. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 306 DONALD P. COSTELLO the entire spiral period of cleavage. However, the first two cleavages in forms such as Nereis have the first two cleavage planes somewhat differently directed than the third through the sixth cleavage planes (see Figure 11). In the somewhat simpler situation of spiral cleavage by duets, as exemplified hy the Polychoerus egg, it is only the first cleavage plane that is slightly anomalous, the others (until the transition to bilateral cleavage begins) being alternately leiotropic and dexiotropic. There
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 306 DONALD P. COSTELLO the entire spiral period of cleavage. However, the first two cleavages in forms such as Nereis have the first two cleavage planes somewhat differently directed than the third through the sixth cleavage planes (see Figure 11). In the somewhat simpler situation of spiral cleavage by duets, as exemplified hy the Polychoerus egg, it is only the first cleavage plane that is slightly anomalous, the others (until the transition to bilateral cleavage begins) being alternately leiotropic and dexiotropic. There are thus two "critical" periods in spiral cleavage. The first is the period before micromere formation (examples: the first cleavage of Polychoerus, and the first two cleavages of Nereis, Crepidula, etc.). The second occurs when there is an inevitable transition from the strict alternation of dexiotropic and leiotropic cleavages to bilateral cleavages which herald the production of the bilaterally sym- metrical acoel, polyclad, annelidan or molluscan larva. We know that in Crepidula (Conklin, 1897) the first cleavage is "prospectively" dexiotropic and the second is. 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 Marine Biological Laboratory (Woods Hole, Mass. ); Marine Biological Laboratory (Woods Hole, Mass. ). Annual report 1907/08-1952; Lillie, Frank Rattray, 1870-1947; Moore, Carl Richard, 1892-; Redfield, Alfred Clarence, 1890-1983. Woods Hole, Mass. : Marine Biological Laboratory
Size: 1567px × 1594px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
