31. Which one of the following about development of sea urchin embryos is TRUE?
(1) Each blastomere of a 4-cell stage possesses a portion of the original animal-vegetal axis and

if isolated and allowed to develop will form a complete but smaller size larva.
(2) Each blastomere of a 8-cell stage has the capacity to form a complete embryo but by the 16-cell stage, blastomeres will develop according to their presumptive fate.
(3) Any blastomere isolated till the pluteus larva formation will regulate to go on and develop into a full sized embryo.
(4) After an intricate recombination at the 16 cell stage, the resulting embryo looses its ability to form a complete larva.

The correct statement about sea urchin embryo development is:

(1) Each blastomere of a 4-cell stage possesses a portion of the original animal-vegetal axis and if isolated and allowed to develop will form a complete but smaller size larva.

Introduction

Sea urchin embryos have long served as a model for studying fundamental developmental biology because they exhibit regulative development and cellular potency. One key feature of their early embryos is how the blastomeres retain a portion of the embryonic axes and the capacity to develop into a complete larva when isolated.

Early Cleavage and Blastomere Potential

• At the 4-cell stage, each blastomere contains a portion of the animal-vegetal axis, meaning positional information and developmental potential are distributed but not overly restricted.

• When isolated, these blastomeres can regulate and compensate, developing into a smaller but structurally complete larva.

• This capacity reflects regulative development where cells adapt their fates based on interactions and positional cues.

Developmental Restriction by Later Stages

• By the 8-cell and especially 16-cell stages, blastomeres become more restricted in their fate.

• At the 16-cell stage, blastomeres begin to show more committed or presumptive fate, reducing their ability to regulate fully if isolated or rearranged.

• Intricate recombinations at late cleavage stages generally compromise complete larval formation due to increased specification.

Importance in Developmental Biology

• Studying sea urchin cleavage demonstrates how early embryos balance potency with fate commitment.

• It illustrates mechanisms behind asymmetry, gastrulation, and the formation of the animal-vegetal axis.

• Understanding these processes informs insights into cell differentiation, embryonic patterning, and regenerative capabilities.

Conclusion

Sea urchin embryos exemplify how early blastomeres retain developmental plasticity, enabling formation of complete larvae from isolated cells at early stages. This highlights the dynamic interplay of potency, cell position, and regulation in embryonic development.

Final Answer:
(1) Each blastomere of a 4-cell stage possesses a portion of the original animal-vegetal axis and if isolated and allowed to develop will form a complete but smaller size larva.