. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. GONOMERY 79 nucleus, so that each of the conjugating amoebae has now a single nucleus. These nuclei undergo a process of meiosis, comparable to the formation of the polar bodies of a Metazoan egg, converting each amoeba into a single gamete. The two gamete cells fuse together to form a zygote, their nuclei, however, remaining unfused. Thus the binucleate condition is restored, to be retained through an indefinite number of cell di\dsions. Fig. 36. Amoeba diploidca. (After Niigler, , 1909.) A, the animal in its a
. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. GONOMERY 79 nucleus, so that each of the conjugating amoebae has now a single nucleus. These nuclei undergo a process of meiosis, comparable to the formation of the polar bodies of a Metazoan egg, converting each amoeba into a single gamete. The two gamete cells fuse together to form a zygote, their nuclei, however, remaining unfused. Thus the binucleate condition is restored, to be retained through an indefinite number of cell di\dsions. Fig. 36. Amoeba diploidca. (After Niigler, , 1909.) A, the animal in its active phase, showing the double nucleus (gonomeres); B, C, division stages showing simultaneous division of the gonomeres ; D, two indi- viduals encysted preparatory to conjugation ; E, in each individual the gonomeres have fused into a single nucleus; F, conjugation has taken place, and the zygote with the two gamete nuclei (gonomeres) is emerging from the cyst, thus bringing the life cycle back to A again. during asexual reproduction, the two gonomeres fusing together for the first time immediately before gamete formation. D. THE GERM-TRACK One more feature of early development remains to be mentioned. In a large number of animals the primitive germ-cells—those cells, that is to say, that will eventually give rise to gametes—are visibly marked out from the remaining or somatic cells at a very early stage of development. The distinguishing marks may be features either of the nucleus or cyto- plasm. The best-known case is that of Ascaris megalocephala (Boveri, 1899, 1904, 1910). Nothing remarkable is to be observed in the first cleavage division,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Agar, Wilfred Eade, 1882-. London Macmillan
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectcells, bookyear1920
