reversal of the processes that occurred in the 

 parent nucleus during the previous pro- 

 phase. Instead of coiling more and more 

 tightly the spiral structure appears to uncoil 

 to a considerable degree; in this extended 

 form (Fig. 2-1 IB) the chromosomes are ex- 

 tremely delicate and difficult to delineate 

 microscopically. 



While nuclear reorganization is progress- 

 ing, cytoplasmic division is achieved. Plant 

 and animal cells differ, however, as to the 

 mechanisms of cytoplasmic division. In the 

 plant cell, a partition, called the cell plate, 

 is built across the center of the cell. The cell 

 plate passes directly across the center of the 

 spindle, at right angles to the spindle axis 

 (Fig. 3-1). The plate, in fact, appears to be 

 constituted of materials derived from the 

 middle section of the central spindle. In any 

 event the cell plate soon splits into two paral- 

 lel plates between which the cell walls of the 

 new cells are deposited. 



The animal cell divides its cytoplasm in 

 another way, however — namely, by furrow- 

 ing (Figs. 3-2 and 3-4). The division furrow 

 (cleavage furrow) appears at the beginning 

 of telophase, encircling the equator of the 

 cell, in the plane that passes through the 

 center of the spindle at right angles to its 

 axis. Gradually the furrow deepens until it 

 cuts through the cytoplasm and spindle rem- 

 nant completely, thus separating the daugh- 

 ter cells. By now, nuclear reorganization has 

 been consummated, the mitotic apparatus, 

 with the exception, perhaps, of the centrioles, 

 has faded from view, and the mitosis is 

 complete. 



The duration of the mitotic process varies 

 in different kinds of cells. It may last a few 

 minutes or several hours, the prophase usu- 

 ally being longest. The duration of the inter- 

 mitotic stage is even more variable. In rap- 

 idly developing tissues, one mitosis may suc- 

 ceed another with practically no interval at 

 all; but in the specialized tissues of a multi- 

 cellular organism many of the mature cells 

 may never divide again during the lifetime 

 of the individual. 



Cell Division in Relation to Reproduction 

 SOME CURRENT PROBLEMS 



47 



The events of mitosis have been observed 

 and described in a wide variety of cells for 

 more than 60 years. Yet a fundamental un- 

 derstanding of the intricate mitotic mech- 

 anisms remains elusive. There are many 

 questions for which only partial answers or 

 no answers have been obtained. What is the 

 "trigger" that initiates mitosis and how is the 

 precise timing of the successive mitotic stages 

 so beautifully regulated? What is the func- 

 tional role of the asters and why is it that 

 these structures are not essential in the divi- 

 sion of plant cells, or even of smaller animal 

 cells? By what means do the genes, chromo- 

 somes, centrioles, and other structures effect 

 a replication of themselves and how does the 

 animal cell obtain energy for furrowing into 

 two cells? These and many other problems 

 are being studied intensively with modern 

 research techniques and more progress can 

 be expected soon. 



Perhaps the best recent progress has been 

 made in reference to the manner in which 

 the DNA proteins, or genie substances, 

 achieve their replication in the chromosomes. 

 Such studies have been aided greatly by the 

 development of radioactive isotopes (p. 141). 

 These provide a means of tracing, step by 

 step, the metabolism of self-templated syn- 

 thesis, as will be explained more fully later 

 (Chap. 27). However, it is necessary to dis- 

 tinguish between self-guided synthesis, which 

 replicates the genie substances, and the divi- 

 sion of a whole chromosome into daughter 

 chromosomes. DNA synthesis, apparently, is 

 completed before mitosis starts, whereas the 

 sister chromosomes are not individually dis- 

 tinguishable until prophase. 



Good progress can also be noted in refer- 

 ence to the manner in which the cell de- 

 velops mechanical energy in achieving the 

 anaphase movements of the chromosomes. 

 The studies of Daniel Mazia and co-workers, 

 at the University of California, hold good 

 promise that this long-standing problem will 

 soon be resolved. 



