Question

13. As cleavage of the fertilized egg progresses through the two-cell, four-cell, 8-cell, and later stages of development, what happens to the size of the cells in relation to the size of the fertilized egg? (NOTE: This can be seen in the images of the sea star embryology series on your handout.)



14. Regarding the previous question, what specifically is the cause of this size difference? In other words, what difference in the cell cycle of the zygote-forming cells (blastomeres, etc.) is occurring as the number of cells is increasing? This may require a little research.

Answer 1

13. The size of the cells decreases as the cleavage progresses compared to the zygote.

During cleavage, the cells divide without an increase in mass, that is, one large single - celled zygote divides into multiple smaller cells . The cytoplasm to volume ratio increases as zygote cleaves and produces smaller cells.

14. In these cells, the cell cycle is restricted to only S->M>S phases and growth phases ( G1 and G2 ) are absent. That is only division is occurring without growth so that you get more cells and dividing up within a ok limited amount of cytoplasm so they get smaller.

One reason is to generate a lot of cells quickly. Need of multiple cells inorder to have multiple cell types, which enable cell signalling and development of specialized tissues.