. Text-book of embryology. Embryology. '^'^Plh'^^'^^^^^^>Cy^'^^^^^''^^'^P. Fig 428.—Stages in tlie segmentation of the egg of Amphioxus lanceolatus. {After Cerfontaine.) A, 32-cell stage seen from the side. B, optical sagittal section of a young blastula. , animal pole of the egg; pb^, second polar body ; , vegetative pole of the egg. 32-cell stage we have actually eight tiers of four cells each. But, as is shown in Fig. 428, this unstable arrangement does not persist. The blastomeres glide on one another, and we get a circle of four cells at both the animal and the vegetative pole,
. Text-book of embryology. Embryology. '^'^Plh'^^'^^^^^^>Cy^'^^^^^''^^'^P. Fig 428.—Stages in tlie segmentation of the egg of Amphioxus lanceolatus. {After Cerfontaine.) A, 32-cell stage seen from the side. B, optical sagittal section of a young blastula. , animal pole of the egg; pb^, second polar body ; , vegetative pole of the egg. 32-cell stage we have actually eight tiers of four cells each. But, as is shown in Fig. 428, this unstable arrangement does not persist. The blastomeres glide on one another, and we get a circle of four cells at both the animal and the vegetative pole, and three intervening tiers of eight cells each. The tier at the animal pole consists of the smallest cells, that at the vegetative pole of the largest, and in the intermediate tiers we have a gradual passage from the one size to the other. At the next period of cleavage the planes of division are no longer parallel to one another in all the cells. The equatorial cells divide into right and left daughters, but the planes of division in the polar cells are oblique. After the next cleavage the egg has divided into 128 cells, and these commence to flatten out against each other and to take on the character of a columnar epithelium which is ciliated. At the next stage, when there are 256 cells, the embryo is a. 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 MacBride, E. W. (Ernest William), 1866-1940; Kerr, John Graham, 1869-; Heape, Walter, 1855-1929. London : Macmillan
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