SELF FERTILIZATION 321 



in plants it is assumed that this is very small, but there is the possibility 

 that, actually, also a larger number may control this mechanism. In ascidians, 

 it is at least possible that the number of determining genes may be consider- 

 able and that in this respect the latter resemble the factors which determine 

 the individuality differentials in the somatic cells of higher organisms. 



In conclusion it may then be stated that while in transplantation of tissues 

 in higher adult organisms incompatibilities in the interaction of host and 

 transplant are avoided only when autogenous relations exist, in fertilization, 

 in general, a homoiogenous relationship produces the most adequate results 

 and the subsequent perfect development of the embryo, although in various 

 plants and animals autogenous fertilization occurs normally, without injurious 

 consequences. As to heterogenous fertilization, this leads, as a rule, to in- 

 jurious results in plants as well as in animals, but in different instances the 

 distance in relationship between spermatozoon and egg, which results in 

 abnormalities, varies. Thus in the case of Zea Mays, Demerec observed that 

 in the variety "everta", homoiogenous as well as autogenous fertilization 

 was successful, but that fertilization with other varieties of Mays did not 

 succeed. Usually we must assume that species differences between male and 

 female plants produce an injurious effect greater than that characteristic of 

 variety differences. On the other hand', certain heterogenous fertilizations in 

 Echinoderms may lead to the formation of normal organisms as far as somatic 

 differentiation is concerned, although the parents belong to different genera or 

 even to different orders. 



In general, homoio- fertilization is the normal process most conducive in 

 plants and animals to an undisturbed development of the embryo and the 

 chromosomes and cytoplasm of the germ cells are not adversely affected by 

 such a relationship. We find, then, in the interaction between spermatozoon 

 or pollen-tube and egg or ovary, reactions which presuppose the presence 

 of individual and species substances or of mechanisms which are characterized 

 by a great sensitiveness to these individual and species differences. The 

 individuality differentials of higher adult animal organisms are not yet de- 

 veloped in these less developed cells and tissues, but there is reason for assum- 

 ing that the latter possess the precursors of organismal differentials. It is pos- 

 sible that such precursor substances are involved in the individual reactions 

 noted in these cells and tissues, at least in some instances ; on the other hand, it 

 is also possible that substances of a special kind are concerned in these re- 

 actions. Such substances and mechanisms of a special kind are present in 

 certain protozoa, and the terms "organismal and individuality differentials" 

 apply to these cases only in a wider sense. In a specific sense, they refer only 

 to the adult tissues of higher vertebrates. 



Inbreeding: There is a condition intermediate between self-fertilization and 

 homoiogenous fertilization, namely, syngenesio-fertilization. This is a process 

 which corresponds to inbreeding. In higher organisms, close inbreeding is 

 effected by means of successive, long-continued brother and sister mating. 

 As a result of this procedure the genetic constitution of such inbred individ- 

 uals becomes gradually more and more similar, until in the end, syngenesio- 



