Reproduction and Development. 37 



the organism — accumulate either within the cell, some- 

 times at one pole, sometimes at the centre, as in the case 

 of the yolk of eggs, or around the cell, as in the case of 

 cartilage or bone. 



Apart from and either preceding or accompanying 

 these phenomena, is the growth or increase of the forma- 

 tive protoplasm itself; concerning which the point to be 

 here observed is that it is not indefinite, but limited. 

 This was first clearly enunciated by Herbert Spencer, and 

 may be called Spencer's law. In simplest expression it 

 may thus be stated : Volume tends to outrun surface. Take 

 a cube measuring one inch in the side ; its volume is one 

 cubic inch, its surface six square inches. Eight such cubes 

 will have a surface of (6 x 8) forty-eight square inches. 

 But let these eight be built into a larger cube, two inches 

 in the side, and it will be found that the surface exposed is 

 now only twenty-four square inches. While the volume 

 has been increased eight times, the surface has been 

 increased only four times. With increase of size, volume 

 tends to outrun surface. But in the organic cell the 

 nutritive material and oxygen are absorbed at the surface, 

 while the explosive changes occur throughout its mass. 

 Increase of size, therefore, cannot be carried beyond certain 

 limits, for the relatively diminished surface is unable to 

 supply the relatively augmented mass with material for 

 elaboration into unstable compounds. Hence the cell 

 divides to afford the same mass increased surface. This 

 process of cell-division is called fission, and in some cases 

 cleavage. 



We will now proceed to pass in review the phenomena 

 of reproduction and development in animals. 



Attention has already been drawn to the difference 

 between those lowly organisms, each of which is composed 

 of a single cell — the protozoa, as they are termed — and 

 those higher organisms, called metazoa, in which there 

 are many cells with varied functions. Confining our 

 attention at first to the former group of unicellular animals, 

 we find considerable diversities of form and habit, from 



