CELL-DIVISION AND GROWTH The relation between cell-division and growth has already been touched upon at pp. 41 and 265. The direction of the divisionplanes in the individual cells evidently stands in some causal relation with the axes of growth in the body, as is especially clear in the case of rapidly elongating structures (apical buds, teloblasts, and the like), where the division-planes are predominantly transverse to the axis of elongation. Which of these is the primary factor, the direction of general growth or the direction of the division-planes ? This question is a difficult one to answer, for the two phenomena are often too closely related to be disentangled. As far as the plants are concerned, however, it has been conclusively shown by Hofmeister, De Bary, and Sachs that the growth of the mass is the primary factor; for the characteristic mode of growth is often shown by the growing mass before it splits up into cells, and the form of cell-division adapts itself to that of the mass : " Die Pflanze bildet Zellen, nicht die Zelle bildet Pflanzen " (De Bary).
The opinion has of late rapidly gained ground that the same is true in principle of animal growth, and this view has been urged by many writers, among whom may be mentioned Rauber, Hertwig, and especially Whitman, whose fine essay on the Inadequacy of the Cell-theory of Development (93) marks a distinct advance in our point of view. It is certain that in the earlier stages of development, and in a less degree in later stages as well, the character of growth and division in the individual cell is but a local manifestation of a formative power pervading the organism as a whole ; and the general truth of this view has been in certain cases conclusively demonstrated by experiment.' It has, however, become clear that this conclusion can be accepted only with certain reservations ; for as development proceeds, the cells may acquire so high a degree of independence that profound modifications may occur in special regions through injury or disease, without affecting the general equilibrium of the body. The most striking proof of this lies in the fact that grafts or transplanted structures may perfectly retain their specific character, though transferred to a different region of the body, or even to another species. Nevertheless the facts of regeneration prove that even in the adult the formative processes in special parts are in many cases definitely correlated with the organization of the entire mass ; and in the following chapter we shall see reason to conclude that such a correlation is a survival, in the adult, of a condition characteristic of the embryonic stages, and that the independence of special parts in the adult is a secondary result of development.