Experimental embryology / by Thomas Hunt Morgan.

  • Thomas Hunt Morgan
Date:
1927
Catalogue details

Licence: In copyright

Credit: Experimental embryology / by Thomas Hunt Morgan. Source: Wellcome Collection.

    139/790 page 121
    CHAPTER X CLEAVAGE AND THE MECHANISM OE CLEAVAGE The cleavage of eggs offers by far the best opportunity for the experimental study of cell-division, because eggs are free cells that can often be procured in immense numbers, all at the same stage and due to divide at a definite time after fertilization at a given temperature. The division into two equal parts is generally regarded as the typical division (Eig. 21&), and while many eggs do divide at first into two equal halves (or blastomeres) other kinds of eggs divide unequally. In many eggs the first division is complete before the second begins, but in other eggs the first division may not have passed completely through the egg before prepara- tions for the second begin. In some eggs the constriction starts as a ring around the egg, but in most eggs of this type the con- striction begins a little sooner at the pole, and passes more rapidly from the pole inwards than it does from other parts of the per- iphery. In a few eggs, as in the ctenophores (Fig. 132), the constriction beginning at the polar field cuts through from that point inward, finally completing itself at the opposite pole. There are other eggs, rather large as a rule, and containing much yolk, in which the segmentation nucleus divides into two, but no super- ficial cytoplasmic division occurs at the time. Many insects and crustaceans show this type of cleavage (Fig. 22&, b). Further nuclear divisions take place as the nuclei gradually move to the surface layer of the egg (Fig. 22c). After the nuclei have reached the surface the superficial layer of protoplasm divides, each nucleus becoming the center of a cell. When an egg cleaves, its blastomeres tend to round up at each division into a sphere. This is, as a rule, interfered with by the confining membrane which keeps the blastomeres pressed against each other (Fig. 21«), but even when the membrane is removed, or is not present immediately around the egg, the
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