Page:Popular Science Monthly Volume 85.djvu/119

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PSM V85 D119 Mouse egg maturation and fertilization diagram.png

Fig. 25. Maturation and Fertilization of the Egg of the Mouse. A, first polar body and second maturation spindle; B, second polar body and maturation spindle; C, entrance of the spermatozoon into the egg; D-G, successive stages in the approach of egg and sperm nuclei; H, formation of chromosomes in each germ nucleus; I, first cleavage spindle showing chromosomes from egg and sperm on opposite sides of spindle. (After Sobotta.)

grows thinner and finally disappears altogether, leaving the chromosomes in the equator of the spindle (Figs. 23 F, 24 E and F, 25 I).

Each of the chromosomes then splits lengthwise into two equal parts, and in the splitting of the chromosomes it is sometimes possible