38 TEXT-BOOK OF EMBRYOLOGY. 



duct is proved by cases of tubal, abdominal and, rarely, ovarian pregnancies. 

 On the other hand Wyder considers the uterus as the normal site of fertiliza- 

 tion, and some other gynecologists say that fertilization may take place in the 

 uterus. Waldeyer also concludes that fertilization may occur in the uterus. 



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. Its chief importance must be considered probably from 

 a standpoint of inheritance and is intimately associated with the interpretation 

 of the maturation processes of the germ cells (p. 25). There are, however, 

 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 reproducing 

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

 stimulation in the eggs of many Molluscs, Echinoderms, Coelenterates, and 

 even in some of the lower Chordates (Teleosts and Amphibians). By fertilizing 

 pieces of egg-cytoplasm containing no nuclear material, parthenogenesis of the 

 sperm has likewise been produced. While cleavage produced in this manner 

 progresses only a short way, the evidence points to the conclusion that fertiliza- 

 tion is not an absolutely necessary factor in reproduction, although it normally 

 occurs in the great majority of cases. 



Another view, advocated by Richard Hertwig and others, is that fertilization 

 induces a rejuvenescence of protoplasm. According to this view protoplasm 

 gradually passes into a state of senescence in which its activity is diminished. 

 With the admixture of new protoplasm during fertilization a new period of 

 vigorous activity is initiated. The life cycles of certain Protozoa are brought 

 to the support of this hypothesis. In these Protozoa a long period of reproduc- 

 tion by a series of cell divisions is followed by some form of conjugation. Two 

 individuals come together and an exchange of nuclear material takes place. 

 As a result a new impetus is given to the protoplasmic activity, and each of the 

 conjugants starts again on a long period of reproduction. It is highly probable 

 that the admixture of new protoplasm in fertilization among Metazoa produces 

 a similar invigorating effect. 



Another interpretation of fertilization is that of Weissman who believed 

 that fertilization or "amphimixis" is important as a source of variation. Since 

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

 duce new combinations and 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. 



