CELL-DIVISION AND GROW Til 293 



pensatory changes in the neighbouring parts ; but it is equally certain 

 that such changes are not a direct mechanical effect of the disturbance, 

 but a highly complex physiological response to it. How complex the 

 problem is, is shown by the fact that even closely related animals 

 may differ widely in this respect. Thus Fraisse has shown that the 

 salamander may completely regenerate an amputated limb, while the 

 frog only heals the wound without further regeneration.^ Again, in 

 the case of ccelenterates, Loeb and Bickford have shown that the 

 tubularian hydroids are able to regenerate the tentacles at both ends 

 of a segment of the stem, while the polyp Cerianthus can regenerate 

 them only at the distal end of a section (Fig. 142). In the latter case, 

 therefore, the body possesses an inherent polarity which cannot be 

 overturned by external conditions. 



D. 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 division- 

 planes in the individual cells evidently stands in some causal rela- 

 tion 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 groioth of tJic mass is the 

 priniary 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 whojii 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 develop- 

 ment, and in a less degree in later stages as well, the character of 

 growth and division in the individual cell is but a local manifesta- 

 tion of a formative power pervading the organism as a whole ; and 



1 In salamanders regeneration only takes place when the bone is cut across, and does not 

 occur if the limb be exarticulated and removed at the joint. 



