Question

The correct sequence of embryonic development is
A) Blastula-morula-zygote-gastrula-embryo
B) Zygote-blastula-morula-gastrula-embryo
C) Zygote-morula-blastula-gastrula-embryo
D) Gastrula-morula-zygote-blastula-embryo

Answer 1

Hint:Embryo is usually defined as the starting of the next generation. It is formed as a result of fusion between male and female gametes. After fertilization implantation occurs in the female uterus the structure is called an embryo. Embryo obtains nutrition from a structure known as placenta.

Complete answer:
During embryo development of embryos many events occur. One such event is cleavage. First cleavage is completed after 30 hours of fertilization. Cleavage furrow passes from animal- vegetal axis as well as centre of zygote (Meridional plane). It divides the zygote completely into two blastomeres (Holoblastic cleavage). Second cleavage is completed after 60 hours of fertilization. It is also meridional but at the right angle to the first one. Next stage is moral.

A) Morula :- Cleavage usually gives rise to a solid ball of cells, Morula having 8-16 cells. Zona pellucida still forms the outer cover. Morula undergoes compaction. The outer/peripheral cells are smaller with tight junctions while the inner cell mass consists of slightly large, rounded cells with gap junctions
B) Blastula :- With the formation of blastocoel, morula is converted into blastula which is called blastocyst in mammals because of the different nature of surface layer and eccentric inner cell mass. It is 128 cell stages.
C) Gastrula :- The gastrulation is movement of cells in small masses or sheets so as to form primary germinal layers. There are three primary germinal layers - endoderm, ectoderm and mesoderm. The cell movements that occur during gastrulation are called morphogenetic movements since they lead to initiation of morphogenesis; the product of gastrulation is called gastrula.
D) After gastrulation, embryos are formed.

Hence the correct answer is ‘C’ i.e, Zygote-morula-blastula-gastrula-embryo.

Note:All cleavage divisions are mitotic and resultant daughter cells are blastomeres. During cleavage, there is no growth in the resulting blastomeres and the total size and volume of the embryo remains the same, because in cleavage divisions interphase is without growth phase.