THE BIOLOGY OF THE CELL SURFACE 



fertilization can not occur because eggs and spermatozoa 

 are not ready for fertilization at the same time. The failure 

 of self-fertilization among hermaphroditic animals is more 

 often assumed than proved.^ None of these modes by 

 which &gg and spermatozoon are brought together can be 

 correlated with the stage in maturation at which the egg- 

 cytoplasm is "ripe" for fertifization. Nor can we say 

 that fertilization and development within the organism 

 are peculiar to higher animals. Among sponges, for exam- 

 ple, the lowest form of animals that produce eggs and 

 spermatozoa, the eggs pass through early development in 

 the parent organism. That tapeworms, members of the 

 third lowest group of multicellular animals, are character- 

 ized by their strong male copulatory organs, is a fact which 

 discredits a popular notion that copulatory organs are 

 found only among the highest animals. Thus, the mode 

 by which the union of egg and spermatozoon is accomplished 

 has no special significance for the events that follow this 

 union. 



In the exposition of these events, which now is given for 

 each class, we shall begin with contact of egg and sperma- 

 tozoon and end with the first cleavage of the egg. These 

 are common points. Between lie those events whose differ- 

 ences call for evaluation. I request the reader to note them 

 in order the better to appreciate the discussion of their sig- 

 nificance. Named in order, these events are: the initial 

 changes at the surface of the &%%'■, the arising of two star- 

 like formations, the asters, associated with sperm- or egg- 

 nucleus (with or without a discrete body, the centriole, at 

 the centre of each aster); the coming together of the egg- 

 and sperm-nuclei (sometimes called pronuclei); and the 

 formation of the cleavage-spindle. 



^ Cf. Just., ig^^b, and earlier workers. 



