130 THE PROTOZOA 



case of those species in which syngamy has not been observed, 

 there are three abstract possibilities : first, that it does occur, 

 but has not yet been seen ; secondly, that it is secondarily in 

 abeyance ; thirdly, that it is primarily absent that is to say, that 

 it has never occurred either in the form in question or in its ancestral 

 lineage. On the whole, the first of these three possibilities is the 

 most probable, though the second must, perhaps, also be taken 

 into account, as will be shown later. 



So far as a generalization is possible or permissible in the present 

 state of knowledge, it appears that sex and syngamy are phenomena 

 of universal occurrence in all truly cellular organisms, but we have 

 no certain knowledge that they exist in any organisms of the 

 bacterial type of organization. With the passage from the bacterial 

 to the cellular type of structure, syngamy became, apparently, a 

 physiological necessity for the organism, and was probably acquired 

 once and for all. 



3. The Significance of Syngamy in the Life-Cycle. In order 

 to appreciate the part that syngamy plays in the life-histories of 

 organisms generally, it is necessary to compare briefly and in 

 general outline the life-cycles of Metazoa and Protozoa in typical 

 cases. 



In the Metazoa the cycle starts from a single cell, the zygote 

 or fertilized ovum, which multiplies by cell-division in the ordinary 

 way. Thus is produced a multicellular individual, composed 

 always of at least two classes of cells tissue-cells (histocytes) and 

 germ-cells. The histocytes are differentiated in various ways, 

 related to various functions, to form tissues, and so build up the 

 soma. The germ-cells are not differentiated for any functions but 

 those of sex and reproduction, and occur primarily as a mass of 

 undifferentiated cells constituting the germen ; they are lodged 

 in the soma and dependent upon it parasitic upon it, so to speak 

 but in a sense distinct from it ; they draw their sustenance from 

 the soma, influence greatly its development and activities, but 

 contribute nothing to the work of the cell-commonwealth. Of 

 these two portions of the Metazoan individual, the soma is neces- 

 sarily mortal, doomed inevitably to ultimate senility and decay. 

 The cells of the germen, on the other hand, are potentially im- 

 mortal, since under favourable conditions they can separate 

 from the soma and give rise in their turn to a new individual of 

 the species with soma and germen complete again. This type of 

 generation is always found in every species, though ion-sexual 

 methods of generation may also occur in many cases. 



In the life-cycle of the Metazoa, as sketched above in its most 

 generalized form, two individualities must be clearly distinguished, 

 the one represented by the soma together with the germen, con- 



