SELF-FERTILIZATION 7 IN CIONA. 329 



that they also may acquire this property. Unless, however, this 

 influence on the eggs were of a peculiarly individual sort, it would 

 prevent all fertilization, cross- as well as self-fertilization, which is 

 not the case. 



There remains still another possibility which may better 

 account for the conditions. It may be that the result does not 

 depend on a reaction between the sperm and the tissues or eggs 

 of the same individual, but to a similarity depending on the com- 

 mon descent of the sperm and the eggs. Owing to their close 

 similarity of composition, the activity of the sperm on coming in 

 contact with its " own " egg-membranes may be decreased, so that 

 the spermatozoon can not force its way into the egg. This point 

 of view offers certain advantages over the others mentioned, espe- 

 cially when extended to some other animals. Thus in the bee, 

 the spermatozoa are stored up in the receptaculum of the female, 

 but the fertilization of the eggs of the female is not thereby pre- 

 vented. In this case the spermatozoa and the eggs have arisen 

 from separate individuals, and, hence, fertilization is possible, de- 

 spite the fact that the sperm is stored in the body of the female 

 that contains the eggs. In some hermaphroditic animals and 

 plants self-fertilization occurs, but this is not a fatal objection to 

 the hypothesis, because, although in these cases also a condition 

 similar to that in dona may be supposed to exist, it may not be 

 sufficiently strong to prevent the sperm from entering the eggs 

 the activity of the sperm being greater, or the reaction being 

 less marked. The fact that in some of these cases self-fertiliza- 

 tion takes place less readily than cross-fertilization is distinctly in 

 favor of the present point of view. 



The greater activity induced in the sperm of Ciona by ether 

 and other exciting substances may make them sufficiently active 

 to break through the barrier around their " own " eggs, and hav- 

 ing once entered the egg, the stimulus caused by the nucleus or 

 by the centrosome may cause the development to proceed. 



If the spermatozoa are brought to rest by substances of some 

 sort on the surface of the egg, or in its membranes, these may 

 be a part of the living substance, and not set free in the sea 

 water. Hence the failure to detect such bodies in the water or 

 extracts under the crude condition of the experiments. 



