232 Studies in the Phystobxjii of Fertilization 



should have bprn fertilized with a like ainount nf the same suspension 

 of "own" sperm, also untreated. Morgan concluded that the variation 

 in the extent of self-fertilization in the dificient experiments of this 

 series was probably due to contamination with sperm contained in the 

 branchial baskets of the animals from which the eggs and sperm had 

 been removed. But this possibility could have been tested by leaving 

 " foreign " unfertilized eggs in sea-water containing a piece of the 

 branchial basket. 



III. Factors influencing the Extent of Self-fertilization. 



From the foregoing it is plain that Ciona intestinalis at Naples is 

 not absolutely self-sterile, but that self-fertilization occurs to a con- 

 siderable degree. Moreover the extent to which it takes place is very 

 variable. The experiments described in this section are attempts to 

 analyse the factors which influence the extent of the self-fertilization ; 

 that is to say, they are exact comparisons between the proportion of 

 eggs to a given individual self-fertilized under a certain set of condi- 

 tions, and the proportion when one of the conditions is altered. At the 

 same time a comparison is made between the percentage of some of 

 the same lot of eggs cross-fertilized by sperm of a "foreign'" individual, 

 or of eggs of a " foreign " individual cross-fertilized by sperm of the first 

 one, under similar conditions to those used in the self-fertilizations. 



1. Sperm-concentration. 



Each of the first series of experiments made to test the influence of 

 sperm-concentration on self-fertilization was carried out as follows. 



Appi-oximately equal amounts of eggs to a given individual were 

 placed in equal quantities of water in four dishes. Fertilization was 

 effected by the addition of sperm as follows : to dish (1), five drops of a 

 milky suspension of " own " sperm ; to dish (2), 4 cc. of ditto ; to (3), 

 five drops of a milky suspension of " foreign " sperm of approximately 



TABLE I. (7.8.1.) 



Eggs selfed Eggs crossed 



Cross 

 Ajm 

 Bjm 

 Cjm 

 Djm 



■ " Own " sperm is used to dsnotc sperm taken from the same individual as the 

 eggs: "foreign " sperm is that from anoilier individual. 



