162 MACROMOLECULAR COMPLEXES 



called the chromosomes. It is not unnatural that a structure of the 

 beauty of the mitotic apparatus should receive so much attention 

 from the early cytologists, and many of the classic observations 

 made by them have not been surpassed to this day. 



One of these observers who speculated about its origin was 

 Flemming ( 1882), who offered two alternatives for the source of the 

 material incorporated into the spindle fibers of the mitotic appara- 

 tus. The first alternative states that the spindle fibers arise from the 

 pale fibers of the achromatic region of the prophase nucleus, which 

 in turn originate from the reticulum and nucleoli of the interphase 

 nucleus. The second alternative imagines that they are derived from 

 the cytoplasm, growing from this into the nucleus. Other distin- 

 guished scholars have held similar views. Boveri (1887), Stras- 

 burger (1880), Wilson (1925), and Hertwig (1909) have all 

 considered as valid the formation of the achromatic figure from 

 pre-existing structures visible in their cytological preparations. 

 There was general agreement that the asters were of cytoplasmic 

 origin but considerable dissension over whether the spindle fibers 

 were of nuclear or of cytoplasmic origin. However, either interpre- 

 tation could be valid, depending upon the material under consider- 

 ation. In its essential form, the precursor concept for the origin of 

 the mitotic apparatus germinated during the fertile "Golden Era" 

 of cytology and, after about 50 years of relative dormancy, it is being 

 revived. Elucidation of the origin of the mitotic apparatus could 

 provide a made-to-order system in which to examine the completely 

 reversible aggregation of molecular units into a transient structure 

 of unquestionable significance to the cell. Important to this studv is 

 the synchronization that exists between the appearance of the 

 mitotic apparatus and other cell activities, which provides the ex- 

 perimenter with a convenient clock. 



The problem of the molecular origin of the mitotic apparatus 

 could not be investigated directly until Mazia and Dan (1952) 

 demonstrated that this structure could be freed of its investing 

 cytoplasm in large numbers from a population of dividing sea-urchin 

 eggs. One could now inquire experimentally into the possible modes 

 of origin of the mitotic apparatus. Two basically different ones at 

 opposite extremes were envisaged. One of these views the formation 

 of the mitotic apparatus as the result of the de novo synthesis from 

 small, non-specific units such as amino acids or small polypeptides. 

 These may arise from the earlier breakdown of some storage pro- 



