Q.44 Which substance determines the type of cleavage during development?
- Nucleus
- Cytoplasm
- Yolk
- Both 1 and 2
Yolk determines the type of cleavage during embryonic development.
This question from developmental biology assesses factors influencing early cleavage patterns in zygotes across species.
Option Analysis
Nucleus: Incorrect. The nucleus provides genetic instructions but does not dictate cleavage mechanics; it directs later differentiation, not cleavage type like holoblastic or meroblastic.
Cytoplasm: Partially relevant but incorrect alone. Cytoplasmic determinants influence cell fate, but cleavage type depends on yolk distribution within the cytoplasm, not cytoplasm itself.
Yolk: Correct. Yolk amount and location control cleavage: abundant yolk (e.g., birds) causes incomplete meroblastic cleavage; sparse yolk (e.g., mammals) allows complete holoblastic cleavage.
Both 1 and 2: Incorrect. Nucleus and cytoplasm do not primarily determine cleavage type; yolk is the key regulator, as seen in comparative embryology.
Substance determines type of cleavage yolk plays a pivotal role in early embryogenesis, shaping how the zygote divides into blastomeres. Yolk distribution dictates cleavage patterns across species, from complete holoblastic in low-yolk eggs to partial meroblastic in high-yolk ones.
Substance Determines Type of Cleavage Yolk Mechanism
Substance determines type of cleavage yolk by influencing physical constraints: heavy yolk slows cleavage furrows in vegetal regions, leading to unequal divisions. In frog eggs (mesolecithal), moderate yolk yields holoblastic but unequal cleavage; bird eggs (polylecithal) restrict it to the blastodisc.
Cytoplasm flows around yolk, but yolk platelets hinder cytokinesis, defining types like radial, spiral, or discoidal.
Cleavage Types by Yolk Content
Yolk Amount Cleavage Type Examples Microlecithal (little yolk) Holoblastic, equal Sea urchin, mammal Mesolecithal (moderate yolk) Holoblastic, unequal Frog, lizard Macrolecithal/Polylecithal (much yolk) Meroblastic, discoidal Bird, reptile Alecithal (no yolk) Holoblastic Some invertebrates Developmental Importance
Understanding substance determines type of cleavage yolk is essential for embryology exams, explaining evolutionary adaptations in egg structure and gastrulation onset.
