SPERMS AND OVA 143 



and worker bees are derived from fertilized, drones from unfertil- 

 ized eggs. There are thus gradations between complete partheno- 

 genesis and conjugation occurring at the start of every cell- 

 community. The reproduction of unicellular forms has been 

 imperfectly studied. Possibly in some, or many, or most cases 

 conjugation never occurs. Even where it has been noted there are 

 always long intervals of asexual multiplication. In paramcecium, 

 for example, at lengthy but seemingly more or less fixed intervals, 

 conjugant individuals appear which differ somewhat from the rest 

 of the population in appearance. They correspond to the germ- 

 cells of multicellular organisms, and their descendants, which 

 intervene between one act of conjugation and the next, to the 

 generations of the cell-community. 1 



234. The ovum is usually a large and passive cell, more or less 

 laden with nutriment for the earliest needs of the new cell-com- 

 munity. The sperm is small and mobile, and is thus enabled to 

 reach and penetrate the ovum. But, as already noted, 2 we have 

 every reason to believe that these differences of size, shape, and 

 function imply nothing as far as the germ-plasm is concerned. 

 Apparently the germ-plasm used in fertilization is situated wholly, 

 or almost wholly in the nuclei of the sex-cells, and the nuclei of 

 sperm and ovum seem equivalent as bearers of heredity. In most 

 cases, conjugating sperms and ova are derived from distinct cell- 

 communities of the same variety. Conjugation between the sex- 

 cells of distinct species, and in a lesser degree of distinct varieties, 

 is hindered or prevented by physical or physiological incapacity, 3 

 and, in the case of the higher animals, by mental disinclination 

 as well. On the other hand, nature has elaborated many 

 devices to prevent conjugation between the germs of the same 

 individual. 



235. In most animal species only sperms or ova are produced 

 by the same individual. Some low animals, however, for example 

 snails and earth-worms, and many plants are hermaphrodite, 

 possessing male and female organs of generation, and producing 

 both sperms and ova. In such cases, self-fertilization is prevented 

 by the ripening of the sperms and ova at different periods, or in 

 other ways. The prevention is usually less perfect in the case of 

 plants than of animals, so that in the former case self-fertiliza- 



1 See i. 2 See 2. 



3 Physical disability prevents, for example, a very large dog mating with a 

 very small one. Physiological disability renders mating sterile. In the latter 

 case the sperms do not fertilize the ova, or, as is probable in some cases, the 

 embryos do not develop beyond a very early stage. 



