The red ooplasm began streaming toward the vegetal pole
soon after release of the 1st polar body, which is consistent with results of experimental work showing that morphogenetic determinants within equally cleaving gastropod eggs become segregated along the animal-vegetal axis between release of the 1st and 2nd polar bodies or between the 2nd polar body release and 1st cleavage (4).
Eggs were fertilized and allowed to undergo the first phase of ooplasmic segregation, in which the cortical myoplasm is moved into the vegetal hemisphere and surrounds the vegetal pole. At a position corresponding to the animal pole, a tear was made in the follicular envelope (FE) that surrounds the egg.
In contrast, when four-celled embryos derived from smaller ME fragments produced from the vegetal pole region were stained with NN18, in most cases (95%; n = 19) more than two blastomeres reacted with the antibody ([ILLUSTRATION FOR FIGURE 13B OMITTED] and Table II).
The main characteristics of this development are (1) incomplete cleavage that becomes holoblastic; (2) external migration of mesenchyme cells, in the perivitelline space, from the animal to the vegetal pole during gastrulation; (3) hatching that occurs at the end of the gastrulation; (4) differentiation of the vestibule as soon as the end of gastrulation is attained; and (5) production of a juvenile directly from the gastrula without any larval stage.
It is analogous to the very beginning of the formation of the polar lobe of some mollusc species in which the first cleavage is accompanied by the formation of a protusion at the vegetal pole of the egg (Verdonk and van den Biggelaar, 1983).
This second contraction heralds the beginning of ooplasmic segregation in the form of streaming of ooplasm toward the animal pole, saltatory movements of parcels toward both polar regions, and movement of oil droplets (a class of ooplasmic inclusions) of various sizes toward the vegetal pole. At [T.sub.n] = 1.0, the blastodisc divides into two blastomeres.
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. In all regions of the egg except the animal pole, all the microtubules were within 2-3 [[micro]meter] of the surface of the egg and were visible in a single optical section.
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).
Sakai's hypothesis was based in part on the observation that when droplets of three oils - vegetable oil, liver oil, and mineral oil - were injected into the ooplasm of medaka eggs, they segregated toward the vegetal pole along with endogenous droplets (Sakai, 1965).
In most experiments, the equatorial region of the egg - that is, a region along a meridian and midway between the animal and vegetal poles
- was illuminated en profil (an edge of the egg was irradiated) via the 10X objective lens.
cells invaginated to form an archenteron.