CHAPTER IV 



FERTILIZATION OF THE OVUM 



" It is conceivable, and indeed probable, that every part of the adult contains molecules 

 derived both from the male and from the female parent; and that, regarded as a mass of 

 molecules, the entire organism may be compared to a web of which the warp is derived from 

 the female and the woof from the male." IIuXLEY.i 



Ix mitototic cell-division we have become acquainted with the means 

 by which, in all higher forms at least, not only the continuity of life, 

 but also the maintenance of the species, is effected ; for through this 

 beautiful mechanism the cell hands on to its descendants an exact dupli- 

 cate of the idioplasm by which its own organization is determined. 

 As far as we can see from an a priori point of view, there is no reason 

 why, barring accident, cell-division should not follow cell-division in 

 endless succession in the stream of life. It is possible, indeed prob- 

 able, that such may be the fact in some of the lower and simpler forms 

 of life where no form of sexual reproduction is known to occur. In 

 the vast majority of living forms, however, the series of cell-divisions 

 tends to run in cycles in each of which the energy of division finally 

 comes to an end and is only restored by an adviixtnrc of living mat- 

 ter derived from another cell. This operation, known as fertiliza- 

 tion or fecundation, is the essence of sexual reproduction ; and in it we 

 behold a process by which on the one hand the energy of division is 

 restored, and by which on the other hand two independent lines of 

 descent are blended into one. Why this dual process should take 

 place we are as yet unable to say, nor do we know which of its two 

 elements is to be regarded as the primary and essential one. 



Harvey and many other of the early embryologists regarded fer- 

 tilization as a stimulus, given by the spermatozoon, through which the 

 ovum was " animated " and thus rendered capable of development. 

 In its modern form this conception appears in the " dynamic " theories 

 of Herbert Spencer, Btitschli, Hertwig, and others, which assume that 

 protoplasm tends gradually to pass into a state of increasingly sta- 

 ble equilibrium in which its activity diminishes, and that fertilization 

 restores it to a labile state, and hence to one of activity, through mix- 

 ture with protoplasm that has been subjected to different conditions. 

 Biitschli ('76) pointed out that the life-cycle of the metazoon is com- 



^ Evolution, in Science and Culture, p. 296, from Enc. Brit., 1878. 



178 



