32 PATTERNS AND PROBLEMS OF DEVELOPMENT 



capable of considerable reconstitution. The original polarity is usually 

 retained, but short pieces may give rise to bipolar forms. Distal nonnu- 

 cleated pieces reconstitute apical parts; proximal pieces, rhizoids more 

 readily or more completely (Hammerling, 1934^, b, c; 1936). To account 

 for the observed results Hammerling postulates two opposed gradients of 

 different formative substances produced by the nucleus and accumulating 

 in concentrations decreasing from each end of the axis. The differences in 

 reconstitution in pieces from different body-levels are not very different 

 from those in certain hydroids. For the hydroids, however, the hypothesis 

 of two opposed gradients of different formative substances advanced by 

 Morgan was later abandoned as inadequate. Since these substances are 

 assumed to accumulate toward opposite poles of the axis, their distribu- 

 tion must be determined by a polarity of some sort to which they react. 

 If this be granted, the possibility that that polarity, rather than the hypo- 

 thetical formative substances, is the essential factor in determining the 

 differences in reconstitution at different body-levels may perhaps be ad- 

 mitted. Undoubtedly, the nucleus is essential for the continued life and 

 activity of the cytoplasm, but whether it is responsible for the axiate pat- 

 tern appears open to question. It does not appear that the nucleus has 

 any mechanism for determining the distribution of the two formative sub- 

 stances in two opposed gradients, even if the hypothesis of the substances 

 and their distribution be accepted. But whatever the interpretation, the 

 experiments show the existence in this single cell body of a gradient pat- 

 tern very similar, as regards its effect on reconstitution, to that in hy- 

 droids and various other animals. 



The alga Caulerpa prolifera, a single multinucleate cell with differentia- 

 tion into rhizomes, rhizoids, and leaflike parts, also exhibits interesting 

 evidences of a gradient pattern in reconstitution." In certain fungi pres- 

 ence of a gradient pattern and dominance of the apical region are indi- 

 cated by the data of reconstitution. 



An interesting example of substitution by formation of adventitious 

 axes appears in the liverwort Marchantia. The thallus pattern resembles 

 that of a bilateral animal in that it consists of a longitudinal and dorsiven- 

 tral axis with the dominant region at one pole (Fig. 11, ^). The notched 

 end may be regarded as anterior, for the apical cell, which constitutes the 

 growing tip and gives rise to other cells, lies in the notch, and this is the 

 dominant pole. The apical cell at times undergoes dichotomous division, 

 giving rise to two apical cells, each of which becomes the anterior pole of 



■< Zimmcrmann, 1929, and references to earlier papers given by him. 



