Isolation of blastomeres. 

 ZojA, on medusae. 

 Wilson, on Amphioxus. 

 Morgan, on Fundulus. 

 Driesch, on Echinus. 

 Driesch and Morgan, on Beroe. 

 LoEB, on sea-urchins. 



c. Summary of the evidence. 



Result unfavorable to the theory of qualita- 

 tive divisions. 

 External conditions a factor in differentiation. 



Hertvvig's theory of development. 



Effects due to the constitution of the egg, 



yoke, shape, etc. 

 New relations established by cleavage. 

 " The differentiation of the cell is a function 



of its position." (Driesch). 

 No preformation of the embryo in the ^g^. 

 Each stage in development determines the 



next stage. 

 Development purely a process of epigenesis. 



Driesch (1894). All nuclei equivalent. 



Reactions of the idioplasm and the cytoplasm 

 upon one another. 



Pre-existing differences of the cytoplasm con- 

 dition the activity of the idioplasm in the 

 different regions of the egg. 



Wilson (1896). 



Relations of blastomeres not purely mechan- 

 ical. 



Differentiation of cytoplasm in one stage has 

 a determining influence upon the next. 



Nucleus undergoes a change during develop- 

 ment, but not because of qualitative divi- 

 sions. 



