Question

Examine the frog models 2-5 which represent the 2-cell, 4-cell,.. morula stages. During cleavage, the large zygote (fertilized egg), a single cell, divides (by mitosis) into two cells (2-cell stage); these two cells divide again yielding four (4-cell stage); these four cells divide again yielding eight (8-cell stage); and so on. After dividing a number of times, the embryo is a solid ball of cells, the morula. Just like in the starfish, a cavity (the blastocoel) then becomes visible in the center of the embryo (which is now called a blastula). The mass of the original zygote is divided into many smaller cells during cleavage, but little rearrangement of cells occurs. Where does the first cleavage furrow form in relation to the sperm entry point and the gray crescent? In the frog, do the cleavage divisions always result in equally At the 8-cell stage, which pole (animal or vegetal) has the larger cells? sized cells? Does this cell size difference between the animal and vegetal poles persist when the embryo is in the morula stage? the blastula stage? i Differences in early cleavage stages of animals often reflect the amount of yolk that is stored in the egg cell. The starfish egg has very little yolk compared to the frog egg. Examine the frog models 6-7, showing the blastula stage. Following the early cleavage stages, a cavity (blastocoel) forms within the embryo and makes it resemble a hollow ball. This "hollow ball" stage is called the blastula, and all animals go through a blastula stage (in one form or another). As with cell division in the early cleavage stages, the amount of yolk in the egg affects the shape of the blastocoel. The starfish has a centrally located, spherical blastocoel. The frog (with more yolk in its vegetal pole) has a hemispherical blastocoel located in its animal hemisphere. Animal pole

Answer 1

Ans a) -the first cleavage furrow  runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the gray crescent.

Ans b) No .

In frog not all cleavage division always result in equally sized cells. Only ist and second cleavage is complete and result in equal sized cells while third cleavage is incomplete and result in unequal sized cells.

Ans C) At 8 celled stage vegetal pole has larger sized cells(macromeres).

Ans d) yes, the cell size difference persist between vegetal and animal pole when the embryo in morula stage and in blastula stage.