. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. T. H. MORGAN AND ALBERT TYLER 64 2, b, and Fig. 2, c) ; similarly when the small cell (^/£) lies to the left (Figs. 3, b, and 3, c). As a matter of fact it was found in these 77 cases of coincidence that when the AB was to the right, only one of the two theoretical types appeared, namely, that shown in Fig. 2, b. When the AB was to the left, again only one of the two theoretical types appeared, namely, that shown in Fig. 3, b. Ordinarily the cleav-. FIG. 3. The same as Fig. 2, except that the AB-ce\l lies to the left of the
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. T. H. MORGAN AND ALBERT TYLER 64 2, b, and Fig. 2, c) ; similarly when the small cell (^/£) lies to the left (Figs. 3, b, and 3, c). As a matter of fact it was found in these 77 cases of coincidence that when the AB was to the right, only one of the two theoretical types appeared, namely, that shown in Fig. 2, b. When the AB was to the left, again only one of the two theoretical types appeared, namely, that shown in Fig. 3, b. Ordinarily the cleav-. FIG. 3. The same as Fig. 2, except that the AB-ce\l lies to the left of the sperm-entrance point. age giving the first type (Fig. 2, b) would be called a leiotropic second cleavage, implying that the third would be dexiotropic. The second type, Fig. 3, b, would be called a dexiotropic second cleavage, implying a leiotropic third. However, a study of the third cleavage of Cumingia has shown that the division is always dexiotropic. This information was obtained from eggs preserved at the time of the oncoming third cleavage. The orientation of the spindles with respect to the poles was determined in 84 eggs, and in every case they showed the cleavage to be dexiotropic (Fig. 5). The observation shows in the first place that it would have been erroneous to conclude that because the third cleavage is dexiotropic, the second must have been leiotropic. It would have been equally erroneous to have concluded from the two types of four-cell stages that the direction of the spiral would be dif- ferent in the two types. By parity of reasoning it would seem unjusti- fiable to infer that because a given egg shows a leiotropic second cleav- age, the first cleavage must have been dexiotropic, and thus to designate the egg as a dexiotropic egg. .. 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 Mari
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
