SUMMARY AND CONCLUSION 85 



Whether this conckision can be accepted without modification 

 remains to be seen. Flemming himself regards it as too extreme, 

 and is incHned to accept Meves' conclusion that amitosis may occur 

 in the sperm-producing cells of the testis. The same conclusion is 

 reached by Preusse in the case of insect-ovaries. There can be no 

 doubt, however, that Flemming's hypothesis in a general way repre- 

 sents 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 



Three distinct elements are involved in the typical mode of cell- 

 division by mitosis ; namely, the centrosome, the chromosome, and the 

 cell-body. Of these, the centrosome may be considered the organ 

 of division par excellence ; for as a rule it leads the way in division, 

 and under its influence, in some unknown manner, is organized the 

 astral system which is the immediate instrument of division. This 

 system appears in the form of two asters, each containing one of the 

 daughter-centrosomes and connected by a spindle to form an aviphi- 

 aster. It arises as a differentiation or morphological rearrangement 

 of the general cell-reticulum, the asters being formed from the extra- 

 nuclear reticulum, the spindle sometimes from the linin-network, 

 sometimes from the cyto-reticulum, sometimes from both. 



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, is formed as a single series of chromatin-granules or 

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

 Every individual chromatin-granulc therefore contributes its quota 

 to each of the daughter-nuclei. 



The mechanism of mitosis is imperfectly understood. There is 

 good reason to believe that the fission of the chromatin-granules, and 

 hence the splitting of the thread, is not caused by division of the 

 centrosome, but only accompanies it as a parallel phenomenon. The 

 divergence of the daughter-chromosomes, on the other hand, is in 

 some manner determined by the spindle-fibres developed under the 

 influence of the centrosomcs. There arc cogent reasons for the view 

 that some at least of these fibres are contractile elements which, like 



