HEREDITY 85 



In all cases, division of the cell as a whole is preceded 

 by the division of its nucleus. In Fig. 46, C, we see that 

 each gamete nucleus (shown on a larger scale in Fig. 

 46, G), contains three long threads, the combined zygote 

 nucleus therefore containing six. These threads are 

 composed mainly of a substance called chromatin, and 

 each of them is called a chromosome. The first step in 

 the division of the nucleus consists in each chromosome 

 splitting along its length into two. At about the same 

 time fine fibres (shown in Fig. 46, D, etc.), make their 

 appearance in the protoplasm, stretching from points at 

 opposite poles of the cell. Of the two chromosomes pro- 

 duced by the splitting of each original one, one becomes 

 attached to a fibre running to one pole, and the other to 

 a fibre running to the opposite pole of the cell. The 

 chromosomes then travel up the lines of these fibres to 

 congregate at the poles, where they form two new nuclei, 

 one at each pole. Thus it will be seen that each gamete 

 introduces three chromosomes, providing the zygote 

 with six, and that each of the two nuclei formed by the 

 division of the zygote nucleus gets a half of each one of 

 these six chromosomes. In other words, the chromatin 

 of each nucleus is derived in equal quantity from the male 

 and female parent. 



The developing egg, or embryo as we may now call it, 

 now consists of two cells, each with its nucleus. By a 

 process similar to that just described each nucleus again 

 divides into two, followed by the division of its cell, so 

 as to give four cells, and this process is repeated again 

 and again until a large number of cells, each with a 

 nucleus, has been formed. Thus the nuclei of all the 

 cells of the embryo and adult Cyclops contain chromatin- 

 or hereditary substance as we may now call it derived 



