246 ROLE OF CYTOPLASM [CH. 



and this fact alone would suggest that the cytoplasm must 

 have an important share in the differentiation of cells and 

 tissues from the relatively unmodified protoplasm of the egg. 

 There is, however, definite evidence that this is true. 



FIG. 30. Segmenting eggs of Nereis, from WILSON (1896). 



i. Normal four-cell stage. 2. Normal eight-cell stage; first quartet of 

 micromeres above. 3. Embryo with four entomeres each containing a large 

 oil drop, and twelve outer cells. 4. Eight-cell stage produced under 

 pressure. The dotted lines show the outlines of the micromeres formed 

 after release from pressure. 



WILSON (1896) in a well-known experiment on the eggs of the 

 worm Nereis showed that nuclei which under natural cir^ 

 cumstances would have been those of ectoderm cells could 

 be made to belong to endoderm cells. If allowed to develop 

 normally, the egg of Nereis segments by two cleavages into 



