178 ESSENTIALS OF BIOLOGY 



In such a multiple reproduction as that of the plaice we say, 

 about the fish, that " it " reproduces some hundreds of thousands 

 of ova during its breeding season. But we should say, about the 

 fish, that " they," that is, the germ-cells, reproduce. 



For the germ-cells, every one of them, is a plaice. An animal 

 is not simply an active, functioning thing : it is a career (see 

 Section 69^) and the egg, embryo, larva and adolescent plaice 

 are phases in the career. In all these phases there is identity of 

 the organism : we could observe it continuously as a unitary thing 

 throughout them all. The plaice-egg is certainly the species, 

 Pleuronectes platessa : it is recognizable as such and no other 

 kind of egg in the sea can be confused with it. The diagnostic 

 characters of the egg are included in the definition of the specific 

 category. 



Similarly, we can observe the gonidial (germ) cell pass continu- 

 ously into the matured egg so that it is identical with the latter 

 in that gonidial cell and egg are phases in the continuous career 

 of a unitary thing that preserves its identity throughout all its 

 phases. The gonidial cell has all the potencies of a plaice-egg — 

 and of no other kind of egg. 



Therefore the gonidial cells are truly organisms that belong 

 to a known category. Each such cell duplicates itself when 

 proliferation of the germ-cells occurs and these doublings, that 

 result from the mitotic divisions of the gonidial cells, are the 

 essential acts of reproduction. Therefore the definition of 

 Section 63 is vahd for the multicellular organism. In the latter, 

 considered as a sub-kingdom of life the simplicity of this con- 

 ception is destroyed by sex. Innumerable modifications in the 

 processes of reproduction occur. Nevertheless, the essential 

 nature of the latter is such as we have indicated above. 



65. ON ASEXUAL REPRODUCTION IN THE 

 MULTICELLULAR ORGANISMS 



In a great number of plant and animal organisms reproduction 

 is essentially the proliferation of germ-cells. In many cases the 

 latter, which are then called spores, conidia, etc., simply divide 

 without conjugation or sex and the resulting cells undergo 

 differentiation into tissue-units, and assembly into the bodily form. 



Thus a germ-cell reproduced in a higher, multicellular plant, 

 for instance, may develop at once, and without the stimulus of 



