Figure 1J-N shows the zygote at early anaphase of the first meiosis viewed from the animal pole. Chromosomes were segregated to both poles (Fig.
Figure 3A shows a schematic representation of polar body formation observed perpendicular to the animal pole. Figure 3B shows a schematic representation of actin distribution in eggs viewed from the animal pole.
Interestingly, in oocytes of invertebrates, such as starfish, fruit fly, jelly fish, oligochaetes, and surf clam, during polar body formation, the actin cortical layer is very thin or almost absent at the animal pole (Shimizu 1997, Pielak et al.
As a result of chlorpyrifos treatment in this sensitive period, a compact extralarval cap, largely comprised of pigmented cells, formed at the animal pole, and the blastocele, indeed the larva itself, was correspondingly diminished because of both cell extrusion and compaction.
Before its loss, the cap was connected to the animal pole of the larva by a thin stalk.
Cell migration itself appears not to be an actual target for the effects of chlorpyrifos, because the movement of cells continued in the normal direction along the animal--vegetal axis, albeit that the cells were migrating into the enormous extralarval cell mass through an abnormal structure, the thin stalk attached to the animal pole. The reverse direction of cell movement (exogastrulation) was seen in some experimental situations, too, simultaneously with formation of the extralarval cap (see, for example, Figure 4C).
Fine structure and early fertilization changes of the animal pole
in eggs of the river lamprey, Lampetra fluviatilis.
Fertilization in the medaka egg is followed by a cortical granule reaction and a contraction, during which the ooplasm and its contents appear to be pulled toward the animal pole. The second meiotic division is completed by [T.sub.n] [approximately equal to] 0.12 and is followed closely by a second animal-pole-directed contraction at [T.sub.n] [approximately equal to] 0.15.
We identified three regions of the medaka egg on the basis of the spatiotemporal pattern of microtubules in them [ILLUSTRATION FOR FIGURES 1-3 OMITTED]: (1) a region within 30 [degrees] ([approximately equal to] 300 [[micro]meter]) arc of the animal pole; (2) a region within 60 [degrees] ([approximately equal to] 600 [[micro]meter]) arc on both sides of the equator; and (3) a region within 30 [degrees] ([approximately equal to] 300 [[micro]meter]) arc of the vegetal pole.
The suggestion that microtubules are involved in the segregation of oil droplets toward the vegetal pole is inconsistent with the hypothesis that this segregation is caused simply by the bulk flow of ooplasm in the opposite direction, that is, toward the animal pole (Sakai, 1965).
Thus, another objective of the present study was to monitor the movement of injected droplets of five fluids that differ widely in their chemical and physical properties; the eggs in the present study were oriented with either their animal pole or their vegetal pole uppermost.
The results illustrated in Figure 1B (a representative example; n = 12), confirm the earlier findings from the animal pole
(3) and the vegetal pole (2).