CLEAVAGE AND DIFFERENTIATION 



we encounter the difficulty of explaining how under the 

 domination of this "half" nucleus the cytoplasm is ordered 

 into regions whence the organs arise. The theory of restric- 

 tion by means of the nucleus removing potencies from the 

 cytoplasm does not encounter this difficulty for accord- 

 ing to it the nuclei extruded as polar bodies relieve the 

 cytoplasm of potencies. 



6. Similar reasoning applies to eggs developing with 

 polyploid nuclei — i.e., with more chromosomes than those 

 contained in the united egg- and sperm-nuclei. In the egg 

 of Nereis, for example, fertilized after treatment with 

 ultra-violet light, ^ the polyploid nucleus, made up of the 

 three nuclei from the suppressed polar bodies plus the egg- 

 and the sperm-nuclei, remove cytoplasmic potencies pre- 

 cisely as these are removed in normal fertilization by polar 

 bodies and egg- and sperm-nuclei. But from the point of 

 view of segregation, conditioned by nuclear material or 

 power escaping into the cytoplasm, development should 

 not take place because of the over-powering influence of 

 the super-abundant nuclear matter. 



7. Certain eggs develop into animals which have the 

 capacity for asexual reproduction by the formation of buds 

 or by the process of breaking up into two or more fragments. 

 Both the buds and the fragments develop into complete 

 organisms similar to the form whence they arose. If sexual 

 and asexual phases regularly succeed each other, they con- 

 stitute a life-history said to reveal alternation of generations. 



Asexual reproduction can be explained by assuming that 

 potencies in the egg, present in excess of those for the forma- 

 tion of a single individual, remain free. The reduplication 

 of the animal by bud or fragmentation though it occurs 

 late in the life-history is thus comparable to poly-embryony. 

 In those cases of alternation of generations, where egg- and 



1 Just, 1933 c. 



