68 



THE REALISATION OF THE NUCLEAR FACTORS 



will influence the composition of the cytoplasm, and, vice versa, 

 the cytoplasm will influence the nuclei and direct the course of 

 their development in divergent ways. We shall first discuss an 

 example of the latter effect. 



In the parasitic nematode, Ascaris megalocephala, the nucleus 

 contains a very small number of chromosomes; in one race, the 

 so-called univalens race, there are only two. Consequently, the 

 division process can very easily be followed in these nuclei, and 



Fig. 25. Chromatin diminution in Ascaris megalocephala. (a-b) 

 second cleavage, diminution in AB, normal division of the chromo- 

 somes in Pi; (c) 4-cell stage; (d) new arrangement of the cleavage 

 cells, diminished nuclei in A and B, diminution taking place now in 

 EMSt, and normal division in P2. After Boveri. 



