136 TROPISMS 



abode. They even remained in the tube when exposed 

 to sunlight which rapidly killed them. 



We find the opposite, negative stereotropism, in many 

 pelagic animals, e.g., larvae of the barnacle or of other 

 crustaceans, which avoid contact with solids. The 

 phenomenon is liable to interfere with heliotropic 

 experiments. 



The importance of stereotropism in animals was first 

 pointed out by the experiments of Dewitz on the sperma- 

 tozoa of the cockroach. 120 . 121 He noticed that when a 

 drop of salt solution containing the spermatozoa was 

 put under a cover glass resting on low supports on a slide, 

 the spermatozoa collect at the solid surfaces of the slide 

 and cover glass, while the liquid between remains free 

 from spermatozoa. When a small glass bead is put into 

 the liquid the spermatozoa will also swim on the surface 

 of the bead, never leaving it again. Dewitz is of the opin- 

 ion that this stereotropism is of assistance in securing 

 the entrance of a spermatozoon into the egg. The egg of 

 the cockroach is rather large and the spermatozoon can 

 enter it only through a micropyle. When the egg is laid 

 it passes by the duct of the seminal pouch in which the 

 female keeps the sperm after copulation. On passing the 

 duct some spermatozoa reach the egg. Dewitz points out 

 that these cannot leave the surface of the egg any more 

 but are compelled to move incessantly on the surface of 

 the egg until one of the spermatozoa by chance gets into 

 the micropyle. 



It is an important fact that different organs of the 

 same organism react differently. We have already men- 

 tioned the tendency of starfish or flatworms to right them- 

 selves, i.e., their ventral surface is positively their dorsal 

 negatively stereotropic. The stolons of hydroids stick 



