202 GENERAL EMBRYOLOGY 



guished from microgamete-f orming or male individuals, some time before 

 the gametes are actually formed ; in a few rare instances this distinction 

 can be made throughout the life cycle, and individuals can be identified 

 at any time as males or females. 



We now come to a consideration of the meaning and theo- 

 retical significance of these processes of fertilization or syngamy. 

 Probably there is, in the whole field of Biology, no process of 

 such widespread occurrence and obvious importance, where 

 the phenomena are so well known, which at the same time is so 

 little understood. Why fertilization should occur, what is 

 effected by it, and how syngamy brings about the results which 

 do follow it, are questions to which to-day, after decades of 

 speculation and research, no sure answers can be given. 

 v Although we may be on uncertain ground, it will be profitable 

 to review some of the suggestions and hypotheses that have 

 been proposed in this connection, even if we accomplish little 

 more than to point out the possibilities and difficulties of this 

 fascinating subject. And furthermore, while no thoroughly 

 demonstrated solutions of the problems of fertilization have 

 been reached, there are several carefully worked out hypotheses 

 in the field, some of which are certainly to be regarded as close 

 approximations toward the correct explanation of some of the 

 problems of fertilization. We may conveniently arrange these 

 current ideas as to the results and primary " purpose" of fertili- 

 zation in four groups. The results of fertilization may be 

 connected with (a) reproduction, (b) rejuvenation, (c) the 

 process of variation, (d) the process of heredity. In considering 

 each of these we shall state as briefly as possible the essentials of 

 the evidence for and against the central idea. 



That fertilization is primarily a reproductive process was the 

 original view held by Harvey and his successors. There are 

 now two forms of the hypothesis. In one form we find the 

 idea that the ovum quite obviously seems to contain only a 

 part (i.e., the cytoplasm and one-half a nucleus) of the mechan- 

 ism necessary for development, and that the spermatozoon 

 brings into the ovum those parts (i.e., one-half a nucleus and 

 the centrosome, or the stimulus to its formation) which com- 



