75 



large ovaries and practically fully developed ova. Those which 

 were caught in August have the ova and the ovaries well developed, 

 larger at all events than the one which was caught in the Beadnell 

 region. It is more than Ukely that this example would have 

 migrated almost immediately if it had not been caught. In this 

 example, however, the ovaries were small and light in colour as 

 compared -udth those which had migrated. 



The impulse therefore for the migration northwards is due to 

 the ripening of the gonads. It is an impulse, however, which in 

 this case only affects the female. 



In the paper in this report on the " Mgrations of the Plaice 

 and Dab," page 52, it has been pointed out that the impulse for 

 the conspicuous migrations of these and other fish is Hkewise due 

 to the ripening of the gonads, but it affects both sexes. It will 

 be at once apparent that there is a fundamental difference between 

 the two cases. The crab carries with her the sperms in her sper- 

 mathecse, often for a long period before they are called into requi- 

 sition. In the case of the common food fish fertilization takes 

 place by the male shedding his milt at the time that the female is 

 spawning. 



I should like to refer here to page 50 of this report, and to say 

 in connexion with what is there stated that the larvae emanating 

 from the female crabs which migrate to the north of the Northum- 

 berland region will be carried southwards along the coast. The 

 young crabs will come to rest on the southern part of the east coast 

 of Scotland, the Northumberland district, and possibly even to 

 the south of the Tjnie. As they grow they join in the seasonal 

 migration offshore and inshore, and in this respect and in their 

 relative numbers along the coast they may be said to behave 

 exactly as a school of plaice or other northern migrant. 



It has already been stated in previous papers that the northern 

 district of Northumberland supports a much larger number of crabs 

 than the southern, and a glance at the returns will show that 

 similarly such successive regions of relative abundance and scarcity 

 occur all along the east and other coasts. It might therefore be 

 thought that the crabs of the east coast were resolved into a series 

 of schools presenting the respective maxima and minima of a 

 northern migrant. Our experiments, however, indicate that the 

 seasonal migration is restricted to an offshore and inshore move 

 ment, and there is no evidence of a migration along the coast. 



