INHERITANCE 



2 93 



succession before our eyes. Nothing could be more convinc- 

 ing of the wonderful refinement of structure of the living 

 substance, or of the precision of its processes, than to watch 

 the behavior of the nucleus in a segmenting egg. And if we 

 supplement what we can see in life by an examination of eggs 

 that have been fixed at different stages, and stained by the 

 precise differential methods of histology, we may discover 

 the chief phenomena of nuclear behavior that regularly 

 recur at every cell division. 



Figure 175 is a diagram of ordinary (indirect) cell division. 

 In the resting stage preceding division (a) the nucleus, in- 

 closed by a nuclear membrane is seen to contain an irregu- 

 larly disposed darker substance (deeply stained in his- 

 tological preparations) called chromatin. This is deposited 

 in a network of excessively fine and almost invisible threads 

 of a substance called linin. Besides these, the watery fluid 

 in which these lie ("nuclear sap") there is also present, more 

 or less constantly a rounded body, the nucleolus, different in 

 character from chromatin, as shown by its staining reactions. 

 Outside of the nucleus but lying close to it is a minute body, 

 the centrosome. 



The first sign of division appears in the division of the 

 centrosome; the resulting daughter centrosomes move apart 

 along the outside of the nuclear wall. The chromatin inside 

 that wall begins to be gathered together in a long convoluted 

 skein called the spireme (b). Before the centrosomes reach 

 opposite sides of the nucleus, the nuclear wall begins to be 

 dissolved. The linin threads take up a position stretched 

 between the two centrosomes and so form the nuclear 

 spindle. Corresponding linin threads in the cytoplasm 

 become radiately arranged about the centrosomes to form 

 the two asters. The chromatin of the spireme becomes 

 broken into segments, that are at first irregularly disposed 

 on the linin threads, and that later are shifted to the middle 



