THE ENERGY OF DIVISION 289 



C. THE ENERGY OF DIVISION 



The causes by which cell-division is incited and by which its cessa- 

 tion is determined are as yet scarcely comprehended, and the ques- 

 tions that they suggest merge into the larger problem of the general 

 control of growth. All animals and plants have a limit of growth, 

 which is, however, much more definite in some forms than in others, 

 and which differs in different tissues. During the individual devel- 

 opment the energy of cell-division is most intense in the early stages 

 (cleavage) and diminishes more and more as the limit of growth is 

 approached. When the limit is attained a more or less definite 

 equilibrium is established, some of the cells ceasing to divide and 

 perhaps losing this power altogether (nerve-cells), others dividing 

 only under special conditions (connective tissue-cells, gland-cells, 

 muscle-cells), while others continue to divide throughout life, and thus 

 replace the worn-out cells of the same tissue (Malpighian layer of 

 the epidermis, etc.). The limit of size at which this state of equi- 

 librium is attained is an hereditary character, which in many cases 

 shows an obvious relation to the environment, and has therefore prob- 

 ably been determined and is maintained by natural selection. From 

 the cytological point of view the limit of body-size appears to be cor- 

 related with the total number of cells formed rather than with their 

 individual size. This relation has been carefully studied by Conklin 

 ('96) in the case of the gasteropod Crepidula, an animal which varies 

 greatly in size in the mature condition, the dwarfs having in some 

 cases not more than ^ the volume of the giants. The eggs are, 

 however, of the same size in all, and their number is proportional to 

 the size of the adult. The same is true of the tissue-cells. Measure- 

 ments of cells from the epidermis, the kidney, the liver, the alimen- 

 tary epithelium, and other tissues show that they are on the whole as 

 large in the dwarfs as in the giants. The body-size therefore depends 

 on the total number of cells rather than on their size individually 

 considered, and the same appears to be the case in plants. 1 



Morgan has examined the same question experimentally through 

 a comparison of normal larvae of echinoderms and Amphioxus with 

 dwarf larvae of the same species developed from egg-fragments ('95, i ; 

 '96). Broadly speaking, his results agree with Conklin's, though they 

 show that the relation is by no means simple or constant. If unseg- 

 mented eggs of sea-urchins (Spharechinus) be shaken to pieces, 

 fragments of all sizes are obtained which may segment and pro- 

 duce blastulas and gastrulas ranging down to ^ the volume of 

 the normal size. Dwarfs are also obtained from isolated blasto- 

 meres of two-, four-, or eight-cell stages. In both cases the num- 



1 See Amelung ('93) and Strasburger ('93). 



