Question

The fourth cleavage plane during the development of a frog’s egg is:
(a) Double meridional
(b) Single meridional
(c) Single latitudinal
(c) Double latitudinal

Answer 1

Hint: The amount of yolk present significantly affects the type of cleavage i.s. holoblastic or meroblastic. Frog eggs are mesolecithal i.e. have a yolk of the intermediate size which is concentrated in one of the hemispheres.

Complete answer:
Some important enzymes present in the sperm initiate the process of cleavage. Repeated mitotic divisions that occur in a single cell are called cleavage and they result in a multicellular structure, also called a morula. The first cleavage in a frog’s egg is meridional i.e. from animal to vegetal pole resulting in a two-celled stage. These cells are also called blastomeres. The second cleavage is also meridional and results in four blastomeres. These all are of the same size. The third cleavage is latitudinal i.e. in the equatorial direction. Out of the eight blastomeres formed, four are large in size and remain near the vegetal pole. These are called macromeres. The rest of the four remain near the animal pole and are smaller in size. They are called micromeres. The fourth cleavage is meridional and unequal holoblastic. The micromeres which do not contain yolk now divide faster than macromeres which contain yolk.
So, the correct answer is ‘Double meridional’.

Note: In microlecithal and isolecithal types of eggs holoblastic equal type of cleavage occurs which results in two equal-sized cells. This type of cleavage occurs in all placental mammals and in amphioxus. In a holoblastic type of furrow will bisect the entire egg whereas, in meroblastic type, the furrow is incomplete and bisects the cell to the point up to which active cytoplasm is found.