FERTILISATION 211 



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, conse- 

 quent upon a reduced vitality in the ova. 1 



Further evidence upon this question is afforded by studying 

 the Protozoa. (See also pp. 601-604.) 



CONJUGATION IN 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 signi- 

 ficance of gametic union in multicellular organisms. 



In the different groups of Protozoa all gradations are to be 

 found between 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 peritrichous Ciliata there is a pronounced sex 

 differentiation in the size and activity of the gametes, which 

 clearly correspond to ova and spermatozoa. Even the matura- 

 tion phenomena, which play so important a part in the develop- 

 mental 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 Paramceiium caudatum, has arrived 



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," &c., Biol. Bull., vol. iii., 11)02) 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. 



* See Enriques, loc. cit. 



3 Pearl, " A Biometrical Study of Conjugation in Paramcecium," Blotne- 

 treka, vol. v., 1907. 



