GENES AND DEVELOPMENT 



153 



the other a cluster of rhizoids. The nucleus always lies near the rhizoid 

 end, and can easily be removed by cutting off this end. Hammerling^ 

 found that enucleate fragments can live for a considerable time, and 

 can also regenerate missing parts. The regeneration of a hat occurs 

 more frequently in fragments originating near the hat end, while 



Fig. 75. Acetabularia. — A. Acetabularia mediterranea, the stem has been some- 

 what shortened in the drawing. 6. A. Wettsteinii. C. Hat regenerated from a piece 

 of mediterranea stem grafted on to Wettsteinii rhizoid containing the nucleus. 



(After Hammerling.) 



rhizoids tend to be regenerated more easily from the other end. By a 

 series of regeneration and grafting experiments, Hammerling demon- 

 strated the existence of two gradients of morphogenetic substances, a 

 "hat substance" concentrated at the hat end, and a "rhizoid substance" 

 concentrated at the other end. These substances must exist indepen- 

 dently of the nucleus in the enucleate fragments, but there is good 

 evidence that the nucleus is ultimately responsible for their production. 

 If, for instance, a piece is isolated from the middle of the stalk, it 

 contains very Httle of either substance, and regeneration only rarely 

 occurs. If, however, the rhizoid end containing the nucleus is left 

 ^ Hammerling 1932, 1934. 



