69 



young are attracted to dilute water, and the succeeding stages 

 make their seasonal migrations in almost direct relationship to 

 temperature. Fish and other larvae are frequently transported 

 across the oceans, and are peculiarly modified to promote the 

 long pelagic life. It is when they reach continental waters that 

 they change into the adult. It may be presumed therefore that 

 in such cases something is at work which inhibits the change 

 until the conditions are favourable. The environment may in this 

 case be said to be the degree of salinity. The change, it may 

 be suggested, is under the control of an organ which has the 

 power of retaining the larval phase, or, in other words, of inhibiting 

 the next phase of life. Those who are familiar with the morphology 

 and physiology of the lower creatures will be able to suggest organs 

 the purpose of which has remained obscure, but with regard to 

 fishes we feel that at all events the adolescent period is controlled 

 by the thymus, and that when the gonad is permitted by the 

 thymus to grow the thyroid is able to control maturity. It is 

 obvious that some such control must take place, for it is not suffi- 

 cient that a state of ripeness be attained, the periodicity of the 

 spawning calls for a seasonal, common culmination of the process 

 of ripening.* 



7.— SUMMARY AND GENERAL CONSIDERATIONS. 



Plaice. — The large spawning grounds of the southern North 

 Sea contribute annually immense numbers of young plaice to 

 the continental shallow waters. The seasonal migrations of these 

 schools, and even the spawning migrations, are mainly confined 

 to the south. On the eastern and western borders of the North 

 Sea, other than those occupied by the southern plaice, the spawning 

 assemblages are smaller and more distinctly isolated ; the pro- 

 ducts therefore form well-defined schools. The Northumberland 

 school is associated with the spawning ground off the Firth of Forth, 

 the Forth school with that off the coast near Aberdeen. In the 

 former the young plaice are spread along the coast of Northum- 

 berland, in the latter from Forfarshire to the north and south coasts 

 of the Firth of Forth. It will be observed that the distance and 

 the area concerned are much the same in each case. Spawning 



* 1920. Meek. Nature, V. 106, p. 532. 



