5: Cleavage: Becoming Multicellular and Development Consequences

first few cellular divisions of a zygote • zygote splits along a longitudinal plane • also longitudinal, but at _ to the plane of the first • _ to the first two and is equatorial in position

the rapid, multiple rounds of cell division are termed

after the cleavage has produced over 100 cells, the embryo is called a

the blastula is usually a spherical layer of cells (_ ) surrounding a fluid-filled or yolk-filled cavity (the _ )

mammals at this stage form a structure called the _ , characterized by an inner cell mass that is distinct from the surrounding blastula

each cell within the blastula is called a

development proceeds at the rate of roughly one cleavage division per day for the first _

when the embryo consists of approximately 16 cells, it is called a

starting after the eight-cell stage, the embryos of placental mammals enter into a phase called

during which the individual outer blastomeres tightly adhere through gap and tight junctions and lose their individual identity when viewed from the surface

compaction mediated by concentration of calcium (Ca++)-activated cell adhesion molecules (such as _ ), in a ring around the apical surface of the blastomeres

through the activity of a sodium (Na+ ), potassium (K+ )–adenosine triphosphatase (ATPase)–based Na+ transport system, Na+ and water (H2O) move across the epitheliumlike outer blastomeres and accumulate in spaces among the inner blastomeres

cavitation occurs about

at the blastocyst stage, the embryo, which is still surrounded by the zona pellucida, consists of two types of cells:

the end of the blastocyst that contains the inner cell mass is known as the _ , and the opposite end is called the _

cells of the inner cell mass give rise to the body of the _ itself in addition to several _ , whereas cells of the trophoblast form only extraembryonic structures, including the _

there is increasing evidence that _ (a growth factor secreted by cells of the inner cell mass), acts to maintain mitotic activity in the overlying trophoblast

The first cleavage of frog’s egg was observed by

The entire process of cleavage in frog’s egg was studied by

planes of cleavage are named depending on the position of the

the plane of the cleavage lies on the animal vegetal axis; it bisects both the poles of the egg; the egg is divided into two equal halves

the cleavage furrows may lie on either side of the meridional plane; the furrows pass from animal to vegetal pole; the cleaved cells may be unequal in size

this cleavage plane bisects the egg at right angles to the main axis; lies on the equatorial plane; divides the egg into two halves

similar to the equatorial plane, but it lies on either side of the equator; also called as transverse or horizontal cleavage