. The cell; outlines of general anatomy and physiology. Cells; Anatomy; Physiology; Cells; Anatomy; Physiology. 262 THE CELL one makes its way in from the surface, whereas just the reverse occurs in Echinoderm eggs ; further, both nuclei are approxi- mately of the same size, and lie close together, although for a time they do not coalesce, but pass through a period of rest. Indeed, even after they have begun to prepare for the formation of the first division spindle, they do not commence to fuse. In consequence of this, and of the further circumstance, that in Ascaris megalocephala during nucl
. The cell; outlines of general anatomy and physiology. Cells; Anatomy; Physiology; Cells; Anatomy; Physiology. 262 THE CELL one makes its way in from the surface, whereas just the reverse occurs in Echinoderm eggs ; further, both nuclei are approxi- mately of the same size, and lie close together, although for a time they do not coalesce, but pass through a period of rest. Indeed, even after they have begun to prepare for the formation of the first division spindle, they do not commence to fuse. In consequence of this, and of the further circumstance, that in Ascaris megalocephala during nuclear division there develop only a few nuclear segments, which are of considerable size, and hence are easy to count, van Beneden (VI. 4a, 4b) was able to supple- Fig. 143.—{I-III). Three diagrams depicting the course of the processes of fertilisation in Ascaris megalocephala hivalens. ment our knowledge of the process of fertilisation by the following fundamental discovery :— During the preparation for the first , the nuclein in the egg- and sperm-nuclei, whilst these are still separated from one another, becomes transformed into a delicate thread which spreads itself out in many coils in the nuclear space. Each thread then divides into two twisted loops of equal size, the nuclear segments (Fig. 142 II). On either side of the pair of nuclei a centrosome makes its appearance; however, up till now, no one has been so fortunate as to observe whence these are derived. The line of demarcation between the two nuclei and the surrounding yolk now disappears. Between the two centrosomes (Fig. 142 III), which are sur- rounded by a radiation sphere, spindle fibrils develop ; these are at first faint, but later on are distinctly visible; they arrange themselves about the four nuclear segments, which have been set free by the breaking up of the nuclear vesicles, so that they rest externally upon the middle of the Please note that these images are extracted
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