25 



Is Amphimixis sufficient to account for the 

 main facts of variation ? 



The mosaic theory of development as evidence 

 for qualitative divisions. 



Experiments of Roux on Frogs' eggs. 



Formation of half embryos. 



Post generation. 



Roux's explanation. 

 Chambry's experiments on eggs of Ascidia as- 



pera. 

 E. B. Wilson on the cell lineage of Nereis. 



Evidence against qualitative divisions. 



1. In normal development. 



a. Lack of correspondence between first 



cleavage plane and any plane of adult 



body. 

 Morgan on frogs and teleosts. 

 Miss Clapp on toad fish. 



b. Variations in cleavage. 



H. V. Wilson on sea-bass. 



Jordan on Amphibia. 



E. B. Wilson on Amphioxus. 



Three types with all grades between. 



Result normal embryos. 



2. Experimental evidence. 



a. Effects of pressure. 



Hertwig, pressure on frogs' eggs, — 

 Abnormal cleavage but normal embryos. 



Morgan, removal of yolk from c^^ of 

 Fnndiilns, — 

 Abnormal cleavage but normal embryos. 



b. Isolation of blastomeres. 



Driesch, eggs of Echinus in 2-cell stage. 

 Result : half blastulas, becoming com- 

 plete dwarf blastulas, gastrulas and 

 plutei. 



LoEB, on eggs of sea-urchins. 



