320 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



tion of hybrids, the germ cells in the latter may have abnormal numbers of 

 chromosomes. Besides, abnormalities in the reduction division and, further- 

 more, a non-disjunction of chromosomes may be observed. Still later, in- 

 compatibilities may become manifest between the nuclei of the male and 

 female germ cells if fertilization between hybrids should be attempted. Con- 

 ditions are, then, in these respects, analogous in plants and in animals; in 

 both, a homoiogenous relationship between the substances produced in cells 

 belonging to the male or female organism, as well as between the cytoplasm 

 and the nuclei of the gametes, is most conducive to the normal development 

 of a new organism. 



II. While in higher plants the prevention of self-fertilization occurs in 

 many species, in hermaphroditic animals it has been observed so far only in 

 several ascidians, and especially in Ciona. In other animals, for instance, 

 the oligochaetae, self-fertilization may occur and lead to the normal de- 

 velopment of the ovum, and even self-copulation may take place in certain 

 species. 



The fact of self-sterility in Ciona was first observed by Castle, and its 

 mechanism has been studied especially by T. H. Morgan. According to Mor- 

 gan self -sterility in this species depends on a condition in the egg membranes, 

 which prevents the entrance of spermatozoa derived from the same individual, 

 whereas spermatozoa from individuals which were not the bearers of the 

 eggs were able to penetrate through the membranes. A short treatment of the 

 egg membrane with acid makes the latter permeable also for the spermato- 

 zoa from the same individual; likewise, the use of dense sperm suspensions 

 renders the chances of self-fertilization better. However, it has not been 

 possible to extract substances from the eggs or sperm which noticeably in- 

 fluenced the results of self- or cross-fertilization. In different ascidian species, 

 the readiness with which self-fertilization succeeds varies; it is greatest in 

 Molgula, intermediate in Styela, and very rare in Ciona. It is the sameness 

 in the genetic constitution of the spermatozoon and the ovum which tends 

 to prevent fertilization and a difference in this constitution which makes 

 possible self-fertilization. Morgan suggests that self-fertilization may be- 

 come possible as a result of a mutation, which alters the genetic constitution 

 of a spermatozoon and makes it unlike that of the egg. It is not known how 

 many genetic factors are involved in this process of self-sterility. 



It seems then that the mechanism which prevents self-fertilization in 

 Nicotiana, Petunia and some other plants, and in ascidians, is not the same, 

 inasmuch as in the latter it depends on the relations between spermatozoon 

 and egg membrane, whereas, in the former, it depends largely on the inter- 

 action between ovary and pollen-tube; but also in plants, self-fertilization 

 may be inhibited in certain instances by incompatibility between the fertilizing 

 element in the pollen-tube and the ovum, as well as by the usual mechanisms. 

 The processes which prevent self-fertilization in these plants and in ascidians 

 agree, in so far as in both, genetic factors determine primarily whether or 

 not self-fertilization can take place; but the effects exerted by these genetic 

 factors differ. As to the number of genetic factors involved in these processes, 



