Question

Which distinguishes a morula from a blastula
A) Presence of more yolk
B) Absence of yolk
C) Presence of a cavity
D) Absence of a cavity

Answer 1

Hint:- Embryogenesis, commonly referred to as embryonic development, is the process of development of an embryo from a zygote. Cell division (mitosis) takes place in the embryo during its early development. Zygote is formed after the fertilisation of the female gamete by the male gametes. DNA of both the gametes combine to form a diploid zygote. This marks the commencement of the germinal stage of embryogenesis. The time from fertilisation through the development of an early embryo until its implantation in the uterus is the germinal stage.

Complete solution:
The first stage of embryonic development is fertilisation. The formation of zygote takes place in this stage.The zygote then undergoes cleavage (division of cells) and the zygote becomes multicellular.
The first division occurs after about 24 hours of fertilisation. The zygote divides to form the morula which consists of a compact mass of 16 cells. The cells are clumped together and there is no cavity present in the morula. The morula contains a bit of yolk.
Cell division or mitosis continues and the morula forms a 64 celled structure called the blastula. In the blastula stage, the embryo is a hollow ball of cells and a cavity is present. This cavity is known as the blastocoel. The blastula contains more yolk than morula. The inner and outer layers of the blastula are distinct. The inner cells are called the inner cell mass (ICM). This ICM forms the pre-embryo containing yolk sac, amnion, and the allantois. The outer layer is called the trophoblast and helps in the implantation of the embryo to the uterus.
Both the morula and the blastula contain yolk therefore the correct answer is D) Absence of cavity.

Note:- After the blastula stage, implantation of the embryo onto the uterine wall occurs. This marks the end of pre-embryonic development. The blastula implanted is now known as the blastocyst. Implantation is the onset of pregnancy. The embryo is nurtured in the uterus and the gastrula is formed. In this stage, formation of the gut occurs and the embryo develops three germ layers called the ectoderm, mesoderm, and endoderm. Finally, the last stage is known as the neurula. In this stage, the ectoderm gives rise to neural tissues.