INHERITANCE 



2 95 



formed by chromosomes, aster and spindle, are known as 

 mitotic figures. These' regularly appear at every cell divi- 

 sion, not only in the embryo, but in almost every growing 

 part of the body, throughout the life of the organisms. 

 They are freely exposed to view in living transparent eggs, 

 but in any developing tissue properly sectioned and stained, 

 nuclei may be seen in some of the division phases above out- 

 lined (fig. 176). These phases follow one another in an 

 inviolable order; each stage conditions the one that is to 

 follow it ; and together they seem admirably fitted for the 

 equivalent distribution of those parts of the nucleus which 

 appear most constant. 



What role these parts may play in inheritance it is as yet 

 impossible to say. They are all minute, and their study is 

 attended with very great difficulty, The centrosome is 

 usually at the limit of vision with the best microscopes, and, 

 hardly anything is known of its structure. The chromoso- 

 mes are the nuclear organs most readily followed, and as we 

 have just seen, between spireme and spireme these are scat- 

 tered in granules on an inconstant linin meshwork, to be 

 reintegrated at each successive division. Only their con- 

 stituent chromatin persists in our view, and this in particles 

 of such minuteness as to be individually unrecognizable. 

 Yet the chromosomes, as integrates of such particles, show 

 such constant features that we are compelled to attribute 

 considerable importance to them. They appear and reap- 

 pear in like number and in similar form. The number dif- 

 fers in different groups but it is constant and characteristic 

 for all the individuals of any given species, in all the cells 

 of the body. The number varies from 2 in a species of 

 round worm (Ascaris) to 168 in the crustacean Artemia, 

 ranging in most cases between 12 and 36. The number 

 appears to be a family characteristic in the grasshoppers, it 

 being 23 in the shorthorned grasshoppers and 33 in the 

 meadow grasshoppers. 



