Biology 687 



then their nuclei, so that one individual is formed from two. But 

 in every case conjugation involves the fusing of nuclei from two 

 individuals. 



Conjugation is the simplest form in which we find the sexual 

 process. Two facts merit remark. First, we see that sexual 

 fusion need not involve difference of sex; the two gametes, as 

 the cells are called which unite during the process, may be alike. 

 Secondly, we see that sex is primitively not associated with 

 reproduction. 



In multicellular animals, however, the gametes are always 

 of two different sorts, the male gametes or spermatozoa, and the 

 female gametes or ova. The former are almost always very 

 small, intensely active, and consist almost entirely of a head which 

 contains the condensed nucleus, and a tail, by whose movements 

 they swim. The latter are large, often very large, cells, and have 

 sacrificed their motility in favour of the storing up of reserve 

 material for the use of the embryo which is to grow out of them. 



When multicellular animals have reached a considerable 

 size and length of life, they can produce the microscopic gametes 

 in enormous quantities and for long periods ; a female sea urchin 

 produces annually about as many eggs as there are human beings 

 in London; and the number of sperms produced by any of the 

 higher animals during its lifetime is considerably greater, not only 

 than the whole present population of the world, but than the 

 total number of human beings that can have existed since 

 man first appeared upon the earth. In such animals therefore, 

 the only form of reproduction is sexual. 



In smaller and shorter-lived multicellular animals, however, 

 sexual reproduction is still attended by certain drawbacks: only 

 a few eggs can be produced at a time by a small female; and if 

 the necessity for fathers could be done away with, twice as many 

 individuals could be engaged in producing offspring. When, 

 therefore, animals are too complex for fission or budding, and too 

 small for the full advantages of sexual reproduction to be felt, 



