13- Epigenesis. 



O. Hertwig (1892, 1894). Doctrine of continuity 



accepted. 

 Control of the cell by the nucleus. 

 Units of idioplasm, idioplasts representing cell 



characters only. 

 Theory of determinants rejected. 

 Comparison of organism to a state. 



Evidence concerning the mosaic theory. 

 For the theory : — 



Experiments of Roux on frogs' eggs. Forma- 

 tion of half embryos. 



Crampton on Ilyanassa. 



E. B. Wilson on normal cleavage of Nereis. 



Presence of independently variable parts. 



Against the theory : — 



a. In normal development. 



Lack of correspondence between first cleav- 

 age plane and any plane of adult bod)-. 

 Miss Clapp on toad fish. 

 Morgan on frogs and tclcosts. 



• 



Variations in cleavage. 



H. V. Wilson on sea bass. 

 Jordan on Amphibia. 

 E. B. Wilson on Amphioxus. 

 Three types with all grades between. 

 All result in normal embryos. 



b. Experimental evidence. 

 Effects of pressure. 



Driescii, on cchinoderm eggs, 

 Hertwig on frogs' eggs. 

 E. B. Wilson on Nereis. 

 Morgan on Fundulus. 



