Hybridization of Echinoids. 39 



Developmental block in this case seems to be associated with what 

 might be described as nuclear indigestion. Fulton (1921, p. 167), in 

 describing the result of continued stimulation of a muscle on its 

 nucleus, says: ''With continued stimulation the nucleus must liter- 

 ally become overwhelmed with material to be oxidized; as a result, 

 it would become fatigued, and would no longer be capable of perform- 

 ing its normal functions." 



The word ''overwhelmed" describes vividly the conditions in- 

 volving the nucleus in these hybrids. 



From the Arbacia-Moira material, the evidence obtained was 

 interpreted as showing that enzymes are emitted through the nuclear 

 membrane and that changes occur in the protoplasm following this 

 emission. The materials formed pass back to and are taken into the 

 nucleus. There is no evidence that the emitted material is chromatin. 



There is nothing in these views that detracts from the chromosomal 

 theory of inheritance. It is evident that what is transmitted from 

 generation to generation is a course of development— what Brooks in 

 "The Foundations of Zoology" calls a "capacity for nurture." 



The results of these cross-activations show that an orderly series 

 of developmental reactions may be disorganized by the introduction 

 of foreign nuclear material. In the cross-activated eggs, in the inter- 

 action between nucleus and cytoplasm, the nucleus seems to be pro- 

 vided with more material than it can handle. In straight-fertilization 

 the accumulation of product would have checked the action of 

 enzymes before that condition arose. The foreign enzymes are 

 specifically different and their action is not checked until a greater 

 than normal supply for this egg has accumulated. Even though the 

 nucleus may take care of products formed, development may come 

 to an end because of an actual exhaustion of substrate. 



