CELL-NUCLEUSFERTILISATION 231 



the chromosomes are constant in qualities 

 and properties from one cell generation to 

 another. (3) Owing to the mode of division, 

 and distribution of the chromosomes at a 

 nuclear division, the two daughter nuclei are, 

 to all appearance, exactly alike, each is the 

 reflected image of the other. Subsequent 

 dissimilarities in size, and likewise in other 

 respects, are not excluded, but these are 

 almost certainly of secondary importance. 



What does all this mean ? We cannot as 

 yet give a complete answer to the question, 

 but a consideration of the events connected 

 with the differentiation of the sexual cells 

 will perhaps serve to throw some light on the 

 problems involved. 



In the first place, we have seen that the 

 sexual act consists essentially in the fusion 

 of two nuclei. How, then, can we reconcile 

 this with the circumstance that the number 

 of chromosomes is constant in the cell nuclei ? 

 For it is evident that the nucleus of each 

 fertilised egg must contain twice as many 

 chromosomes as those present in the nucleus 

 of each of the fusing gametes. 



The solution of this problem is furnished 

 by a most remarkable nuclear division which 

 is invariably intercalated somewhere in the 

 series of nuclear divisions that intervene 

 between the first formation of the embryo 

 at fertilisation and the final production of 

 sexual cells which closes the life cycle of the 

 organism (Fig. 26, l-6i). 



