6. The reason that an egg once fertilized with sperm cannot be fertilized again may be found in a group of facts which we will now discuss, namely, the self-sterility of many hermaphrodites. The fact that hermaphrodites are often self-sterile, while their eggs can be fertilized with sperm from a different individual of the same species has played a great rôle in the theories of evolution. We are here only concerned with the mechanism which determines the block to the entrance of a spermatozoön into an egg of the same hermaphroditic individual.
Castle[77] observed and studied the phenomenon of self-sterility in an Ascidian, _Ciona intestinalis_, which is hermaphroditic. Animals which were kept isolated discharged both eggs and sperm into the surrounding sea water. Often no egg was fertilized, but in some cases five, ten, or as many as fifty per cent. of the eggs could be successfully fertilized with sperm from the same individual; while if several individuals were put into the same dish as a rule one hundred per cent. of the eggs which were discharged segmented. Morgan[78] found that the eggs of various females differ in their power of being fertilized by sperm of the same individual while one hundred per cent. could usually be fertilized with sperm of a different individual. He found in addition that if the eggs of _Ciona_ are put for about ten minutes into a two per cent. ether solution in sea water in a number of cases the percentage of eggs fertilized by sperm of the same individual shows a slight increase. Fuchs[79] has reported results similar to those of Castle and Morgan.
[77] Castle, W. E., _Bull. Mus. Comp. Zoöl._, Harvard, 1896, xxvii., 203.
[78] Morgan, T. H., _Jour. Exper. Zoöl._, 1904, i., 135; _Arch. f. Entwcklngsmech._, 1910, xxx., 206.
[79] Fuchs, H. M., _Jour. Genet._, 1915, iv., 215.
A new point of attack has been introduced into the work of self-sterility in plants by the consideration of heredity. Darwin found that in _Reseda_ which is monoecious (or hermaphroditic) certain individuals are either completely self-sterile or completely self-fertile; and Compton showed that apparently self-fertility is a Mendelian dominant to self-sterility.[80]
According to Jost this self-sterility in hermaphroditic plants is due to the fact that if pollen of the same plant is used the normal growth of the pollen tube is inhibited, while this inhibition does not exist for pollen from a different individual. Correns calls these substances which prevent the adequate growth of pollen, "inhibitory" substances, and finds that they can apparently be transmitted to the offspring. He made experiments on _Cardamine pratensis_ which is self-sterile.[81] He fertilized two individuals of _Cardamine_ crosswise and raised sixty plants of the first generation. He compared the fertility of these F{ 1} plants toward (_a_) their parents, and (_b_) foreign plants. All the fertilizations with the foreign plants were successful, but the fertilizations with the parents were only partly successful. According to their reaction they could be divided into four groups: