Question

Which of the following statements are true about protostomes?
(A) Cleavage spindles or planes are oblique to the polar axis of the zygote.
(B) Cleavage spindles or planes are at right angles to the polar axis of the zygote.
(C) The fate of blastomeres is fixed early in the development.
(D) The fate of blastomeres is fixed relatively late in the development.

• A. (B) and (D)
• B. (A) and (C)
• C. (C) and (D)
• D. (A) and (B)

Answer 1

Answer: (B)

(A) and (C)

Protostomes are a groups of invertebrate animals with a true body cavity or coelom. These include the mollusks, annelids, and arthropods. The protostomes are distinguished from deuterostomes on the basis of their embryonic cleavage pattern (division of the zygote to become a multicellular animal). The developing zygote undergoes spiral cleavage in which the cells divide at a 45° angle to one another due to a realignment of the mitotic spindle. This realignment causes each cell to divide unequally, resulting in a spiral displacement of small cells, the micromeres. The blastomeres develop into the mouth quite early in the development stage.