At the sixth division, each of the posterior 16 cells divides symmetrically to produce 32 similarly shaped cells. As a result, the 16 gonidial initials have about 30 times the volume of the somatic cells. By using a temperature-sensitive mutation that can interrupt cell division and cause large cells to arise randomly throughout the embryo, they found that any cell larger than 9 m in diameter at the end of cleavage became a gonidium, whereas any cell below 8 m became a biflagellated somatic cell. Rather, any cell that becomes large can become a gonidium. The quantity , rather than the quality , of the cytoplasm was the important factor.
Early development is normal, and the young adults resemble wild-type embryos of the same age.
They produce progeny that repeat this pattern. It appears that the wild-type regA locus is normally expressed in the somatic cells and prevents their differentiating into gonidia. Whereas regA probably functions in somatic cells, the wild-type lag gene probably works in the gonidial initals to suppress the soamtic differentiation pathway. Choi and colleagues interpret this result as showing as reflecting the fact that gonidia grow faster and need more energy than the somatic cells. The lag proteins might activate these genes to enablethese cells to grow more rapidly.