. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 256 BIOLOGICAL LECTURES. rical cleavage is necessary for the symmetrical distribution of the precociously segregated material, so that it is to be expected that the axes of symmetry in the cleaving egg should correspond zvith the axes of symmetry in the adnlt. It is probable, however, that the establishment of these axes is significant as an incident connected zvith the mechanics of this form of cleavage, rather than as indicating differentiation. The time of division in the different blastomeres
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 256 BIOLOGICAL LECTURES. rical cleavage is necessary for the symmetrical distribution of the precociously segregated material, so that it is to be expected that the axes of symmetry in the cleaving egg should correspond zvith the axes of symmetry in the adnlt. It is probable, however, that the establishment of these axes is significant as an incident connected zvith the mechanics of this form of cleavage, rather than as indicating differentiation. The time of division in the different blastomeres is also a factor in the result attained. As mentioned above, the time at which the spiral form of cleavage ends is closely connected with the process of gastrulation, etc. All through the bilateral period of cleavage the divisions seem to occur at the right time to serve as a factor in morphogenesis. In the case of symmet- rical divisions on the two sides of the ^^g this fact is especially noticeable, and the best illustration is found in the later divisions of the somatic plate (Figs. 11-17). The symmetrical divisions occur at the same time, or nearly, although they may be almost on opposite sides of the ^g^. Sometimes one side is slightly ahead of the other, but in any case the variation is not great, and the arrangement of the material is the same in every case. In general, it may be stated that the larger cells divide more rapidly than the smaller, but this rule does not hold good in all cases, and does not apply of course to yolk-laden cells. In- deed, as Conklin ('97) and Lillie ('95) have suggested, ttie rapidity of division is apparently regulated in some cases by the time at which the portion of material concerned is to become functional, but neither does this rule apply to all cases. For instance, the primary trochoblasts in Arenicola are rel- ^;«.«.//«. Small size of crossed^ Smaller than in Amphi- and large size of primary trochoblasts (after V -j'"
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