. The development of the frog's egg; an introduction to experimental embryology. Embryology. 38 DEVELOPMENT OF THE FROG'S EGG [Ch. IV larsfer, and two are intermediate in size. The smallest blasto- meres of the upper four ahvays lie nearest the summit of the tvhite crescent; the largest is its vis-d-vis. If we think of the third planes of cleavage as lying in a single plane not quite at right angles to the first and second, but tilted a little, we get a clearer conception of the conditions present. The fourth cleavage- period comes in from a half to three-quarters of an hour later. As an ideal
. The development of the frog's egg; an introduction to experimental embryology. Embryology. 38 DEVELOPMENT OF THE FROG'S EGG [Ch. IV larsfer, and two are intermediate in size. The smallest blasto- meres of the upper four ahvays lie nearest the summit of the tvhite crescent; the largest is its vis-d-vis. If we think of the third planes of cleavage as lying in a single plane not quite at right angles to the first and second, but tilted a little, we get a clearer conception of the conditions present. The fourth cleavage- period comes in from a half to three-quarters of an hour later. As an idealized form, we may think of the new planes as form- ing two great circles at right angles to each other, and lying vertically and between the planes of the first and second cleav-. B Fig. 14.—a. Diagram of four-cell stage to show cross-line. B, C. Sixteen-cell stage of two eggs. (After Rauber.) ages (Fig. 13, C). This regular form is rarely if ever attained, and the greatest amount of variation is found to exist. Remak said that frog's eggs divide much more regularly when carried in from places wher5 tbey were normally laid. If allowed to stand quietly after being laid, they soon begin to divide irregtdarly. Vogt has also observed that those eggs of the salmon develop most regularly that have been kept in motion. It does not seem probable, however, that the motion itself could have anything to do directly with the matter; but if the egg be not supplied with a sufficient amount of fresh Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Morgan, Thomas Hunt, 1866-1945. New York, The Macmillan company; [etc. , etc. ]
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