24 COMPARATIVE ANATOMY CHAP. 



origin and meaning of this phenomenon are not certainly ascertained. 

 Many see in them a strengthening of the product of the fusion, i.e. the 

 young new individual. 



2. The different sizes, or the the sexual dimorphism of the 

 fusing reproductive cells. This is to be explained by the principle 

 of the division of labour. The reproductive cells have a double object 

 to fulfil : (1) such a cell must mingle with another (fertilisation) ; and 

 (2) must, after mingling, form a new organism like that of its parents. 

 To secure the first object free locomotion is useful, so that the repro- 

 ductive cells may seek each other and meet ; and further, in certain 

 circumstances a power of resistance to external influences is needed. 

 To fulfil the second object the cell must be of a considerable size, and 

 contain, if possible, nutritive material which can be used during 

 development. Both these objects cannot be fulfilled by each of the 

 reproductive cells without disadvantage. Here division of labour 

 steps in. Some cells fulfil the first object ; they move about with great 

 ease ; they are resistent, and moreover very small (the smallest cells 

 of the organism). Their smallness has a further advantage ; they 

 are produced in greater numbers, and can easily penetrate the second 

 sort of reproductive cell. These are called male reproductive cells, 

 sperm cells, sperm filaments, or spermatozoa. 



Other cells fulfil the second object. They are large and often 

 filled with much reserve material (the largest cells of the organism). 

 They substitute size and mass for free locomotion. These are called 

 female reproductive cells, or egg cells. 



Male and female reproductive cells are either formed in one and 

 the same metazoan individual (hermaphroditism), or in two different 

 individuals, male and female (gonoehorism, separation of the sexes). 

 The latter appears among the Metazoa generally as the rule, the 

 former as the exception. The causes which determined the separation 

 of the sexes are most probably quite similar to those which brought 

 about fertilisation in the animal kingdom. If one remembers that even 

 among hermaphrodite animals a copulation of two individuals often 

 takes place, or adaptations are present which prevent the fertilisation 

 of the eggs of an animal by the spermatozoa of the same individual 

 (self-fertilisation), the recent opinion that all Metazoa were originally 

 sexually separated, and that hermaphroditism has developed secondarily 

 from the male or female condition gains in probability. 



The utility of cross-fertilisation places in a new light the utility of 

 a sexual differentiation of the reproductive cells into freely movable 

 spermatozoa and massive eggs. So as still further to secure cross- 

 fertilisation we find copulation in very many Metazoa. These animals 

 possess special copulatory organs, by means of which the spermatozoa 

 from the male body are carried into the sexual organ of the female, 

 and thus into the neighbourhood of the egg. 



