CELLS AND TISSUES 37 



cell growth is provided by the experiments of Hammerling with the sin- 

 gle-celled plant Acetabularia. This marine alga, which is 4 to 5 cm. long, 

 is mushroom-shaped, with "roots" and a stalk surmounted by a flattened, 

 disc-shaped umbrella. The single nucleus is located near the base of the 

 stalk. Hammerling cut across the stalk (Fig. 3.3) and found that although 

 the lower part, containing the nucleus, could live and regenerate an 

 umbrella, the upper part would eventually die without regenerating a 

 stalk and roots. In further experiments, Hammerling first severed the 

 stalk just above the nucleus (cut 1, Fig. 3.3), then made a second cut just 

 below the umbrella (cut 2). The section of stalk thus isolated, when 

 replaced in sea water, was able to grow a partial or complete umbrella. 

 This might seem to show that a nucleus is not necessary for regenera- 

 tion; however, when Hammerling cut oft this second uniDrella the stalk 

 was unable to form a new one. From experiments such as these, Ham- 

 merling concluded that the nucleus supplies some substance necessary 

 for umbrella formation. This substance passes up the stalk and instigates 

 umbrella growth. In the experiments described here, some of this sub- 

 stance remained in the stalk after cuts 1 and 2, enough to produce one 

 new umbrella. After that amount of "umbrella substance" was exhausted 

 by the regeneration of an umbrella, no second regeneration was possible 

 in the absence of a nucleus. 



Dr. Jean Brachet, of the University of Brussels, found that both 

 nucleatecl and non-nucleated fragments of Acetabularia kept in radio- 

 active carbon dioxide in the ligiu would incorporate the radioactive 

 carbon into proteins at rates which were identical lor the first ten days. 

 Even thirty days after the removal of the nucleus, non-nucleated frag- 

 ments synthesized protein, as measured by the incorporation of radio- 

 active carbon, at a rate which was 70 per cent as great as that of the 

 nucleated fragments. Dr. Brachet concluded that the nuclear control of 

 protein synthesis is not an immediate one but an indirect one. He be- 

 lieves that protein synthesis is a function of the microsomes and the 

 multiplication of the microsomes is under the control of the nucleus. 



^Vhen a cell has been killed by fixation with the proper chemicals, 

 and then stained with the appropriate dyes, several structures— strands 

 of chromatin and one or more nucleoli— are visible within the nucleus 

 (Fig. 3.4). These are difficult to see in a living cell with an ordinary light 

 microscope but are evident by phase microscopy. Strands of chromatin, 

 composed of nucleoproteins with a strong affinity for basic dyes, run 

 irregularly through the nucleus and exhibit a netlike or gianular appear- 

 ance. W^hen the cell divides, the chromatin threads condense and form 

 the dark-staining, rod-shaped chromosomes which contain the hereditary 

 units called genes. A nucleolus is a small, spherical body found within 

 the nucleus. There may be more than one nucleolus per nucleus, but the 

 cells of any particular animal have the same number of nucleoli. The 

 nucleolus disappears when a cell is about to divide and reappears after 

 division is complete. It has been postulated that the nucleolus plays some 

 role in the synthesis of proteins and ribonucleic acids, but its function 

 is not known. 



