20 



stages suggestive of mitosis, where attempts appear to be 

 made to first break up the thread into chromosomes. And 

 experiments with cultures of spirogyra showed that either 

 mode of division might be induced in the nuclei by altering 

 the medium of growth. When "5 to i per cent, of ether was 

 added to water containing the plants they began to divide 

 amitotically, and mitosis was resumed on replacing them in 

 water under normal conditions.^* 



Amitosis " is characteristic of highly-specialised or 

 degenerating cells in which development is approaching its 

 end." It appears to be a process needing quick accom- 

 plishment, due in a measure in degenerating cells to their 

 lower vitality and probable lack of food supplies (Fig. 30). 

 On the other hand, where these supplies are abundant and 

 rich, they may possibly incite to very rapid growth and 

 attendant direct division of the nuclei, e.g., tapetum cells 

 (Fig. 31). Here also the physiological conditions surround- 

 ing the sporogenous cells are unique, initiating, as they do, 

 those remarkably striking phenomena, which result in the 

 biological transition of cells with sporophyte characters 

 into those of the gametophyte. The change results as a 

 consequence of the inter-relations between the physio- 

 logical and biological conditions in the cells accompanying 

 this aspect of the alternation of generations. The contra- 

 aspect attendant upon the phenomena of fertilisation, by 

 which the sporophyte characters are restored in the 

 fertilised ^^g, has also its equally complex nutritive and 

 biological changes. 



THE CENTROSOME 



is an extremely minute body with dark centre, usually lying 

 in a hyaline matrix ; it is situated just outside the nuclear 

 membrane of animal cells, and is freely described also in 

 those of many cellular plants. It is very active in connec- 



