Question

Name the general process by which blastulas are formed (hint: it is a series of cell divisions). Then describe whether this process is holoblastic or meroblastic in a sea urchin, frog, and chicken.

Answer 1

Cleavage is the process by which blastulas are formed. After the formation of a zygote when an ovum is fertilized by a cell of sperm, the zygote undergoes division called mitotic. This division of zygote does not make any compelling effect on growth. However, this forms a multicellular cluster which is nothing but a blastula.  

  

Figure 1: Showing a series of cell divisions

Cleavage in sea urchin is holoblastic which shows the complete segmentation of yolk. This cleavage divides the yolk into two equal embryo cells. The line of segmentation or division is perpendicular and meridional to each other. The yolk of an egg is inadequate amount.

  

Figure 2: Cleavage in sea urchin

Cleavage in the frog is the holoblastic and partial meroblastic. Segmentation increases the cell division but not increases the cell mass. Frog cell contains no yolk or sparse amount of yolk. The line of segmentation is passing all through the zygote. This type of holoblastic cleavage seen in humans also. In this holoblastic cleavage nourishment given to offspring through the placenta.

  

Figure 3: Cleavage in Frog

Cleavage in chicken is meroblastic and discoidal because the line of division doesn't cross completely through egg so it the part of the egg. The bird has a megalicicithal egg which has a huge amount of yolk. Birds have discoidal egg mean the formation of a disc of the cell at the animal pole of the zygote. The bird egg has megalicicithal quality which has large yolk so it resists the segmentation or cleavage.

Figure 4: Cleavage in Chicken