FERTILISATION 207 



of the ova and spermatozoa is reduced below a certain point, 

 assortative mating as a result of affinity between gametes bearing 

 similar characters no longer occurs. It thus happens that a 

 reduction of vitality is frequently correlated with an increased 

 tendency towards cross-fertilisation, which, on this view, is a 

 source of renewal of vitahty. This theory was adopted to ex- 

 plain certain phenomena of cross-fertilisation occurring among 

 plants, by Fritz Miiller, who wrote as follows : — 



" Every plant requires for the production of the strongest 

 possible and most proHfic progeny, a certain amount of difference 

 between male and female elements which unite. FertiUty is 

 diminished as well when this degree is too low (in relatives too 

 closely allied) as when it is too high (in those too httle related)." 

 And, further, " species which are wholly sterile with poUen of 

 the same stock, and even with pollen of nearly allied stocks, 

 wiU generally be fertilised very readily by the pollen of another 

 species. The self-sterile species of the genus Ahutilon, which 

 are, on the other hand, so much inclined to hybridisation, 

 afford a good example of this theory, which appears to be con- 

 firmed also by Lobelia, Passifiora, and Oncidium." ^ 



Castle ^ found that the eggs of the hermaphrodite Ascidian, 

 Cionaintestinalis,could not, as a rule, be fertilised by spermatozoa 

 derived from the same individual, while they could be fertilised 

 readily with the spermatozoa of another individual. This rule, 

 however, was not without exceptions, for in some cases as many 

 as fifty per cent, of the eggs of one Ciona could be fertihsed with 

 sperms of the same individual, although this was very unusual. 

 Morgan, who confirmed Castle's observations, states that the 

 failure to conjugate is due to the inabiUty of the sperms to 

 enter the eggs. If the sperm succeeds in entering, as in the 

 exceptional cases, the fertilised egg develops normally. Morgan 

 found, further, that if the sperms are stimulated to greater 

 activity by alcohol, ether, ammonia, or certain salt solutions, 

 self-fertihsation may in some cases be induced. In another 

 Ascidian, Cynthia partita, Morgan observed that self-fertihsa- 



' Miiller, " Investigations respecting the Fertilisation of Ahutilon," 

 English Translation in American Naturalist, vol. viii., 1874. 



'' Castle, " The Early Embryology of Ciona intestinalis," Bull. Mus. Comp. 

 Zool., vol. xxvii., 1896. 



