SUMMARY AND CONCLUSION 



119 



There is, however, strong evidence that this conchision is too 

 extreme. Meves ('94) has given good reason for the conchision that 

 in the salamander the nuclei of the sperm-producing cells (spermato- 

 gonia) may divide by amitosis yet afterward undergo normal mitotic 

 division, and Preusse ('95) has reached a similar result in the case of 

 insect-ovaries. Perhaps the most convincing evidence in this direc- 

 tion is afforded by Pfeffer's ('99) recent experiments on Spirogyra. 

 If this plant be placed in water containing 0.5 to 1.0% of ether, active 

 growth and division continue, but only by amitosis. If, however, the 

 same individuals be replaced in water, mitotic division is resumed and 

 entirely normal growth continues. This seems to show conclusively 

 that amitosis, in lower forms of life at least, does not necessarily mean 

 the approach of degeneration, but is a result of special conditions. 

 Nevertheless, there can be no doubt that Flemming's hypothesis in a 

 general way represents the truth, and that in the vast majority of cases 

 amitosis is a secondary process which does not fall in the generative 

 series of cell-divisions. 



F. Summary and Conclusion 



All cells arise by division from preexisting cells, cell-body from 

 cell-body, nucleus from nucleus, plastids (when these bodies are pres- 

 ent) from plastids, and in some cases centrosomes from centrosomes. 

 The law of genetic continuity thus applies not merely to the cell con- 

 sidered as a whole, but also to some of its structural constituents. 



In mitosis, the usual and typical mode of division, the nucleus under- 

 goes a complicated transformation, and, together with some of the 

 cytoplasmic material, gives rise to the -mitotic figuTc. Of this, the 

 most characteristic features are the cJiromatic figure, consisting of 

 chromosomes derived from the chromatin, and the (XcJirouiatic figure, 

 derived from the cytoplasm, the nucleus, or from both, and consisting 

 of a spindle, at each pole of which, as a rule, is a centrosome and 

 aster. There is, however, strong evidence that both these latter struc- 

 tures may in some cases be wanting, and the spindle is therefore prob- 

 ably to be regarded as the most essential element. 



The chromosomes, always of the same number in a given species 

 (with only apparent exceptions), arise by the transformation of the 

 chromatin-reticulum into a thread which breaks into segments and 

 splits lengthwise throughout its whole extent. The two halves are 

 thereupon transported in opposite directions along the spindle to 

 its respective poles and there enter into the formation of the two 

 corresponding daughter-nuclei. The spireme-thread, and hence the 

 chromosome, arises from a single series of chromatin-granules or 

 chromomeres which, by their fission, cause the splitting of the thread. 



