124 ON THE DEVELOPMENT OP THE COMMON PROG. 



from the same mass of frog-spawn differ considerably at 

 all stages of their growth, both in size and degree of deve- 

 lopment ; and this difference is seen from the commence- 

 ment. The ova on the surface of the mass develop, and 

 the tadpoles escape, whilst those in the centre still remain 

 unchanged, at least as regards their round shape. The 

 hatching of the ova, in fact, instead of being simultaneous, 

 takes place successively from the surface to the centre ; so 

 that if the earliest tadpoles escape in about a fortnight, 

 the latest will not follow until a week or more afterwards, 

 the cause apparently being that they require direct con- 

 tact with water for their development. And this fact is of 

 interest, as it may be some advantage to the tadpoles by 

 allowing them gradually to disperse, and thus preventing 

 the excessive crowding which would result from such 

 immense numbers being hatched at once at the same spot. 

 This difference in the time of hatching accounts to some 

 extent for the difference in size and development seen in 

 the tadpoles afterwards ; but a further cause of difference 

 appears to have reference to feeding, the greater natural 

 vigour of some enabling them to take a much larger quan- 

 tity of food, whilst weakness of constitution, or, perhaps, 

 some defect of the alimentary apparatus, throws others 

 far behind. But, at the same time, the great majority of 

 the tadpoles do not differ very widely in development, 

 about a week or ten days representing the amount of their 

 variation. 



For the purpose of bringing the successive periods as 

 nearly as possible to one standard, the largest and most 

 advanced tadpole was always selected from a considerable 

 number taken out of the pond for each daily observation, 

 the others being put back ; and in consequence of this 

 method all the periods as given are rather shorter than 

 the averages would be. The extremes will be sufficiently 



