The main target of chromosome manipulation in these studies was inhibition of polar body (PB) formation or first cleavage division. Most of these studies observed and described chromosome movements using normal light (Crassostrea virginica: Longwell & Stiles 1968), electron microscopy (Crassostrea gigas: Longo et al.
The objectives of this work were to document early development starting with meiosis in newly fertilized eggs through the first and second cleavage divisions, including centrosome inheritance and duplication observed by confocal microscopy.
The second cleavage division, which begins shortly before the first is complete, also occurs in a distal-proximal direction and is perpendicular to the first one.
By the fourth to fifth cleavage division, a cavity begins to form in the interior of the cell mass [ILLUSTRATION FOR FIGURE 11 OMITTED], while the surface remains uneven [ILLUSTRATION FOR FIGURE 8 OMITTED].
Thereafter, the cleavage divisions are very irregular.
Dynamics of filamentous actin organization in the sea urchin egg cortex during early cleavage divisions
: implications for the mechanism of cytokinesis.
The concentration of ACh exhibits distinct peaks during early cleavage divisions, but the major, sustained increases occur after the beginning of gastrulation, in tandem with transcription of zygotic genes and attendant rises in choline acetyltransferase, the enzyme that synthesizes ACh (Buznikov et al.
DMAE added to the embryos at the two-cell stage in concentrations of 100-400 [micro]M did not alter cleavage divisions or blastulation, but at very high concentrations (600-800 [micro]M), cleavages were inhibited (Figures 2 and 3).
Like those of a large majority of taxa, sea urchin embryos establish a spatial coordinate system for the initial body plan from one axis, the animal-vegetal (A-V), that is fixed during oogenesis by asymmetric deposition of maternal molecules (the embryologists' "determinants") and a second axis, dorsal-ventral (or, more descriptively, oral-aboral), that is specified sometime during the first few cleavage divisions
(reviewed by Davidson, 1989).
In some species the first and second cleavage divisions are equal, and the blastomeres of the four-celled embryo are identical in size.
The first and second cleavage divisions normally correlation to the frontal plane and the plane of bilateral symmetry, generating left- and right-ventral and left- and right-dorsal quadrants.