388 CEiJ.-nins/ox axd development 



to the periphery, the deutoj)lasm become massed at one pole, and 

 the polarity of the egg come into xxo.w {Xercis, Figs. 60 and 97).^ \\\ 

 such cases the axis of the egg may i:)erhaps be predetermined by 

 the i^osition of the centrosome, and we have still to seek the causes 

 bv which the position is established in the ovarian history of the Qg^. 

 These considerations show that this problem is a complex one, involv- 

 ing, as it does, the whole question of cell-polarity ; and I know of 

 no more promising held of investigation than the ovarian history of 

 the ovum with reference to this question. That Mark's view is cor- 

 rect in {M-inci])le is indicated by a great array of general evidence 

 considered in the following chapter, where its bearing on the general 

 theory of development is more fully dealt with. 



C. Cell-division and Growth 



The general relations between cell-division and growth, which have 

 already been briefly considered at page 58 and in the course of this 

 chapter, may now be more critically examined, together with some 

 account of the causes that incite or inhibit division. It has been 

 shown above that every precise inquiry into the rate form, or direc- 

 tion of cell-division, inevitably merges into the larger problem of the 

 general determination of growth. We may conveniently approach 

 this subject by considering first the energy of division and the limita- 

 tion of growth. 



All animals and plants have a limit of growth, which is, how- 

 ever, much more definite in some forms than in others, and differs 

 in different tissues. During the individual development 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 estab- ■ 

 lished, some of the cells ceasing to divide and perhaps losing this 

 power altogether (nerve-cells), others dividing only under special con- 

 ditions (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 equilibrium is attained is an heredi- 

 tary character, which in many cases shows an obvious relation to the 

 environment, and has therefore probably been determined and is 

 maintained by natural selection. From the cytological point of view 

 the limit of body-size appears to be correlated 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 gastero- 



^ The immature egg of Xereis shows, however, a distinct polarity in the arrangement of 

 the fat-drops, which form a ring in the e([uatorial regions. 



