70 



THE CELL AND PROTOPLASM 



lism of the cell or cells concerned. Some of 

 the external factors which determine such 

 patterns in one kind or another of proto- 

 plasm are: differential ilUumination by 

 visible light, definitely directed electric 

 current, a gradient of oxygen content or of 

 hydrogen-ion concentration. The resulting 

 patterns persist after the external differ- 

 ential has ceased to act. In any case the 

 external factor can be regarded only as 

 initiator; the characteristics of the estab- 

 lished pattern are determined by the con- 

 stitution and condition of the protoplasm 

 in which they appear. In many of the 

 simpler animals a new developmental pat- 

 tern may result from a transverse section 

 of the body, from a partial section or even 

 from a local wound sufficient to produce a 

 certain degree of cell activation. Here 

 there is unquestionably excitation, but the 

 possibility of production of specific sub- 

 stances at the wound and their transport 

 to other regions also exists. The result, 

 however, is a gradient of greater or less 

 length which, in its physiological character- 

 istics and as a basis for development and 

 differentiation, appears to be exactly like 

 that resulting from an external gradient of 

 oxygen concentration or from directed 

 electric current. At any rate, it is beyond 

 question that a purely quantitative external 

 differential or a local activation can deter- 

 mine in a cell or cell mass a pattern which 

 becomes the basis for organismic differen- 

 tiation. A few examples will serve to 

 illustrate the fact. 



It has long been known that the pattern 

 of branching and even the polarity of vari- 

 ous algae can be determined by differential 

 illumination. One of the most interesting 

 cases is the egg of Fucus and related forms. 

 Here light is apparently an inhibiting fac- 

 tor, for with directed illumination the first 

 visible evidence of developmental activity, 

 the primary rhizoid outgrowth, occurs on 

 the less illuminated side. However, the 

 polarity of this egg can be determined by 

 other external differentials. Lund found 

 electric current effective in a certain range 

 of density and action period. In groups 

 of eggs lying close together the rhizoids on 



each develop toward the center of the 

 group. More recently Whitaker has shown 

 that the polarity of this egg can be de- 

 termined by a hydrogen-ion-concentration 

 gradient, the rhizoid developing either at 

 the high or low end of the external gradi- 

 ent, according to range of concentration. 

 Also increase of hydrogen-ion concentration 

 to a certain limit increases the group effect. 

 Centrifuging may also be effective when 

 the displaced egg contents are not too 

 rapidly redistributed, the rhizoid develop- 

 ing centrifugally. In eggs remaining elon- 

 gated after deformation in a capillary tube 

 the rhizoid develops at or near one end of 

 the long axis, but with increased hydrogen- 

 ion concentration the gradient between free 

 surface and surface in contact becomes more 

 effective than the deformation, and the 

 rhizoid may develop from the region in con- 

 tact. Lowrance, working with a tempera- 

 ture gradient, found the rhizoid developing 

 on the warmer side. Various methods show 

 the presence of a gradient in the egg, at 

 least as soon as the rhizoid outgrowth be- 

 comes evident, with the tip of the rhizoid 

 indicated as the high end. 



In animal eggs the foundations of de- 

 velopmental pattern are laid before the 

 eggs are isolated from the parent body, 

 presumably during the ovarian develop- 

 ment of the oocyte. At present we are 

 unable to substitute controlled external 

 factors for the ovarian environment during 

 the oocyte stage, but it is possible with 

 many animal species to alter or obliterate 

 the pattern established and to determine 

 new patterns in the course of development, 

 or even in adult individuals by means of 

 external differentials. 



In the development of the multicellular 

 organism the single cell becomes merely a 

 part of a development pattern on a larger 

 scale. This pattern is characterized by 

 axiate spatial and chronological orders, 

 whether the developing system is a bud, 

 an isolated piece, an embryo or an organ 

 system. The axis concept is, of course, 

 merely a convenient abstraction. The ac- 

 tual pattern may be meridional rather than 

 linear, present only in the superficial cyto- 



