Distribution of germ plasm during cleavage of (A) normal and (B) centrifuged zygotes of Parascaris. (A) The germ plasm is normally conserved in the most vegetal blastomere, as shown by the lack of chromosomal diminution in that particular cell. Thus, at the 4-cell stage, the embryo has one stem cell for its gametes. (B) When the first cleavage is displaced 90 degrees by centrifugation, both resulting cells have vegetal germ plasm, and neither cell undergoes chromosome diminution. After the second cleavage, these two cells give rise to germinal stem cells. (After Waddington 1966.)