THE REALISATION OF THE NUCLEAR FACTORS 69 



this animal constitutes an excellent subject for cytological 

 research. The fertilised egg of the worm contains two long, 

 curved chromosomes. At the first cleavage, the egg divides into 

 an animal and a vegetative blastomere. At the second cleavage, 

 each of these divides once more into two cells, and, during this 

 division, a peculiar process, known as chromatin diminution, 

 take place in the animal blastomeres. The ends of the chromo- 

 somes are thrown off into the cytoplasm, where they dis- 

 integrate, and the middle part of each chromosome divides into 

 a number of fragments (Fig. 25a). At this stage, no diminution 

 has yet occurred in the vegetative blastomere. In the next 

 cleavage, however, this will take place in one of its daughter- 

 cells (Fig. 25d). The process is repeated several times, and in 

 the end all cells contain diminished chromatin, except for a 

 single cell, Vv^hich still has two chromosomes of the original 

 shape. This cell contains cytoplasm that was originally located 

 at some distance from the vegetative pole of the egg, and 

 it is from this cell that the primary germ-cells will develop, 

 which supply the building material for the gonads of the new 

 individual. 



Boveri (1910) investigated and experimented on both normal 

 and abnormal eggs of Ascaris. He put forward the hypothesis 

 that the nature of the cytoplasm into which the nucleus finds 

 its way at cleavage determines whether or not diminution will 

 occur. The cytoplasm originally located at the vegetative pole 

 has different properties from that of the rest of the egg. For 

 this reason, chromatin diminution would not take place in this 

 cytoplasm. Although later observations by Von Ubisch (1943) 

 and Pasteels (1948) have led to a modification of this view, the 

 fact remains that evidently the development of the nuclei is 

 influenced by local differences in the egg cytoplasm. 



The dragon fly Platycnemis pennipes, investigated by Seidel 

 (1929-34), provides another clear-cut example of the interaction 

 of nucleus and cytoplasm. This insect has oblong eggs, the 

 hinder ends of which have special properties. Seidel named this 

 region the activating centre, because it is of decisive importance 

 for the further development of the egg. If at an early stage of 

 development this centre is killed with a hot needle, or separated 



