220 THE PHYSIOLOGY OF REPRODUCTION 



fertilisation. There is some evidence, therefore, that a reduction 

 of vigour among the gametes, whether occurring naturally as a 

 consequence of in-breeding or produced artificially as in Doncaster's 

 experiments, may lead to a similar result, since both conditions may 

 bring about an increased tendency towards the union of dissimilar 

 gametes. On another view, the tendency towards cross-fertilisation 

 in Doncaster's experiments may be looked upon as evidence of a 

 diminished power of resistance, on the part of the ova, to the 

 entrance of foreign spermatozoa, consequent upon a reduced vitality 

 in the ova. 1 



Further evidence upon this question is afforded by studying the 

 Protozoa (see also pp. 639-642). 



CONJUGATION ix THE PROTOZOA 



The phenomenon of conjugation in the Protozoa possesses a 

 special interest, inasmuch as it is undoubtedly the forerunner of 

 fertilisation in the Metazoa. It is clear, therefore, that a complete 

 understanding of the changes which attend the former process 

 cannot fail to throw great light on the nature and significance of 

 gametic union in multicellular organisms. 



In the different groups of Protozoa all gradations are to be found 

 between the conjugation in the general sense (i.e. the union, either 

 temporary or permanent, of two similar unicellular organisms), and 

 a process identical with metazoan fertilisation. Thus, in the peri- 

 trichous Ciliata there is a pronounced sex differentiation in the size 

 and activity of the gametes, which clearly correspond to ova and 

 spermatozoa. Even the maturation phenomena, which play so 

 important a part in the developmental history of the metazoan 

 gametes, are represented in some sort by comparable processes which 

 have been observed in certain Protozoa. 2 There can be no doubt, 

 therefore, as to the essential similarity of conjugation in unicellular 

 organisms and fertilisation in multicellular ones. 



Raymond Pearl, 3 as a result of a biometrical study of the process 

 of conjugation in Pdram&cinm caudatum, has arrived at the conclusion 

 that in this protozoon there is a definite tendency for like individuals 



1 It may be mentioned also that Loeb has shown that, whereas mature 

 starfish eggs soon die if fertilisation is prevented, eggs in which maturation 

 is artificially hindered through lack of oxygen or the addition of an acid to 

 the sea- water, remain alive much longer than when allowed to become mature. 

 (Loeb, "Maturation, Natural Death, and the Prolongation of the Life of 

 Unfertilised Starfish Eggs, etc.," Biol. Buff., vol. iii., 1902.) It would appear, 

 therefore, that the mature eggs have suffered a loss of vitality which ordinarily 

 can only be increased by the act of fertilisation. 



2 See Enriques, loc. cit. See footnote, p. 198. 



3 Pearl, "A Biometrical Study of Conjugation in Paramoecium" Biometreika, 

 vol. v., 1907. 



