bucco-hypophysial canal; h.v. blood-vessel; c. continuation with 



pharynx; i. infundibulum; n.h. neurohypophysis;/)^, parasphenoid. 



(After Olsson.) 



They probably involve endocrine changes, for example the thyroid 

 is very active in the smolt as they begin to migrate. The return to the 

 home river may be a result of olfactory conditioning (see p. 221). 



11. Endocrine glands of bony fishes 



The pituitary gland occupies the same central part in the endocrine 

 signalling system that it has in mammals. Neural and glandular 

 regions are present and the adenohypophysis has three parts, the 

 two more posterior corresponding to the mammalian intermediate and 

 anterior lobes. The most anterior glandular region may be comparable 

 to the pars tuberalis. Experiments by removal and injection have 

 shown that the middle portion produces hormones that stimulate 

 growth, the gonads, the thyroid, adrenal, and probably the pancreas. 

 The posterior lobe produces a melanophore-dispersing hormone 



