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 C Y T 0 L O G Y may be motile (some Algae) or non-motile, as in Spirogyra, Mucor, Basidiobolus, &c. In many of the lower plants and in all higher plants there is a difference in size in the fusing cells, the male cell being the smaller. The reduction in size is due to the absence of cytoplasm, which is in some cases so small in amount that the cell consists mainly of a nucleus. In all cases of complete sexual differentiation the egg-cell is quiescent, but the male cell may be motile or non-motile. In many of the Fungi the non-motile male cell or nucleus is carried by means of a fertilizing tube actually into the interior of the egg-cell, and is extruded through the apex in close proximity to the egg nucleus. In the Floridese, Lichens, and Laboulbeniacem the male cell is a non-motile spermatium, which is carried to the female organ by movements in the water. In Monoblepharis, one of the lower Fungi, in some Algae, in the Vascular Cryptogams, in Cycads {Zamia and Cycas), and in Ginkgo, an isolated genus of Gymnosperms, the Fig. 3.—Fertilization in the Lily. (From Wilson, after Guignard and Mottier.) A, male cell is a motile Two vermiform nuclei in the embryo sac; one approaching the egg-nucleus, the other spermatozoid with uniting with the upper polar nucleus. B, two or more cilia. In Union of the vermiform nuclei with the eggnucleus and the two polar nuclei. C, Fusion the Algae such as of the germ nuclei in the egg-cell; a, antipodal cells; p, polar nuclei; pt, pollen Fucus, Volvox, Oedotube. gonium, Bulbochcete, and in the Fungus Monoblepharis, the spermatozoid is a small oval or elongate cell containing nucleus, cytoplasm, and sometimes plastids. In the Characeae, the Vascular Cryptogams, in Zamia and Gycas, and in Ginkgo, the spermatozoids are more or less highly modified cells with two or more cilia, and resemble in many respects, both in their 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 of 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 Gingko, 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 Gingko 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

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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 Cystopus,. Peronospora, and Vaucheria, it is usually a uninucleate cell differentiated by separation of the nuclei from a multinucleate cell, but in Cystopus Bliti it is multinucleate, and in Sphwroplea 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 {CEdogonium, Vaucheria, &c.), or by a receptive papilla consisting of hyaline cytoplasm (Peronosporeae). Fertilization is effected by the union of two nuclei only in all those cases which have been carefully investigated. Even in the multinucleate oosphere of Cystopus Bliti the nuclei fuse in pairs; and in the oospheres of Sphceroplea, 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 Fig. 4.—Spermatozoid and fertilization in higher Fungi nuZamia. (After Webber.) clear fusions take place in Basidia or Asci which involve the union of two and sometimes more 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