THE SYNTHESIS OF NEW SUBSTANCES 



377 



which we are justified in leaping direct to the gene itself: there is always 

 the possibihty, which is indeed often a probability, that there are several 

 intervening steps between the gene and the most deep-lying precursor 

 which we have been able to find. 



Most gene-controlled substances which can be easily identified are 

 found in the cytoplasm, and are probably produced in it, so that the genes 

 must be involved only at second hand in their formation. Direct evidence 

 of the production of developmentally active substances by the nucleus 

 itself, or its immediate neighbourhood, is, however, available in some 

 cases. One of the most striking of these occurs in the unicellular alga 

 Acetabularia (Fig. 17.3). During most of its hfe-cycle this organism consists 

 of a rhizoid, which is attached to the ground, from which arises a stalk 

 which terminates in an umbrella-shaped hat. There is only a single nucleus, 

 although the whole alga may attain the size of several centimeters or more. 

 Haemmerling (1934, 1953) showed that if the hat is removed, a new one 

 will regenerate. He then cut off the nucleus-containing rhizome from an 

 alga of one species {A. mediterranea) and substituted a similar piece con- 



FiGxjRE 17.3 

 Nuclear grafts in Acetabularia. {A) Acetabularia mediterranea; the stem has 

 been somewhat shortened in the drawing; the single nucleus lies in the 

 rhizoid at the bottom. (B) A. Wettsteinii. (C) the Wettsteitiii-XiVt 'hat' re- 

 generated from a short piece of mediterranea stem grafted on to the nucleus- 

 containing rhizoid of Wettsteinii. (After Haemmerling I934-) 



