It was found that approximately fifteen of the sixty children belonged to each of the four groups. This should be expected if the inhibitory substance to each parent is transmitted to the children independently. Half of the children will thus inherit the inhibitory substance of one parent and the other half will inherit the inhibitory substance of the other parent. This agrees with the assumption that there are definite determiners for the inhibitory substances in the children which will be transmitted to half of the children. Rather complicated assumptions are needed to explain all the facts observed by Correns on this basis and since the subject is still under investigation we need not go further into the details. To us the assumption and experimental support of the idea that self-sterility is caused by the presence of a substance inhibitory to the entrance of a spermatozoön is important. Should it be possible that the block created by the entrance of a spermatozoön into the egg is also due to an inhibitory substance carried by a spermatozoön into the egg; and furthermore that the effect of the inhibitory substance should be the prevention of further agglutination of the spermatozoön with the egg or of the growth of the pollen tube in plants? On such an assumption self-sterility would be due to a lack of agglutination between the egg of a hermaphrodite and a spermatozoön of the same individual. The experiments on the agglutinins have shown that while isoagglutinins (_i. e._, agglutinins for other individuals of the same species) are common auto-agglutinins (_i. e._, agglutinins for cells of the same individual) rarely if ever occur. 7. A positive chemotropism of the spermatozoa toward an egg of the same species has been demonstrated in a few cases, but it seems that this phenomenon is not determined by that type of substances which give rise to species specificity. The famous experiment of Pfeffer on the spermatozoa of ferns inaugurates this line of investigation. He found that such spermatozoa when moving in a straight line through the water will be deviated in their course if they come near an archegonium; they will then turn toward it, enter it, and enter the egg. Pfeffer showed that 0.01 per cent. malic acid if put into a capillary tube will attract the spermatozoa of ferns. When the liquid in the tube contains only 0.01 per cent. malic acid the spermatozoa of ferns very soon move toward the opening of the capillary tube and within from five to ten minutes many hundreds of spermatozoa may accumulate in the tube. The malic acid acts as well in the form of a free acid as in the form of salts.[82] [82] Pfeffer, _Untersuchungen aus dem botanischen Institut zu Tübingen_, 1881-1885, i., 363.