FERTILISATION 215 



althougk in some cases such incipient degrees of cross-infertility are 

 further shown by the number or quality of the seeds being fewer or 

 inferior." 



It would appear, however, that when the aggregate vitality 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 

 vitality. This theory was adopted to explain certain phenomena of 

 cross-fertilisation occurring among plants, by Fritz Mtiller, who wrote 

 as follows : — 



" Every plant requires for the production of the strongest possible 

 and most prolific progeny, a certain amoimt of difference between 

 male and female elements which unite. Fertility is diminished as 

 well when this degree is too low (in relatives too closely allied) 

 as when it is too high (in those too little related)." And, further, 

 " species which are wholly sterile with pollen of the same stock, and 

 even with pollen of nearly alUed stocks, will generally be fertilised 

 very readily by the pollen of another species. The self-sterile species 

 of the gtous Abutilon, which are, on the other hand, so much inclined 

 to hybridisation, afford a ^ood example of this theory, which appears 

 to be confirmed also by Lobelia, Passiflora, and Oncidimn." ^ 



Castle ^ found that the eggs of the hermaphrodite Ascidian, Ciona 

 intestinalis, 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 fertilised 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 inability 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 byalcohol, ether, ammonia, or certain salt solutions, 

 self-fertilisation may in some cases be induced. In another Ascidian, 

 Cynthia partita, Morgan observed that self -fertilisation frequently 

 occurs, but that the eggs in this species also are most usually 

 fertilised by spermatozoa from another individual.^ The foregoing 



' MUller, " Investigations respecting the Fertilisation of Abutilon," English 

 Translation in Ainei-ican Naturalist, vol. viii., 1874. 



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

 ZooL, vol. xxvii., 1896. 



^ Morgan, " Self-Fertilisation induced by Artificial Means," Jour, of Exp. 

 ZooL, vol. i., 1904. ' " Some Further Experiments on Self -Fertilisation in Cionn," 

 Biol. Bull., vol. viii., 1905. 



