THE PLAN OF CELLULAR REPRODUCTION 163 



power of self -reproduction and moving in very regular paths after their 

 reproduction. 



That centrioles arise from pre-existing centrioles is a fact of obser- 

 vation. It was demonstrated a long time ago (Boveri, 1903) that the 

 multiplication of centrioles may proceed in the absence of the nucleus. 

 We are unable to apply one important criterion of self-reproduction— 

 mutability— to the centrioles, because we can give no meaning, at 

 present, to the mutation of a pole. The statement that centrioles are 

 self -reproducing units is surely more than an interesting hypothesis. 



Where the structure of centrioles has been resolved by electron 

 microscopy, they appear as cylindrical bodies, about 1500 angstroms in 

 diameter and 300 to 500 angstroms long, the "wall" of the cylinder being 

 made up of nine groups of tubules, each tubule about 150 to 200 ang- 

 stroms in diameter. This structural pattern has been observed in a 

 variety of cells (summarized by Bernhard and deHarven, 1960), but 

 much more complex structures may be associated with the centriolar 

 function in other cases— the flagellates, for instance (Cleveland, 1957). 



Here we have a genuine self -reproducing particle, large in relation 

 to molecular dimensions but possessing a relatively simple structure so 

 far as we can tell. In mitosis its function is to organize a pole, but here 

 we may be seeing only one of its functions. There are good reasons for 

 relating it to the kinetochores, and to the basal particles or kinetosomes 

 of ciha and flagella and through the latter, to a variety of structural 

 specializations of cells, such as those that have been detected in visual 

 receptors. It would not be surprising if we were dealing with the arche- 

 type of the cytoplasmic self-reproducing particles which have so often 

 been invoked in the theory of diflFerentiation ( e.g., LwoflF, 1950 ) . 



Let us consider some of the reproductive habits of the centriole. 

 One interesting finding of many microscopic observers is that it is gen- 

 erally a paired structure. The electron microscope supports this ( Bern- 

 hard and deHarven, 1960). There are two centriolar units at each 

 mitotic pole, and the reproduction might be described as a 2-4 doubhng 

 rather than a 1-2 doubling. This might mean that the double unit is 

 actually required for the function of establishing a mitotic pole. Let 

 me describe some recent experiments leading to quite a diflPerent 

 interpretation. 



Studies on the chemistry of the mitotic apparatus (Mazia, 1955) 

 had suggested that sulfur bonding was important in its organization. It 

 was predicted that an excess of -SH, introduced experimentally, would 

 interfere with sulfur-bonding of the proteins forming the mitotic ap- 

 paratus, and therefore would block division. The agent selected was 

 mercaptoethanol, and the prediction was borne out. But it was also ob- 

 served that if the cells— sea-urchin and sand-dollar eggs— were blocked 

 at metaphase, were permitted to remain blocked in mercaptoedianol 



