VEGETATIVE PROPAGATION 317 



not living, e.g. firm supporting structures and pigment granules. 

 In early stages more food is taken in and utilized than is neces- 

 sary to counterbalance the processes of waste that are constantly 

 going on, and the result is growth. But when a certain size, 

 varying with the species, is attained, the individual becomes adult, 

 after which the chief use of food is to maintain the weight of the 

 body by preventing the animal from gradually wasting away 

 owing to the constant formation of various products of waste. 

 In other words, the income and expenditure balance one another, 

 in so far as the individual itself is concerned. But in all cases 

 more food than is necessary for this particular purpose is taken 

 into the body, and this is employed to build up living substance 

 that becomes detached from the parent form and develops into 

 new individuals. We have, in fact, a kind of overgrowth. A 

 hen, for example, after it becomes adult, lays eggs, which may 

 be regarded as detached parts of its body, and which, by develop- 

 ing into chickens, prevent the fowl species from becoming extinct. 

 It will be convenient to consider under separate headings 

 Vegetative Propagation and Development from Eggs. 



VEGETATIVE PROPAGATION 



We have elsewhere (vol. i, p. 490) had occasion to note that 

 the simple animals known as Animalcules (Protozoa) differ, as 

 a rule, from all higher forms (Metazoa) in one very important 

 particular. For any one of the latter is built up of more or less 

 numerous units of structure technically known as cells, each of 

 which is a fragment of living substance or protoplasm, part of 

 this being specialized into a particle known as the nucleus. An 

 Animalcule, on the contrary, consists of but one cell, though the 

 parts of this may be very highly specialized. These facts are 

 embodied in the technical statement that Protozoa are one-celled, 

 while the Metazoa are many-celled. Any member of the latter 

 group, e.g. a Worm, a Snail, or a Fish, may therefore be regarded 

 as a cell-community, and the physiological work of the body is 

 more or less divided among the members of this community, 

 there being, in other words, a division of physiological labour. 

 It invariably happens that some of the units or cells of the com- 

 munity are especially concerned with propagation, and either 

 produce the eggs which develop into fresh individuals or else 



