4IO SENESCENCE AND REJUVENESCENCE 



cases and by certain methods, and no criticism can detract from 

 the importance and interest of this fact. 



The questions which have been most widely discussed in con- 

 nection with this field of investigation, viz., the nature of the 

 changes produced in the egg and the manner in which the experi- 

 mental conditions act to produce them, are outside the range of 

 the present discussion. The point to which I desire particularly 

 to call attention is the difference in the reaction of the eggs of 

 different animals to the experimental conditions. Some eggs react 

 readily to a variety of experimental conditions and give loo per 

 cent, or nearly, of normal embryos or larvae, while others, even in 

 the most favorable cases, give only a small percentage of normal 

 forms, or react only to certain experimental conditions, and still 

 others are refractory to all methods and have never been known 

 to develop except when fertiHzed. In the egg of the starfish, for 

 example, which is on the verge of natural parthenogenesis, develop- 

 ment can apparently be initiated by ahnost any sHght stimulus, 

 while the egg of the sea-urchin is somewhat less susceptible to the 

 various agents and conditions employed to initiate development, 

 and many other eggs are only sHghtly or not at all susceptible. 

 Our knowledge along this Hne is as yet somewhat fragmentary, for, 

 although changes of some kind and degree have been experimentally 

 induced in the eggs of many different species of invertebrates and a 

 few vertebrates, no systematic comparative study along these hnes 

 has yet been attempted. But that great differences in the capacity 

 to begin development without fertilization exist in different "eggs 

 is a demonstrated fact, and the probabiHty that these differences 

 are associated with the different degrees of speciahzation and differ- 

 entiation of eggs at once suggests itself. If the eggs of different 

 species represent various degrees of specialization, all gradations 

 from natural parthenogenesis through the various degrees of sus- 

 ceptibiHty to experimental parthenogenic agents to the strictly 

 zygogenic condition, in which the egg reacts only to the entrance 

 of the sperm, must be expected to occur. Apparently some eggs 

 can be aroused from their quiescent condition and started along the 

 course of development in a great variety of ways, some of which may 

 differ widely from the process of fertilization, while others can be 



