202 COMPARATIVE ANATOMY 



any other example, that the essential thing in conjugation is 

 the mingling of nuclear material derived from two different 

 individuals. It is possible that more than this is necessary ; 

 that the nuclear material must be derived from two different 

 stocks. Maupas has shown that, in one infusorian, individuals 

 derived from the repeated division of a single parent will not 

 conjugate with one another but only with the progeny of 

 another parent. Nothing is certainly known on this point 

 in Paramecium, but if it should prove to be a general rule 

 it makes the correspondence between the conjugation of the 

 Protozoa and the process of fertilisation in the Metazoa very 

 striking. For we know that even in those members of the 

 latter group which are hermaphrodite there are numerous 

 devices for avoiding self-fertilisation, and fertilisation, as has 

 been seen in the case of the frog, is essentially the same thing 

 as conjugation, the mingling of nuclear material of two different 

 cells. 



In a Protozoon like Paramecium or Polytoma there is a 

 distinct life cycle, which comprises the alternation of a sexual 

 or conjugate generation with a series of asexual generations 

 reproducing their like by simple binary fission. The products 

 of fission of a zygote separate from one another, lead free 

 individual existences, and form a stock or family which has a 

 limited term of existence, for its members degenerate and die 

 if they do not conjugate at the proper time. In a Metazoon 

 the product of fertilisation is the fertilised ovum or oosperm, 

 comparable to the zygote of a Protozoon. The oosperm re- 

 produces itself, just as Protozoa reproduce themselves, by 

 binary division, but its progeny, instead of separating, remain 

 adherent, forming an integral whole or individual. Countless 

 numbers of cells are formed by repeated binary division, and 

 the whole, of which they form a part, grows into what we call 

 the adult organism. This we may compare with the sum of 

 the numerous individuals forming a stock or family of Protozoa. 

 Even as the Protozoon family has a limit of existence, degenera- 

 ting and eventually dying if that limit is overpassed, so the 

 Metazoon body, composed of the innumerable progeny of the 

 oosperm, has its well-marked phases of growth, maturity, decay, 

 and death. In both cases the extinction of the race is provided 

 against by the act of conjugation or fertilisation, by means of 

 which the forces of life to use a vague expression are as it 



