FERTILIZATION 165 



buds, etc. This is particularly true of the Begonias, where 

 even a few cells, from almost any part of the growing plant, 

 may be removed and, under proper conditions, be made to 

 form a new complete organism capable of producing typical 

 germ cells. 



The questions why fertilization should be necessary, and how 

 fertilization actually accomplishes the results which obviously 

 follow it, are not easy to answer. But the essential facts 

 regarding the process of syngamic fusion, and the visible 

 results of it, are clear. We shall, therefore, confine our atten- 

 tion first to these; then, having described the phenomena of 

 fertilization we may consider briefly some of the more theoret- 

 ical aspects of the process. 



The word "fertilization" is a general, inclusive term, used to 

 denote all of the various phenomena concerned in the meeting 

 and fusion of the germ cells (gametes) or germ plasms, and even 

 some of the results of such a fusion. The simpler fact of the 

 mere fusion of gametes is more precisely termed syngamy. In 

 all of the Metazoa syngamy may be defined as the meeting of 

 two completely specialized, unicellular gametes, an ovum and 

 a spermatozoon, derived in most cases from two individuals, 

 and their subsequent fusion, nucleus with nucleus, and cyto- 

 plasm with cytoplasm, into a single, uninucleate cell, the zygote. 

 This definition is not completely applicable to the unicellular 

 organisms, for in these the gametes are usually not completely 

 specialized, sometimes indeed not especially differentiated at all. 

 Such forms may offer some suggestions as to the history and 

 significance of the fertilization process, and we shall return to 

 consider this subject later in this chapter. 



We have seen in Chapter III some of the methods by which 

 it is ensured that eggs and sperm shall be brought into the same 

 general region or into fairly close proximity, but it remains to 

 be seen how the ovum is actually encountered by the sperm 

 cell. Taken altogether, the processes leading to this result often 

 become very complicated and special, and in most species the 

 probability is very high that practically every normal egg 

 produced will be fertilized. The gametes are completely 



