The Behavior of the Chromosomes in Fertilization.

The problems of heredity are intimately related to the two critical stages in the life cycle of the higher organisms, the reduction divisions of sporogenesis in plants and of gametogenesis in animals, and the cytological processes of fertilization. The first of these has received much attention and is generally considered to be closely correlated with the segregation of the factors which determine the characters of the next generation. The union of the gametes a t the time of fertilization is equally important, although we know little of the behavior of the chromosomes at this period. Recent research has made it desirable from both a cytological and a genetical standpoint to examine critically their behavior in the higher plants. The behavior of the chromosomes in animals at the time of fertilization is comparatively well known. In some cases the sexual nuclei unite while in the resting condition so that the male and female contribution of chromosomes can not be distinguished. It has been shown, however, that in many species the male and female chromosomes are formed before the fusion of the sexual nuclei and that they may maintain the two distinct groups during the first division of the zygote. In some species the independence of male and female chromosomes has been traced through several divisions and even to later cleavage stages. Indeed there are reasons for believing that the male and female chromosomes may maintain their independence until gametogenesis. In all cases the first division of the fertilized egg appears to be essentially similar to any other normal somatic division. In plants there is a great deal of variation in the behavior of the sexual nuclei in fertilization. In the algae it has been shown that in some cases the first division of the zygote is a reduction division. In Spirogyra (KARSTEN 1908) it has been found that the first division of the zygospore reduces the chromosome number from twenty-eight to fourteen.