ORIGIN OF AXIATE PATTERN 51 



of Fucaceae differential illumination of the two sides of 

 the egg determines the polarity of the plant developing 

 from the egg, though in the absence of light polarity 

 appears and germination occurs, but more slowly. 

 Stahl (1885) showed that in the spore of Equisetum 

 polarity is determined in the same way. Winkler 

 (1900^) also showed that polarity in the alga Bryopsis 

 could be determined by light. In various bilaterally 

 symmetrical plants, such as liverworts, light determines 

 the dorsiventrality and some algae develop a radially 

 symmetrical thallus w r hen the illumination is equal on 

 all sides of the polar axis and are bilateral when the 

 illumination is from one side. In the spermatophytes 

 conditions which determine egg polarity are undoubt- 

 edly intraorganismic. The embryo sac shows a definite 

 polarity with respect to surrounding parts and the ovum 

 is attached to one end of the sac. The unattached end 

 of the ovum becomes the apical, the attached end the 

 basal pole of the embryo. What particular factors are 

 concerned in this case is not known. 



In the simpler animals we see new polarities arise at 

 cut ends of pieces, e.g., the development of new apical 

 regions and axes from the aboral ends of pieces in various 

 hydroids, the development of heads from posterior cut 

 surfaces in Planaria (see Figs. 30-34, p. 125). In such 

 cases the new axis is always represented by a new 

 gradient and the relation between the new axis and the 

 occurrence of differential exposure is obvious, though 

 whether the wound stimulus, oxygen supply, or some 

 other factor is chiefly concerned in determining the new 

 gradient is not known. Loeb (1892) has maintained 

 that new polarities are determined by gravity in the 



