THE REALISATION OF THE NUCLEAR FACTORS 71 



sets the development of the embryo going. The diffusion of this 

 substance is not impeded by an incomplete constriction of the 

 egg. Evidently, the production of this essential activating sub- 

 stance is due to an interaction between the cytoplasm of the 

 activating centre and a cleavage nucleus. All cleavage nuclei 

 are equivalent also in this respect. Seidel demonstrated this 

 as follows. He killed the hindmost nucleus, a descendant of 

 which would in the normal course of events move into the 

 activating centre, by irradiation with a narrow beam of ultra- 

 violet light. The place of this nucleus was then taken by one 

 of the other cleavage nuclei, and development proceeded in the 

 normal way. 



These experiments give us a clue as to the way in which the 

 nuclear factors influence development. During cleavage, the 

 nuclei have become located in areas of different physico- 

 chemical properties. Once they begin to interact with their 

 environment, in certain parts of the egg chemical reactions 

 will occur between the nuclear factors and the cytoplasm of 

 those areas. Nuclear factors, which have so far remained in- 

 active, can begin to unfold their activity once they have found 

 a suitable substrate on which to act, or with which to react. 

 It may be said, therefore, that they are activated by the sur- 

 rounding cytoplasm. In other parts of the egg, where the cyto- 

 plasm is of a different composition, the reaction in question 

 will not occur, but other nuclear factors may be activated there. 



We have not yet entered into the question of the nature of 

 these nuclear factors. It is natural in this connection to think 

 of the genes, which we may regard as localized in the chromo- 

 somes. An experiment by Boveri (1907) demonstrates that 

 the chromosomes do indeed play an important role in devel- 

 opment. If sea urchin eggs are fertilised with concentrated 

 semen, into many eggs two sperms will penetrate simultaneously 

 (so-called double fertilisation). As a rule both sperm nuclei 

 fuse with the egg nucleus in such eggs, forming a "triploid" 

 zygote nucleus, containing three sets of chromosomes, instead 

 of two as in normally fertilised, "diploid", eggs. At the first 

 cleavage the majority of such abnormal eggs divides at once 

 into three or four daughter-cells simultaneously. This, according 



