CLEAVAGE AND DIFFERENTIATION 



Finally, consider the experimental evidence. This, 

 although not embracing all eggs, Indicates clearly that 

 fertilized eggs may reach a certain degree of differentiation 

 without cytoplasmic cleavage. 



Differentiation without cleavage has been noted by sev- 

 eral observers to take place in eggs of annulated worms and 

 of ascldlans having been subjected to experimental treat- 

 ment. The most thorough-going description of this type 

 of development is that given for eggs of Chaetopterus having 

 been exposed to hypertonic sea-water.^ These eggs develop 

 into most abnormal swimming forms made up of either 

 uninucleated or multinucleated undivided cytoplasm due 

 either to complete failure of cytoplasmic cleavage or to the 

 disappearance of cleavages and fusion of the blastomeres. 



As development progresses, the cytoplasmic Inclusions 

 take up new positions in almost the same way and at the 

 same time as in fertilized normally developing (untreated) 

 eggs. But the ectoplasm behaves in a quite different 

 manner. A portion of it moves to the animal pole, a 

 smaller amount rests at the opposite pole. Later the ecto- 

 plasm at the animal pole moves toward the vegetal pole 

 so that the endoplasm Is covered, thus simulating the over- 

 growth of the larger yolk-containing blastomeres by the 

 smaller ones in normal development. 



Another characteristic feature in differentiation without 

 cleavage In the egg of Chaetopterus Is the behavior of the 

 nuclei in uninucleated eggs. In these eggs when the nucleus 

 reaches the size of the germinal vesicle of the normal egg 

 there appears a strong mutual attraction between It 

 and the ectoplasm: the ectoplasm Is drawn Into the egg 

 to form a mass that lies close to the nucleus or the chromatic 

 part of the nucleus Is drawn out to the ectoplasm. Eggs 

 which show ectoplasm drawn Into the interior do not 



^ Lillie, F. R., igo6. 



2Q9 



