Fertilization,— One of the most important problems of cytology is the problem of fertiliza tion and attendant phenomena. While new in dividuals, even in the more highly organized plants and animals, may arise occasionally with out fertilization (by parthenogenesis, chemical stimulus, vegetable multiplication, etc.), such cases are so rare that fertilization is assumed to occur unless its absence is definitely established. To the cytologist, fertilization consists in the union of definitely organized male and female elements. The cytological details of fertilization are in plants and animals. (Fig. 3). Fertilization, both in plants and in animals, is preceded by a reduction in the number of chromosomes, so that the number of chromo somes found in the male nucleus or in the fe male nucleus is just one-half the number which is found in the body cells of a given plant or animal. Consequently, when the two sexual nuclei fuse during fertilization, the number of chromosomes which characterizes the body cells is restored. The complicated details of the proc esses by which this reduction in the number of chromosomes is effected are essentially alike in animals and plants. The closing paragraphs of this article show that the chromosome con tributions of the two parents are not quite so identical as stated above, and that the differ ences are effective in determining the sex of the offspring. Those cytologists who have in vestigated most thoroughly the phenomena of fertilization have come to the conclusion that heredity is referable to a definite cytological basis.
A Cytological View of without exception, cytologists believe that chro matin is the physical basis of heredity. The reasons for this belief are, briefly, as follows: The male and female parents are equally potent in transmitting characters to offspring; an equal amount of chromatin and an equal number of chromosomes are contributed by each parent; nothing but chromatin is contributed equally by the two parents. (Fig. 4). There is usually a great difference in size between the male and female germ cells. The sperm cell (spermato zoan) of the ostrich is almost invisible to the naked eye, while the egg is as large as a coco nut, and such differences in size are usual both in plants and animals. The egg contains a large amount of protoplasm and vanous food stuffs, while the male cell contains very little protoplasm or foodstuffs and in many cases even that little is left outside at the time of fertilization, only the nucleus entering the egg. Hence, protoplasm and foodstuffs do not trans mit hereditary characters. While the male nu cleus is usually the smaller at the time of its entrance into the egg, it increases in size so that, at the time of fusion, the sex nuclei are alike in size. (See Fig. 3). Investigators of this subject are usually tempted into speculative phi losophy. Philosophizing over the facts is com mendable, but the wild philosophizing in ad vance of the facts which has attended this subject has been of little value, except as it has stimulated investigation.
The organization of embryos and mature or ganisms from eggs is a cytological problem which has not yet been solved. The visible p stages in development have been observed and described ad its nituns. Both experimental cy tology dealing ergely with living material, and anatomical studies of thin sections, stained so as to show the most minute details of structure, are contributing to the solution of the problem, but the fundamental causes underlying thep he nomena are still unknown and seem as difficult of solution as the problem of the origin of life. The eggs of the sunflower and the willow, like the eggs of all flowering plants, are too small to be examined with the naked eye, but even when examined with the aid of modern tech nique and the most powerful microscopes, they present no essential differences in external ap pearance or internal structure, and yet one al ways develops into a sunflower, and the other into a willow. Within the fertilized egg are all the potentialities of the adult, even to the color of the flower or the markings on the wings of the butterfly. Some have believed that each part of the adult comes from a certain predeter mined part of the fertilized egg. This is called the theory of germinal localization. Others have made experiments to show that this theory is not true. Most botanists and zoologists ac cept, as a working hypothesis, that chromatin is the physical basis of heredity, an hypothesis which assumes that there is no prelocalization in the protoplasm of the egg. The problem of organization is so bound up with that of hered ity that much of the literature of the subject will be found in treatises on heredity.
At present, the determination of sex is re ceiving a great deal of attention from students of both animal and plant cytology. In various animals it has been shown that the chromosome contributions of the two parents are not identi cal. In the grasshopper, all the eggs have six chromosomes; but, in the male, two of the four sperms developed from each sperm-mother-cell have six cromosomes and the other two have only five. Eggs fertilized by sperms containing six chromosomes produce females, while those fertilized by sperms with only five chromosomes produce males. Similar results have been re corded for other animals. The problem in plants is much more difficult because the separa tion of the sexes is not so prevalent and be cause cases are rare in which the four sperms formed from one mother-cell remain together for any length of time. However, some prog ress is being made and it seems probable that differences in the chromosome contributions of the two parents are responsible for the deter mination of sex in plants.