438 HEREDITY AND DEVELOPMENT 



Minute inquiries have gone so far that it is possible to assert 

 that in some cases the young germ-cell has an equal number 

 of paternal and maternal chromosomes. And similar minute 

 inquiries — which almost baffle us with their intricacy — make 

 it exceedingly probable that in the reduction divisions maternal 

 chromosomes separate from paternal chromosomes, and yet not 

 so thoroughly that all the paternal chromosomes pass into one 

 cell and all the maternal into another. If this be true, we can 

 better appreciate the importance of the reduction-divisions which 

 occur in maturation, for they afford opportunity for new per- 

 mutations and combinations of hereditary qualities. They do 

 not originate anything new, but they shuffle the cards, so to speak. 



Fertilisation. — Recent work has forcibly suggested that there 

 are in fertilisation two more or less distinct processes : on the 

 one hand, the process by which the gametes, bearing the 

 hereditary characters, unite to form the beginning of a new in- 

 dividuality ; on the other hand, the process by which the sperma- 

 tozoon supplies some stimulus, prompting the ovum to divide. 

 The first aspect is that of amphimixis, believed by many to be 

 of importance in initiating — and, it may be, also in checking — 

 variations, but in any case effecting the union of hereditary 

 qualities contained in the two gametes. The second aspect 

 is that of mitotic stimulus, believed by some to be afforded by 

 an enzyme — for which the name of " ovulase " has been suggested 

 — and by others to be localised in the sperm-centrosome. It is 

 seen in many cases that equivalent numbers of chromosomes 

 are contributed by the two nuclei ; it is evident that the ovum 

 contributes by far the larger quantity of cytoplasm ; it seems 

 to have been securely demonstrated in some cases that " from 

 the father comes the centrosome to organise the machinery of 

 mitotic division by which the egg splits up into the elements 

 of the tissues, and by which each of these elements receives its 

 quota of the common heritage of chromatin." " Huxley hit 

 the mark two-score years ago when he compared the organism 



