Page:EB1911 - Volume 21.djvu/801

Rh structure and mode of formation, the spermatozoids of animals. In Characeae and Muscineae they are of elongate spiral form, and consist of an elongate dense nucleus and a small quantity of cytoplasm. At the anterior end are attached two cilia or flagella. In the Vascular Cryptogams the structure is much the same, but a more or less spherical mass of cytoplasm remains attached to the posterior spirals, and a large number of cilia are grouped along the cytoplasmic anterior portion of the spiral. In Zamia (fig. 4, A), Cycas and Ginkgo they consist of large spherical or oval cells with a coiled band of cilia at one end, and a large nucleus which nearly fills the cell. They are carried by the pollen tube to the apex of the prothallus, where they are extruded, and by means of their cilia swim through a small quantity of liquid, contained in a slight depression to the oosphere. In the other Phanerogams the male cell, which is non-motile, is carried to the oosphere by means of a pollen tube. In the spermatozoids of Chara, Vascular Cryptogams, and in those of Cycas, Zamia and Ginkgo, the cilia arise from a centrosome-like body which is found on one side of the nucleus of the spermatozoid mother-cell. This body has been called a blepharoplast, and in the Pteridophytes, Cycads and Ginkgo it gives rise to the spiral band on which the cilia are formed. Belajeff regards it as a true centrosome; but this is doubtful, for while in some cases it appears to be connected with the division of the cell, in others it is independent of it. The egg-cell or oosphere is a large cell containing a single large nucleus, and in the green plants the rudiments of plastids. In plants with multinucleate cells, such as Albugo, Peronospora and Vaucheria, it is usually a uninucleate cell differentiated by separation of the nuclei from a multinucleate cell, but in Albugo bliti it is multinucleate, and in Sphaeroplea it may contain more than one nucleus. In some cases the region where the penetration of the male organ takes place is indicated on the oosphere by a hyaline receptive spot (Oedogonium, Vaucheria, &c.), or by a receptive papilla consisting of hyaline cytoplasm (Peronosporeae). Fertilization is effected by the union of two nuclei in all those cases which have been carefully investigated. Even in the multinucleate oosphere of Albugo bliti the nuclei fuse in pairs; and in the oospheres of Sphaeroplea, which may contain more than one nucleus, the egg nucleus is formed by the fusion of one only of these with the spermatozoid nucleus (Klebahn). In the higher Fungi nuclear fusions take place in basidia or asci which involve the union of two (fig. 7, A) nuclei, which may be regarded as physiologically equivalent to a sexual fusion. The union of the germ nuclei has now been observed in all the main groups of Angiosperms, Gymnosperms, Ferns, Mosses, Algae and Fungi, and presents a striking resemblance in all. In nearly all cases the nuclei appear to fuse in the resting stage (fig. 3, C). In many Gymnosperms the male nucleus penetrates the female nucleus before fusing with it (Blackman, Ikeno). In other cases the two nuclei place themselves side by side, the nuclear membrane between them disappears, and the contents fuse together—nuclear thread

with nuclear thread, and nucleolus with nucleolus—so completely that the separate constituents of the nuclei are not visible. It was at one time thought that the centrosomes played an important part in the fertilization of plants, but recent researches seem to indicate that this is not so. Even in those cases where the cilia band, which is the product of the centrosome-like body or blepharoplast, enters the ovum, as in Zamia (c in fig. 4, B, C, D), it appears to take no part in the fertilization phenomena, nor in the subsequent division of the nucleus. During the process of fertilization in the Angiosperms it has been shown by the researches of Nawaschin and Guignard that in Lilium and Fritillaria both generative nuclei enter the embryo sac, one fusing with the oosphere nucleus, the other with the polar nuclei (fig. 3, A, B). A double fertilization thus takes place. Both nuclei are elongated vermiform structures, and as they enter the embryo sac present a twisted appearance like a spermatozoid without cilia (fig. 3, A, B). It has since been shown by other observers that this double fertilization occurs in many other Angiosperms, both Dicotyledons and Monocctyledons, so that it is probably of general occurrence throughout the group (see ).

The Nucleus in Relation to Heredity.—There is a certain amount of cytological evidence to show that the nucleus is largely concerned with the transmission of hereditary characters. Whether this is entirely confined to the nucleus is, however, not certain. The strongest direct evidence seems to be that the nuclear substances are the only parts of the cells which are always equivalent in quantity, and that in the higher plants and animals the male organ or spermatozoid is composed almost entirely of the nucleus, and that the male nucleus is carried into the female cell without a particle of cytoplasm.

Since, however, the nucleus of the female cell is always accompanied by a larger or smaller quantity of cytoplasm, and that in a large majority of the power plants and animals the male cell also contains cytoplasm, it cannot yet be definitely stated that the cytoplasm does not play some part in the process. On the other hand, the complex structure of the nucleus with its separate units, the chromosomes, and possibly even smaller units represented by the chromatin granules, and the means taken through the complex phenomena of mitosis to ensure that an exact and equal division of the chromosomes shall take place, emphasizes the importance of the nucleus in heredity. Further, it is only in the nucleus and in its chromosomes that we have any visible evidence to account for the Mendelian segregation of characters in hybrids which are known to occur. Visible differences in the chromosomes have even been observed, especially in insects, which are due apparently to an unequal division by which an additional or accessory chromosome is produced, or in some cases one or two extra chromosomes which differ in size from the others. These differences indicate a separation of different elements in the formation of the chromosomes and have been definitely associated with the determination of sex. It is possible, however, that the segregation of characters in the gametes may depend upon something far more subtle and elusive than the chromosomes or even of possible combinations of units within the chromosomes, but so far as we can see at present these are the only structures in the cell with which it can be satisfactorily associated. Boveri in fact has put forward the view that the chromosomes are elementary units which maintain an organic continuity and independent existence in the cell. The cytological evidence for this appears to be made stronger for animal than for plant cells. From numerous investigations which have been made to trace the chromosomes through the various stages of the nuclear ontogeny of plant cells, it appears that the individuality and continuity of the chromosomes can only be conceived as possible if we assume the existence of something like chromosome centres in the resting nucleus around which the chromosomes become organized for purposes of division. Rosenberg (1909) adduces evidence for