THE PLAN OF CELLULAR REPRODUCTION 167 



ter. This generative model explains the normal twoness of the cen- 

 trioles, which, as we have seen, does not appear to be a necessity for 

 forming a pole. The explanation is simple and familiar: If reproduction 

 proceeds generatively, and there is a period of development between 

 conception, parturition, and the next conception, obviously at least two 

 generations have to be in existence simultaneously. This is one way to 

 explain structures which are consistently double. All that was done in 

 the quadripartition experiment was to suspend the operations of the 

 older generation while the newer one grew and could operate. 



It is implied that only a small part of the centriole is capable of 

 self-replication— that its replication plants the "seed" for the assembly 

 of a second unit. Admittedly such a diflPerentiation has not been ob- 

 served in the structure of the centriole, but it has been observed 

 ( microscopic observations of Cleveland, 1957; Bernhard and deHarven, 

 1960; and the remarkable conclusion by Heidenhain, 1907, that the 

 centrioles reproduce by "budding") that centrioles do not undergo 

 binary fission but do give rise to "baby" daughters. In molecular terms, 

 the question would focus on the presence and distribution of nucleic 

 acids, if we feel compelled to associate r-eproduction with nucleic acids. 

 There are scattered observations of staining of the centers for nucleic 

 acids in the literature. In our laboratory we have tried every way we 

 could find to demonstrate that the self-reproducing centers were foci 

 of nucleic-acid activity by means of staining methods and by autora.di- 

 ography employing a variet}^ of tritiated precursors of high specific 

 activity. So far as sea-urchin eggs are concerned, the results have 

 been negative so far. But the centriole is a rather simple structure, and 

 we need not imagine that it calls for much structural information. It 

 can be imagined that only a few nucleic-acid molecules are involved in 

 the replicative events of the conception of a new center, that most of 

 what we see with the microscope is the "soma" that has developed from 

 this seed. 



So far as the reproductive plan of the cell as a whole is concerned, 

 it is of interest to consider the timing of the events in the reproduction 



Figure 3. (See opposite page.) A. A diagrammatic representation of the 

 generative reproduction of centrioles. Bars connecting the units imply that 

 the daughter units have not yet split away from the parent units. Chromo- 

 somes are shown only to identify the stages, and the figure is not meant to 

 represent the reproductive cycle of the chromosomes as discussed in the text. 

 B. An interpretation of the experiments discussed in the text. The dia- 

 grams show the behavior of the centers during blockage, the four-way divi- 

 sion after removal of the block, the following monopolar mitosis, and the 

 bipolar mitosis resulting in the division of the four cells to eight. 

 (After Mazia, Harris, and Bibring, 1960.) 



