. Cytology, with special reference to the metazoan nucleus. Cells. 82 CYTOLOGY CHAP. thrown out and the middle pieces break up, as in A. megalocephala, except that only two instead of about thirty small chromosomes are produced from each original one. In many animals the germ-track is marked out from the undivided egg onwards by characteristics of the cytoplasm instead of the nucleus. The fresh-water crustacean Cyclops furnishes an example. The germ- track in this animal was first worked out by Hacker in C. viridis (1897 a), his results being confirmed in all essentials by Amma in 1911, who al
. Cytology, with special reference to the metazoan nucleus. Cells. 82 CYTOLOGY CHAP. thrown out and the middle pieces break up, as in A. megalocephala, except that only two instead of about thirty small chromosomes are produced from each original one. In many animals the germ-track is marked out from the undivided egg onwards by characteristics of the cytoplasm instead of the nucleus. The fresh-water crustacean Cyclops furnishes an example. The germ- track in this animal was first worked out by Hacker in C. viridis (1897 a), his results being confirmed in all essentials by Amma in 1911, who also. Germ Cells Fig. 38. Scheme of the cleavage divisions in Ascaris megalocephala, (Boveri, Ergebnisse, 1904.) The uppermost cell is the fertilized ovum. 0 Cell in which chromatin diminution has not taken place. 'O* Cell in which chromatin diminution takes place. O Cell with diminished chromatin. discovered the same process in several different species of Cyclops and in the allied genera Diaptomus and Canthocamptus. The account given by the latter author for Cyclops fuscus will serve for an example (Fig. 39). In the prophase of the first cleavage division one attraction sphere is distinguished from the other by a group of granules which surrounds it, these being completely absent from the other sphere. Consequently, at cell division all the granules pass into one of the first two cells or blastomeres and none into the other ; nor do they ever appear in the descendants of the latter cell. In the case of the blastomere containing the granules the process is repeated in the following mitosis. After the first cell division is completed the granules become clumped together. 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, W. E. (Wilfred Eade), 1882-1951. London, Macmillan and Co. , limited
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