404 SENESCENCE AND REJUVENESCENCE 



after union of the gametes, and in the cessation of development of 

 the "winter eggs" of flatworms, rotifers, Crustacea, and insects, 

 the presence of shells or envelopes of some sort is undoubtedly the 

 chief factor in retarding or inhibiting the metabolic activity. Even 

 in those animal eggs which, like some seeds, must before they will 

 hatch be subjected to certain external conditions, such as freez- 

 ing temperature or desiccation, or in the case of Branchipus, the 

 fairy shrimp, apparently to both, there is good reason to beHeve 

 that the effect of these conditions in altering or disintegrating the 

 egg envelope is much more important than any effect which they 

 may have upon the protoplasm itself. These, however, are cases 

 of the cessation of development rather than of its failure to begin. 



There are some cases where gametic union does not result in a 

 period of increased activity and where internal rather than external 

 factors seem to be responsible. Jennings ('13), for example, has 

 found that in Paramecium the effects of conjugation are by no 

 means uniform, for many of the ex-conjugants show decreased rather 

 than increased activity and some die, while others do exhibit an 

 increased rate of growth and division. It is probable that this lack 

 of uniformity in the results of gametic union is connected with the 

 fact that the body and the gamete are the same cell. Different 

 individuals become specialized in different directions and the 

 physiological effect of gametic union must vary widely, for in some 

 cases the two protoplasms are incompatible in some way, or a sum- 

 mation of their physiological defects occurs, while in others the 

 result is the opposite. In the multicellular organisms, however, 

 where the gametes develop as speciahzed parts of the body more or 

 less remote from the influence of factors external to the organism, 

 their course of development and consequently the effects of their 

 union are much more definite and uniform, but even here the results 

 of gametic union may vary to some extent, although increased 

 dynamic activity following union is the usual result. 



There are in fact very few exceptions to the rule that gametic 

 union is followed by increased dynamic activity, and it is probable 

 that most, if not all, of these exceptions will prove to be apparent 

 rather then real. We may say then with Loeb that fertilization 

 in some way saves the Ufe of the gametes. If the gametes are highly 



