I GROWTH OF THE GERM-CELLS 33 



impinge upon the nuclear membranes, while in other direc- 

 tions similar outgrowths constitute the asters. The spindle 

 quickly elongates while its fibres break through the nuclear 

 membranes and pass continuously from pole to pole. The 

 chromosomes are formed independently in each nucleus, 

 and the two sets, paternal and maternal, each to the 

 number of n, lie side by side in the equator. There they 

 are divided lengthways in the usual fashion and their 

 halves pass to the spindle-poles. The nucleus of each of 

 the first two blastomeres and eventually of each cell in 

 the body thus receives a full set of chromosomes from each 

 parent. 



It is not unnatural that the union of the two pronuclei 

 should have led the discoverers of the fact to regard it as 

 being the essence of fertilization. The opinion was further 

 supported by the phenomena of conjugation in certain 

 Infusoria, where apparently, there is merely an exchange of 

 micronuclear material between the gametes, and by the 

 similarity of the two germ-cells in respect of their nuclei, 

 the latter being conceived as the sole vehicles for the trans- 

 mission of the inheritable characters of the species, a task 

 believed to be performed equally by the two sexes. 



Now whatever views we may come to hold as to the role of 

 the nucleus in inheritance, it is assuredly not true that both 

 nuclei are necessary for the production of a normal individual. 

 For in the first place in parthenogenesis, natural and artificial, 

 only the female nucleus is present, and yet a normal embryo 

 or larva is developed and reared. 



In the second place in what is called merogony , the enucleate 

 fragment of an ovum may be fertilized and give rise to a 

 normal larva. 



We are obliged therefore to look for the meaning of fertiliza- 

 tion elsewhere, and we find it in the restoration of the lost 

 power of nuclear and cell-division. The ripe germ-cells are 

 cells which have come to the end of their power of reproduc- 

 tion by division: in the act of fertilization that power is 

 restored. It is mutually restored, for in ordinary fertiliza- 

 tion we see the egg- cell with both sets of chromosomes divide 



1963 D 



