408 SENESCENCE AND REJUVENESCENCE 



note that it is relatively frequent in the lower plant s, the algae and 

 fungi. To what extent it may occur among the lower animals is 

 not fully known, though apparently it appears c hiefly as a charac- 

 teristic of certain groups without relation to their systematic posi- 

 tion. Finally, we cannot expect to find parthenogenesis ne cessarily 

 associated with a high degree of reconstitutional capa city in other 

 parts of the body, for the physiological condition of the primitive 

 germ cells from which eggs are formed, the rate of gr owth of the 

 egg, the character and amount of its nutrition, and doubtless many 

 other factors, must be concerned in determining whether it shall be 

 parthenogenic or zygogenic. 



From this point of view the parthenogenic egg is a cell which 

 has undergone more or less development as a gamete but still re- 

 tains the capacity to initiate dedifferentiation and re constitution 

 independently of union with a male gamete. In this respect it 

 resembles the less highly specialized cells of other tissues rather 

 than the gametes. 



Much evidence has accumulated to show that in the higher seed 

 plants reproduction of a new sporophyte generation very often 

 occurs in various other ways than by the fertilization of a zygogenic 

 egg. In some cases the reproductive cell is not the egg cell, but a 

 vegetative cell of the gametophyte and the reproductive process is 

 known as apogamy; in other cases the maturation divisions char- 

 acteristic of spore formation do not occur, i.e., there is apospory, 

 but a gametophyte containing a parthenogenic egg is formed; in 

 still other cases the reproductive cell is not even a part of the 

 gametophyte, but a cell of the nucellus which corresponds to the 

 sporangium. There can be little doubt that in such cases the 

 reproductive cell does not attain the specialized condition and 

 advanced age characteristic of the zygogenic egg. The final stages 

 of progressive development are omitted in one generation or the 

 other. 



THE EXPERIMENTAL INITIATION OF DEVELOPMENT 



Through the extensive investigations of Loeb, Delage, Bataillon, 

 and many others during the last twenty years it has been demon- 

 strated that the eggs of various species of animals which in nature 



