158 SCIENCE PROGRESS. 



which have been advanced when there appears to be definite 

 evidence opposed to them. 



One of the most remarkable facts which confront one in 

 connection with chromosome reduction is that /:l'0 divisions 

 should be almost invariably associated with the process. 

 The only exceptions I can recall are those in which, as in 

 Lily embryosacs, the mitoses in which it would have nor- 

 mally been accomplished have been telescoped into a shorter 

 course, owing to the suppression of the divisions of an em- 

 bryosac mother cell. 



And the second point lies in the rapidity with which 

 the two divisions follow on each other ; the daughter nuclei 

 resulting from the first mitosis rarely go into the complete 

 resting stage before they commence to divide once more. 

 And this perhaps helps us to understand how the process 

 may be so compressed as to admit of the telescoping of Oiie 

 or both divisions, such as is seen in the Lily. 



The third point of general importance is the very long 

 period of rest and growth through which the nuclei pass 

 just before entering on these divisions. In pollen mother 

 cells, and in the sporogonia of liverworts for example, the 

 final number of spore mother cells is reached tolerably early, 

 and then a very long interval elapses before they rush 

 throuorh the final mitoses. The same thingr thouQ^h in a 

 somewhat less degree, is to be seen in the reproductive 

 glands of animals. 



Fourthly, wiien the nuclei are preparing lor the impor- 

 tant divisions, after their long period of growth, they go 

 through several remarkable phases, which are common both 

 to plants and animals. I have seen them in all the cases I 

 have investigated or had the opportunity of examining. 

 The cells become extremely sensitive to injurious sub- 

 stances, and it is to this cause that the well-known difficulty 

 of killing and fixing properly is due. Moveover. the linin 

 framework in which the chromatin is increasing in quantity 

 becomes lumped together in a highly characteristic manner ; 

 this is a transient phenomenon, and most people have been 

 inclined to regard it as due to imperfect fi.xation. But I 

 have myself seen it in the living cell in som.e animals, and 



