224 University of California Publications in Zoology. [Vol.6 



the horizontal. As soon as the embryos began to emerge, record 

 was made of the numbers emerging respectively above and below 

 the horizontal. The angle of divergence from the horizontal was 

 usually small ; very few approximated ninety degrees in either 

 direction, and the neutrals were numerous. On one plate 20 

 emerged above, 20 below the horizontal; in another 19 above. 

 16 below. These were typical. 



Gravity appeared to play no definite part in these phenomena. 

 Over the frustules, however, it exerted a definite control. The 

 frustules arose on the embryo always opposite the point of emerg- 

 ence, a position obviously not determined by gravity. But with 

 elongation the clavate extremity of each frustule showed a dis- 

 tinct tendency to migrate downward, in many cases directly, even 

 though the frustule arose at a point above the horizontal, as in 

 the accompanying figure. The frustules accordingly exhibited a 

 positive geotropism of definite mechanical value in the estab- 

 lishment of a holdfast firmly anchored over a considerable area. 



La Jolla, Cal. 



June 14, 1910. 



