. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. JENNINGS : DEVELOPMENT OF ASPLANCHNA HEKRICKII. 41 the factors mentioned. Thus, Figure 56 (Plate 7) shows that the cell b^-^ is dividing before the cells c®*^ and b^-'^, though the cells are appar- ently of the same size, and from the sequence of preceding cleavages the cell c^-^ would be expected to divide first. However, such variations mat/ be correlated with differences in the size of the cells, since it is impossible to calculate precisely the volume of cells which have such irregular forms, and are subj
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. JENNINGS : DEVELOPMENT OF ASPLANCHNA HEKRICKII. 41 the factors mentioned. Thus, Figure 56 (Plate 7) shows that the cell b^-^ is dividing before the cells c®*^ and b^-'^, though the cells are appar- ently of the same size, and from the sequence of preceding cleavages the cell c^-^ would be expected to divide first. However, such variations mat/ be correlated with differences in the size of the cells, since it is impossible to calculate precisely the volume of cells which have such irregular forms, and are subjected to varying conditions with the chan- ging positions of the surrounding cells. Certain general facts appear from the preceding discussion of the sixth cleavage. (Compare the table of this cleavage, on page 39.) (1) Every cell of any quadrant cleaves with its spindle in the same direction as the corresponding cell of any other quadrant (except the large interior cell d^-^). The cleavage of a single typical quadrant up to this time is shown iu the annexed diagram (Diagram I.).. Diagram I. Diagram of quadrant A, B, or C in the seventh generation. Only the second exponent designating the cells appears in the diagram, the first being in all cases 7. Thus, the cell labelled 5 represents (n, b, or c)"'5. The arrows connect cells of common origin, and show the direction of the spindles at the preceding division. R signifies right; L, left; D, dorsal; V, ventral; according to the plan of orientation explained at page 14. (2) All the cells in any layer (series of cells occupying the same relative position between the dorsal and ventral poles of the egg) cleave with spindles in the same direction (except <P-^). (.3) All the cleavages are equal except in the dorsal (fifth) layer, and in d^-\ c^-\ and d^-\ (\) There is a tendency for the largest cells to cleave fastest. The minute cell d^-'^ does not cleave at all. (o) The cell rf®-^ cleaves in such a manner as to form a mark
Size: 1369px × 1826px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorha, bookcentury1800, bookdecade1860, booksubjectzoology
