40 TEXT-BOOK OF EMBRYOLOGY. 



when the activities of some eggs are so interfered with, by subjecting them to 

 unfavorable chemical or thermal conditions, they are unable to construct a 

 vitelline membrane; such eggs can no longer prevent the entrance of several 

 spermatozoa. 



Significance of Fertilization.* 



Mitotic cell division constitutes the wonderful mechanism by which not only the con- 

 tinuity of life but also the maintainance of the species is accomplished. "From an a priori 

 point of view there is no reason why, barring accidents, cell division should not follow cell 

 division in endless succession." And it is probable that such is the case among the lower 

 forms of animal and vegetable life, where no true sexual reproduction occurs. A one-celled 

 animal or plant may divide and produce two individuals; each of these two may produce 

 two more, and so on ad infinitum. "In the vast majority of forms, however, the series of 

 cell divisions tend to run in cycles in which the energy of division comes to an end and is 

 restored only by an admixture of living matter from another cell." 



This admixture of the living matter of two cells is known as fertilization and is the 

 essential feature of sexual reproduction. It is the process which on the one hand restores 

 to the cell the energy necessary to continue its division and on the other hand accomplishes 

 the blending of two independent lines of descent. 



Certain views regarding the significance of fertilization may be grouped together as the 

 rejuvenescence theory. The earlier embryologists regarded fertilization as "a stimulus given 

 by the spermatozoon, by which the ovum is 'animated' and made capable of development." 

 The more modern "dynamic" theorists express practically the same conception. They hold 

 that protoplasm tends to pass gradually into a state of equilibrium in which activity dimin- 

 ishes, and that fertilization restores to it a state of activity through the admixture of protoplasm 

 which has been subjected to different conditions. 



Certain known facts tend in a general way to support these theories. For example, 

 among the Protozoa or one-celled animals, a long series of cell divisions is followed by con- 

 jugation. In conjugation two individuals come together and fuse permanently, or inter- 

 change substances and separate again. This process results in the restoration of the energy 

 of growth and division and a new life-cycle is begun. Among the higher animals and 

 plants fertilization is always necessary for the initiation of a new life-cycle with its subsequent 

 cell division and growth. In some lower forms, however, parthenogenesis occurs and a new 

 life-cycle is initiated without the union of cells from two sexually different individuals. It 

 must therefore be admitted that whether or not the tendency toward senescence and the need 

 of fertilization are primary attributes of living matter is unknown. 



Parallel with the rejuvenescence theory are other views which do not necessarily oppose 

 or confirm it. One view in particular is that fertilization is in some way concerned with 

 variations in the individuals of a species. Brooks and Weismann developed the hypothesis 

 that fertilization, the admixture of germ plasms from two individuals, is a source of varia- 

 tion "a conclusion suggested by the experience of practical breeders of plants and animals." 

 Weismann himself holds that the need of fertilization is a secondary matter, but that the 

 admixture of different germ plasms insures the mingling and renewal of variations. 



Spencer and Darwin, on the other hand, believe that although crossing among animals or 

 plants may lead to variability within certain limits, it tends in the long run to hold in check 

 any wide digression from a norm and hold the species true to type. 



*The quotations in the following paragraphs are from "The Cell in Development and 

 Inheritance," by E. B. Wilson. 



