Question

In humans, identical twins are possible because

(A) extraembryonic cells interact with the zygote nucleus.

(B) convergent extension occurs.

(C) early blastomeres can form a complete embryo if isolated.

(D) the gray crescent divides the dorsal-ventral axis into new cells.

Short answer

(A) The statement“extraembryonic cells interact with the zygote nucleus” is false.

(B) The statement “convergent extension occurs” is false.

(C) The statement “early blastomeres can form a complete embryo if isolated” is true.

(D) The statement “the gray crescent divides the dorsal-ventral axis into new cells” is false.

Step-by-step solution

Human beings

Human beings are primate mammals that exhibit bipedal locomotion due to their straight and upright skeleton. These are the only mammals with the ability to communicate with words and socialize. The intellectual efficiency of humans allows them to study other organisms and the environment.

Explanation of option ‘(A)’

The extraembryonic cells are developed from the cells of the blastocyst’s trophoblast layer soon after the completion of embryonic implantation. The function of these cells is to support and facilitate the development of the embryo, and it does not interact with the nucleus of other cells.

Thus, extraembryonic cells facilitate the development of the embryo.

Therefore, the given statement is false.

Explanation of option ‘(B)’

The process of convergent extension is a part of organogenesis (specifically, the cytoskeleton development). In this process, the cells converge their width and extend their length, causing an overall extension.

Thus, the convergent extension causes a morphological change in the developing embryo.

Therefore, the given statement is false.

Explanation of option ‘(C)’

The blastomeres are cells developed from the cleavage of the fertilized egg cell and have a totipotent nature. This means that each blastomere can develop into an individual organism if isolated and supplied with all the essential factors; these organisms will be twins.

Thus, the totipotent nature of early blastomeres can be the reason behind the development of identical twins in humans.

Therefore, the given statement is true.

Explanation of option ‘(D)’

In embryonic development, the gray crescent is the region opposite to the site from where the male gamete (sperm) entered the egg cell. This site directs the development and marks the embryonic mass's dorsal/ventral and anterior/posterior regions.

Thus, the gray crescent has no role in the development of twins, and it only suggests the embryonic developmental pattern.

Therefore, the given statement is false.