FERTILIZATION IN THE LIFE-HISTORY 35 



of senescence and rejuvenescence, we gain a physiological 

 conception of the sense in which the mature gametes 

 may be termed old or senescent, and of the sense in which 

 fertilization may be said to induce a process of rejuvenes- 

 cence. 



The gametes are old in the sense of possessing 

 a high degree of differentiation. The egg cell is more or 

 less loaded with a highly inert material, the yolk, and the 

 spermatozoon is highly differentiated in a different sense. 

 With the activation of the ovum by fertilization there 

 begins a process of resorption and utilization of yolk, 

 and the cleavage cells gradually consume, and thus 

 free themselves from, this encumbrance. The differen- 

 tiated parts of the spermatozoon are lost, and the nucleus 

 enters into the nucleoplasmic activities of the zygote. 

 It is improbable that this is the whole story of dedif- 

 ferentiation, and Child lays strong emphasis in this 

 definition and elsewhere on changes in the colloidal 

 system, but the nature of such changes in relation 

 to the process of rejuvenescence remains unknown. 



The inner significance of sex and fertilization in the 

 life-history cannot, however, be confined within these 

 boundaries. Sex seems to be very nearly a universal 

 attribute of living beings, either actual or potential, 

 even in organisms that do not utilize it invariably for 

 purposes of reproduction; and it hardly suffices to say 

 that in such cases the condition is an inherited one that 

 has lost partially or wholly its function in the life- 

 history. Sexual differentiation seems to represent an 

 inevitable dimorphism of living matter for which we 

 possess no satisfactory analogy, and fertilization appears 

 to be a consequence of such dimorphism. The gametes 



