25. Following statements are made regarding animal development:
    A. The cell is first specified towards a given fate, suggesting that it would develop into this cell type, even in a neutral environment.
    B. Holoblastic rotational cleavage is observed in tunicates.
    C. Infolding of sheet of cells is called ingression.
    D. Conditional specification can be observed in sea urchin embryos.
    Which one of the following options represents the combination of all correct statements?
    (1) A and B (2) B and C
    (3) A and D (4) C and D

The evaluation of the statements is as follows:

A. “The cell is first specified towards a given fate, suggesting that it would develop into this cell type, even in a neutral environment.”

• This describes autonomous specification, where cells have predetermined fates independent of external signals. This is a correct statement.

B. “Holoblastic rotational cleavage is observed in tunicates.”

• Tunicates typically exhibit holoblastic bilateral cleavage, not rotational. So, this statement is incorrect.

C. “Infolding of sheet of cells is called ingression.”

• Infolding of a sheet of cells is actually invagination; ingression refers to individual cells leaving a sheet to migrate as mesenchymal cells. So, C is incorrect.

D. “Conditional specification can be observed in sea urchin embryos.”

• Sea urchins typically display conditional specification, where cell fate depends on interactions with neighbors and signaling. This is correct.

Hence, the correct combination of statements is:
(3) A and D

Introduction

Animal development involves tightly regulated processes guiding cells towards specific fates, shapes, and functions. Two prominent mechanisms of cell fate specification are autonomous (intrinsic) and conditional (extrinsic) specification. Alongside, cleavage patterns provide insights into developmental strategies used across species.

Autonomous vs. Conditional Specification

• Autonomous specification means a cell’s fate is predetermined by inherited factors and unfolds even when isolated from other cells.

• Conditional specification relies on external cues and cell-cell interactions, guiding cells to adopt fates based on their environment.

• Sea urchins are a classic example of conditional specification, whereas some invertebrates show autonomous specification.

Cleavage Patterns

• Tunicates show holoblastic bilateral cleavage, characterized by cleavage planes producing bilateral symmetry, differing from holoblastic rotational cleavage observed in mammals.

• This difference highlights diversity in early embryogenesis within chordates and other groups.

Cell Movement: Invagination vs. Ingression

• Invagination is the inward folding of a cell sheet forming structures such as the blastopore or archenteron during gastrulation.

• Ingression involves individual cells detaching from an epithelial sheet to migrate as independent mesenchymal cells.

• Mislabeling invagination as ingression is incorrect and can confuse developmental processes.

Conclusion

Correct understanding of cell fate specification and cleavage patterns aids comprehension of developmental biology. Among the given options, autonomous fate specification (A) and conditional specification in sea urchins (D) represent accurate statements, making option (3) the correct choice.

Final Answer:
(3) A and D