FERTILIZATION 189 



is not supposed to represent a phyletic relation. It is now too late to 

 state with any considerable degree of probability the course of evolution 

 of the germ cells and the process of syngamy. Apart from this, how- 

 ever, the consideration of such a series as this brings out many important 

 and interesting facts regarding the general process of fertilization, and 

 emphasizes the idea that this complicated process as we see it in the 

 higher organisms to-day, is all the product of an evolutionary history. 



As a preliminary distinction of a general and underlying character we 

 should note that among the unicellular forms the cells which meet or 

 fuse may be of the same race or family, that is, closely related by descent 

 from a comparatively recent common ancestor; or they may be only 

 distantly related, so distantly as to be regarded as unrelated, coming 

 from races or families that have long been distinct, and that have had 

 different histories. The former condition is termed endogamy, the latter 

 exogamy. These two relations may be distinguished in all forms of 

 syngamy or conjugation; exogamy is much the more usual, and 

 involves the more complicated reproductive processes among the 

 Protozoa, but no such relation seems really necessary, for conjugation 

 may occur with equal facility between any two different individual 

 protoplasms, whether closely related or not. 



In a general way we may arrange the varied phenomena of conjuga- 

 tion and syngamy in three classes with reference to the nature and 

 extent of the fusion which occurs. In its simplest form this fusion is 

 not morphological, but is expressed by the congregation of cells in 

 groups; this is to be regarded as a form of cytotropy. Occasionally 

 large collections of cells result and the elements come into close and 

 extensive contact without really fusing or losing cell limits. Whatever 

 exchange of substance there may be occurs through an osmotic process. 

 After a temporary association of this kind the cells scatter and resume 

 vegetative and reproductive processes. Such a process of cytotropy 

 has been observed in Amoeba (Rhumbler) . 



The simplest form in which a real fusion of plasmas occurs is that 

 known as plastogamy. Here two or sometimes more (2-30 in Actino- 

 phrys) vegetative cells meet and flow together so that the cytoplasms 

 mingle completely; the cell nuclei remain separate, though osmotically 

 they may affect one another and the fused cytoplasms. The result of 

 this is the formation of a physiologically bi- or multinucleate cell. 

 Plastogamy may be only temporary; in such a case the cells come into 

 relation only through comparatively limited contact surfaces and the 

 original cell outlines are not lost. Then after a brief period, during 

 which chemical interchanges may occur, the cells separate again. 

 More frequently, however, plastogamy is permanent, and the fusion of 

 the cells is so complete that the original cell outlines are completely lost. 

 Then, following plastogamy, the nuclei of the combined cells usually 



