314 THE RECAPITULATION THEORY 



establish itself permanently in fresh water an animal 

 must either be fixed, or else be strong enough to 

 withstand and make headway against the currents 

 of the streams or rivers it inhabits, for otherwise 

 it will in the long run be swept out to sea, and 

 this consideration applies to larval forms equally 

 with adults. 



The majority of marine Invertebrates leave the 

 egg as minute ciliated larvae, and such larvae are 

 quite incapable of holding their own in currents of 

 any strength. Hence it is only forms which have 

 got rid of the free swimming ciliated larval stage, 

 and which leave the egg of considerable size and 

 strength, that can establish themselves as fresh- 

 water animals. This is effected most readily by 

 the acquisition of food yolk hence the large size 

 of the eggs of fresh-water animals and is often 

 supplemented, as Sollas has shown, by special 

 protective devices of a most interesting nature. For 

 this reason fresh-water forms are not so well 

 adapted as their marine allies for the study of 

 ancestral history as revealed in larval or embryonic 

 development. 



Before leaving the question of food yolk, refer- 

 ence must be made to the proposal of the brothers 

 Sarasin, to regard the yolk cells as forming a dis- 

 tinct embryonic layer, the lecithoblast,* distinct 

 from the blastoderm. I do not desire to speak 

 dogmatically on a point the full bearings of which 

 are not yet apparent, but I venture to think that 



* P. and F. Sarasin, " Ergebnisse naturwissenschaftlicher 

 Forschungen auf Ceylon," Bd. ii. Heft iii. 1889. 



