SPECIFICITY IN FERTILIZATION 205 



for different individuals and localities (Castle, 1903; 

 Morgan, 1904, 1905, 1910; Fuchs, 1914, 1915). 



In plants the problem of self-fertilization was 

 brought to the forefront of investigation by Darwin's 

 classical " Studies on Cross- and Self-Fertilization in 

 the Vegetable Kingdom," and the problem of selfing 

 has been very carefully and extensively studied in 

 recent years by Jost (1907), Correns (1912), East 

 (1915(1, 19156), East and Parke (1917), Stout (1916 

 and 1917), and others. The problem in plants is simi- 

 lar in many respects to that in animals, but it should 

 be remembered that in plants the incompatibility that 

 occurs in many cases concerns the growth of the pollen 

 tube, which is more or less abortive on the stigma and 

 in the style of the same flower in such cases, and not 

 the reaction of the actual gametes, which appear usually 

 not to meet. Of course in many plants the pollen of the 

 same flower is perfectly compatible, and in the case of 

 cleistogamous flowers that never open but nevertheless 

 produce perfect seed there is no chance for cross- 

 fertilization. The phenomenon of physiological incom- 

 patibility of own pollen is more or less sporadic in its 

 occurrence, and in fact the plants in which this occurs 

 form a relatively small class. 



The ascidian Ciona is the only known and carefully 

 studied example of physiological self-incompatibility of 

 gametes in the animal kingdom. The various authors 

 who have studied this case (Castle, Morgan, Fuchs) 

 all found certain individuals in which the eggs are not 

 susceptible of fertilization with the sperm of the same 

 individual, although they may be fertilized with sperm 

 of other individuals; and the sperm thus impotent on 



