130 PROBLEMS OF FERTILIZATION 



reactions, so that the entrance of more than one spermat- 

 ozoon does not usually occur. With reference to rever- 

 sibility the ovum differs from most cells. A nerve cell or 

 a gland cell or a muscle cell after functioning returns 

 to the functional condition again; but, so far as our 

 evidence goes, this is not true of the egg cell or of the 

 spermatozoon; their functioning is essentially a pro- 

 gressive process, the cycle of which occupies an entire 

 generation. From this point of view the lack of 

 reversibility is readily understood. 



Fertilization is a specific reaction in the sense that 

 it occurs usually only between the gametes of the same 

 species. Hybridization is possible within certain indef- 

 inite limits, which do not follow any invariable taxo- 

 nomic rule, though as a general thing it is regarded as 

 increasingly improbable with remoteness of taxonomic 

 relationship. In the echinids and teleosts, however, 

 hybrid fertilization is possible even between different 

 families or suborders. Resistance to hybridization may 

 also be broken down by certain experimental proce- 

 dures considered later. On the other hand we have 

 the special phenomenon of self-infertility in certain 

 hermaphrodites (see chap. vi). 



Certain eggs are normally parthenogenetic, and in 

 others artificial parthenogenesis may be produced by 

 various experimental procedures. It is obvious that 

 in such cases the activating effects of fertilization are 

 produced, and that the analysis of parthenogenesis must 

 have very direct bearings on this problem of fertili- 

 zation. We shall therefore utilize the results of such 

 studies to a certain extent, but only as an aid in the 

 analysis of fertilization (see chap. vii). 



