64 EIGHTEENTH REPORT. 



Just as the cytoplasm and plastids divide iuto similar halves so 

 we find division going- on within the nucleus. It is, however, only 

 the chromatin lumps that divide. This division occurs in the 

 Myxophyceae where the nucleus is less highly organized than I 

 have described above, apparently by the pulling apart of the 

 chromatin masses. In the better organized nuclei, however, a 

 cleavage plane is jtroduced by the formation of vacuoles which 

 separate the lumps of chromatin into equal halves. This is the 

 essential feature of nuclear division. The matter is not so simple 

 as this statement would make it appear for this cleavage is organ- 

 ized and controlled in a very complicated manner. In brief the 

 process is as follows : the separate chromatin masses crowd to- 

 gether into a definite number of more or less elongated bodies, the 

 chromosomes. Special structures arise in the cytoplasm and en- 

 tering the nucleus arrange the chromosomes in a definite order 

 and, after they have undergone cleavage, pull apart the halves. 

 These half chromosomes represent merely the crowded together and 

 perhaps partially fused halves of the chromatin lumps of the rest- 

 ing nucleus. As all protoplasm has the power of regenerating it- 

 self so each lump regenerates itself exactly (probably using up the 

 food previously stored up in the nucleolus for the purpose) and 

 then the lumps separate and the daughter nuclei are reorganized. 



Careful study of a great many animals and plants by investiga- 

 tors in all parts of the world makes it clear that the chromatin 

 lumps always crowd together into the same number of chromo- 

 somes, at each nuclear division for the same species of organism. 

 These chromosomes, too, often have characteristic shapes and 

 sizes which are constant for the species. It seems probable that 

 not only are the shape and size of the individual chromosomes con- 

 stant, but even the relative position in the nucleus. This forces us 

 to the inevitable conclusion that the individual lumps of chromatin 

 which are united into the chromosomes are themselves permanent 

 cell organs and that the complicated mechanism of mitosis is an 

 arrangement hj which the halving and distribution of these chro- 

 matin masses to the daughter nuclei is made more certain. 



These facts have led biologists to assume that the control of 

 the development and functioning of the cells and consequently the 

 structure and physiological nature of the individual resides in 

 these various chromatin masses, in other words, we speak of them 

 as the "carriers of heredity." The further bearings of this theory 

 need not be mentioned here. 



When two nuclei fuse in sexual reproduction we find that the 



