. Embryology. Embryology; Embryology; genealogy. 116 EMBRYOLOGY. Fig. 57. A four-cell and an eight-cell salamander egg. Left: The intersection of the first and second cleavage furrows at the animal pole. Compare with Figure 55, which shows the four-cell stage with both the animal and vegetal hemispheres in view. Right: An eight-cell stage with four small cells on top and four large cells underneath. (Courtesy Professor ]ohn A. Moore, Columbia University.) The third cleavage gives rise to eight cells and is horizontal—that is, it follows a line parallel to the equator. After this cleavage the e
. Embryology. Embryology; Embryology; genealogy. 116 EMBRYOLOGY. Fig. 57. A four-cell and an eight-cell salamander egg. Left: The intersection of the first and second cleavage furrows at the animal pole. Compare with Figure 55, which shows the four-cell stage with both the animal and vegetal hemispheres in view. Right: An eight-cell stage with four small cells on top and four large cells underneath. (Courtesy Professor ]ohn A. Moore, Columbia University.) The third cleavage gives rise to eight cells and is horizontal—that is, it follows a line parallel to the equator. After this cleavage the egg is composed of four animal cells and four vegetal cells, with the vegetal cells distinctly larger (Fig. 55). The third cleavage is, therefore, holoblastic, unequal, and horizontal. An eight-cell salamander egg showing the four relatively small cells in the animal hemisphere is shown in Figure 57. These four cells are in direct contact with four larger cells in the vegetal hemisphere. The fourth cleavage of the frog egg passes through the animal-vegetal axis and follows the meridians of the egg. The result is a cluster of eight relatively small cells in the animal hemisphere and eight relatively large cells in the vegetal hemisphere. In Figure 55, stage 6, the fourth cleavage passed through the animal cells but is only part way through the vegetal cells. Late cleavage through gastrulation The next series of changes (Fig. 58) takes us from the 6th hour to the 28th hour of development. From 6 through 16 hours cleavage continues and the cells at the animal pole become very small; at 12 and 16 hours, therefore, the individual cells are no longer shown. The vegetal cells are relatively larger and are easily seen at 16 hours. Photographs of salamander eggs at. 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 Ba
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