192 PHYSIOLOGICAL GENETICS 



like beaded and beadex. 1 This problem has already been 

 discussed in the chapter on interaction of the genes (page 161). 

 Of a different type bu1 very important in connection with the 



problems of this and the last chapter is (he work of Baemmerling 

 (1934) on Acetabularia (See also page 179). This alga has the 

 form of an umbrella with a long stem growing from a root like 

 base. The umbrella-like spread is formed after a series of wreaths 

 of hairs have been formed at the distal end of the stem and have 

 disappeared. The umbrella itself consists of a number of indi- 

 vidual radial chambers, in which the gametes will be formed later. 

 This whole plant is only a single cell with a large nucleus situated 

 at the base of the stem in the rhizoid. By cutting the stem, a 

 uucleated part containing the rhizoid and an anucleated one 

 ending in the umbrella, or "hat," will be obtained. The 

 anucleated fragment will live a very long time but is finally 

 doomed. Such a nucleated segment easily regenerates the lost 

 parts. But an anucleated fragment may also regenerate the hat 

 at the anterior as well as at the posterior cut end. It was then 

 shown that this regeneration of anucleated fragments takes place 

 more readily at the anterior end and that its success depends upon 

 the presence of a substance that is used up during regeneration 

 and shows a gradient of concentration, increasing toward the 

 end of the stem. The same piece will also regenerate a rhizoid, 

 and the stuffs responsible for this show the opposite gradient 

 with a maximum concentration toward the base of the stem. 

 The influence of the nucleus could then be tested in the following 

 type of experiment. A piece with a nucleus was allowed to 

 begin regeneration, and after it had been started the nucleus 

 was removed. The piece now left had complete power of regener- 

 ation though derived from the part of the stem that otherwise 

 would not form a hat. The nucleus then had produced the 

 formative stuffs which spread along the stem. Such experiments 

 show the nuclear origin of the formative stuffs but do not prove 

 that it is the genie material within the nucleus that is involved 

 here. This problem was attacked by using a second species 

 (1. A. mediterranea, 2. A. Wettsteinii) , differing from the first 

 mainly by the number and shape of the chambers constituting 

 the hat, the color, and a number of other characters. It was 

 possible to transplant anucleated fragments of one species to 

 1 Unpublished work. 



