PHYSIOLOGICAL GRADIENTS. 



bilaterally symmetrical plants, such as liverworts, light deter- 

 mines the dorsiventrality, and some algae develop a radially sym- 

 metrical thallus when 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 undoubtedly 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 develop- 

 ment of heads from posterior cut surfaces in Planaria. In such 

 cases the new axis is always represented by a new gradient, and 

 the relation between the new axis and the occurrence of differ- 

 ential exposure is obvious, though whether the wound stimulus, 

 oxygen supply, or some other factor is chiefly concerned in de- 

 termining the new gradient is not known. Loeb ('92) has main- 

 tained that new polarities are determined by gravity in the regu- 

 latory development of the hydroid Antennularia antennina, but 

 Morgan ('01) and Stevens ('02, '10), while not disputing Loeb's 

 results, showed that other factors besides gravity were concerned 

 in determining polarity. 



In the development of sponges from dissociated tissue cells 

 described by H. V. Wilson ('07, 'n) the polarity of the new 

 individual is determined by some sort of differential between 

 free and attached surfaces of the cell mass, the osculum develop- 

 ing on the free surface. Similarly in the experiments on oblitera- 

 tion of a preexisting polarity and the establishment of a new 

 polar axis in hydroids (Child, 'i$c, pp. 142-146) the apical region 

 of the new axis arises in the region of greatest exposure to the 

 environment, but the particular factors chiefly concerned have 

 not been determined. In both these cases the sponge and the 

 hydroid, the difference in oxygen supply between the exposed 

 surface of the cell mass and the surface in contact suggests a 

 probable factor in determining the new axis. 



