7. The pattern of embryonic cleavage specific to a species is determined by two major parameters.
   (A) The amount and distribution of yolk protein within the cytoplasm.
   (B) The factors in the cytoplasm that influence the angle mitotic spindles and the timings of its
   formation.
   Which of the following statements are true?
   (1) Species having teloecithal egg follow a holoblastic cleavage.
   (2) Species having isolecithal egg follow a holoblastic cleavage.
   (3) Species having centrolecithal egg follow a holoblastic cleavage.
   (4) Species having isolecithal egg follow a meroblastic cleavage.

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Introduction

Embryonic cleavage patterns are species-specific and highly influenced by intrinsic factors within the egg. Two key determinants guide how the early embryo divides: the amount and distribution of yolk within the cytoplasm, and the cytoplasmic components that regulate the orientation and timing of mitotic spindle formation.

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The Role of Yolk Content and Distribution

• Yolk is the nutritional resource stored in the egg for embryo development.

• Its amount and localization affect the mechanical feasibility and pattern of cleavage.

• Eggs with isolecithal yolk have small, evenly distributed yolk, allowing complete (holoblastic) cleavage.

• Teloecithal eggs contain dense yolk concentrated at one end (vegetal pole), leading to partial (meroblastic) cleavage where cell divisions occur only in yolk-free regions.

• Centrolecithal eggs have centrally-located yolk, undergoing superficial cleavage.

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Influence of Cytoplasmic Factors

• The mitotic spindle apparatus dictates the angle and timing of cleavage furrow formation.

• Cytoplasmic factors orchestrate spindle orientation, ensuring stereotyped cleavage patterns in different species.

• Proper spindle regulation leads to characteristic cleavage patterns like radial, spiral, bilateral, or rotational cleavage.

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Correct Matching of Egg Types and Cleavage Patterns

• Isolecithal eggs → Holoblastic cleavage: Observed in mammals, echinoderms.

• Teloecithal eggs → Meroblastic cleavage: Seen in birds, reptiles.

• Centrolecithal eggs → Superficial cleavage: Typical in many arthropods.

Therefore, the only correct option among the given is:

• (2) Species having isolecithal egg follow a holoblastic cleavage.

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Why Other Options Are Incorrect

• (1) Species having teloecithal egg follow a holoblastic cleavage: Incorrect because dense yolk prevents complete cleavage.

• (3) Species having centrolecithal egg follow a holoblastic cleavage: They undergo superficial cleavage, not holoblastic.

• (4) Species having isolecithal egg follow a meroblastic cleavage: Isolecithal eggs undergo holoblastic cleavage, not meroblastic.

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Conclusion

Embryonic cleavage patterns hinge critically on yolk distribution and cytoplasmic spindle dynamics. Isolecithal eggs characterized by sparse yolk undergo complete holoblastic cleavage, a fundamental pattern supporting efficient embryo formation across many species.

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This article illuminates the biological principles dictating cleavage patterns, foundational for developmental biology.

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Final Answer:
(2) Species having isolecithal egg follow a holoblastic cleavage