206 PROBLEMS OF FERTILIZATION 



eggs of the same individual may fertilize perfectly the 

 eggs of other individuals. The failure to self -fertilize 

 in these cases is not due to immobility of the spermat- 

 ozoa in the presence of own eggs, or inability to reach 

 the membrane of the egg, but it is due to absence of 

 the reaction that leads to penetration of the egg by the 

 spermatozoon. Such incompatibility is by no means 

 universal in Ciona, for all authors have found certain 

 individuals in which self-fertilization may occur to a 

 certain extent. 



The determination of the occurrence of self- 

 fertilization obviously requires much care to avoid con- 

 tamination with the sperm of other individuals. The 

 method which was originally employed by Castle in 

 his determinations consisted in comparing the percent- 

 ages of fertilized eggs from isolated individuals with 

 the percentages from pairs of individuals placed to- 

 gether. Observations were made on the same individ- 

 uals for five successive days, and the fertilized eggs of 

 each day were separately estimated. The result was 

 that of fifty estimates from ten isolated individuals 

 thirty-seven contained no eggs fertilized, nine from 4 

 per cent to 25 per cent fertilized, two contained 90 

 per cent of fertilized eggs, and in two cases no eggs 

 were deposited. The paired individuals yielded twenty- 

 five estimates, of which twenty-three showed 100 per 

 cent fertilized, one yielded 20 per cent, and one none 

 fertilized. 



To this method of determining the extent of self- 

 fertilization the objection has been made that spermat- 

 ozoa of foreign origin may remain in the atrial cavity 

 or tangled in the branchial basket and give the effect 



