FERTILIZATION. 33 



Significance of Fertilization. 



The meaning of such a widely occurring phenomenon as fertilization has 

 been interpreted differently by different scientists, and the question is still 

 far from definite solution. There are several views which may be briefly 

 mentioned. 



The earlier belief that fertilization was a necessary antecedent to cleavage 

 of the ovum has been destroyed by the evidence of recent years. Loeb and 

 others have been able to induce artificial parthenogenesis in forms reproduc- 

 ing normally by sexual reproduction. Thus cleavage has been started by 

 chemical stimulation in the eggs of many molluscs, echinoderms, ccelenter- 

 ates, and even in some of the chordates (teleosts and amphibians). By 

 fertilizing pieces of egg cytoplasm containing no nuclear material, partheno- 

 genesis of the sperm has likewise been induced. While cleavage induced in 

 this manner progresses only a short way, the evidence points to the con- 

 clusion that fertilization is not an absolutely necessary factor in reproduction 

 although it normally occurs in the great majority of cases. 



Another view is that fertilization rejuvenates protoplasm. According to 

 this view protoplasm tends gradually to pass into a state of senility in which 

 its activity is diminished. With the admixture of new protoplasm when 

 fertilization occurs a new period of vigor is initiated. The life cycles of 

 certain Protozoa are brought to the support of this hypothesis. In these 

 Protozoa a long period of reproduction by a series of cell divisions is followed 

 by some form of conjugation in which two individuals come together and 

 exchange a part of their nuclear material. After conjugation protoplasmic 

 activity is renewed and each of the conjugants starts again on a long period 

 of reproduction. It is probable that the admixture of new protoplasm in 

 fertilization among Metazoa produces a similar invigorating effect. 



Another interpretation of fertilization is that this process, called amphi- 

 mixis in this connection, is important as a source of variation. Since the 

 chromatin of different individuals varies more or less, fertilization will pro- 

 duce new combinations and therefore tend to the production of new forms. 

 However, there is very little evidence that forms which reproduce sexually 

 show more variations than those reproducing by parthenogenesis. 



In the opinion of most modern investigators the union of the two germ 

 cells, one from each parent, may result in rejuvenation of the protoplasm, it 

 may be a stimulus to reproduction, a controlling factor in variation; but 

 probably no one of these things expresses the whole significance of fertiliza- 

 tion, nor can any one of them necessarily be ruled out. The chief interest 

 of the process at the present time is centered around its relation to the phe- 

 nomena of heredity and is intimately associated with the interpretation of the 



