. Cytology, with special reference to the metazoan nucleus. Cells. V CONTINUITY OF THE CHROMOSOMES 129 type D produce the nuclei shown in E, which have two equal projections, each containing two chromosome ends. In the next prophases (C, F) the chromosomes reappear in the same arrangement as they exhibited in the previous telophases. It will be noticed that the orientation of the nuclei towards each other in the two daughter cells has changed slightly owing to the rotation of the nuclei within the cells. This however does not affect their internal architecture. It must be understood that what
. Cytology, with special reference to the metazoan nucleus. Cells. V CONTINUITY OF THE CHROMOSOMES 129 type D produce the nuclei shown in E, which have two equal projections, each containing two chromosome ends. In the next prophases (C, F) the chromosomes reappear in the same arrangement as they exhibited in the previous telophases. It will be noticed that the orientation of the nuclei towards each other in the two daughter cells has changed slightly owing to the rotation of the nuclei within the cells. This however does not affect their internal architecture. It must be understood that what we have here described as a process is, like all similar work in cytology, really pieced together from a series of fixed stages. Thus it cannot actually be observed that the prophase nuclei of types C and F are the outcome of telophase nuclei of types A and D respectively, but this can be inferred without reasonable doubt since (1) they are connected up by a close series of intermediate stages of which B and E are examples; and (2) in the prophase, as in the. A B Fig. 59. Showing similar orientation of telophase (A) and prophase (B) in the epidermis of the salamander. (Rabl, , 1885.) telophase, the arrangement of the chromosomes in the two sister nuclei derived from the previous mitosis is the same. An orientation of the chromosomes in telophase similar to that in the succeeding prophase has been described by many cytologists from Rabl (1885) onwards (Fig. 59), though the conditions are seldom so favourable for observation as in A scar is megalocephala. Another fact directly supporting the hypothesis of the genetic continuity of the chromosomes is that each chromosome may undergo its telophase metamorphosis in a more or less separate vesicle within the nucleus. This is especially characteristic of Orthopteran spermato- genesis (Fig. 60). Sutton (1903) described the larger chromosomes of the spermatogonial telophase of Brachystola magna as forming each its own reticulum in a
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