52 THE ORIGIN OF THE NERVOUS SYSTEM 



regulatory development of the hydroid Antennularia 

 antennina, but Morgan (1901) and Stevens (1902, 1910), 

 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 (1907, 1911) the 

 polarity of the new individual is determined by some 

 sort of differential between free and attached surfaces of 

 the cell mass, the osculum developing on the free surface. 

 Similarly in the experiments on obliteration of pre- 

 existing polarity and the establishment of a new polar 

 axis in hydroids (Child, 1915^, pp. 142-46) 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 on the exposed surface of the cell mass 

 and the surface in contact suggests a probable factor in 

 determining the new axis. 



In the actinian Harenactis the localization of a 

 region of more rapid and more extensive growth by 

 injury is sufficient under certain conditions to deter- 

 mine the position and development of new polarities 

 (Child, iqiob, 191 5c, Figs. 79-83). The axial grada- 

 tion results in this case from the fact that the activity 

 of the cells is greatest in the middle region of such an 

 area and decreases*toward its borders. In this connec- 

 tion a recent statement of Harrison's concerning the 

 limb rudiment of Amblystoma is of interest. Harrison 

 says: : The limb rudiment may be thus regarded, not 

 as a definite circumscribed area like a stone in a mosaic, 



