SYNGAMY AND SEX IN THE PROTOZOA 161 



minute bodies reproduction by fission does not produce differentia- 

 tion in the fission-products. With increased size such differences 

 may arise, at first to a minor extent, and capable of being adjusted 

 by internal rearrangements of the living substance such as have 

 been described in the larger Bacteria. Not until the process of 

 natural evolution had gone so far as to produce the full complica- 

 tion of structure seen in a true cell would localized differences in 

 the living substance be brought about to a sufficient extent to 

 lead to differences between the daughter-cells produced by fission, 

 as a consequence of the imperfections of the process of cell-division. 

 The differences produced in this way might be changes in the 

 nucleo-cytoplasmic balance, as Hertwig supposes, or in the relative 

 proportions of substances exerting different physiological activities, 

 as suggested by Biitschli, Geddes and Thomson, Schaudinn and 

 Doflein, or possibly of all these and other changes yet unknown. 

 In any case it is reasonable to suppose that the imperfect character 

 of the primitive types of cell-division, described in the last chapter, 

 might produce accumulated material or structural inequalities in 

 the daughter-cells, such as could only be rectified by the union of 

 two cells differentiated in opposite directions, thus making syngamy 

 a necessity for the continued existence of the species. This theory 

 explains the necessity for syngamy recurring with greater frequency 

 in forms having a high degree of structural differentiation than in 

 forms of a primitive and simple type of organization. 



With increasing perfection in the process of the division of the 

 cell, and especially of the nucleus, the primary cause of, or necessity 

 for, syngamy might be expected to disappear ; but at this stage in 

 evolution other benefits to the species consequent on the process 

 of amphimixis might be a sufficient cause for the retention of 

 a process already well established. This conclusion appears to 

 receive some support from the fact that intensive culture, whether 

 artificial, or natural as in parasitism, seems to diminish the necessity 

 for syngamy. It can hardly be supposed that intensive culture 

 can diminish consequences arising from defective cell-division ; but 

 it might conceivably produce a strengthening effect equal to, and 

 capable of supplanting, the benefits derived from amphimixis. 

 Enriques (113) has stated that in Infusoria ex-conjugants may 

 proceed to conjugation again, so that between one act of syngamy 

 and the next there may not be a single cell-division intervening. 

 In this case neither cell-division nor any consequences of cell- 

 division can be the factor bringing about sexual union, but some other 

 explanation must be sought. Enriques considers that the function 

 of syngamy in Infusoria is to maintain the fixity of the species. 



Bibliography. For references see p. 479. 



