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.

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 monœcious (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 F1 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:

(A) fertile with both parents. Type bg
(B) fertile with one (B), sterile with the other parent (G).
(a) fertile with B, sterile with G. Type bG
(b) fertile with G, sterile with B. Type Bg
(C) sterile with both parents. Type BG

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]

These experi­ments were continued and amplified by Shibata. Bruchmann[83] found that the spermatozoa of Lycopodium are positively chemotactic to citric acid and salts of this acid, although no citric acid could be shown in the contents of the archegonia. They are also positively chemotactic to the watery extract from archegonia.