. The biology of the frog . G H FlG. 16. — Segmentation of egg. (After Schultze.) A, two-cell stage,with the beginning of the second furrow; B, eight-cell stage, showingthe cross furrow at the animal pole; E, eight-cell stage; C, D, F, G,sixteen-cell stages, showing variations in the plan of cleavage ; //, thirty-two-cell stage. (From Morgans Development of the Frogs Egg.) V THE DEVELOPMENT OF THE FROG 91 rarely it lies at right angles to it, although it may occur inalmost any intermediate position. The second cleavage appears about three quarters of anhour after the first; the furrow extends
. The biology of the frog . G H FlG. 16. — Segmentation of egg. (After Schultze.) A, two-cell stage,with the beginning of the second furrow; B, eight-cell stage, showingthe cross furrow at the animal pole; E, eight-cell stage; C, D, F, G,sixteen-cell stages, showing variations in the plan of cleavage ; //, thirty-two-cell stage. (From Morgans Development of the Frogs Egg.) V THE DEVELOPMENT OF THE FROG 91 rarely it lies at right angles to it, although it may occur inalmost any intermediate position. The second cleavage appears about three quarters of anhour after the first; the furrow extends gradually from theanimal to the vegetal pole, at right angles to the first furrow,and divides the egg into four cells. The first and secondcleavage planes stand in a tolerably constant relation to theaxes of the body of the embryo, the first cleavage planemarking the median or sagittal plane of the future animal;but this is a rule not without exceptions. The third cleavage furrow comes in a little above theequator of the
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