. Outlines of zoology. Zoology. CELL DLVISION. 45 Protozoa, a more definite sheath is formed around the cell substance. Again, animal cells may die off superficially into a "cuticle," sometimes of known chemical composition, as, for instance, the chitin formed by the ectoderm cells in Insects, Crustaceans, and other Arthro- pods. In animals, as well as in plants, adjacent cells are often linked by „ „. f ,, intercellular bridsres of livins t: IG. 4. — Diagram of cell 00 division. (After Boveri.) matter. , ^, . Cell Division.—Though the cliT. Chromosomes forming an j- - • r ii i 1 - i
. Outlines of zoology. Zoology. CELL DLVISION. 45 Protozoa, a more definite sheath is formed around the cell substance. Again, animal cells may die off superficially into a "cuticle," sometimes of known chemical composition, as, for instance, the chitin formed by the ectoderm cells in Insects, Crustaceans, and other Arthro- pods. In animals, as well as in plants, adjacent cells are often linked by „ „. f ,, intercellular bridsres of livins t: IG. 4. — Diagram of cell 00 division. (After Boveri.) matter. , ^, . Cell Division.—Though the cliT. Chromosomes forming an j- - • r ii i 1 - i n equntorial plate; M. centrosoma. dlVlSlOH Ot CCUs, by whlch all growth is affected, is a subject with which the physiologist is as much concerned as the morphologist, it will be convenient to discuss it here. The following facts are most important. (i) We know that there is a striking unity in all cases, and that the nucleus plays an essential part in the process. In most cases the dividing nucleus passes through a series of complex changes known as karyokinesis or mitosis, and these are much the same everywhere, though different kinds of cells have their specific peculiarities. Occasionally, how- ever, both in Protozoa and Metazoa the nucleus divides by simple constriction (direct or amitotic division). (2) The eventful changes of karyokinesis are as follows;— [a] The resti}ig stage of the nucleus shows a network or complete coil of filaments (chromatin elements) (Fig, 2). (/') First Stage.—As division begins, the membrane separating the nucleus from the cell substance disappears, and the chromatin elements are seen as a tangled or broken coil (Fig. S, !)• [c] Astroid-stage.—The chromatin elements bend into looped pieces, which are disposed in a star, the free ends of the U-shaped loops being directed outwards. Meanwhile, a centrosome has appeared and divided into two separating halves, between which a spindle of fine achromatin threads is formed. This see
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
